diff --git a/packages/worker/src/oauth-handlers.ts b/packages/worker/src/oauth-handlers.ts
index 8fdc350ce0..c3da72894f 100644
--- a/packages/worker/src/oauth-handlers.ts
+++ b/packages/worker/src/oauth-handlers.ts
@@ -25,6 +25,8 @@ import { invalidClientIdMismatchMessage } from '@kody-internal/shared/oauth-mess
 import { getUsernameFormatValidationError } from '#app/username.ts'
 import { getPkceValidationError } from '#worker/oauth-pkce.ts'
 import { oauthPaths } from '#app/oauth-paths.ts'
+import { getAppBaseUrl } from '#app/app-base-url.ts'
+import { mcpResourcePath } from './mcp-auth.ts'
 
 export { oauthPaths }
 
@@ -214,7 +216,28 @@ function createSetCookieHeaders(cookies: Array<string | null | undefined>) {
 	return hasCookie ? headers : undefined
 }
 
-async function resolveAuthRequest(helpers: OAuthHelpers, request: Request) {
+function defaultMcpResourceForAuthRequest(
+	authRequest: AuthRequest,
+	request: Request,
+	env: Env,
+) {
+	// Gemini Spark (and some other MCP hosts) omit RFC 8707 `resource` on
+	// authorize. Without a grant resource, token exchange can mint a token with
+	// no audience, and `/mcp` rejects it. Default to the MCP resource when the
+	// client did not send one; explicit `resource` values are left unchanged.
+	if (authRequest.resource !== undefined) return
+	const origin = getAppBaseUrl({
+		env,
+		requestUrl: request.url,
+	})
+	authRequest.resource = `${origin}${mcpResourcePath}`
+}
+
+async function resolveAuthRequest(
+	helpers: OAuthHelpers,
+	request: Request,
+	env: Env,
+) {
 	try {
 		const authRequest = await helpers.parseAuthRequest(request)
 		if (!authRequest.clientId || !authRequest.redirectUri) {
@@ -227,6 +250,7 @@ async function resolveAuthRequest(helpers: OAuthHelpers, request: Request) {
 		if (!client) {
 			return { error: 'Unknown OAuth client.' }
 		}
+		defaultMcpResourceForAuthRequest(authRequest, request, env)
 		return { authRequest, client }
 	} catch (error) {
 		const message =
@@ -593,7 +617,7 @@ export async function loadOAuthAuthorizeData(
 	env: Env,
 ): Promise<OAuthAuthorizeDataResult> {
 	const helpers = getOAuthHelpers(env)
-	const resolution = await resolveAuthRequest(helpers, request)
+	const resolution = await resolveAuthRequest(helpers, request, env)
 	if ('error' in resolution) {
 		const { allowClientReset, setCookie } =
 			await resolveAuthorizeInfoResetState(
@@ -736,7 +760,7 @@ export async function handleAuthorizeRequest(
 		return handleResetClientRequest(request, env, helpers, requestIp)
 	}
 
-	const resolution = await resolveAuthRequest(helpers, request)
+	const resolution = await resolveAuthRequest(helpers, request, env)
 	if ('error' in resolution) {
 		return respondAuthorizeError(
 			request,
diff --git a/packages/worker/src/oauth-handlers.workers.test.ts b/packages/worker/src/oauth-handlers.workers.test.ts
index 41d135a8fb..1188fa5c97 100644
--- a/packages/worker/src/oauth-handlers.workers.test.ts
+++ b/packages/worker/src/oauth-handlers.workers.test.ts
@@ -56,6 +56,25 @@ const claudeClient: ClientInfo = {
 	clientName: 'Claude',
 	tokenEndpointAuthMethod: 'none',
 }
+// Gemini Spark custom MCP apps omit RFC 8707 `resource` on authorize.
+const geminiAuthorizeUrl =
+	'https://heykody.dev/oauth/authorize?response_type=code&client_id=QuXak4ugdtPZncjp&redirect_uri=https%3A%2F%2Foauth-redirect.googleusercontent.com%2Fr%2Fuser_bound_custom-mcp-106664623666703652842-heykody_dev&scope=profile+email&code_challenge=uMIae0HtFCz1_e_KxVlJ_W2oz1eT87iC0h4WkBB4tEc&code_challenge_method=S256&state=gemini-demo-state'
+const geminiAuthRequestWithoutResource: AuthRequest = {
+	responseType: 'code',
+	clientId: 'QuXak4ugdtPZncjp',
+	redirectUri:
+		'https://oauth-redirect.googleusercontent.com/r/user_bound_custom-mcp-106664623666703652842-heykody_dev',
+	scope: ['profile', 'email'],
+	state: 'gemini-demo-state',
+	codeChallenge: 'uMIae0HtFCz1_e_KxVlJ_W2oz1eT87iC0h4WkBB4tEc',
+	codeChallengeMethod: 'S256',
+}
+const geminiClient: ClientInfo = {
+	clientId: geminiAuthRequestWithoutResource.clientId,
+	redirectUris: [geminiAuthRequestWithoutResource.redirectUri],
+	clientName: 'Gemini',
+	tokenEndpointAuthMethod: 'none',
+}
 
 function createHelpers(overrides: Partial<OAuthHelpers> = {}): OAuthHelpers {
 	return {
@@ -500,6 +519,46 @@ test('Claude-shaped authorize requests render and approve without throwing', asy
 	expect(capturedOptions?.request.scope).toEqual(['profile', 'email'])
 })
 
+test('Gemini-shaped authorize requests default resource to /mcp when omitted', async () => {
+	let capturedOptions: CompleteAuthorizationOptions | null = null
+	const helpers = createHelpers({
+		// Return a fresh object each time so the defaulting mutation stays local.
+		parseAuthRequest: async () => ({ ...geminiAuthRequestWithoutResource }),
+		lookupClient: async () => geminiClient,
+		async completeAuthorization(options) {
+			capturedOptions = options
+			return {
+				redirectTo:
+					'https://oauth-redirect.googleusercontent.com/r/user_bound_custom-mcp-106664623666703652842-heykody_dev?code=demo&state=gemini-demo-state',
+			}
+		},
+	})
+	const postResponse = await handleAuthorizeRequest(
+		new Request(geminiAuthorizeUrl, {
+			method: 'POST',
+			headers: {
+				Accept: 'application/json',
+				'Content-Type': 'application/x-www-form-urlencoded',
+			},
+			body: new URLSearchParams({
+				decision: 'approve',
+				email: 'user@example.com',
+				password: 'password123',
+			}),
+		}),
+		createEnv(helpers, await createDatabase('password123')),
+	)
+
+	expect(postResponse.status).toBe(200)
+	await expect(postResponse.json()).resolves.toEqual({
+		ok: true,
+		redirectTo:
+			'https://oauth-redirect.googleusercontent.com/r/user_bound_custom-mcp-106664623666703652842-heykody_dev?code=demo&state=gemini-demo-state',
+	})
+	expect(capturedOptions?.request.resource).toBe('https://heykody.dev/mcp')
+	expect(geminiAuthRequestWithoutResource.resource).toBeUndefined()
+})
+
 test('session approval uses database user id when cookie email is stale', async () => {
 	const currentEmail = `changed-oauth-${crypto.randomUUID()}@example.com`
 	const userId = await seedWorkerUser(currentEmail, 'password123')