diff --git a/packages/protocol/src/cbor-decoder.ts b/packages/protocol/src/cbor/decoder.ts
similarity index 99%
rename from packages/protocol/src/cbor-decoder.ts
rename to packages/protocol/src/cbor/decoder.ts
index db19154d0..979a3c044 100644
--- a/packages/protocol/src/cbor-decoder.ts
+++ b/packages/protocol/src/cbor/decoder.ts
@@ -5,7 +5,7 @@ import {
 	resolveOptions,
 	textDecoder,
 	UINT32_BASE,
-} from "./cbor-options.ts";
+} from "./types.ts";
 
 class CborReader {
 	private readonly bytes: Uint8Array;
diff --git a/packages/protocol/src/cbor-encoder.ts b/packages/protocol/src/cbor/encoder.ts
similarity index 99%
rename from packages/protocol/src/cbor-encoder.ts
rename to packages/protocol/src/cbor/encoder.ts
index 081b919de..0713d28dc 100644
--- a/packages/protocol/src/cbor-encoder.ts
+++ b/packages/protocol/src/cbor/encoder.ts
@@ -7,7 +7,7 @@ import {
 	textDecoder,
 	textEncoder,
 	UINT32_BASE,
-} from "./cbor-options.ts";
+} from "./types.ts";
 
 class CborWriter {
 	private buffer: Uint8Array;
diff --git a/packages/protocol/src/cbor.ts b/packages/protocol/src/cbor/index.ts
similarity index 51%
rename from packages/protocol/src/cbor.ts
rename to packages/protocol/src/cbor/index.ts
index 87cd405e4..63f94b838 100644
--- a/packages/protocol/src/cbor.ts
+++ b/packages/protocol/src/cbor/index.ts
@@ -1,9 +1,9 @@
-export { decodeCbor } from "./cbor-decoder.ts";
-export { encodeCbor } from "./cbor-encoder.ts";
+export { decodeCbor } from "./decoder.ts";
+export { encodeCbor } from "./encoder.ts";
 export {
 	CborError,
 	type CborOptions,
 	DEFAULT_MAX_CBOR_BYTE_LENGTH,
 	DEFAULT_MAX_CBOR_CONTAINER_LENGTH,
 	DEFAULT_MAX_CBOR_DEPTH,
-} from "./cbor-options.ts";
+} from "./types.ts";
diff --git a/packages/protocol/src/cbor-options.ts b/packages/protocol/src/cbor/types.ts
similarity index 100%
rename from packages/protocol/src/cbor-options.ts
rename to packages/protocol/src/cbor/types.ts
diff --git a/packages/protocol/src/codec.ts b/packages/protocol/src/codec.ts
index 249f21dbb..b83ae9853 100644
--- a/packages/protocol/src/codec.ts
+++ b/packages/protocol/src/codec.ts
@@ -1,5 +1,5 @@
 import { Check } from "typebox/value";
-import { decodeCbor, encodeCbor } from "./cbor.ts";
+import { decodeCbor, encodeCbor } from "./cbor/index.ts";
 import {
 	assertCompleteFrame,
 	DEFAULT_MAX_FRAME_LENGTH,
@@ -7,8 +7,13 @@ import {
 	FrameDecoder,
 	type FrameDecoderOptions,
 } from "./framing.ts";
-import { type ClientMessage, ClientMessageSchema, type ServerMessage, ServerMessageSchema } from "./messages.ts";
-import { PROTOCOL_VERSION } from "./schemas.ts";
+import {
+	type ClientMessage,
+	ClientMessageSchema,
+	PROTOCOL_VERSION,
+	type ServerMessage,
+	ServerMessageSchema,
+} from "./schemas.ts";
 
 export class ProtocolValidationError extends Error {
 	constructor(message: string, _value?: unknown) {
@@ -17,25 +22,31 @@ export class ProtocolValidationError extends Error {
 	}
 }
 
-function isProtocolValue(value: unknown, optionalProperty = false): boolean {
+function isProtocolValue(value: unknown, optionalProperty = false, ancestors = new Set<object>()): boolean {
 	if (value === undefined) return optionalProperty;
 	if (value === null || typeof value === "boolean" || typeof value === "number" || typeof value === "string") {
 		return true;
 	}
-	if (Array.isArray(value)) return value.every((item) => isProtocolValue(item));
-	if (typeof value !== "object" || Object.getPrototypeOf(value) !== Object.prototype) return false;
-	return Object.values(value).every((item) => isProtocolValue(item, true));
+	if (typeof value !== "object" || ancestors.has(value)) return false;
+	ancestors.add(value);
+	try {
+		if (Array.isArray(value)) return value.every((item) => isProtocolValue(item, false, ancestors));
+		if (Object.getPrototypeOf(value) !== Object.prototype) return false;
+		return Object.values(value).every((item) => isProtocolValue(item, true, ancestors));
+	} finally {
+		ancestors.delete(value);
+	}
 }
 
 export function parseClientMessage(value: unknown): ClientMessage {
-	if (!Check(ClientMessageSchema, value) || !isProtocolValue(value)) {
+	if (!isProtocolValue(value) || !Check(ClientMessageSchema, value)) {
 		throw new ProtocolValidationError("Invalid client protocol message");
 	}
 	return value;
 }
 
 export function parseServerMessage(value: unknown): ServerMessage {
-	if (!Check(ServerMessageSchema, value) || !isProtocolValue(value)) {
+	if (!isProtocolValue(value) || !Check(ServerMessageSchema, value)) {
 		throw new ProtocolValidationError("Invalid server protocol message");
 	}
 	return value;
diff --git a/packages/protocol/src/index.ts b/packages/protocol/src/index.ts
index 2c230e19a..411641ef8 100644
--- a/packages/protocol/src/index.ts
+++ b/packages/protocol/src/index.ts
@@ -1,5 +1,4 @@
-export * from "./cbor.ts";
+export * from "./cbor/index.ts";
 export * from "./codec.ts";
 export * from "./framing.ts";
-export * from "./messages.ts";
 export * from "./schemas.ts";
diff --git a/packages/protocol/src/messages.ts b/packages/protocol/src/messages.ts
deleted file mode 100644
index 48dfbd29e..000000000
--- a/packages/protocol/src/messages.ts
+++ /dev/null
@@ -1,182 +0,0 @@
-import Type, { type Static } from "typebox";
-import {
-	ModelRefSchema,
-	PROTOCOL_VERSION,
-	ProtocolErrorSchema,
-	ServerSnapshotSchema,
-	SessionSnapshotSchema,
-	SessionSummarySchema,
-	ThinkingLevelSchema,
-	TranscriptProgressSchema,
-} from "./schemas.ts";
-
-const IdSchema = Type.String({ minLength: 1 });
-const StrictObject = <const T extends Parameters<typeof Type.Object>[0]>(properties: T) =>
-	Type.Object(properties, { additionalProperties: false });
-
-const PromptPayloadProperties = {
-	sessionId: IdSchema,
-	text: Type.String(),
-} as const;
-
-export const ListCommandSchema = StrictObject({ command: Type.Literal("list") });
-export const CreateCommandSchema = StrictObject({
-	command: Type.Literal("create"),
-	cwd: Type.Optional(Type.String({ minLength: 1 })),
-	name: Type.Optional(Type.String()),
-	model: Type.Optional(ModelRefSchema),
-	thinkingLevel: Type.Optional(ThinkingLevelSchema),
-});
-export const AttachCommandSchema = StrictObject({ command: Type.Literal("attach"), sessionId: IdSchema });
-export const DetachCommandSchema = StrictObject({ command: Type.Literal("detach"), sessionId: IdSchema });
-export const PromptCommandSchema = StrictObject({ command: Type.Literal("prompt"), ...PromptPayloadProperties });
-export const SteerCommandSchema = StrictObject({ command: Type.Literal("steer"), ...PromptPayloadProperties });
-export const AbortCommandSchema = StrictObject({ command: Type.Literal("abort"), sessionId: IdSchema });
-export const SetModelCommandSchema = StrictObject({
-	command: Type.Literal("set_model"),
-	sessionId: IdSchema,
-	model: ModelRefSchema,
-});
-export const SetThinkingCommandSchema = StrictObject({
-	command: Type.Literal("set_thinking"),
-	sessionId: IdSchema,
-	thinkingLevel: ThinkingLevelSchema,
-});
-export const CommandSchema = Type.Union([
-	ListCommandSchema,
-	CreateCommandSchema,
-	AttachCommandSchema,
-	DetachCommandSchema,
-	PromptCommandSchema,
-	SteerCommandSchema,
-	AbortCommandSchema,
-	SetModelCommandSchema,
-	SetThinkingCommandSchema,
-]);
-export type Command = Static<typeof CommandSchema>;
-export type CommandName = Command["command"];
-
-export const CreateResultSchema = StrictObject({
-	command: Type.Literal("create"),
-	session: SessionSnapshotSchema,
-});
-export const AttachResultSchema = StrictObject({
-	command: Type.Literal("attach"),
-	session: SessionSnapshotSchema,
-});
-export const PromptResultSchema = StrictObject({
-	command: Type.Literal("prompt"),
-	session: SessionSnapshotSchema,
-});
-export const SteerResultSchema = StrictObject({
-	command: Type.Literal("steer"),
-	session: SessionSnapshotSchema,
-});
-export const AbortResultSchema = StrictObject({
-	command: Type.Literal("abort"),
-	session: SessionSnapshotSchema,
-});
-export const SetModelResultSchema = StrictObject({
-	command: Type.Literal("set_model"),
-	session: SessionSnapshotSchema,
-});
-export const SetThinkingResultSchema = StrictObject({
-	command: Type.Literal("set_thinking"),
-	session: SessionSnapshotSchema,
-});
-
-export const ListResultSchema = StrictObject({
-	command: Type.Literal("list"),
-	sessions: Type.Array(SessionSummarySchema),
-});
-export const DetachResultSchema = StrictObject({
-	command: Type.Literal("detach"),
-	sessionId: IdSchema,
-});
-export const CommandResultSchema = Type.Union([
-	ListResultSchema,
-	CreateResultSchema,
-	AttachResultSchema,
-	DetachResultSchema,
-	PromptResultSchema,
-	SteerResultSchema,
-	AbortResultSchema,
-	SetModelResultSchema,
-	SetThinkingResultSchema,
-]);
-export type CommandResult = Static<typeof CommandResultSchema>;
-
-export type ResultForCommand<TCommand extends Command> = TCommand["command"] extends "list"
-	? Static<typeof ListResultSchema>
-	: TCommand["command"] extends "detach"
-		? Static<typeof DetachResultSchema>
-		: Extract<CommandResult, { command: TCommand["command"] }>;
-
-/** Must be the first frame sent by a client. Version is intentionally an integer, not a coercible string. */
-export const ClientHelloSchema = StrictObject({
-	type: Type.Literal("hello"),
-	version: Type.Integer({ minimum: 0 }),
-	token: Type.String({ minLength: 1 }),
-});
-export type ClientHello = Static<typeof ClientHelloSchema>;
-
-export const RequestEnvelopeSchema = StrictObject({
-	type: Type.Literal("request"),
-	id: IdSchema,
-	request: CommandSchema,
-});
-export type RequestEnvelope = Static<typeof RequestEnvelopeSchema>;
-export const ClientMessageSchema = Type.Union([ClientHelloSchema, RequestEnvelopeSchema]);
-export type ClientMessage = Static<typeof ClientMessageSchema>;
-
-export const ServerEventSchema = Type.Union([
-	StrictObject({ type: Type.Literal("server_snapshot"), snapshot: ServerSnapshotSchema }),
-	StrictObject({ type: Type.Literal("session_snapshot"), snapshot: SessionSnapshotSchema }),
-	StrictObject({
-		type: Type.Literal("session_progress"),
-		sessionId: IdSchema,
-		progress: TranscriptProgressSchema,
-	}),
-	StrictObject({ type: Type.Literal("session_removed"), sessionId: IdSchema }),
-]);
-export type ServerEvent = Static<typeof ServerEventSchema>;
-
-export const ServerHelloSchema = StrictObject({
-	type: Type.Literal("hello"),
-	version: Type.Literal(PROTOCOL_VERSION),
-	connectionId: IdSchema,
-	snapshot: ServerSnapshotSchema,
-});
-export const ServerHelloErrorSchema = StrictObject({
-	type: Type.Literal("hello_error"),
-	error: ProtocolErrorSchema,
-});
-export const ResponseEnvelopeSchema = Type.Union([
-	StrictObject({
-		type: Type.Literal("response"),
-		id: IdSchema,
-		ok: Type.Literal(true),
-		result: CommandResultSchema,
-	}),
-	StrictObject({
-		type: Type.Literal("response"),
-		id: IdSchema,
-		ok: Type.Literal(false),
-		error: ProtocolErrorSchema,
-	}),
-]);
-export const EventEnvelopeSchema = StrictObject({
-	type: Type.Literal("event"),
-	event: ServerEventSchema,
-});
-export const ServerMessageSchema = Type.Union([
-	ServerHelloSchema,
-	ServerHelloErrorSchema,
-	ResponseEnvelopeSchema,
-	EventEnvelopeSchema,
-]);
-export type ServerHello = Static<typeof ServerHelloSchema>;
-export type ServerHelloError = Static<typeof ServerHelloErrorSchema>;
-export type ResponseEnvelope = Static<typeof ResponseEnvelopeSchema>;
-export type EventEnvelope = Static<typeof EventEnvelopeSchema>;
-export type ServerMessage = Static<typeof ServerMessageSchema>;
diff --git a/packages/protocol/src/schemas.ts b/packages/protocol/src/schemas.ts
index c9ef63518..16cde6320 100644
--- a/packages/protocol/src/schemas.ts
+++ b/packages/protocol/src/schemas.ts
@@ -243,3 +243,170 @@ export const ProtocolErrorSchema = StrictObject({
 });
 export type ProtocolErrorCode = Static<typeof ProtocolErrorCodeSchema>;
 export type ProtocolError = Static<typeof ProtocolErrorSchema>;
+
+const PromptPayloadProperties = {
+	sessionId: IdSchema,
+	text: Type.String(),
+} as const;
+
+export const ListCommandSchema = StrictObject({ command: Type.Literal("list") });
+export const CreateCommandSchema = StrictObject({
+	command: Type.Literal("create"),
+	cwd: Type.Optional(Type.String({ minLength: 1 })),
+	name: Type.Optional(Type.String()),
+	model: Type.Optional(ModelRefSchema),
+	thinkingLevel: Type.Optional(ThinkingLevelSchema),
+});
+export const AttachCommandSchema = StrictObject({ command: Type.Literal("attach"), sessionId: IdSchema });
+export const DetachCommandSchema = StrictObject({ command: Type.Literal("detach"), sessionId: IdSchema });
+export const PromptCommandSchema = StrictObject({ command: Type.Literal("prompt"), ...PromptPayloadProperties });
+export const SteerCommandSchema = StrictObject({ command: Type.Literal("steer"), ...PromptPayloadProperties });
+export const AbortCommandSchema = StrictObject({ command: Type.Literal("abort"), sessionId: IdSchema });
+export const SetModelCommandSchema = StrictObject({
+	command: Type.Literal("set_model"),
+	sessionId: IdSchema,
+	model: ModelRefSchema,
+});
+export const SetThinkingCommandSchema = StrictObject({
+	command: Type.Literal("set_thinking"),
+	sessionId: IdSchema,
+	thinkingLevel: ThinkingLevelSchema,
+});
+export const CommandSchema = Type.Union([
+	ListCommandSchema,
+	CreateCommandSchema,
+	AttachCommandSchema,
+	DetachCommandSchema,
+	PromptCommandSchema,
+	SteerCommandSchema,
+	AbortCommandSchema,
+	SetModelCommandSchema,
+	SetThinkingCommandSchema,
+]);
+export type Command = Static<typeof CommandSchema>;
+export type CommandName = Command["command"];
+
+export const CreateResultSchema = StrictObject({
+	command: Type.Literal("create"),
+	session: SessionSnapshotSchema,
+});
+export const AttachResultSchema = StrictObject({
+	command: Type.Literal("attach"),
+	session: SessionSnapshotSchema,
+});
+export const PromptResultSchema = StrictObject({
+	command: Type.Literal("prompt"),
+	session: SessionSnapshotSchema,
+});
+export const SteerResultSchema = StrictObject({
+	command: Type.Literal("steer"),
+	session: SessionSnapshotSchema,
+});
+export const AbortResultSchema = StrictObject({
+	command: Type.Literal("abort"),
+	session: SessionSnapshotSchema,
+});
+export const SetModelResultSchema = StrictObject({
+	command: Type.Literal("set_model"),
+	session: SessionSnapshotSchema,
+});
+export const SetThinkingResultSchema = StrictObject({
+	command: Type.Literal("set_thinking"),
+	session: SessionSnapshotSchema,
+});
+
+export const ListResultSchema = StrictObject({
+	command: Type.Literal("list"),
+	sessions: Type.Array(SessionSummarySchema),
+});
+export const DetachResultSchema = StrictObject({
+	command: Type.Literal("detach"),
+	sessionId: IdSchema,
+});
+export const CommandResultSchema = Type.Union([
+	ListResultSchema,
+	CreateResultSchema,
+	AttachResultSchema,
+	DetachResultSchema,
+	PromptResultSchema,
+	SteerResultSchema,
+	AbortResultSchema,
+	SetModelResultSchema,
+	SetThinkingResultSchema,
+]);
+export type CommandResult = Static<typeof CommandResultSchema>;
+
+export type ResultForCommand<TCommand extends Command> = TCommand["command"] extends "list"
+	? Static<typeof ListResultSchema>
+	: TCommand["command"] extends "detach"
+		? Static<typeof DetachResultSchema>
+		: Extract<CommandResult, { command: TCommand["command"] }>;
+
+/** Must be the first frame sent by a client. Version is intentionally an integer, not a coercible string. */
+export const ClientHelloSchema = StrictObject({
+	type: Type.Literal("hello"),
+	version: Type.Integer({ minimum: 0 }),
+	token: Type.String({ minLength: 1 }),
+});
+export type ClientHello = Static<typeof ClientHelloSchema>;
+
+export const RequestEnvelopeSchema = StrictObject({
+	type: Type.Literal("request"),
+	id: IdSchema,
+	request: CommandSchema,
+});
+export type RequestEnvelope = Static<typeof RequestEnvelopeSchema>;
+export const ClientMessageSchema = Type.Union([ClientHelloSchema, RequestEnvelopeSchema]);
+export type ClientMessage = Static<typeof ClientMessageSchema>;
+
+export const ServerEventSchema = Type.Union([
+	StrictObject({ type: Type.Literal("server_snapshot"), snapshot: ServerSnapshotSchema }),
+	StrictObject({ type: Type.Literal("session_snapshot"), snapshot: SessionSnapshotSchema }),
+	StrictObject({
+		type: Type.Literal("session_progress"),
+		sessionId: IdSchema,
+		progress: TranscriptProgressSchema,
+	}),
+	StrictObject({ type: Type.Literal("session_removed"), sessionId: IdSchema }),
+]);
+export type ServerEvent = Static<typeof ServerEventSchema>;
+
+export const ServerHelloSchema = StrictObject({
+	type: Type.Literal("hello"),
+	version: Type.Literal(PROTOCOL_VERSION),
+	connectionId: IdSchema,
+	snapshot: ServerSnapshotSchema,
+});
+export const ServerHelloErrorSchema = StrictObject({
+	type: Type.Literal("hello_error"),
+	error: ProtocolErrorSchema,
+});
+export const ResponseEnvelopeSchema = Type.Union([
+	StrictObject({
+		type: Type.Literal("response"),
+		id: IdSchema,
+		ok: Type.Literal(true),
+		result: CommandResultSchema,
+	}),
+	StrictObject({
+		type: Type.Literal("response"),
+		id: IdSchema,
+		ok: Type.Literal(false),
+		error: ProtocolErrorSchema,
+	}),
+]);
+export const EventEnvelopeSchema = StrictObject({
+	type: Type.Literal("event"),
+	event: ServerEventSchema,
+});
+export const ServerMessageSchema = Type.Union([
+	ServerHelloSchema,
+	ServerHelloErrorSchema,
+	ResponseEnvelopeSchema,
+	EventEnvelopeSchema,
+]);
+export type ServerHello = Static<typeof ServerHelloSchema>;
+export type ServerHelloError = Static<typeof ServerHelloErrorSchema>;
+export type ResponseEnvelope = Static<typeof ResponseEnvelopeSchema>;
+export type EventEnvelope = Static<typeof EventEnvelopeSchema>;
+export type ServerMessage = Static<typeof ServerMessageSchema>;
diff --git a/packages/protocol/test/cbor.test.ts b/packages/protocol/test/cbor/cbor.test.ts
similarity index 99%
rename from packages/protocol/test/cbor.test.ts
rename to packages/protocol/test/cbor/cbor.test.ts
index 0bb944eed..20da27b2f 100644
--- a/packages/protocol/test/cbor.test.ts
+++ b/packages/protocol/test/cbor/cbor.test.ts
@@ -6,7 +6,7 @@ import {
 	DEFAULT_MAX_CBOR_DEPTH,
 	decodeCbor,
 	encodeCbor,
-} from "../src/index.ts";
+} from "../../src/index.ts";
 
 function fromHex(hex: string): Uint8Array {
 	if (hex.length % 2 !== 0) throw new Error("Hex fixture must contain whole bytes");
diff --git a/packages/protocol/test/protocol.test.ts b/packages/protocol/test/protocol.test.ts
index 9b0d9c209..23621ab47 100644
--- a/packages/protocol/test/protocol.test.ts
+++ b/packages/protocol/test/protocol.test.ts
@@ -121,6 +121,19 @@ describe("protocol validation", () => {
 		expect(parseServerMessage(message)).toEqual(message);
 	});
 
+	test("rejects cyclic protocol values with a protocol validation error", () => {
+		const details: Record<string, unknown> = {};
+		details.self = details;
+		const message = {
+			type: "response",
+			id: "request-1",
+			ok: false,
+			error: { code: "invalid_request", message: "invalid", details },
+		};
+
+		expect(() => parseServerMessage(message)).toThrow(ProtocolValidationError);
+	});
+
 	test("validation errors do not retain rejected payloads", () => {
 		let thrown: unknown;
 		try {