diff --git a/packages/coding-agent/examples/capability-capnweb-spike/README.md b/packages/coding-agent/examples/capability-capnweb-spike/README.md
new file mode 100644
index 000000000..55347355f
--- /dev/null
+++ b/packages/coding-agent/examples/capability-capnweb-spike/README.md
@@ -0,0 +1,102 @@
+# Cap'n Web client binding spike
+
+A separate version of the client-capability spike using [Cap'n Web](https://github.com/cloudflare/capnweb) for symmetric, object-capability RPC.
+
+It demonstrates a server extension invoking a typed client binding without knowing whether the originating client is a TUI or browser. The TUI renders notifications in yellow. The browser displays them centrally and dramatically spins them away.
+
+## Architecture
+
+```text
+browser/TUI exports ClientBindingApi       server exports ServerApi
+┌──────────────────────────────┐          ┌───────────────────────────┐
+│ event(event)                 │◀─────────│ prompt(text)              │
+│ notify({ message })          │─────────▶│ command(name, input)      │
+└──────────────────────────────┘ Cap'n Web└───────────────────────────┘
+             ▲                                  │
+             │                                  ▼
+             └──── originating-client stub ─ notify extension
+                                                ├─ notify command
+                                                ├─ notify LLM tool
+                                                └─ exact-Paris hook
+```
+
+Every WebSocket connection is a symmetric Cap'n Web session:
+
+- The server exports a connection-scoped `ServerApi` as its main RPC interface.
+- The client exports `ClientBindingApi` as its main RPC interface.
+- The client calls `connect()` with binding IDs and versions.
+- The server retains that connection's remote client stub.
+- A prompt arrives through the connection-scoped server object, so the originating client is known without trusting a caller-supplied client ID on every call.
+- Commands, tools, and hooks call `context.clientBindings.notify()`. Cap'n Web handles reverse invocation, request correlation, promise resolution, and remote errors.
+
+Pi still owns semantic concerns that Cap'n Web does not provide: binding names and versions, originating-client selection, runtime validation, timeouts, and extension-to-tool exposure. TypeBox validates RPC values because Cap'n Web's TypeScript interfaces do not perform runtime validation.
+
+Unlike the HTTP/SSE spike, prompts, events, commands, and client binding calls all use one WebSocket. Plain HTTP only serves the static browser files.
+
+This remains a local spike. Authentication, persistent sessions, reconnect replay, controller leases, and production authorization are intentionally omitted. Cap'n Web is also currently described by its maintainers as experimental.
+
+## Install
+
+The experimental dependencies are isolated from the coding-agent package:
+
+```bash
+npm install --prefix packages/coding-agent/examples/capability-capnweb-spike --ignore-scripts
+```
+
+## Run
+
+From the repository root:
+
+```bash
+OPENAI_API_KEY=... npm run server --prefix packages/coding-agent/examples/capability-capnweb-spike
+```
+
+The default model is `openai/gpt-5.4-nano`. Select another registered OpenAI or Anthropic model with:
+
+```bash
+PI_CAPNWEB_SPIKE_MODEL=anthropic/claude-haiku-4-5 ANTHROPIC_API_KEY=... \
+  npm run server --prefix packages/coding-agent/examples/capability-capnweb-spike
+```
+
+The server starts without provider credentials, which is enough to exercise direct binding calls. LLM prompts require the selected provider's environment credential.
+
+Open [http://127.0.0.1:8788](http://127.0.0.1:8788) for the browser client.
+
+In another terminal, run the TUI:
+
+```bash
+npm run tui --prefix packages/coding-agent/examples/capability-capnweb-spike
+```
+
+Try the direct binding path:
+
+```text
+/notify TUI binding confirmed.
+```
+
+Or prompt the model:
+
+```text
+Notify me that Cap'n Web works, then briefly explain what happened.
+```
+
+When multiple clients are connected, notifications target only the client that submitted the prompt.
+
+For a deterministic extension-initiated call without an LLM tool call, ask:
+
+```text
+What is the capital of France?
+```
+
+The system prompt makes the assistant answer exactly `Paris`. The extension observes that completed response and calls the originating client's `notify()` binding with `❤️`.
+
+## Files
+
+- `protocol.ts`: shared TypeScript RPC interfaces, binding metadata, and TypeBox schemas
+- `binding-registry.ts`: connected-client routing, negotiation, validation, timeout, and stub invocation
+- `extension.ts`: binding-facing command, LLM tool, and exact-`Paris` response hook
+- `server.ts`: `AgentHarness`, HTTP static server, and connection-scoped Cap'n Web APIs
+- `ws-transport.ts`: typed adapter between Node's `ws` server and `RpcSession`
+- `tui.ts`: Cap'n Web TUI client
+- `web/`: static Cap'n Web browser client
+- `package.json`: isolated, exact-version Cap'n Web and WebSocket dependencies
diff --git a/packages/coding-agent/examples/capability-capnweb-spike/binding-registry.ts b/packages/coding-agent/examples/capability-capnweb-spike/binding-registry.ts
new file mode 100644
index 000000000..904bee2dd
--- /dev/null
+++ b/packages/coding-agent/examples/capability-capnweb-spike/binding-registry.ts
@@ -0,0 +1,137 @@
+import type { RpcStub } from "capnweb";
+import { Value } from "typebox/value";
+import {
+	type ClientBindingApi,
+	type ClientRegistration,
+	type NotifyInput,
+	type NotifyOutput,
+	notifyBinding,
+	type ServerEvent,
+} from "./protocol.ts";
+
+export class BindingError extends Error {
+	readonly code:
+		| "client_unavailable"
+		| "binding_unavailable"
+		| "invalid_input"
+		| "invalid_output"
+		| "aborted"
+		| "timeout";
+
+	constructor(
+		code: "client_unavailable" | "binding_unavailable" | "invalid_input" | "invalid_output" | "aborted" | "timeout",
+		message: string,
+	) {
+		super(message);
+		this.name = "BindingError";
+		this.code = code;
+	}
+}
+
+export interface ConnectedClient {
+	readonly id: string;
+	readonly bindings: ReadonlyMap<string, number>;
+	readonly rpc: RpcStub<ClientBindingApi>;
+	connected: boolean;
+}
+
+export class BindingRegistry {
+	readonly #clientsById = new Map<string, ConnectedClient>();
+
+	register(registration: ClientRegistration, rpc: RpcStub<ClientBindingApi>): ConnectedClient {
+		if (this.#clientsById.has(registration.clientId)) {
+			throw new BindingError("client_unavailable", `Client ${registration.clientId} is already connected`);
+		}
+		const client: ConnectedClient = {
+			id: registration.clientId,
+			bindings: new Map(registration.bindings.map((binding) => [binding.id, binding.version])),
+			rpc,
+			connected: true,
+		};
+		this.#clientsById.set(client.id, client);
+		return client;
+	}
+
+	unregister(client: ConnectedClient): void {
+		if (this.#clientsById.get(client.id) !== client) return;
+		client.connected = false;
+		this.#clientsById.delete(client.id);
+	}
+
+	async send(client: ConnectedClient, event: ServerEvent): Promise<void> {
+		this.#requireConnected(client);
+		const call = client.rpc.event(event);
+		try {
+			await call;
+		} finally {
+			call[Symbol.dispose]();
+		}
+	}
+
+	async broadcast(event: ServerEvent): Promise<void> {
+		await Promise.all(
+			[...this.#clientsById.values()].map(async (client) => {
+				try {
+					await this.send(client, event);
+				} catch {
+					this.unregister(client);
+				}
+			}),
+		);
+	}
+
+	async notify(
+		client: ConnectedClient,
+		input: unknown,
+		options: { signal?: AbortSignal; timeoutMs?: number } = {},
+	): Promise<NotifyOutput> {
+		this.#requireConnected(client);
+		if (client.bindings.get(notifyBinding.id) !== notifyBinding.version) {
+			throw new BindingError(
+				"binding_unavailable",
+				`Client ${client.id} does not provide ${notifyBinding.id}@${notifyBinding.version}`,
+			);
+		}
+		if (!Value.Check(notifyBinding.input, input)) {
+			throw new BindingError("invalid_input", `Input does not match ${notifyBinding.id}@${notifyBinding.version}`);
+		}
+		if (options.signal?.aborted) {
+			throw new BindingError("aborted", `${notifyBinding.id} was aborted`);
+		}
+
+		const rpcCall = client.rpc.notify(input as NotifyInput);
+		const timeoutMs = options.timeoutMs ?? 10_000;
+		let rejectCancellation: ((error: Error) => void) | undefined;
+		const cancellation = new Promise<never>((_resolve, reject) => {
+			rejectCancellation = reject;
+		});
+		const timer = setTimeout(() => {
+			rejectCancellation?.(new BindingError("timeout", `${notifyBinding.id} timed out after ${timeoutMs}ms`));
+		}, timeoutMs);
+		const onAbort = () => {
+			rejectCancellation?.(new BindingError("aborted", `${notifyBinding.id} was aborted`));
+		};
+		options.signal?.addEventListener("abort", onAbort, { once: true });
+
+		try {
+			const output = await Promise.race([rpcCall, cancellation]);
+			if (!Value.Check(notifyBinding.output, output)) {
+				throw new BindingError(
+					"invalid_output",
+					`Output does not match ${notifyBinding.id}@${notifyBinding.version}`,
+				);
+			}
+			return { displayed: true };
+		} finally {
+			clearTimeout(timer);
+			options.signal?.removeEventListener("abort", onAbort);
+			rpcCall[Symbol.dispose]();
+		}
+	}
+
+	#requireConnected(client: ConnectedClient): void {
+		if (!client.connected || this.#clientsById.get(client.id) !== client) {
+			throw new BindingError("client_unavailable", `Client ${client.id} is not connected`);
+		}
+	}
+}
diff --git a/packages/coding-agent/examples/capability-capnweb-spike/extension.ts b/packages/coding-agent/examples/capability-capnweb-spike/extension.ts
new file mode 100644
index 000000000..a3dc1050b
--- /dev/null
+++ b/packages/coding-agent/examples/capability-capnweb-spike/extension.ts
@@ -0,0 +1,77 @@
+import type { AgentHarnessEvent, AgentHarnessTool } from "@earendil-works/pi-agent-core";
+import { contentText } from "@earendil-works/pi-ai";
+import { Type } from "typebox";
+import type { NotifyInput, NotifyOutput } from "./protocol.ts";
+
+export interface ClientBindings {
+	notify(input: NotifyInput, options?: { signal?: AbortSignal; timeoutMs?: number }): Promise<NotifyOutput>;
+}
+
+export interface NotifyExtensionContext {
+	clientBindings: ClientBindings;
+}
+
+export type ServerCommand = (input: unknown, context: NotifyExtensionContext, signal?: AbortSignal) => Promise<unknown>;
+
+export type MessageEndEvent = Extract<AgentHarnessEvent, { type: "message_end" }>;
+
+export type MessageEndHandler = (
+	event: MessageEndEvent,
+	context: NotifyExtensionContext,
+	signal?: AbortSignal,
+) => Promise<void> | void;
+
+export interface ServerExtensionApi {
+	registerTool(tool: AgentHarnessTool<NotifyExtensionContext>): void;
+	registerCommand(name: string, command: ServerCommand): void;
+	on(type: "message_end", handler: MessageEndHandler): void;
+}
+
+async function notifyClient(
+	input: unknown,
+	context: NotifyExtensionContext,
+	signal?: AbortSignal,
+): Promise<NotifyOutput> {
+	if (
+		typeof input !== "object" ||
+		input === null ||
+		!("message" in input) ||
+		typeof input.message !== "string" ||
+		input.message.trim().length === 0
+	) {
+		throw new Error("notify requires a non-empty message");
+	}
+	return context.clientBindings.notify({ message: input.message.trim() }, { signal });
+}
+
+const notifyParameters = Type.Object(
+	{
+		message: Type.String({ description: "Notification text", minLength: 1, maxLength: 500 }),
+	},
+	{ additionalProperties: false },
+);
+
+export function notifyExtension(pi: ServerExtensionApi): void {
+	pi.registerCommand("notify", notifyClient);
+	pi.on("message_end", async (event, context, signal) => {
+		if (event.message.role !== "assistant") return;
+		if (contentText(event.message.content, "").trim() === "Paris") {
+			await notifyClient({ message: "❤️" }, context, signal);
+		}
+	});
+	pi.registerTool({
+		name: "notify",
+		label: "Notify client",
+		description: "Show a brief notification on the client that submitted the current prompt.",
+		parameters: notifyParameters,
+		async execute(_toolCallId, params, signal, _onUpdate, context) {
+			const input = typeof params === "object" && params !== null ? params : {};
+			const message = "message" in input && typeof input.message === "string" ? input.message : "";
+			await notifyClient({ message }, context, signal);
+			return {
+				content: [{ type: "text", text: `Notified the originating client: ${message}` }],
+				details: { displayed: true },
+			};
+		},
+	});
+}
diff --git a/packages/coding-agent/examples/capability-capnweb-spike/package-lock.json b/packages/coding-agent/examples/capability-capnweb-spike/package-lock.json
new file mode 100644
index 000000000..c76516a0e
--- /dev/null
+++ b/packages/coding-agent/examples/capability-capnweb-spike/package-lock.json
@@ -0,0 +1,595 @@
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+				"capnweb": "0.10.0",
+				"ws": "8.21.1"
+			},
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+}
diff --git a/packages/coding-agent/examples/capability-capnweb-spike/package.json b/packages/coding-agent/examples/capability-capnweb-spike/package.json
new file mode 100644
index 000000000..e89a48186
--- /dev/null
+++ b/packages/coding-agent/examples/capability-capnweb-spike/package.json
@@ -0,0 +1,18 @@
+{
+	"name": "pi-capability-capnweb-spike",
+	"private": true,
+	"type": "module",
+	"scripts": {
+		"server": "tsx server.ts",
+		"tui": "tsx tui.ts"
+	},
+	"dependencies": {
+		"capnweb": "0.10.0",
+		"ws": "8.21.1"
+	},
+	"devDependencies": {
+		"@types/node": "24.12.4",
+		"@types/ws": "8.18.1",
+		"tsx": "4.22.1"
+	}
+}
diff --git a/packages/coding-agent/examples/capability-capnweb-spike/protocol.ts b/packages/coding-agent/examples/capability-capnweb-spike/protocol.ts
new file mode 100644
index 000000000..a81f9c3b7
--- /dev/null
+++ b/packages/coding-agent/examples/capability-capnweb-spike/protocol.ts
@@ -0,0 +1,76 @@
+import { type Static, Type } from "typebox";
+
+export const protocolVersion = 1;
+
+export const notifyBinding = {
+	id: "pi.notify",
+	version: 1,
+	input: Type.Object(
+		{
+			message: Type.String({ minLength: 1, maxLength: 500 }),
+		},
+		{ additionalProperties: false },
+	),
+	output: Type.Object(
+		{
+			displayed: Type.Literal(true),
+		},
+		{ additionalProperties: false },
+	),
+} as const;
+
+export type NotifyInput = Static<typeof notifyBinding.input>;
+export type NotifyOutput = Static<typeof notifyBinding.output>;
+
+export interface BindingRegistration {
+	id: string;
+	version: number;
+}
+
+export interface ClientRegistration {
+	clientId: string;
+	protocolVersion: number;
+	bindings: BindingRegistration[];
+}
+
+export const clientRegistrationSchema = Type.Object(
+	{
+		clientId: Type.String({ minLength: 1, maxLength: 80, pattern: "^[a-zA-Z0-9_-]+$" }),
+		protocolVersion: Type.Literal(protocolVersion),
+		bindings: Type.Array(
+			Type.Object(
+				{
+					id: Type.String({ minLength: 1, maxLength: 100 }),
+					version: Type.Integer({ minimum: 1 }),
+				},
+				{ additionalProperties: false },
+			),
+		),
+	},
+	{ additionalProperties: false },
+);
+
+export type ServerEvent =
+	| { type: "ready"; clientId: string; bindings: BindingRegistration[] }
+	| { type: "user_prompt"; clientId: string; text: string }
+	| { type: "run_start" }
+	| { type: "run_end" }
+	| { type: "assistant_start" }
+	| { type: "assistant_delta"; text: string }
+	| { type: "assistant_end" }
+	| { type: "tool_start"; name: string }
+	| { type: "tool_end"; name: string; isError: boolean }
+	| { type: "error"; message: string };
+
+/** The main Cap'n Web interface exported by each connected client. */
+export interface ClientBindingApi {
+	event(event: ServerEvent): Promise<void>;
+	notify(input: NotifyInput): Promise<NotifyOutput>;
+}
+
+/** The main Cap'n Web interface exported by the server for one connection. */
+export interface ServerApi {
+	connect(registration: ClientRegistration): Promise<void>;
+	prompt(text: string): Promise<void>;
+	command(name: string, input: unknown): Promise<void>;
+}
diff --git a/packages/coding-agent/examples/capability-capnweb-spike/server.ts b/packages/coding-agent/examples/capability-capnweb-spike/server.ts
new file mode 100644
index 000000000..e35037eae
--- /dev/null
+++ b/packages/coding-agent/examples/capability-capnweb-spike/server.ts
@@ -0,0 +1,292 @@
+import { readFile } from "node:fs/promises";
+import { createServer, type ServerResponse } from "node:http";
+import { dirname, join } from "node:path";
+import { fileURLToPath } from "node:url";
+import {
+	AgentHarness,
+	type AgentHarnessEvent,
+	type AgentHarnessTool,
+	InMemorySessionStorage,
+	Session,
+} from "@earendil-works/pi-agent-core";
+import { createModels, type Model } from "@earendil-works/pi-ai";
+import { anthropicProvider } from "@earendil-works/pi-ai/providers/anthropic";
+import { openaiProvider } from "@earendil-works/pi-ai/providers/openai";
+import { RpcSession, type RpcStub, RpcTarget } from "capnweb";
+import { Value } from "typebox/value";
+import { WebSocketServer } from "ws";
+import { BindingRegistry, type ConnectedClient } from "./binding-registry.ts";
+import {
+	type MessageEndHandler,
+	type NotifyExtensionContext,
+	notifyExtension,
+	type ServerCommand,
+	type ServerExtensionApi,
+} from "./extension.ts";
+import {
+	type ClientBindingApi,
+	type ClientRegistration,
+	clientRegistrationSchema,
+	type ServerApi,
+	type ServerEvent,
+} from "./protocol.ts";
+import { NodeWebSocketTransport } from "./ws-transport.ts";
+
+const host = process.env.PI_CAPNWEB_SPIKE_HOST ?? "127.0.0.1";
+const port = Number(process.env.PI_CAPNWEB_SPIKE_PORT ?? "8788");
+const modelSpec = process.env.PI_CAPNWEB_SPIKE_MODEL ?? "openai/gpt-5.4-nano";
+const [providerId, modelId] = modelSpec.split("/", 2);
+if (!providerId || !modelId) {
+	throw new Error(`PI_CAPNWEB_SPIKE_MODEL must use provider/model syntax, received ${modelSpec}`);
+}
+
+const models = createModels();
+models.setProvider(openaiProvider());
+models.setProvider(anthropicProvider());
+
+function createFallbackModel(provider: string, id: string): Model<"openai-responses" | "anthropic-messages"> {
+	const common = {
+		id,
+		name: id,
+		reasoning: false,
+		cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+		contextWindow: 128_000,
+		maxTokens: 16_384,
+	};
+	if (provider === "openai") {
+		return {
+			...common,
+			api: "openai-responses",
+			provider: "openai",
+			baseUrl: "https://api.openai.com/v1",
+			input: ["text"],
+		};
+	}
+	if (provider === "anthropic") {
+		return {
+			...common,
+			api: "anthropic-messages",
+			provider: "anthropic",
+			baseUrl: "https://api.anthropic.com",
+			input: ["text"],
+		};
+	}
+	throw new Error(`Unsupported provider ${provider}. This spike registers openai and anthropic.`);
+}
+
+const model = models.getModel(providerId, modelId) ?? createFallbackModel(providerId, modelId);
+const registry = new BindingRegistry();
+const tools: AgentHarnessTool<NotifyExtensionContext>[] = [];
+const commands = new Map<string, ServerCommand>();
+const messageEndHandlers: MessageEndHandler[] = [];
+const extensionApi: ServerExtensionApi = {
+	registerTool(tool) {
+		tools.push(tool);
+	},
+	registerCommand(name, command) {
+		commands.set(name, command);
+	},
+	on(type, handler) {
+		if (type === "message_end") messageEndHandlers.push(handler);
+	},
+};
+notifyExtension(extensionApi);
+
+let activeOriginClient: ConnectedClient | undefined;
+let promptInFlight = false;
+
+function contextFor(client: ConnectedClient): NotifyExtensionContext {
+	return {
+		clientBindings: {
+			notify: (input, options) => registry.notify(client, input, options),
+		},
+	};
+}
+
+const harness = new AgentHarness({
+	session: new Session(new InMemorySessionStorage()),
+	models,
+	model,
+	thinkingLevel: "off",
+	tools,
+	toolContext: () => {
+		if (!activeOriginClient) throw new Error("No originating client is bound to this turn");
+		return contextFor(activeOriginClient);
+	},
+	systemPrompt: [
+		"You are a concise assistant.",
+		"When asked for the capital of France, respond with exactly Paris and nothing else.",
+		"When the user asks for a notification, call the notify tool with the exact message they want shown.",
+	].join(" "),
+});
+
+harness.subscribe(async (event, signal) => {
+	for (const wireEvent of toServerEvents(event)) await registry.broadcast(wireEvent);
+	if (event.type === "message_end" && messageEndHandlers.length > 0) {
+		if (!activeOriginClient) throw new Error("No originating client is bound to this assistant response");
+		const context = contextFor(activeOriginClient);
+		for (const handler of messageEndHandlers) await handler(event, context, signal);
+	}
+});
+
+function toServerEvents(event: AgentHarnessEvent): ServerEvent[] {
+	switch (event.type) {
+		case "agent_start":
+			return [{ type: "run_start" }];
+		case "agent_end":
+			return [{ type: "run_end" }];
+		case "message_start":
+			return event.message.role === "assistant" ? [{ type: "assistant_start" }] : [];
+		case "message_update":
+			return event.assistantMessageEvent.type === "text_delta"
+				? [{ type: "assistant_delta", text: event.assistantMessageEvent.delta }]
+				: [];
+		case "message_end":
+			if (event.message.role !== "assistant") return [];
+			return [
+				{ type: "assistant_end" },
+				...(event.message.errorMessage ? [{ type: "error" as const, message: event.message.errorMessage }] : []),
+			];
+		case "tool_execution_start":
+			return [{ type: "tool_start", name: event.toolName }];
+		case "tool_execution_end":
+			return [{ type: "tool_end", name: event.toolName, isError: event.isError }];
+		default:
+			return [];
+	}
+}
+
+async function runPrompt(client: ConnectedClient, text: string): Promise<void> {
+	if (promptInFlight) throw new Error("The shared session is already processing a prompt");
+	promptInFlight = true;
+	activeOriginClient = client;
+	await registry.broadcast({ type: "user_prompt", clientId: client.id, text });
+	try {
+		const assistant = await harness.prompt(text);
+		if (assistant.stopReason === "error") throw new Error(assistant.errorMessage ?? "Agent run failed");
+	} finally {
+		activeOriginClient = undefined;
+		promptInFlight = false;
+	}
+}
+
+class PiServerConnection extends RpcTarget implements ServerApi {
+	readonly #remoteClient: () => RpcStub<ClientBindingApi>;
+	#client: ConnectedClient | undefined;
+
+	constructor(remoteClient: () => RpcStub<ClientBindingApi>) {
+		super();
+		this.#remoteClient = remoteClient;
+	}
+
+	async connect(registration: ClientRegistration): Promise<void> {
+		if (this.#client) throw new Error("This RPC session is already registered");
+		if (!Value.Check(clientRegistrationSchema, registration)) throw new Error("Invalid client registration");
+		const client = registry.register(registration, this.#remoteClient());
+		this.#client = client;
+		try {
+			await registry.send(client, { type: "ready", clientId: client.id, bindings: registration.bindings });
+		} catch (error) {
+			registry.unregister(client);
+			this.#client = undefined;
+			throw error;
+		}
+	}
+
+	async prompt(text: string): Promise<void> {
+		const client = this.#requireClient();
+		if (typeof text !== "string" || text.trim().length === 0) throw new Error("Prompt is required");
+		await runPrompt(client, text.trim());
+	}
+
+	async command(name: string, input: unknown): Promise<void> {
+		const client = this.#requireClient();
+		if (typeof name !== "string") throw new Error("Command name must be a string");
+		const command = commands.get(name);
+		if (!command) throw new Error(`Unknown command ${name}`);
+		await command(input, contextFor(client));
+	}
+
+	[Symbol.dispose](): void {
+		if (!this.#client) return;
+		registry.unregister(this.#client);
+		this.#client = undefined;
+	}
+
+	#requireClient(): ConnectedClient {
+		if (!this.#client?.connected) throw new Error("RPC client has not connected");
+		return this.#client;
+	}
+}
+
+function sendStatic(response: ServerResponse, path: string, type: string): Promise<void> {
+	return readFile(path).then((body) => {
+		response.writeHead(200, { "content-type": type, "cache-control": "no-store" });
+		response.end(body);
+	});
+}
+
+function sendJson(response: ServerResponse, status: number, body: unknown): void {
+	response.writeHead(status, {
+		"content-type": "application/json; charset=utf-8",
+		"cache-control": "no-store",
+	});
+	response.end(JSON.stringify(body));
+}
+
+const exampleRoot = dirname(fileURLToPath(import.meta.url));
+const webRoot = join(exampleRoot, "web");
+const capnwebBrowserModule = fileURLToPath(import.meta.resolve("capnweb"));
+const staticFiles = new Map([
+	["/", { path: join(webRoot, "index.html"), type: "text/html; charset=utf-8" }],
+	["/app.js", { path: join(webRoot, "app.js"), type: "text/javascript; charset=utf-8" }],
+	["/styles.css", { path: join(webRoot, "styles.css"), type: "text/css; charset=utf-8" }],
+	["/vendor/capnweb.js", { path: capnwebBrowserModule, type: "text/javascript; charset=utf-8" }],
+]);
+
+const httpServer = createServer(async (request, response) => {
+	try {
+		const url = new URL(request.url ?? "/", `http://${request.headers.host ?? `${host}:${port}`}`);
+		const staticFile = request.method === "GET" ? staticFiles.get(url.pathname) : undefined;
+		if (staticFile) {
+			await sendStatic(response, staticFile.path, staticFile.type);
+			return;
+		}
+		sendJson(response, 404, { ok: false, error: "Not found" });
+	} catch (error) {
+		sendJson(response, 500, { ok: false, error: error instanceof Error ? error.message : String(error) });
+	}
+});
+
+const wsServer = new WebSocketServer({
+	server: httpServer,
+	path: "/api/rpc",
+	maxPayload: 64 * 1024,
+	perMessageDeflate: false,
+});
+wsServer.on("connection", (socket) => {
+	let remoteClient: RpcStub<ClientBindingApi> | undefined;
+	const connection = new PiServerConnection(() => {
+		if (!remoteClient) throw new Error("Remote client binding is not ready");
+		return remoteClient;
+	});
+	const session = new RpcSession<ClientBindingApi>(new NodeWebSocketTransport(socket), connection, {
+		limits: { maxMessageSize: 64 * 1024 },
+	});
+	remoteClient = session.getRemoteMain();
+	socket.once("close", () => connection[Symbol.dispose]());
+});
+
+httpServer.listen(port, host, async () => {
+	const auth = await models.checkAuth(providerId);
+	console.log(`Cap'n Web capability spike listening at http://${host}:${port}`);
+	console.log(`Model: ${modelSpec}${auth ? ` (${auth.source})` : " (provider auth not configured)"}`);
+	console.log("Run the TUI with: npm run tui --prefix packages/coding-agent/examples/capability-capnweb-spike");
+});
+
+for (const signal of ["SIGINT", "SIGTERM"] as const) {
+	process.once(signal, () => {
+		wsServer.close();
+		httpServer.close(() => process.exit(0));
+	});
+}
diff --git a/packages/coding-agent/examples/capability-capnweb-spike/tui.ts b/packages/coding-agent/examples/capability-capnweb-spike/tui.ts
new file mode 100644
index 000000000..fe175d8f3
--- /dev/null
+++ b/packages/coding-agent/examples/capability-capnweb-spike/tui.ts
@@ -0,0 +1,136 @@
+import { randomUUID } from "node:crypto";
+import { createInterface } from "node:readline";
+import { newWebSocketRpcSession, type RpcPromise, RpcTarget } from "capnweb";
+import { Value } from "typebox/value";
+import {
+	type ClientBindingApi,
+	type NotifyInput,
+	type NotifyOutput,
+	notifyBinding,
+	protocolVersion,
+	type ServerApi,
+	type ServerEvent,
+} from "./protocol.ts";
+
+const baseUrl = process.env.PI_CAPNWEB_SPIKE_URL ?? "http://127.0.0.1:8788";
+const rpcUrl = new URL("/api/rpc", baseUrl);
+rpcUrl.protocol = rpcUrl.protocol === "https:" ? "wss:" : "ws:";
+const clientId = `tui-${randomUUID()}`;
+
+const readline = createInterface({ input: process.stdin, output: process.stdout, prompt: "\x1b[1mYou ›\x1b[0m " });
+let assistantLineOpen = false;
+let busy = false;
+let runActive = false;
+
+function printLine(text: string): void {
+	if (assistantLineOpen) {
+		process.stdout.write("\n");
+		assistantLineOpen = false;
+	}
+	process.stdout.write(`\r\x1b[2K${text}\n`);
+}
+
+function restorePromptIfIdle(): void {
+	if (!runActive && !busy) readline.prompt(true);
+}
+
+function handleEvent(event: ServerEvent): void {
+	switch (event.type) {
+		case "run_start":
+			runActive = true;
+			process.stdout.write("\r\x1b[2K");
+			break;
+		case "run_end":
+			runActive = false;
+			restorePromptIfIdle();
+			break;
+		case "assistant_start":
+			if (assistantLineOpen) process.stdout.write("\n");
+			process.stdout.write("\r\x1b[2K\x1b[36mPi  ›\x1b[0m ");
+			assistantLineOpen = true;
+			break;
+		case "assistant_delta":
+			process.stdout.write(event.text);
+			break;
+		case "assistant_end":
+			if (assistantLineOpen) process.stdout.write("\n");
+			assistantLineOpen = false;
+			break;
+		case "tool_start":
+			printLine(`\x1b[2m[tool: ${event.name}]\x1b[0m`);
+			break;
+		case "tool_end":
+			break;
+		case "error":
+			printLine(`\x1b[31m${event.message}\x1b[0m`);
+			restorePromptIfIdle();
+			break;
+		default:
+			break;
+	}
+}
+
+class TuiClientBindings extends RpcTarget implements ClientBindingApi {
+	async event(event: ServerEvent): Promise<void> {
+		handleEvent(event);
+	}
+
+	async notify(input: NotifyInput): Promise<NotifyOutput> {
+		if (!Value.Check(notifyBinding.input, input)) throw new Error("Invalid notify input");
+		printLine(`\x1b[33m${input.message}\x1b[0m`);
+		restorePromptIfIdle();
+		return { displayed: true };
+	}
+}
+
+async function awaitRpc(call: RpcPromise<void>): Promise<void> {
+	try {
+		await call;
+	} finally {
+		call[Symbol.dispose]();
+	}
+}
+
+const server = newWebSocketRpcSession<ServerApi>(rpcUrl.toString(), new TuiClientBindings(), {
+	limits: { maxMessageSize: 64 * 1024 },
+});
+server.onRpcBroken((error: unknown) => {
+	printLine(`\x1b[31mRPC connection closed: ${error instanceof Error ? error.message : String(error)}\x1b[0m`);
+});
+await awaitRpc(
+	server.connect({
+		clientId,
+		protocolVersion,
+		bindings: [{ id: notifyBinding.id, version: notifyBinding.version }],
+	}),
+);
+
+console.log(`\x1b[2mConnected as ${clientId}. Use /notify <message> to invoke the binding without the LLM.\x1b[0m`);
+readline.prompt();
+
+readline.on("line", (line) => {
+	const text = line.trim();
+	if (!text) {
+		readline.prompt();
+		return;
+	}
+	if (busy) {
+		printLine("\x1b[33mThe shared session is busy.\x1b[0m");
+		return;
+	}
+	busy = true;
+	const operation = text.startsWith("/notify ")
+		? server.command("notify", { message: text.slice("/notify ".length) })
+		: server.prompt(text);
+	void awaitRpc(operation)
+		.catch((error: unknown) => printLine(`\x1b[31m${error instanceof Error ? error.message : String(error)}\x1b[0m`))
+		.finally(() => {
+			busy = false;
+			restorePromptIfIdle();
+		});
+});
+
+readline.once("close", () => {
+	server[Symbol.dispose]();
+	process.stdout.write("\n");
+});
diff --git a/packages/coding-agent/examples/capability-capnweb-spike/web/app.js b/packages/coding-agent/examples/capability-capnweb-spike/web/app.js
new file mode 100644
index 000000000..5a7a6a227
--- /dev/null
+++ b/packages/coding-agent/examples/capability-capnweb-spike/web/app.js
@@ -0,0 +1,171 @@
+import { newWebSocketRpcSession, RpcTarget } from "/vendor/capnweb.js";
+
+const protocolVersion = 1;
+const notifyBinding = { id: "pi.notify", version: 1 };
+const clientId = sessionStorage.getItem("pi-capnweb-client") ?? `web-${crypto.randomUUID()}`;
+sessionStorage.setItem("pi-capnweb-client", clientId);
+
+const connection = document.querySelector(".connection");
+const connectionLabel = document.querySelector("#connection-label");
+const transcript = document.querySelector("#transcript");
+const emptyState = document.querySelector("#empty-state");
+const form = document.querySelector("#prompt-form");
+const prompt = document.querySelector("#prompt");
+const send = document.querySelector("#send");
+const testNotify = document.querySelector("#test-notify");
+const notification = document.querySelector("#notification");
+
+let connected = false;
+let busy = false;
+let assistantContent;
+let notificationTimer;
+
+function setConnection(state, label) {
+	connection.dataset.state = state;
+	connectionLabel.textContent = label;
+	connected = state === "connected";
+	send.disabled = !connected || busy;
+	testNotify.disabled = !connected || busy;
+}
+
+function appendMessage(role, text, className = role.toLowerCase()) {
+	emptyState?.remove();
+	const message = document.createElement("article");
+	message.className = `message ${className}`;
+	const label = document.createElement("p");
+	label.className = "role";
+	label.textContent = role;
+	const content = document.createElement("p");
+	content.className = "content";
+	content.textContent = text;
+	message.append(label, content);
+	transcript.append(message);
+	transcript.scrollTop = transcript.scrollHeight;
+	return content;
+}
+
+function showNotification(message) {
+	clearTimeout(notificationTimer);
+	notification.hidden = false;
+	notification.textContent = message;
+	notification.classList.remove("is-flying");
+	void notification.offsetWidth;
+	notification.classList.add("is-flying");
+	notificationTimer = setTimeout(() => {
+		notification.hidden = true;
+		notification.classList.remove("is-flying");
+	}, 2400);
+}
+
+function handleEvent(event) {
+	switch (event.type) {
+		case "ready":
+			setConnection("connected", "Binding online");
+			break;
+		case "user_prompt":
+			appendMessage(event.clientId === clientId ? "You" : "Peer", event.text, "user");
+			break;
+		case "assistant_start":
+			assistantContent = undefined;
+			break;
+		case "assistant_delta":
+			if (!assistantContent) assistantContent = appendMessage("Pi", "", "assistant");
+			assistantContent.textContent += event.text;
+			transcript.scrollTop = transcript.scrollHeight;
+			break;
+		case "assistant_end":
+			assistantContent = undefined;
+			break;
+		case "tool_start":
+			appendMessage("Tool", event.name, "tool");
+			break;
+		case "error":
+			appendMessage("Error", event.message, "error");
+			break;
+	}
+}
+
+class BrowserClientBindings extends RpcTarget {
+	async event(event) {
+		handleEvent(event);
+	}
+
+	async notify(input) {
+		if (typeof input?.message !== "string" || input.message.length === 0) {
+			throw new Error("Invalid notify input");
+		}
+		showNotification(input.message);
+		return { displayed: true };
+	}
+}
+
+async function awaitRpc(call) {
+	try {
+		return await call;
+	} finally {
+		call[Symbol.dispose]();
+	}
+}
+
+const rpcUrl = new URL("/api/rpc", window.location.href);
+rpcUrl.protocol = rpcUrl.protocol === "https:" ? "wss:" : "ws:";
+const server = newWebSocketRpcSession(rpcUrl.toString(), new BrowserClientBindings(), {
+	limits: { maxMessageSize: 64 * 1024 },
+});
+server.onRpcBroken((error) => {
+	setConnection("error", "Disconnected");
+	appendMessage("Error", error instanceof Error ? error.message : String(error), "error");
+});
+
+try {
+	await awaitRpc(
+		server.connect({
+			clientId,
+			protocolVersion,
+			bindings: [notifyBinding],
+		}),
+	);
+} catch (error) {
+	setConnection("error", error instanceof Error ? error.message : String(error));
+}
+
+form.addEventListener("submit", async (event) => {
+	event.preventDefault();
+	const text = prompt.value.trim();
+	if (!text || busy) return;
+	busy = true;
+	setConnection("connected", "Pi is working");
+	prompt.value = "";
+	try {
+		await awaitRpc(server.prompt(text));
+	} catch (error) {
+		appendMessage("Error", error instanceof Error ? error.message : String(error), "error");
+	} finally {
+		busy = false;
+		setConnection("connected", "Binding online");
+		prompt.focus();
+	}
+});
+
+prompt.addEventListener("keydown", (event) => {
+	if (event.key === "Enter" && !event.shiftKey) {
+		event.preventDefault();
+		form.requestSubmit();
+	}
+});
+
+testNotify.addEventListener("click", async () => {
+	if (busy) return;
+	busy = true;
+	setConnection("connected", "Invoking binding");
+	try {
+		await awaitRpc(server.command("notify", { message: "Browser binding confirmed." }));
+	} catch (error) {
+		appendMessage("Error", error instanceof Error ? error.message : String(error), "error");
+	} finally {
+		busy = false;
+		setConnection("connected", "Binding online");
+	}
+});
+
+window.addEventListener("beforeunload", () => server[Symbol.dispose]());
diff --git a/packages/coding-agent/examples/capability-capnweb-spike/web/index.html b/packages/coding-agent/examples/capability-capnweb-spike/web/index.html
new file mode 100644
index 000000000..e04bb29d3
--- /dev/null
+++ b/packages/coding-agent/examples/capability-capnweb-spike/web/index.html
@@ -0,0 +1,51 @@
+<!doctype html>
+<html lang="en">
+	<head>
+		<meta charset="UTF-8" />
+		<meta name="viewport" content="width=device-width, initial-scale=1" />
+		<meta name="color-scheme" content="light" />
+		<link rel="icon" href="data:," />
+		<title>Pi Cap'n Web capability spike</title>
+		<link rel="stylesheet" href="/styles.css" />
+	</head>
+	<body>
+		<div class="page-grid" aria-hidden="true"></div>
+		<header class="masthead">
+			<div>
+				<p class="eyebrow">Client binding / Cap'n Web</p>
+				<h1>Pi, called remotely.</h1>
+			</div>
+			<div class="connection" data-state="connecting">
+				<span class="connection-dot"></span>
+				<span id="connection-label">Connecting</span>
+			</div>
+		</header>
+
+		<main>
+			<section class="transcript" id="transcript" aria-live="polite" aria-label="Conversation">
+				<div class="empty-state" id="empty-state">
+					<p class="index">01</p>
+					<p>Ask Pi to notify you, or call the client binding directly.</p>
+				</div>
+			</section>
+
+			<form class="composer" id="prompt-form">
+				<label for="prompt">Prompt</label>
+				<div class="input-row">
+					<textarea id="prompt" name="prompt" rows="2" placeholder="Notify me through Cap'n Web" required></textarea>
+					<button type="submit" id="send" disabled>Send</button>
+				</div>
+				<div class="composer-meta">
+					<span>Enter to send · Shift+Enter for newline</span>
+					<button type="button" class="text-button" id="test-notify" disabled>Call notify binding</button>
+				</div>
+			</form>
+		</main>
+
+		<div class="notification-stage" aria-live="assertive" aria-atomic="true">
+			<div class="notification" id="notification" hidden></div>
+		</div>
+
+		<script type="module" src="/app.js"></script>
+	</body>
+</html>
diff --git a/packages/coding-agent/examples/capability-capnweb-spike/web/styles.css b/packages/coding-agent/examples/capability-capnweb-spike/web/styles.css
new file mode 100644
index 000000000..4a821f15a
--- /dev/null
+++ b/packages/coding-agent/examples/capability-capnweb-spike/web/styles.css
@@ -0,0 +1,376 @@
+@layer reset, base, components, motion;
+
+@layer reset {
+	*,
+	*::before,
+	*::after {
+		box-sizing: border-box;
+	}
+
+	body,
+	h1,
+	p {
+		margin: 0;
+	}
+
+	button,
+	textarea {
+		font: inherit;
+	}
+}
+
+@layer base {
+	:root {
+		--paper: oklch(0.96 0.025 86);
+		--ink: oklch(0.19 0.025 54);
+		--muted: oklch(0.49 0.04 70);
+		--rule: oklch(0.78 0.055 76);
+		--signal: oklch(0.78 0.18 77);
+		--hot: oklch(0.59 0.23 34);
+		--success: oklch(0.55 0.13 151);
+		font-family: "Avenir Next", "Century Gothic", sans-serif;
+		color: var(--ink);
+		background: var(--paper);
+	}
+
+	body {
+		min-block-size: 100svh;
+		display: grid;
+		grid-template-rows: auto 1fr;
+		padding: clamp(1rem, 3vw, 2.75rem);
+		overflow-x: hidden;
+	}
+
+	button,
+	textarea {
+		color: inherit;
+	}
+
+	button:focus-visible,
+	textarea:focus-visible {
+		outline: 3px solid var(--hot);
+		outline-offset: 3px;
+	}
+
+	.page-grid {
+		position: fixed;
+		inset: 0;
+		z-index: -1;
+		background-image:
+			linear-gradient(to right, color-mix(in oklch, var(--rule) 34%, transparent) 1px, transparent 1px),
+			linear-gradient(to bottom, color-mix(in oklch, var(--rule) 25%, transparent) 1px, transparent 1px);
+		background-size: 2rem 2rem;
+		mask-image: linear-gradient(to bottom, black, transparent 78%);
+	}
+}
+
+@layer components {
+	.masthead {
+		display: flex;
+		align-items: start;
+		justify-content: space-between;
+		gap: 2rem;
+		padding-block-end: clamp(2rem, 7vh, 5rem);
+
+		& h1 {
+			max-inline-size: 15ch;
+			font-family: "Iowan Old Style", "Palatino Linotype", Georgia, serif;
+			font-size: clamp(2.75rem, 8vw, 7.5rem);
+			font-weight: 600;
+			font-style: italic;
+			letter-spacing: -0.065em;
+			line-height: 0.84;
+			text-wrap: balance;
+		}
+	}
+
+	.eyebrow,
+	.connection,
+	.composer label,
+	.composer-meta {
+		font-size: 0.72rem;
+		font-weight: 700;
+		letter-spacing: 0.12em;
+		text-transform: uppercase;
+	}
+
+	.eyebrow {
+		margin-block-end: 1rem;
+		color: var(--hot);
+	}
+
+	.connection {
+		display: flex;
+		align-items: center;
+		gap: 0.55rem;
+		padding-block: 0.4rem;
+		white-space: nowrap;
+
+		& .connection-dot {
+			inline-size: 0.65rem;
+			block-size: 0.65rem;
+			border: 1px solid currentColor;
+			border-radius: 50%;
+			background: var(--signal);
+		}
+
+		&[data-state="connected"] .connection-dot {
+			background: var(--success);
+		}
+
+		&[data-state="error"] .connection-dot {
+			background: var(--hot);
+		}
+	}
+
+	main {
+		inline-size: min(100%, 68rem);
+		margin-inline-start: auto;
+		display: grid;
+		grid-template-rows: minmax(14rem, 1fr) auto;
+		gap: clamp(2rem, 5vh, 4rem);
+	}
+
+	.transcript {
+		align-self: end;
+		max-block-size: 42vh;
+		overflow-y: auto;
+		scrollbar-gutter: stable;
+		display: grid;
+		gap: 1.2rem;
+		padding-inline: 0.25rem 1rem;
+	}
+
+	.empty-state {
+		display: grid;
+		grid-template-columns: 4rem minmax(0, 26rem);
+		gap: 1rem;
+		align-items: start;
+		color: var(--muted);
+		font-family: "Iowan Old Style", "Palatino Linotype", Georgia, serif;
+		font-size: clamp(1.15rem, 2.5vw, 1.65rem);
+		line-height: 1.25;
+
+		& .index {
+			font-family: "Avenir Next", "Century Gothic", sans-serif;
+			font-size: 0.72rem;
+			font-weight: 700;
+			letter-spacing: 0.12em;
+		}
+	}
+
+	.message {
+		display: grid;
+		grid-template-columns: 4rem minmax(0, 42rem);
+		gap: 1rem;
+		line-height: 1.55;
+		white-space: pre-wrap;
+
+		& .role {
+			padding-block-start: 0.25rem;
+			color: var(--muted);
+			font-size: 0.67rem;
+			font-weight: 800;
+			letter-spacing: 0.1em;
+			text-transform: uppercase;
+		}
+
+		&.user .content {
+			font-family: "Iowan Old Style", "Palatino Linotype", Georgia, serif;
+			font-size: 1.2rem;
+		}
+
+		&.tool .content {
+			color: var(--muted);
+			font-size: 0.82rem;
+			font-weight: 700;
+			letter-spacing: 0.06em;
+			text-transform: uppercase;
+		}
+	}
+
+	.composer {
+		border-block-start: 2px solid var(--ink);
+		padding-block-start: 0.75rem;
+
+		& label {
+		display: block;
+		margin-block-end: 0.6rem;
+		}
+	}
+
+	.input-row {
+		display: grid;
+		grid-template-columns: 1fr auto;
+		gap: 0.75rem;
+		align-items: stretch;
+	}
+
+	textarea {
+		min-block-size: 5.5rem;
+		resize: vertical;
+		border: 0;
+		background: transparent;
+		padding: 0.65rem 0;
+		font-family: "Iowan Old Style", "Palatino Linotype", Georgia, serif;
+		font-size: clamp(1.15rem, 2.2vw, 1.55rem);
+		line-height: 1.35;
+
+		&::placeholder {
+			color: color-mix(in oklch, var(--muted) 65%, transparent);
+		}
+	}
+
+	button {
+		border: 0;
+		cursor: pointer;
+	}
+
+	#send {
+		min-inline-size: 6.5rem;
+		background: var(--ink);
+		color: var(--paper);
+		font-size: 0.8rem;
+		font-weight: 800;
+		letter-spacing: 0.08em;
+		text-transform: uppercase;
+		transition: transform 160ms ease-out, background-color 160ms ease-out;
+
+		&:hover:not(:disabled) {
+			background: var(--hot);
+			transform: translateY(-2px);
+		}
+
+		&:disabled {
+			cursor: not-allowed;
+			opacity: 0.35;
+		}
+	}
+
+	.composer-meta {
+		display: flex;
+		justify-content: space-between;
+		gap: 1rem;
+		margin-block-start: 0.75rem;
+		color: var(--muted);
+	}
+
+	.text-button {
+		padding: 0;
+		background: transparent;
+		color: var(--hot);
+		font-weight: 800;
+		text-decoration: underline;
+		text-decoration-thickness: 1px;
+		text-underline-offset: 0.25rem;
+
+		&:disabled {
+			cursor: not-allowed;
+			opacity: 0.4;
+		}
+	}
+
+	.notification-stage {
+		position: fixed;
+		inset: 0;
+		z-index: 10;
+		pointer-events: none;
+		overflow: hidden;
+	}
+
+	.notification {
+		position: absolute;
+		inset-block-start: 50%;
+		inset-inline-start: 50%;
+		max-inline-size: min(80vw, 46rem);
+		padding: clamp(1rem, 3vw, 2rem) clamp(1.25rem, 5vw, 3.5rem);
+		background: var(--signal);
+		color: var(--ink);
+		box-shadow: 0.8rem 0.8rem 0 var(--hot);
+		font-family: "Iowan Old Style", "Palatino Linotype", Georgia, serif;
+		font-size: clamp(1.75rem, 6vw, 5rem);
+		font-weight: 700;
+		font-style: italic;
+		line-height: 0.95;
+		text-align: center;
+		text-wrap: balance;
+	}
+
+	@media (width < 44rem) {
+		body {
+			padding: 1rem;
+		}
+
+		.masthead {
+			align-items: end;
+			padding-block-end: 3rem;
+		}
+
+		.connection {
+			& #connection-label {
+				display: none;
+			}
+		}
+
+		.message,
+		.empty-state {
+			grid-template-columns: 2.5rem minmax(0, 1fr);
+		}
+
+		.input-row {
+			grid-template-columns: 1fr;
+		}
+
+		#send {
+			min-block-size: 3.25rem;
+		}
+
+		.composer-meta {
+			align-items: start;
+			flex-direction: column;
+		}
+	}
+}
+
+@layer motion {
+	.notification.is-flying {
+		animation: notify-spin-out 2.35s cubic-bezier(0.77, 0, 0.22, 1) both;
+	}
+
+	@keyframes notify-spin-out {
+		0% {
+			opacity: 0;
+			transform: translate(-50%, -50%) rotate(-18deg) scale(0.35);
+		}
+		13% {
+			opacity: 1;
+			transform: translate(-50%, -50%) rotate(2deg) scale(1.08);
+		}
+		25%,
+		52% {
+			opacity: 1;
+			transform: translate(-50%, -50%) rotate(0deg) scale(1);
+		}
+		100% {
+			opacity: 0;
+			transform: translate(calc(-50% + 112vw), calc(-50% - 82vh)) rotate(1600deg) scale(0.08);
+		}
+	}
+
+	@media (prefers-reduced-motion: reduce) {
+		*,
+		*::before,
+		*::after {
+			scroll-behavior: auto !important;
+		}
+
+		.notification.is-flying {
+			animation: notify-fade 2.35s ease-out both;
+		}
+
+		@keyframes notify-fade {
+			0%, 100% { opacity: 0; transform: translate(-50%, -50%); }
+			15%, 75% { opacity: 1; transform: translate(-50%, -50%); }
+		}
+	}
+}
diff --git a/packages/coding-agent/examples/capability-capnweb-spike/ws-transport.ts b/packages/coding-agent/examples/capability-capnweb-spike/ws-transport.ts
new file mode 100644
index 000000000..559e8b3b2
--- /dev/null
+++ b/packages/coding-agent/examples/capability-capnweb-spike/ws-transport.ts
@@ -0,0 +1,69 @@
+import type { RpcTransport } from "capnweb";
+import type WebSocket from "ws";
+
+interface Receiver {
+	resolve(message: string): void;
+	reject(error: Error): void;
+}
+
+/** Adapts the Node `ws` server socket to Cap'n Web without an unsafe type cast. */
+export class NodeWebSocketTransport implements RpcTransport {
+	readonly #socket: WebSocket;
+	readonly #messages: string[] = [];
+	readonly #receivers: Receiver[] = [];
+	#closedError: Error | undefined;
+
+	constructor(socket: WebSocket) {
+		this.#socket = socket;
+		socket.on("message", (data, isBinary) => {
+			if (isBinary) {
+				this.abort(new Error("Binary Cap'n Web frames are not supported"));
+				return;
+			}
+			this.#deliver(data.toString());
+		});
+		socket.once("close", () => this.#fail(new Error("WebSocket closed")));
+		socket.once("error", (error) => this.#fail(error));
+	}
+
+	send(message: string): Promise<void> {
+		if (this.#closedError || this.#socket.readyState !== 1) {
+			return Promise.reject(this.#closedError ?? new Error("WebSocket is not open"));
+		}
+		return new Promise<void>((resolve, reject) => {
+			this.#socket.send(message, (error) => {
+				if (error) reject(error);
+				else resolve();
+			});
+		});
+	}
+
+	receive(): Promise<string> {
+		const message = this.#messages.shift();
+		if (message !== undefined) return Promise.resolve(message);
+		if (this.#closedError) return Promise.reject(this.#closedError);
+		return new Promise<string>((resolve, reject) => {
+			this.#receivers.push({ resolve, reject });
+		});
+	}
+
+	abort(reason: unknown): void {
+		const error = reason instanceof Error ? reason : new Error(String(reason));
+		this.#fail(error);
+		if (this.#socket.readyState === 0 || this.#socket.readyState === 1) {
+			this.#socket.close(1011, "RPC session aborted");
+		}
+	}
+
+	#deliver(message: string): void {
+		const receiver = this.#receivers.shift();
+		if (receiver) receiver.resolve(message);
+		else this.#messages.push(message);
+	}
+
+	#fail(error: Error): void {
+		if (this.#closedError) return;
+		this.#closedError = error;
+		for (const receiver of this.#receivers.splice(0)) receiver.reject(error);
+	}
+}
diff --git a/packages/coding-agent/examples/capability-spike/README.md b/packages/coding-agent/examples/capability-spike/README.md
new file mode 100644
index 000000000..a8fd5d678
--- /dev/null
+++ b/packages/coding-agent/examples/capability-spike/README.md
@@ -0,0 +1,79 @@
+# Client capability spike
+
+A deliberately small server/client spike built on the new `AgentHarness` API. It demonstrates that a server extension can invoke one semantic client action without knowing whether the originating client is a TUI or a browser.
+
+## Architecture
+
+```text
+notify extension ─┬─ notify tool ───────┐
+                  ├─ notify command ────┤
+                  └─ response hook ─────┼─ pi.notify@1 ── originating client
+                                        │
+AgentHarness ─── shared in-memory session
+```
+
+The clients dynamically advertise `pi.notify@1` after connecting. Registrations are scoped to the SSE connection and disappear on disconnect. The model-facing tool, direct command, and assistant-response event hook call the same extension function and capability registry.
+
+The transport is intentionally plain HTTP plus server-sent events:
+
+- SSE carries session events and server-to-client action requests.
+- HTTP requests carry prompts, capability registration, and action results.
+- The server also serves the static web client, so a Cloudflare Worker is unnecessary for this local spike.
+
+This only prototypes the capability boundary. Authentication, persistent sessions, client-controller leases, reconnect replay, and a production protocol are intentionally omitted.
+
+## Run
+
+From the repository root:
+
+```bash
+OPENAI_API_KEY=... npx tsx packages/coding-agent/examples/capability-spike/server.ts
+```
+
+The default model is `openai/gpt-5.4-nano`. Select another registered OpenAI or Anthropic model with:
+
+```bash
+PI_SPIKE_MODEL=anthropic/claude-haiku-4-5 ANTHROPIC_API_KEY=... \
+  npx tsx packages/coding-agent/examples/capability-spike/server.ts
+```
+
+The server starts without provider credentials, which is enough to exercise direct capability calls. LLM prompts require the selected provider's environment credential.
+
+Open [http://127.0.0.1:8787](http://127.0.0.1:8787) for the web client.
+
+In another terminal, run the TUI client:
+
+```bash
+npx tsx packages/coding-agent/examples/capability-spike/tui.ts
+```
+
+Try:
+
+```text
+/notify TUI capability confirmed.
+```
+
+Or prompt the model:
+
+```text
+Notify me that the capability RPC works, then briefly explain what happened.
+```
+
+When both clients are connected, a notification requested by a prompt is delivered only to the client that submitted that prompt.
+
+For a deterministic extension-initiated action that does not use the tool wrapper, ask:
+
+```text
+What is the capital of France?
+```
+
+The server system prompt makes the assistant answer exactly `Paris`. The extension observes the completed assistant message and invokes `pi.notify@1` with `❤️` on the originating client.
+
+## Files
+
+- `server.ts`: HTTP/SSE server and `AgentHarness` session
+- `extension.ts`: server extension with the command, model-facing tool, and assistant-response hook
+- `capability-registry.ts`: connection-scoped action registration and invocation
+- `protocol.ts`: typed `pi.notify@1` input/output contract and wire events
+- `tui.ts`: readline client whose notify implementation prints yellow text
+- `web/`: static browser client with the spinning notification animation
diff --git a/packages/coding-agent/examples/capability-spike/capability-registry.ts b/packages/coding-agent/examples/capability-spike/capability-registry.ts
new file mode 100644
index 000000000..c9aabc7f9
--- /dev/null
+++ b/packages/coding-agent/examples/capability-spike/capability-registry.ts
@@ -0,0 +1,190 @@
+import { randomUUID } from "node:crypto";
+import type { Static, TSchema } from "typebox";
+import { Value } from "typebox/value";
+import type { ActionRegistration, ActionResultMessage, ClientAction, ServerEvent } from "./protocol.ts";
+
+interface ClientConnection {
+	id: string;
+	actions: Map<string, number>;
+	send(event: ServerEvent): void;
+}
+
+interface PendingInvocation {
+	clientId: string;
+	action: ClientAction<TSchema, TSchema>;
+	resolve(value: unknown): void;
+	reject(error: Error): void;
+	timer: NodeJS.Timeout;
+	removeAbortListener?: () => void;
+}
+
+export class CapabilityError extends Error {
+	public code:
+		| "client_unavailable"
+		| "action_unavailable"
+		| "invalid_input"
+		| "invalid_output"
+		| "aborted"
+		| "timeout"
+		| "remote_error";
+
+	constructor(
+		code:
+			| "client_unavailable"
+			| "action_unavailable"
+			| "invalid_input"
+			| "invalid_output"
+			| "aborted"
+			| "timeout"
+			| "remote_error",
+		message: string,
+	) {
+		super(message);
+		this.name = "CapabilityError";
+		this.code = code;
+	}
+}
+
+export class CapabilityRegistry {
+	private readonly clients = new Map<string, ClientConnection>();
+	private readonly pending = new Map<string, PendingInvocation>();
+
+	connect(clientId: string, send: (event: ServerEvent) => void): () => void {
+		this.disconnect(clientId, "Client reconnected");
+		const connection: ClientConnection = { id: clientId, actions: new Map(), send };
+		this.clients.set(clientId, connection);
+		connection.send({ type: "ready", clientId });
+
+		return () => {
+			if (this.clients.get(clientId) === connection) {
+				this.disconnect(clientId, "Client disconnected");
+			}
+		};
+	}
+
+	hasClient(clientId: string): boolean {
+		return this.clients.has(clientId);
+	}
+
+	register(clientId: string, actions: ActionRegistration[]): void {
+		const client = this.clients.get(clientId);
+		if (!client) {
+			throw new CapabilityError("client_unavailable", `Client ${clientId} is not connected`);
+		}
+		client.actions = new Map(actions.map((action) => [action.id, action.version]));
+		client.send({ type: "capabilities_updated", actions });
+	}
+
+	async invoke<TInputSchema extends TSchema, TOutputSchema extends TSchema>(
+		clientId: string,
+		action: ClientAction<TInputSchema, TOutputSchema>,
+		input: unknown,
+		options: { signal?: AbortSignal; timeoutMs?: number } = {},
+	): Promise<Static<TOutputSchema>> {
+		if (!Value.Check(action.input, input)) {
+			throw new CapabilityError("invalid_input", `Input does not match ${action.id}@${action.version}`);
+		}
+		const client = this.clients.get(clientId);
+		if (!client) {
+			throw new CapabilityError("client_unavailable", `Client ${clientId} is not connected`);
+		}
+		if (client.actions.get(action.id) !== action.version) {
+			throw new CapabilityError(
+				"action_unavailable",
+				`Client ${clientId} does not provide ${action.id}@${action.version}`,
+			);
+		}
+		if (options.signal?.aborted) {
+			throw new CapabilityError("aborted", `${action.id} was aborted`);
+		}
+
+		const requestId = randomUUID();
+		const timeoutMs = options.timeoutMs ?? 10_000;
+		const result = new Promise<unknown>((resolve, reject) => {
+			const timer = setTimeout(() => {
+				this.rejectInvocation(
+					requestId,
+					new CapabilityError("timeout", `${action.id} timed out after ${timeoutMs}ms`),
+				);
+				client.send({ type: "action_cancel", requestId });
+			}, timeoutMs);
+			const pending: PendingInvocation = {
+				clientId,
+				action,
+				resolve,
+				reject,
+				timer,
+			};
+			if (options.signal) {
+				const onAbort = () => {
+					this.rejectInvocation(requestId, new CapabilityError("aborted", `${action.id} was aborted`));
+					client.send({ type: "action_cancel", requestId });
+				};
+				options.signal.addEventListener("abort", onAbort, { once: true });
+				pending.removeAbortListener = () => options.signal?.removeEventListener("abort", onAbort);
+			}
+			this.pending.set(requestId, pending);
+		});
+
+		client.send({
+			type: "action_request",
+			requestId,
+			action: { id: action.id, version: action.version },
+			input,
+		});
+		const output = await result;
+		if (!Value.Check(action.output, output)) {
+			throw new CapabilityError("invalid_output", `Output does not match ${action.id}@${action.version}`);
+		}
+		return output;
+	}
+
+	handleResult(requestId: string, message: ActionResultMessage): void {
+		const pending = this.pending.get(requestId);
+		if (!pending) return;
+		if (pending.clientId !== message.clientId) {
+			throw new CapabilityError("remote_error", `Client ${message.clientId} cannot answer request ${requestId}`);
+		}
+		this.pending.delete(requestId);
+		clearTimeout(pending.timer);
+		pending.removeAbortListener?.();
+		if (!message.ok) {
+			pending.reject(new CapabilityError("remote_error", message.error || `${pending.action.id} failed`));
+			return;
+		}
+		if (!Value.Check(pending.action.output, message.result)) {
+			pending.reject(
+				new CapabilityError(
+					"invalid_output",
+					`Output does not match ${pending.action.id}@${pending.action.version}`,
+				),
+			);
+			return;
+		}
+		pending.resolve(message.result);
+	}
+
+	broadcast(event: ServerEvent): void {
+		for (const client of this.clients.values()) {
+			client.send(event);
+		}
+	}
+
+	private rejectInvocation(requestId: string, error: Error): void {
+		const pending = this.pending.get(requestId);
+		if (!pending) return;
+		this.pending.delete(requestId);
+		clearTimeout(pending.timer);
+		pending.removeAbortListener?.();
+		pending.reject(error);
+	}
+
+	private disconnect(clientId: string, reason: string): void {
+		if (!this.clients.delete(clientId)) return;
+		for (const [requestId, pending] of this.pending) {
+			if (pending.clientId === clientId) {
+				this.rejectInvocation(requestId, new CapabilityError("client_unavailable", reason));
+			}
+		}
+	}
+}
diff --git a/packages/coding-agent/examples/capability-spike/extension.ts b/packages/coding-agent/examples/capability-spike/extension.ts
new file mode 100644
index 000000000..078253acd
--- /dev/null
+++ b/packages/coding-agent/examples/capability-spike/extension.ts
@@ -0,0 +1,86 @@
+import type { AgentHarnessEvent, AgentHarnessTool } from "@earendil-works/pi-agent-core";
+import { contentText } from "@earendil-works/pi-ai";
+import { type Static, type TSchema, Type } from "typebox";
+import { type NotifyOutput, notifyAction } from "./protocol.ts";
+
+export interface ClientActions {
+	invoke<TInputSchema extends TSchema, TOutputSchema extends TSchema>(
+		action: {
+			id: string;
+			version: number;
+			input: TInputSchema;
+			output: TOutputSchema;
+		},
+		input: unknown,
+		options?: { signal?: AbortSignal; timeoutMs?: number },
+	): Promise<Static<TOutputSchema>>;
+}
+
+export interface NotifyExtensionContext {
+	clientActions: ClientActions;
+}
+
+export type ServerCommand = (input: unknown, context: NotifyExtensionContext, signal?: AbortSignal) => Promise<unknown>;
+
+export type MessageEndEvent = Extract<AgentHarnessEvent, { type: "message_end" }>;
+
+export type MessageEndHandler = (
+	event: MessageEndEvent,
+	context: NotifyExtensionContext,
+	signal?: AbortSignal,
+) => Promise<void> | void;
+
+export interface ServerExtensionApi {
+	registerTool(tool: AgentHarnessTool<NotifyExtensionContext>): void;
+	registerCommand(name: string, command: ServerCommand): void;
+	on(type: "message_end", handler: MessageEndHandler): void;
+}
+
+async function notifyClient(
+	input: unknown,
+	context: NotifyExtensionContext,
+	signal?: AbortSignal,
+): Promise<NotifyOutput> {
+	if (
+		typeof input !== "object" ||
+		input === null ||
+		!("message" in input) ||
+		typeof input.message !== "string" ||
+		input.message.trim().length === 0
+	) {
+		throw new Error("notify requires a non-empty message");
+	}
+	return context.clientActions.invoke(notifyAction, { message: input.message.trim() }, { signal });
+}
+
+const notifyParameters = Type.Object(
+	{
+		message: Type.String({ description: "Notification text", minLength: 1, maxLength: 500 }),
+	},
+	{ additionalProperties: false },
+);
+
+export function notifyExtension(pi: ServerExtensionApi): void {
+	pi.registerCommand("notify", notifyClient);
+	pi.on("message_end", async (event, context, signal) => {
+		if (event.message.role !== "assistant") return;
+		if (contentText(event.message.content, "").trim() === "Paris") {
+			await notifyClient({ message: "❤️" }, context, signal);
+		}
+	});
+	pi.registerTool({
+		name: "notify",
+		label: "Notify client",
+		description: "Show a brief notification on the client that submitted the current prompt.",
+		parameters: notifyParameters,
+		async execute(_toolCallId, params, signal, _onUpdate, context) {
+			const input = typeof params === "object" && params !== null ? params : {};
+			const message = "message" in input && typeof input.message === "string" ? input.message : "";
+			await notifyClient({ message }, context, signal);
+			return {
+				content: [{ type: "text", text: `Notified the originating client: ${message}` }],
+				details: { displayed: true },
+			};
+		},
+	});
+}
diff --git a/packages/coding-agent/examples/capability-spike/protocol.ts b/packages/coding-agent/examples/capability-spike/protocol.ts
new file mode 100644
index 000000000..1bfd16667
--- /dev/null
+++ b/packages/coding-agent/examples/capability-spike/protocol.ts
@@ -0,0 +1,55 @@
+import { type Static, type TSchema, Type } from "typebox";
+
+export interface ClientAction<TInputSchema extends TSchema, TOutputSchema extends TSchema> {
+	id: string;
+	version: number;
+	input: TInputSchema;
+	output: TOutputSchema;
+}
+
+export const notifyAction = {
+	id: "pi.notify",
+	version: 1,
+	input: Type.Object(
+		{
+			message: Type.String({ minLength: 1, maxLength: 500 }),
+		},
+		{ additionalProperties: false },
+	),
+	output: Type.Object(
+		{
+			displayed: Type.Literal(true),
+		},
+		{ additionalProperties: false },
+	),
+} satisfies ClientAction<TSchema, TSchema>;
+
+export type NotifyInput = Static<typeof notifyAction.input>;
+export type NotifyOutput = Static<typeof notifyAction.output>;
+
+export interface ActionRegistration {
+	id: string;
+	version: number;
+}
+
+export type ServerEvent =
+	| { type: "ready"; clientId: string }
+	| { type: "capabilities_updated"; actions: ActionRegistration[] }
+	| { type: "user_prompt"; clientId: string; text: string }
+	| { type: "run_start" }
+	| { type: "run_end" }
+	| { type: "assistant_start" }
+	| { type: "assistant_delta"; text: string }
+	| { type: "assistant_end" }
+	| { type: "tool_start"; name: string }
+	| { type: "tool_end"; name: string; isError: boolean }
+	| { type: "error"; message: string }
+	| { type: "action_request"; requestId: string; action: ActionRegistration; input: unknown }
+	| { type: "action_cancel"; requestId: string };
+
+export interface ActionResultMessage {
+	clientId: string;
+	ok: boolean;
+	result?: unknown;
+	error?: string;
+}
diff --git a/packages/coding-agent/examples/capability-spike/server.ts b/packages/coding-agent/examples/capability-spike/server.ts
new file mode 100644
index 000000000..1e1e616ed
--- /dev/null
+++ b/packages/coding-agent/examples/capability-spike/server.ts
@@ -0,0 +1,317 @@
+import { readFile } from "node:fs/promises";
+import { createServer, type IncomingMessage, type ServerResponse } from "node:http";
+import { dirname, join } from "node:path";
+import { fileURLToPath } from "node:url";
+import {
+	AgentHarness,
+	type AgentHarnessEvent,
+	type AgentHarnessTool,
+	InMemorySessionStorage,
+	Session,
+} from "@earendil-works/pi-agent-core";
+import { createModels, type Model } from "@earendil-works/pi-ai";
+import { anthropicProvider } from "@earendil-works/pi-ai/providers/anthropic";
+import { openaiProvider } from "@earendil-works/pi-ai/providers/openai";
+import { CapabilityError, CapabilityRegistry } from "./capability-registry.ts";
+import {
+	type MessageEndHandler,
+	type NotifyExtensionContext,
+	notifyExtension,
+	type ServerCommand,
+	type ServerExtensionApi,
+} from "./extension.ts";
+import type { ActionRegistration, ActionResultMessage, ServerEvent } from "./protocol.ts";
+
+const host = process.env.PI_SPIKE_HOST ?? "127.0.0.1";
+const port = Number(process.env.PI_SPIKE_PORT ?? "8787");
+const modelSpec = process.env.PI_SPIKE_MODEL ?? "openai/gpt-5.4-nano";
+const [providerId, modelId] = modelSpec.split("/", 2);
+if (!providerId || !modelId) {
+	throw new Error(`PI_SPIKE_MODEL must use provider/model syntax, received ${modelSpec}`);
+}
+
+const models = createModels();
+models.setProvider(openaiProvider());
+models.setProvider(anthropicProvider());
+function createFallbackModel(provider: string, id: string): Model<"openai-responses" | "anthropic-messages"> {
+	const common = {
+		id,
+		name: id,
+		reasoning: false,
+		cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+		contextWindow: 128_000,
+		maxTokens: 16_384,
+	};
+	if (provider === "openai") {
+		return {
+			...common,
+			api: "openai-responses",
+			provider: "openai",
+			baseUrl: "https://api.openai.com/v1",
+			input: ["text"],
+		};
+	}
+	if (provider === "anthropic") {
+		return {
+			...common,
+			api: "anthropic-messages",
+			provider: "anthropic",
+			baseUrl: "https://api.anthropic.com",
+			input: ["text"],
+		};
+	}
+	throw new Error(`Unsupported provider ${provider}. This spike registers openai and anthropic.`);
+}
+const model = models.getModel(providerId, modelId) ?? createFallbackModel(providerId, modelId);
+
+const registry = new CapabilityRegistry();
+const tools: AgentHarnessTool<NotifyExtensionContext>[] = [];
+const commands = new Map<string, ServerCommand>();
+const messageEndHandlers: MessageEndHandler[] = [];
+const extensionApi: ServerExtensionApi = {
+	registerTool(tool) {
+		tools.push(tool);
+	},
+	registerCommand(name, command) {
+		commands.set(name, command);
+	},
+	on(type, handler) {
+		if (type === "message_end") messageEndHandlers.push(handler);
+	},
+};
+notifyExtension(extensionApi);
+
+let activeOriginClientId: string | undefined;
+let promptInFlight = false;
+
+function contextFor(clientId: string): NotifyExtensionContext {
+	return {
+		clientActions: {
+			invoke: (action, input, options) => registry.invoke(clientId, action, input, options),
+		},
+	};
+}
+
+const harness = new AgentHarness({
+	session: new Session(new InMemorySessionStorage()),
+	models,
+	model,
+	thinkingLevel: "off",
+	tools,
+	toolContext: () => {
+		if (!activeOriginClientId) throw new Error("No originating client is bound to this turn");
+		return contextFor(activeOriginClientId);
+	},
+	systemPrompt: [
+		"You are a concise assistant.",
+		"When asked for the capital of France, respond with exactly Paris and nothing else.",
+		"When the user asks for a notification, call the notify tool with the exact message they want shown.",
+	].join(" "),
+});
+
+harness.subscribe(async (event, signal) => {
+	for (const wireEvent of toServerEvents(event)) registry.broadcast(wireEvent);
+	if (event.type === "message_end" && messageEndHandlers.length > 0) {
+		if (!activeOriginClientId) throw new Error("No originating client is bound to this assistant response");
+		const context = contextFor(activeOriginClientId);
+		for (const handler of messageEndHandlers) await handler(event, context, signal);
+	}
+});
+
+function toServerEvents(event: AgentHarnessEvent): ServerEvent[] {
+	switch (event.type) {
+		case "agent_start":
+			return [{ type: "run_start" }];
+		case "agent_end":
+			return [{ type: "run_end" }];
+		case "message_start":
+			return event.message.role === "assistant" ? [{ type: "assistant_start" }] : [];
+		case "message_update":
+			return event.assistantMessageEvent.type === "text_delta"
+				? [{ type: "assistant_delta", text: event.assistantMessageEvent.delta }]
+				: [];
+		case "message_end":
+			if (event.message.role !== "assistant") return [];
+			return [
+				{ type: "assistant_end" },
+				...(event.message.errorMessage ? [{ type: "error" as const, message: event.message.errorMessage }] : []),
+			];
+		case "tool_execution_start":
+			return [{ type: "tool_start", name: event.toolName }];
+		case "tool_execution_end":
+			return [{ type: "tool_end", name: event.toolName, isError: event.isError }];
+		default:
+			return [];
+	}
+}
+
+function sendJson(response: ServerResponse, status: number, body: unknown): void {
+	response.writeHead(status, {
+		"content-type": "application/json; charset=utf-8",
+		"cache-control": "no-store",
+	});
+	response.end(JSON.stringify(body));
+}
+
+async function readJson(request: IncomingMessage): Promise<unknown> {
+	const chunks: Buffer[] = [];
+	let size = 0;
+	for await (const chunk of request) {
+		const buffer = Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk);
+		size += buffer.length;
+		if (size > 64 * 1024) throw new Error("Request body exceeds 64 KiB");
+		chunks.push(buffer);
+	}
+	if (chunks.length === 0) return {};
+	return JSON.parse(Buffer.concat(chunks).toString("utf8"));
+}
+
+function objectRecord(value: unknown): Record<string, unknown> | undefined {
+	return typeof value === "object" && value !== null && !Array.isArray(value)
+		? (value as Record<string, unknown>)
+		: undefined;
+}
+
+function requireClientId(value: unknown): string {
+	if (typeof value !== "string" || !/^[a-zA-Z0-9_-]{1,80}$/.test(value)) {
+		throw new Error("Invalid clientId");
+	}
+	return value;
+}
+
+function parseActions(value: unknown): ActionRegistration[] {
+	if (!Array.isArray(value)) throw new Error("actions must be an array");
+	return value.map((candidate) => {
+		const action = objectRecord(candidate);
+		if (!action || typeof action.id !== "string" || typeof action.version !== "number") {
+			throw new Error("Invalid action registration");
+		}
+		return { id: action.id, version: action.version };
+	});
+}
+
+function errorStatus(error: unknown): number {
+	if (!(error instanceof CapabilityError)) return 400;
+	return error.code === "client_unavailable" || error.code === "action_unavailable" ? 409 : 400;
+}
+
+function errorMessage(error: unknown): string {
+	return error instanceof Error ? error.message : String(error);
+}
+
+const webRoot = join(dirname(fileURLToPath(import.meta.url)), "web");
+const staticFiles = new Map([
+	["/", { path: join(webRoot, "index.html"), type: "text/html; charset=utf-8" }],
+	["/app.js", { path: join(webRoot, "app.js"), type: "text/javascript; charset=utf-8" }],
+	["/styles.css", { path: join(webRoot, "styles.css"), type: "text/css; charset=utf-8" }],
+]);
+
+const server = createServer(async (request, response) => {
+	try {
+		const url = new URL(request.url ?? "/", `http://${request.headers.host ?? `${host}:${port}`}`);
+		const staticFile = request.method === "GET" ? staticFiles.get(url.pathname) : undefined;
+		if (staticFile) {
+			response.writeHead(200, { "content-type": staticFile.type, "cache-control": "no-store" });
+			response.end(await readFile(staticFile.path));
+			return;
+		}
+
+		if (request.method === "GET" && url.pathname === "/api/events") {
+			const clientId = requireClientId(url.searchParams.get("clientId"));
+			response.writeHead(200, {
+				"content-type": "text/event-stream; charset=utf-8",
+				"cache-control": "no-cache, no-transform",
+				connection: "keep-alive",
+			});
+			response.flushHeaders();
+			const disconnect = registry.connect(clientId, (event) => {
+				response.write(`data: ${JSON.stringify(event)}\n\n`);
+			});
+			const heartbeat = setInterval(() => response.write(": heartbeat\n\n"), 15_000);
+			request.once("close", () => {
+				clearInterval(heartbeat);
+				disconnect();
+			});
+			return;
+		}
+
+		if (
+			request.method === "PUT" &&
+			url.pathname.startsWith("/api/clients/") &&
+			url.pathname.endsWith("/capabilities")
+		) {
+			const clientId = requireClientId(url.pathname.slice("/api/clients/".length, -"/capabilities".length));
+			const body = objectRecord(await readJson(request));
+			registry.register(clientId, parseActions(body?.actions));
+			sendJson(response, 200, { ok: true });
+			return;
+		}
+
+		if (request.method === "POST" && url.pathname.startsWith("/api/action-results/")) {
+			const requestId = url.pathname.slice("/api/action-results/".length);
+			const body = objectRecord(await readJson(request));
+			const message: ActionResultMessage = {
+				clientId: requireClientId(body?.clientId),
+				ok: body?.ok === true,
+				result: body?.result,
+				error: typeof body?.error === "string" ? body.error : undefined,
+			};
+			registry.handleResult(requestId, message);
+			sendJson(response, 200, { ok: true });
+			return;
+		}
+
+		if (request.method === "POST" && url.pathname === "/api/prompt") {
+			const body = objectRecord(await readJson(request));
+			const clientId = requireClientId(body?.clientId);
+			if (!registry.hasClient(clientId)) throw new CapabilityError("client_unavailable", "Client is not connected");
+			if (typeof body?.text !== "string" || body.text.trim().length === 0) throw new Error("Prompt is required");
+			if (promptInFlight) {
+				sendJson(response, 409, { ok: false, error: "The shared session is already processing a prompt" });
+				return;
+			}
+			promptInFlight = true;
+			activeOriginClientId = clientId;
+			registry.broadcast({ type: "user_prompt", clientId, text: body.text.trim() });
+			try {
+				const assistant = await harness.prompt(body.text.trim());
+				sendJson(response, 200, { ok: assistant.stopReason !== "error", error: assistant.errorMessage });
+			} finally {
+				activeOriginClientId = undefined;
+				promptInFlight = false;
+			}
+			return;
+		}
+
+		if (request.method === "POST" && url.pathname.startsWith("/api/commands/")) {
+			const name = url.pathname.slice("/api/commands/".length);
+			const command = commands.get(name);
+			if (!command) {
+				sendJson(response, 404, { ok: false, error: `Unknown command ${name}` });
+				return;
+			}
+			const body = objectRecord(await readJson(request));
+			const clientId = requireClientId(body?.clientId);
+			const result = await command(body, contextFor(clientId));
+			sendJson(response, 200, { ok: true, result });
+			return;
+		}
+
+		sendJson(response, 404, { ok: false, error: "Not found" });
+	} catch (error) {
+		sendJson(response, errorStatus(error), { ok: false, error: errorMessage(error) });
+	}
+});
+
+server.listen(port, host, async () => {
+	const auth = await models.checkAuth(providerId);
+	console.log(`Capability spike listening at http://${host}:${port}`);
+	console.log(`Model: ${modelSpec}${auth ? ` (${auth.source})` : " (provider auth not configured)"}`);
+	console.log("Open the web URL or run: npx tsx packages/coding-agent/examples/capability-spike/tui.ts");
+});
+
+for (const signal of ["SIGINT", "SIGTERM"] as const) {
+	process.once(signal, () => {
+		server.close(() => process.exit(0));
+	});
+}
diff --git a/packages/coding-agent/examples/capability-spike/tui.ts b/packages/coding-agent/examples/capability-spike/tui.ts
new file mode 100644
index 000000000..ef133fa51
--- /dev/null
+++ b/packages/coding-agent/examples/capability-spike/tui.ts
@@ -0,0 +1,182 @@
+import { randomUUID } from "node:crypto";
+import { createInterface } from "node:readline";
+import type { ActionResultMessage, ServerEvent } from "./protocol.ts";
+import { notifyAction } from "./protocol.ts";
+
+const baseUrl = process.env.PI_SPIKE_URL ?? "http://127.0.0.1:8787";
+const clientId = `tui-${randomUUID()}`;
+const eventsAbort = new AbortController();
+
+interface JsonResponse {
+	ok?: boolean;
+	error?: string;
+}
+
+async function request(path: string, init: RequestInit): Promise<unknown> {
+	const response = await fetch(`${baseUrl}${path}`, {
+		...init,
+		headers: { "content-type": "application/json", ...init.headers },
+	});
+	const body = (await response.json()) as JsonResponse;
+	if (!response.ok || body.ok === false) throw new Error(body.error ?? `Request failed with ${response.status}`);
+	return body;
+}
+
+function isServerEvent(value: unknown): value is ServerEvent {
+	return typeof value === "object" && value !== null && "type" in value && typeof value.type === "string";
+}
+
+const eventResponse = await fetch(`${baseUrl}/api/events?clientId=${encodeURIComponent(clientId)}`, {
+	headers: { accept: "text/event-stream" },
+	signal: eventsAbort.signal,
+});
+if (!eventResponse.ok || !eventResponse.body) {
+	throw new Error(`Could not connect to ${baseUrl}: ${eventResponse.status} ${eventResponse.statusText}`);
+}
+
+await request(`/api/clients/${encodeURIComponent(clientId)}/capabilities`, {
+	method: "PUT",
+	body: JSON.stringify({ actions: [{ id: notifyAction.id, version: notifyAction.version }] }),
+});
+
+const readline = createInterface({ input: process.stdin, output: process.stdout, prompt: "\x1b[1mYou ›\x1b[0m " });
+let assistantLineOpen = false;
+let busy = false;
+let runActive = false;
+
+function printLine(text: string): void {
+	if (assistantLineOpen) {
+		process.stdout.write("\n");
+		assistantLineOpen = false;
+	}
+	process.stdout.write(`\r\x1b[2K${text}\n`);
+}
+
+function restorePromptIfIdle(): void {
+	if (!runActive && !busy) readline.prompt(true);
+}
+
+async function answerAction(event: Extract<ServerEvent, { type: "action_request" }>): Promise<void> {
+	if (event.action.id !== notifyAction.id || event.action.version !== notifyAction.version) {
+		await postActionResult(event.requestId, { clientId, ok: false, error: "Unsupported action" });
+		return;
+	}
+	const input = typeof event.input === "object" && event.input !== null ? event.input : undefined;
+	const message = input && "message" in input && typeof input.message === "string" ? input.message : undefined;
+	if (!message) {
+		await postActionResult(event.requestId, { clientId, ok: false, error: "Invalid notify input" });
+		return;
+	}
+	printLine(`\x1b[33m${message}\x1b[0m`);
+	await postActionResult(event.requestId, { clientId, ok: true, result: { displayed: true } });
+	restorePromptIfIdle();
+}
+
+async function postActionResult(requestId: string, result: ActionResultMessage): Promise<void> {
+	await request(`/api/action-results/${encodeURIComponent(requestId)}`, {
+		method: "POST",
+		body: JSON.stringify(result),
+	});
+}
+
+function handleEvent(event: ServerEvent): void {
+	switch (event.type) {
+		case "run_start":
+			runActive = true;
+			process.stdout.write("\r\x1b[2K");
+			break;
+		case "run_end":
+			runActive = false;
+			restorePromptIfIdle();
+			break;
+		case "assistant_start":
+			if (assistantLineOpen) process.stdout.write("\n");
+			process.stdout.write("\r\x1b[2K\x1b[36mPi  ›\x1b[0m ");
+			assistantLineOpen = true;
+			break;
+		case "assistant_delta":
+			process.stdout.write(event.text);
+			break;
+		case "assistant_end":
+			if (assistantLineOpen) process.stdout.write("\n");
+			assistantLineOpen = false;
+			break;
+		case "tool_start":
+			printLine(`\x1b[2m[tool: ${event.name}]\x1b[0m`);
+			break;
+		case "tool_end":
+			break;
+		case "error":
+			printLine(`\x1b[31m${event.message}\x1b[0m`);
+			restorePromptIfIdle();
+			break;
+		case "action_request":
+			void answerAction(event).catch((error: unknown) => printLine(`\x1b[31m${String(error)}\x1b[0m`));
+			break;
+		default:
+			break;
+	}
+}
+
+async function consumeEvents(): Promise<void> {
+	const decoder = new TextDecoder();
+	let buffer = "";
+	for await (const chunk of eventResponse.body!) {
+		buffer += decoder.decode(chunk, { stream: true });
+		let boundary = buffer.indexOf("\n\n");
+		while (boundary !== -1) {
+			const frame = buffer.slice(0, boundary);
+			buffer = buffer.slice(boundary + 2);
+			for (const line of frame.split("\n")) {
+				if (!line.startsWith("data: ")) continue;
+				const parsed: unknown = JSON.parse(line.slice(6));
+				if (isServerEvent(parsed)) handleEvent(parsed);
+			}
+			boundary = buffer.indexOf("\n\n");
+		}
+	}
+}
+
+void consumeEvents().catch((error: unknown) => {
+	if (!eventsAbort.signal.aborted) {
+		printLine(`\x1b[31mEvent stream closed: ${String(error)}\x1b[0m`);
+		runActive = false;
+		restorePromptIfIdle();
+	}
+});
+
+console.log(`\x1b[2mConnected as ${clientId}. Use /notify <message> to invoke the capability without the LLM.\x1b[0m`);
+readline.prompt();
+
+readline.on("line", (line) => {
+	const text = line.trim();
+	if (!text) {
+		readline.prompt();
+		return;
+	}
+	if (busy) {
+		printLine("\x1b[33mThe shared session is busy.\x1b[0m");
+		return;
+	}
+	busy = true;
+	const operation = text.startsWith("/notify ")
+		? request("/api/commands/notify", {
+				method: "POST",
+				body: JSON.stringify({ clientId, message: text.slice("/notify ".length) }),
+			})
+		: request("/api/prompt", {
+				method: "POST",
+				body: JSON.stringify({ clientId, text }),
+			});
+	void operation
+		.catch((error: unknown) => printLine(`\x1b[31m${error instanceof Error ? error.message : String(error)}\x1b[0m`))
+		.finally(() => {
+			busy = false;
+			restorePromptIfIdle();
+		});
+});
+
+readline.once("close", () => {
+	eventsAbort.abort();
+	process.stdout.write("\n");
+});
diff --git a/packages/coding-agent/examples/capability-spike/web/app.js b/packages/coding-agent/examples/capability-spike/web/app.js
new file mode 100644
index 000000000..079205611
--- /dev/null
+++ b/packages/coding-agent/examples/capability-spike/web/app.js
@@ -0,0 +1,176 @@
+const notifyAction = { id: "pi.notify", version: 1 };
+const clientId = sessionStorage.getItem("pi-capability-client") ?? `web-${crypto.randomUUID()}`;
+sessionStorage.setItem("pi-capability-client", clientId);
+
+const connection = document.querySelector(".connection");
+const connectionLabel = document.querySelector("#connection-label");
+const transcript = document.querySelector("#transcript");
+const emptyState = document.querySelector("#empty-state");
+const form = document.querySelector("#prompt-form");
+const prompt = document.querySelector("#prompt");
+const send = document.querySelector("#send");
+const testNotify = document.querySelector("#test-notify");
+const notification = document.querySelector("#notification");
+
+let connected = false;
+let busy = false;
+let assistantContent;
+let notificationTimer;
+
+function setConnection(state, label) {
+	connection.dataset.state = state;
+	connectionLabel.textContent = label;
+	connected = state === "connected";
+	send.disabled = !connected || busy;
+	testNotify.disabled = !connected || busy;
+}
+
+function appendMessage(role, text, className = role.toLowerCase()) {
+	emptyState?.remove();
+	const message = document.createElement("article");
+	message.className = `message ${className}`;
+	const label = document.createElement("p");
+	label.className = "role";
+	label.textContent = role;
+	const content = document.createElement("p");
+	content.className = "content";
+	content.textContent = text;
+	message.append(label, content);
+	transcript.append(message);
+	transcript.scrollTop = transcript.scrollHeight;
+	return content;
+}
+
+function showNotification(message) {
+	clearTimeout(notificationTimer);
+	notification.hidden = false;
+	notification.textContent = message;
+	notification.classList.remove("is-flying");
+	void notification.offsetWidth;
+	notification.classList.add("is-flying");
+	notificationTimer = setTimeout(() => {
+		notification.hidden = true;
+		notification.classList.remove("is-flying");
+	}, 2400);
+}
+
+async function api(path, options) {
+	const response = await fetch(path, {
+		...options,
+		headers: { "content-type": "application/json", ...options?.headers },
+	});
+	const body = await response.json();
+	if (!response.ok || body.ok === false) throw new Error(body.error ?? `Request failed with ${response.status}`);
+	return body;
+}
+
+async function registerCapabilities() {
+	await api(`/api/clients/${encodeURIComponent(clientId)}/capabilities`, {
+		method: "PUT",
+		body: JSON.stringify({ actions: [notifyAction] }),
+	});
+	setConnection("connected", "Capability online");
+}
+
+async function answerAction(event) {
+	if (event.action.id !== notifyAction.id || event.action.version !== notifyAction.version) {
+		await postActionResult(event.requestId, { clientId, ok: false, error: "Unsupported action" });
+		return;
+	}
+	const message = event.input?.message;
+	if (typeof message !== "string" || !message) {
+		await postActionResult(event.requestId, { clientId, ok: false, error: "Invalid notify input" });
+		return;
+	}
+	showNotification(message);
+	await postActionResult(event.requestId, { clientId, ok: true, result: { displayed: true } });
+}
+
+async function postActionResult(requestId, result) {
+	await api(`/api/action-results/${encodeURIComponent(requestId)}`, {
+		method: "POST",
+		body: JSON.stringify(result),
+	});
+}
+
+function handleEvent(event) {
+	switch (event.type) {
+		case "ready":
+			void registerCapabilities().catch((error) => setConnection("error", error.message));
+			break;
+		case "user_prompt":
+			appendMessage(event.clientId === clientId ? "You" : "Peer", event.text, "user");
+			break;
+		case "assistant_start":
+			assistantContent = undefined;
+			break;
+		case "assistant_delta":
+			if (!assistantContent) assistantContent = appendMessage("Pi", "", "assistant");
+			assistantContent.textContent += event.text;
+			transcript.scrollTop = transcript.scrollHeight;
+			break;
+		case "assistant_end":
+			assistantContent = undefined;
+			break;
+		case "tool_start":
+			appendMessage("Tool", event.name, "tool");
+			break;
+		case "error":
+			appendMessage("Error", event.message, "error");
+			break;
+		case "action_request":
+			void answerAction(event).catch((error) => appendMessage("Error", error.message, "error"));
+			break;
+	}
+}
+
+const events = new EventSource(`/api/events?clientId=${encodeURIComponent(clientId)}`);
+events.onmessage = ({ data }) => handleEvent(JSON.parse(data));
+events.onerror = () => setConnection("error", "Reconnecting");
+
+form.addEventListener("submit", async (event) => {
+	event.preventDefault();
+	const text = prompt.value.trim();
+	if (!text || busy) return;
+	busy = true;
+	setConnection("connected", "Pi is working");
+	prompt.value = "";
+	try {
+		await api("/api/prompt", {
+			method: "POST",
+			body: JSON.stringify({ clientId, text }),
+		});
+	} catch (error) {
+		appendMessage("Error", error instanceof Error ? error.message : String(error), "error");
+	} finally {
+		busy = false;
+		setConnection("connected", "Capability online");
+		prompt.focus();
+	}
+});
+
+prompt.addEventListener("keydown", (event) => {
+	if (event.key === "Enter" && !event.shiftKey) {
+		event.preventDefault();
+		form.requestSubmit();
+	}
+});
+
+testNotify.addEventListener("click", async () => {
+	if (busy) return;
+	busy = true;
+	setConnection("connected", "Invoking directly");
+	try {
+		await api("/api/commands/notify", {
+			method: "POST",
+			body: JSON.stringify({ clientId, message: "Browser capability confirmed." }),
+		});
+	} catch (error) {
+		appendMessage("Error", error instanceof Error ? error.message : String(error), "error");
+	} finally {
+		busy = false;
+		setConnection("connected", "Capability online");
+	}
+});
+
+window.addEventListener("beforeunload", () => events.close());
diff --git a/packages/coding-agent/examples/capability-spike/web/index.html b/packages/coding-agent/examples/capability-spike/web/index.html
new file mode 100644
index 000000000..97ad67766
--- /dev/null
+++ b/packages/coding-agent/examples/capability-spike/web/index.html
@@ -0,0 +1,51 @@
+<!doctype html>
+<html lang="en">
+	<head>
+		<meta charset="UTF-8" />
+		<meta name="viewport" content="width=device-width, initial-scale=1" />
+		<meta name="color-scheme" content="light" />
+		<link rel="icon" href="data:," />
+		<title>Pi capability spike</title>
+		<link rel="stylesheet" href="/styles.css" />
+	</head>
+	<body>
+		<div class="page-grid" aria-hidden="true"></div>
+		<header class="masthead">
+			<div>
+				<p class="eyebrow">Client 02 / browser</p>
+				<h1>Pi, loosely coupled.</h1>
+			</div>
+			<div class="connection" data-state="connecting">
+				<span class="connection-dot"></span>
+				<span id="connection-label">Connecting</span>
+			</div>
+		</header>
+
+		<main>
+			<section class="transcript" id="transcript" aria-live="polite" aria-label="Conversation">
+				<div class="empty-state" id="empty-state">
+					<p class="index">01</p>
+					<p>Ask Pi to notify you, or fire the capability directly.</p>
+				</div>
+			</section>
+
+			<form class="composer" id="prompt-form">
+				<label for="prompt">Prompt</label>
+				<div class="input-row">
+					<textarea id="prompt" name="prompt" rows="2" placeholder="Notify me that the browser capability works" required></textarea>
+					<button type="submit" id="send" disabled>Send</button>
+				</div>
+				<div class="composer-meta">
+					<span>Enter to send · Shift+Enter for newline</span>
+					<button type="button" class="text-button" id="test-notify" disabled>Test notify directly</button>
+				</div>
+			</form>
+		</main>
+
+		<div class="notification-stage" aria-live="assertive" aria-atomic="true">
+			<div class="notification" id="notification" hidden></div>
+		</div>
+
+		<script type="module" src="/app.js"></script>
+	</body>
+</html>
diff --git a/packages/coding-agent/examples/capability-spike/web/styles.css b/packages/coding-agent/examples/capability-spike/web/styles.css
new file mode 100644
index 000000000..4a821f15a
--- /dev/null
+++ b/packages/coding-agent/examples/capability-spike/web/styles.css
@@ -0,0 +1,376 @@
+@layer reset, base, components, motion;
+
+@layer reset {
+	*,
+	*::before,
+	*::after {
+		box-sizing: border-box;
+	}
+
+	body,
+	h1,
+	p {
+		margin: 0;
+	}
+
+	button,
+	textarea {
+		font: inherit;
+	}
+}
+
+@layer base {
+	:root {
+		--paper: oklch(0.96 0.025 86);
+		--ink: oklch(0.19 0.025 54);
+		--muted: oklch(0.49 0.04 70);
+		--rule: oklch(0.78 0.055 76);
+		--signal: oklch(0.78 0.18 77);
+		--hot: oklch(0.59 0.23 34);
+		--success: oklch(0.55 0.13 151);
+		font-family: "Avenir Next", "Century Gothic", sans-serif;
+		color: var(--ink);
+		background: var(--paper);
+	}
+
+	body {
+		min-block-size: 100svh;
+		display: grid;
+		grid-template-rows: auto 1fr;
+		padding: clamp(1rem, 3vw, 2.75rem);
+		overflow-x: hidden;
+	}
+
+	button,
+	textarea {
+		color: inherit;
+	}
+
+	button:focus-visible,
+	textarea:focus-visible {
+		outline: 3px solid var(--hot);
+		outline-offset: 3px;
+	}
+
+	.page-grid {
+		position: fixed;
+		inset: 0;
+		z-index: -1;
+		background-image:
+			linear-gradient(to right, color-mix(in oklch, var(--rule) 34%, transparent) 1px, transparent 1px),
+			linear-gradient(to bottom, color-mix(in oklch, var(--rule) 25%, transparent) 1px, transparent 1px);
+		background-size: 2rem 2rem;
+		mask-image: linear-gradient(to bottom, black, transparent 78%);
+	}
+}
+
+@layer components {
+	.masthead {
+		display: flex;
+		align-items: start;
+		justify-content: space-between;
+		gap: 2rem;
+		padding-block-end: clamp(2rem, 7vh, 5rem);
+
+		& h1 {
+			max-inline-size: 15ch;
+			font-family: "Iowan Old Style", "Palatino Linotype", Georgia, serif;
+			font-size: clamp(2.75rem, 8vw, 7.5rem);
+			font-weight: 600;
+			font-style: italic;
+			letter-spacing: -0.065em;
+			line-height: 0.84;
+			text-wrap: balance;
+		}
+	}
+
+	.eyebrow,
+	.connection,
+	.composer label,
+	.composer-meta {
+		font-size: 0.72rem;
+		font-weight: 700;
+		letter-spacing: 0.12em;
+		text-transform: uppercase;
+	}
+
+	.eyebrow {
+		margin-block-end: 1rem;
+		color: var(--hot);
+	}
+
+	.connection {
+		display: flex;
+		align-items: center;
+		gap: 0.55rem;
+		padding-block: 0.4rem;
+		white-space: nowrap;
+
+		& .connection-dot {
+			inline-size: 0.65rem;
+			block-size: 0.65rem;
+			border: 1px solid currentColor;
+			border-radius: 50%;
+			background: var(--signal);
+		}
+
+		&[data-state="connected"] .connection-dot {
+			background: var(--success);
+		}
+
+		&[data-state="error"] .connection-dot {
+			background: var(--hot);
+		}
+	}
+
+	main {
+		inline-size: min(100%, 68rem);
+		margin-inline-start: auto;
+		display: grid;
+		grid-template-rows: minmax(14rem, 1fr) auto;
+		gap: clamp(2rem, 5vh, 4rem);
+	}
+
+	.transcript {
+		align-self: end;
+		max-block-size: 42vh;
+		overflow-y: auto;
+		scrollbar-gutter: stable;
+		display: grid;
+		gap: 1.2rem;
+		padding-inline: 0.25rem 1rem;
+	}
+
+	.empty-state {
+		display: grid;
+		grid-template-columns: 4rem minmax(0, 26rem);
+		gap: 1rem;
+		align-items: start;
+		color: var(--muted);
+		font-family: "Iowan Old Style", "Palatino Linotype", Georgia, serif;
+		font-size: clamp(1.15rem, 2.5vw, 1.65rem);
+		line-height: 1.25;
+
+		& .index {
+			font-family: "Avenir Next", "Century Gothic", sans-serif;
+			font-size: 0.72rem;
+			font-weight: 700;
+			letter-spacing: 0.12em;
+		}
+	}
+
+	.message {
+		display: grid;
+		grid-template-columns: 4rem minmax(0, 42rem);
+		gap: 1rem;
+		line-height: 1.55;
+		white-space: pre-wrap;
+
+		& .role {
+			padding-block-start: 0.25rem;
+			color: var(--muted);
+			font-size: 0.67rem;
+			font-weight: 800;
+			letter-spacing: 0.1em;
+			text-transform: uppercase;
+		}
+
+		&.user .content {
+			font-family: "Iowan Old Style", "Palatino Linotype", Georgia, serif;
+			font-size: 1.2rem;
+		}
+
+		&.tool .content {
+			color: var(--muted);
+			font-size: 0.82rem;
+			font-weight: 700;
+			letter-spacing: 0.06em;
+			text-transform: uppercase;
+		}
+	}
+
+	.composer {
+		border-block-start: 2px solid var(--ink);
+		padding-block-start: 0.75rem;
+
+		& label {
+		display: block;
+		margin-block-end: 0.6rem;
+		}
+	}
+
+	.input-row {
+		display: grid;
+		grid-template-columns: 1fr auto;
+		gap: 0.75rem;
+		align-items: stretch;
+	}
+
+	textarea {
+		min-block-size: 5.5rem;
+		resize: vertical;
+		border: 0;
+		background: transparent;
+		padding: 0.65rem 0;
+		font-family: "Iowan Old Style", "Palatino Linotype", Georgia, serif;
+		font-size: clamp(1.15rem, 2.2vw, 1.55rem);
+		line-height: 1.35;
+
+		&::placeholder {
+			color: color-mix(in oklch, var(--muted) 65%, transparent);
+		}
+	}
+
+	button {
+		border: 0;
+		cursor: pointer;
+	}
+
+	#send {
+		min-inline-size: 6.5rem;
+		background: var(--ink);
+		color: var(--paper);
+		font-size: 0.8rem;
+		font-weight: 800;
+		letter-spacing: 0.08em;
+		text-transform: uppercase;
+		transition: transform 160ms ease-out, background-color 160ms ease-out;
+
+		&:hover:not(:disabled) {
+			background: var(--hot);
+			transform: translateY(-2px);
+		}
+
+		&:disabled {
+			cursor: not-allowed;
+			opacity: 0.35;
+		}
+	}
+
+	.composer-meta {
+		display: flex;
+		justify-content: space-between;
+		gap: 1rem;
+		margin-block-start: 0.75rem;
+		color: var(--muted);
+	}
+
+	.text-button {
+		padding: 0;
+		background: transparent;
+		color: var(--hot);
+		font-weight: 800;
+		text-decoration: underline;
+		text-decoration-thickness: 1px;
+		text-underline-offset: 0.25rem;
+
+		&:disabled {
+			cursor: not-allowed;
+			opacity: 0.4;
+		}
+	}
+
+	.notification-stage {
+		position: fixed;
+		inset: 0;
+		z-index: 10;
+		pointer-events: none;
+		overflow: hidden;
+	}
+
+	.notification {
+		position: absolute;
+		inset-block-start: 50%;
+		inset-inline-start: 50%;
+		max-inline-size: min(80vw, 46rem);
+		padding: clamp(1rem, 3vw, 2rem) clamp(1.25rem, 5vw, 3.5rem);
+		background: var(--signal);
+		color: var(--ink);
+		box-shadow: 0.8rem 0.8rem 0 var(--hot);
+		font-family: "Iowan Old Style", "Palatino Linotype", Georgia, serif;
+		font-size: clamp(1.75rem, 6vw, 5rem);
+		font-weight: 700;
+		font-style: italic;
+		line-height: 0.95;
+		text-align: center;
+		text-wrap: balance;
+	}
+
+	@media (width < 44rem) {
+		body {
+			padding: 1rem;
+		}
+
+		.masthead {
+			align-items: end;
+			padding-block-end: 3rem;
+		}
+
+		.connection {
+			& #connection-label {
+				display: none;
+			}
+		}
+
+		.message,
+		.empty-state {
+			grid-template-columns: 2.5rem minmax(0, 1fr);
+		}
+
+		.input-row {
+			grid-template-columns: 1fr;
+		}
+
+		#send {
+			min-block-size: 3.25rem;
+		}
+
+		.composer-meta {
+			align-items: start;
+			flex-direction: column;
+		}
+	}
+}
+
+@layer motion {
+	.notification.is-flying {
+		animation: notify-spin-out 2.35s cubic-bezier(0.77, 0, 0.22, 1) both;
+	}
+
+	@keyframes notify-spin-out {
+		0% {
+			opacity: 0;
+			transform: translate(-50%, -50%) rotate(-18deg) scale(0.35);
+		}
+		13% {
+			opacity: 1;
+			transform: translate(-50%, -50%) rotate(2deg) scale(1.08);
+		}
+		25%,
+		52% {
+			opacity: 1;
+			transform: translate(-50%, -50%) rotate(0deg) scale(1);
+		}
+		100% {
+			opacity: 0;
+			transform: translate(calc(-50% + 112vw), calc(-50% - 82vh)) rotate(1600deg) scale(0.08);
+		}
+	}
+
+	@media (prefers-reduced-motion: reduce) {
+		*,
+		*::before,
+		*::after {
+			scroll-behavior: auto !important;
+		}
+
+		.notification.is-flying {
+			animation: notify-fade 2.35s ease-out both;
+		}
+
+		@keyframes notify-fade {
+			0%, 100% { opacity: 0; transform: translate(-50%, -50%); }
+			15%, 75% { opacity: 1; transform: translate(-50%, -50%); }
+		}
+	}
+}