diff --git a/packages/coding-agent/src/core/mcp-servers.ts b/packages/coding-agent/src/core/mcp-servers.ts
index 858d8f2fa..a30a7a727 100644
--- a/packages/coding-agent/src/core/mcp-servers.ts
+++ b/packages/coding-agent/src/core/mcp-servers.ts
@@ -93,6 +93,11 @@ export interface McpHttpServerConfig extends McpServerConfigBase {
 	/** Values may reference environment variables (`${NAME}`) or commands (`!cmd`). */
 	headers?: Record<string, string>;
 	oauth?: McpOAuthConfig;
+	/**
+	 * Send the token of a pi provider (`/login <provider>`) instead of using OAuth. Not allowed in project
+	 * `mcp.json` files, and requires https except on loopback hosts, since it sends the credential to `url`.
+	 */
+	auth?: { provider: string };
 }
 
 export type McpServerConfig = McpStdioServerConfig | McpHttpServerConfig;
@@ -217,6 +222,15 @@ export function validateMcpServerConfig(name: string, raw: unknown): McpServerCo
 		}
 		const oauthError = validateOAuth(value.oauth);
 		if (oauthError) return `server "${name}": ${oauthError}`;
+		if (value.auth !== undefined) {
+			if (!isRecord(value.auth) || typeof value.auth.provider !== "string" || !value.auth.provider) {
+				return `server "${name}": auth.provider must be a provider name`;
+			}
+			const url = new URL(value.url);
+			if (url.protocol !== "https:" && !LOOPBACK_HOSTS.includes(url.hostname)) {
+				return `server "${name}": auth requires an https URL, or http on localhost, 127.0.0.1, or [::1]`;
+			}
+		}
 		return value as unknown as McpHttpServerConfig;
 	}
 	if (typeof value.command === "string" && (type === undefined || type === "stdio")) {
diff --git a/packages/coding-agent/src/extensions/mcp/config.ts b/packages/coding-agent/src/extensions/mcp/config.ts
index b1b685779..c2b7efb8e 100644
--- a/packages/coding-agent/src/extensions/mcp/config.ts
+++ b/packages/coding-agent/src/extensions/mcp/config.ts
@@ -17,6 +17,8 @@
  * ```
  *
  * HTTP servers without an `Authorization` header use OAuth when they answer 401 (sign in with `/mcp`).
+ * `"auth": { "provider": "<provider>" }` sends the token of a `/login` provider instead. Project files
+ * cannot use it, so a repository cannot pick where the credential goes.
  *
  * The top-level `autoEnableCodemode` (default true) activates the codemode tool when a server
  * with `codemode` exposure connects. A project value overrides the global one.
@@ -98,6 +100,10 @@ function readConfigFile(path: string, scope: "global" | "project", state: McpCon
 			errors.push(`${path}: server "${name}" conflicts with "${clash}"`);
 			continue;
 		}
+		if (scope === "project" && "url" in config && config.auth) {
+			errors.push(`${path}: server "${name}": auth is only allowed in the global mcp.json`);
+			continue;
+		}
 		servers.set(name, { name, config, source: path, scope });
 	}
 }
diff --git a/packages/coding-agent/src/extensions/mcp/index.ts b/packages/coding-agent/src/extensions/mcp/index.ts
index 14b81a82c..0248112b9 100644
--- a/packages/coding-agent/src/extensions/mcp/index.ts
+++ b/packages/coding-agent/src/extensions/mcp/index.ts
@@ -38,6 +38,7 @@ import type {
 	ToolDefinition,
 } from "../../core/extensions/types.ts";
 import { mcpNamespace } from "../../core/mcp-servers.ts";
+import type { ModelRegistry } from "../../core/model-registry.ts";
 import { openBrowser } from "../../utils/open-browser.ts";
 import { CODEMODE_TOOL_NAME, isCodemodeTool } from "../codemode/tool.ts";
 import { isToolSearchTool, TOOL_SEARCH_TOOL_NAME } from "../tool-search/tool.ts";
@@ -287,6 +288,8 @@ export function createMcpExtension(options: McpExtensionOptions = {}): Extension
 		/** Working directory of the session, for stdio servers. */
 		let sessionCwd = process.cwd();
 		let credentials = options.credentials;
+		/** The session's model registry, which resolves `auth.provider` tokens. */
+		let modelRegistry: ModelRegistry | undefined;
 		let serverLog: McpServerLog | undefined;
 		const openUrl = options.openUrl ?? openBrowser;
 		const updateConfig =
@@ -507,6 +510,7 @@ export function createMcpExtension(options: McpExtensionOptions = {}): Extension
 				cwd: sessionCwd,
 				createTransport: options.createTransport ?? runtime.createDefaultTransport,
 				credentials: getCredentials(runtime),
+				providerToken: async (provider) => modelRegistry?.getApiKeyForProvider(provider),
 				log: getServerLog(runtime),
 				onTools: registerTools,
 				onChange: onConnectionChange,
@@ -945,6 +949,7 @@ export function createMcpExtension(options: McpExtensionOptions = {}): Extension
 			warnedUnreachable = false;
 			waitedForStartup = false;
 			sessionCwd = ctx.cwd;
+			modelRegistry = ctx.modelRegistry;
 			const current = ++generation;
 			sessionActive = true;
 			configuredEntries = loaded.servers;
diff --git a/packages/coding-agent/src/extensions/mcp/runtime.ts b/packages/coding-agent/src/extensions/mcp/runtime.ts
index f848854f1..01e8b5b1d 100644
--- a/packages/coding-agent/src/extensions/mcp/runtime.ts
+++ b/packages/coding-agent/src/extensions/mcp/runtime.ts
@@ -73,14 +73,15 @@ function isTransientError(error: unknown): boolean {
 	return error instanceof TypeError;
 }
 
-function signInRequiredMessage(name: string): string {
-	return `MCP server "${name}" requires sign-in. Run /mcp to sign in.`;
+function signInRequiredMessage(entry: McpServerEntry): string {
+	const provider = "url" in entry.config ? entry.config.auth?.provider : undefined;
+	return `MCP server "${entry.name}" requires sign-in. Run ${provider ? `/login ${provider}` : "/mcp"} to sign in.`;
 }
 
-/** HTTP servers authenticate with OAuth unless the config supplies an `Authorization` header. */
+/** HTTP servers authenticate with OAuth unless the config supplies an `Authorization` header or `auth`. */
 function usesOAuth(entry: McpServerEntry): boolean {
 	const { config } = entry;
-	if (!("url" in config)) return false;
+	if (!("url" in config) || config.auth) return false;
 	return !Object.keys(config.headers ?? {}).some((header) => header.toLowerCase() === "authorization");
 }
 
@@ -184,6 +185,8 @@ export class McpServerConnection implements McpToolCaller, McpResourceServer {
 		cwd: string;
 		createTransport: McpTransportFactory;
 		credentials: McpOAuthCredentialStore;
+		/** The current token of a pi provider, for servers with `auth.provider`. */
+		providerToken?: (provider: string) => Promise<string | undefined>;
 		onTools: (connection: McpServerConnection) => void;
 		/** Called when `state`, `error`, or `tools` change. */
 		onChange?: (connection: McpServerConnection) => void;
@@ -197,6 +200,7 @@ export class McpServerConnection implements McpToolCaller, McpResourceServer {
 		this.onChange = options.onChange;
 		this.log = options.log;
 		const url = this.oauthUrl;
+		const provider = "url" in this.entry.config ? this.entry.config.auth?.provider : undefined;
 		this.authProvider = url
 			? createMcpAuthProvider({
 					serverUrl: url,
@@ -206,7 +210,10 @@ export class McpServerConnection implements McpToolCaller, McpResourceServer {
 						this.challenge = challenge;
 					},
 				})
-			: undefined;
+			: provider
+				? // Read on every request, so the provider's refreshes apply; MCP stores no copy.
+					{ token: async () => options.providerToken?.(provider), settled: async () => {} }
+				: undefined;
 	}
 
 	get name(): string {
@@ -299,7 +306,7 @@ export class McpServerConnection implements McpToolCaller, McpResourceServer {
 				if (!this.needsSignIn(error)) throw error;
 				await this.dropClient(client);
 				this.markNeedsAuth();
-				throw new Error(signInRequiredMessage(this.entry.name));
+				throw new Error(signInRequiredMessage(this.entry));
 			}
 		}
 	}
@@ -322,7 +329,7 @@ export class McpServerConnection implements McpToolCaller, McpResourceServer {
 	private needsSignIn(error: unknown): boolean {
 		return (
 			error instanceof McpOAuthAuthorizationRequiredError ||
-			(this.oauthUrl !== undefined && error instanceof McpAuthRequiredError)
+			(this.authProvider !== undefined && error instanceof McpAuthRequiredError)
 		);
 	}
 
@@ -412,7 +419,7 @@ export class McpServerConnection implements McpToolCaller, McpResourceServer {
 	private connectFailed(error: unknown): Error {
 		if (this.needsSignIn(error) && !this.closed) {
 			this.markNeedsAuth();
-			return new Error(signInRequiredMessage(this.entry.name));
+			return new Error(signInRequiredMessage(this.entry));
 		}
 		this.state = this.closed ? "closed" : "failed";
 		this.error = this.stderrTail ? `${errorMessage(error)}\n${this.stderrTail}` : errorMessage(error);
diff --git a/packages/coding-agent/test/mcp-extension.test.ts b/packages/coding-agent/test/mcp-extension.test.ts
index 4afc5a909..b94fc8174 100644
--- a/packages/coding-agent/test/mcp-extension.test.ts
+++ b/packages/coding-agent/test/mcp-extension.test.ts
@@ -1,4 +1,6 @@
 import { mkdirSync, mkdtempSync, readFileSync, realpathSync, rmSync, writeFileSync } from "node:fs";
+import { createServer } from "node:http";
+import type { AddressInfo } from "node:net";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import {
@@ -179,6 +181,33 @@ describe("MCP config", () => {
 			"codemode",
 		);
 	});
+
+	it("validates provider auth and accepts it only in the global mcp.json", () => {
+		const paths = setup(
+			{
+				mcpServers: {
+					radius: { url: "https://radius.example/mcp", auth: { provider: "radius" } },
+					local: { url: "http://localhost:8788/mcp", auth: { provider: "radius-dev" } },
+					plain: { url: "http://radius.example/mcp", auth: { provider: "radius" } },
+					empty: { url: "https://radius.example/mcp", auth: { provider: "" } },
+				},
+			},
+			{ mcpServers: { radius: { url: "https://evil.example/mcp", auth: { provider: "radius" } } } },
+		);
+		const { servers, errors } = loadMcpConfig({ ...paths, projectTrusted: true });
+		// The project entry cannot replace the global one: it would send the credential to its own URL.
+		expect(servers.map((server) => [server.name, server.scope, "url" in server.config && server.config.url])).toEqual(
+			[
+				["radius", "global", "https://radius.example/mcp"],
+				["local", "global", "http://localhost:8788/mcp"],
+			],
+		);
+		expect(errors).toEqual([
+			expect.stringContaining('server "plain": auth requires an https URL'),
+			expect.stringContaining('server "empty": auth.provider must be a provider name'),
+			expect.stringContaining('server "radius": auth is only allowed in the global mcp.json'),
+		]);
+	});
 });
 
 describe("MCP tools", () => {
@@ -535,6 +564,40 @@ for await (const line of createInterface({ input: process.stdin })) {
 		await connection.close();
 	});
 
+	it("sends the provider token and asks for the provider login when the server rejects it", async () => {
+		const authorizations: (string | undefined)[] = [];
+		const server = createServer((request, response) => {
+			authorizations.push(request.headers.authorization);
+			request.resume();
+			response.writeHead(401).end();
+		});
+		await new Promise<void>((resolve) => server.listen(0, "127.0.0.1", resolve));
+		const { port } = server.address() as AddressInfo;
+		const connection = new McpServerConnection({
+			entry: {
+				name: "radius",
+				config: { url: `http://127.0.0.1:${port}/mcp`, auth: { provider: "radius" } },
+				source: "test",
+			},
+			cwd: process.cwd(),
+			createTransport: createDefaultTransport,
+			credentials: new McpOAuthCredentialStore(new InMemoryAuthStorageBackend()),
+			providerToken: async (provider) => (provider === "radius" ? "tok" : undefined),
+			onTools: () => {},
+		});
+		try {
+			expect(connection.oauthUrl).toBeUndefined();
+			await expect(connection.getClient()).rejects.toThrow(
+				'MCP server "radius" requires sign-in. Run /login radius to sign in.',
+			);
+			expect(connection.state).toBe("needs-auth");
+			expect(authorizations).toEqual(["Bearer tok"]);
+		} finally {
+			await connection.close();
+			await new Promise((resolve) => server.close(resolve));
+		}
+	});
+
 	it("appends server log messages to the log file", async () => {
 		const dir = mkdtempSync(join(tmpdir(), "pi-mcp-log-"));
 		try {