diff --git a/apps/ai/src/mcp/lib/resolve-tenant.oauth.test.ts b/apps/ai/src/mcp/lib/resolve-tenant.oauth.test.ts
index 783f3dcc54..11c9f3e7db 100644
--- a/apps/ai/src/mcp/lib/resolve-tenant.oauth.test.ts
+++ b/apps/ai/src/mcp/lib/resolve-tenant.oauth.test.ts
@@ -110,6 +110,16 @@ describe("an MCP OAuth token, resolved by the AI worker", () => {
 		}).pipe(Effect.provide(makeLayer(db)))
 	})
 
+	it.effect("treats a request with cookies and no bearer as carrying no credential", () => {
+		const db = createTestDb(createdDbs)
+		return Effect.gen(function* () {
+			const failure = yield* resolveMcpTenantContext(
+				new Request(resource, { headers: { cookie: "__session=anything" } }),
+			).pipe(Effect.flip)
+			expect(failure._tag).toBe("@maple/mcp/errors/McpAuthMissingError")
+		}).pipe(Effect.provide(makeLayer(db)))
+	})
+
 	it.effect("is refused for a resource it was not bound to", () => {
 		const db = createTestDb(createdDbs)
 		return Effect.gen(function* () {
diff --git a/apps/ai/src/mcp/lib/resolve-tenant.ts b/apps/ai/src/mcp/lib/resolve-tenant.ts
index 87f4bb36db..35f52cbaf5 100644
--- a/apps/ai/src/mcp/lib/resolve-tenant.ts
+++ b/apps/ai/src/mcp/lib/resolve-tenant.ts
@@ -122,8 +122,8 @@ export const describeMalformedBearer = (token: string | undefined): string | und
  */
 export const mcpResourceUrl = (apiBaseUrl: string) => `${Str.replace(/\/+$/, "")(apiBaseUrl)}/mcp`
 
-/** A request with neither an `Authorization` header nor a cookie sent nothing a session reads. */
-const carriesCredential = (headers: Headers): boolean => headers.has("authorization") || headers.has("cookie")
+/** A request without an `Authorization` header sent nothing a session reads. */
+const carriesCredential = (headers: Headers): boolean => headers.has("authorization")
 
 /**
  * The session resolver's failures as the MCP ones. A provider outage is retryable (503), and a
diff --git a/apps/web/src/api-proxy-forward.ts b/apps/web/src/api-proxy-forward.ts
index c980cd28cc..e75990635e 100644
--- a/apps/web/src/api-proxy-forward.ts
+++ b/apps/web/src/api-proxy-forward.ts
@@ -73,6 +73,8 @@ const forwardOverBinding = (
 			// Copying the request copies the browser's `Host: app.…`, and a service binding passes it
 			// through as-is. The API builds OAuth redirect URIs from Host, so it must name the API.
 			forwarded.headers.set("host", target.host)
+			// The app authenticates with a bearer. The page's cookies are not the API's to read.
+			forwarded.headers.delete("cookie")
 			// @effect-diagnostics-next-line preferEffectRecord:off -- not a record type; Rec.toEntries would need a cast
 			Arr.forEach(Object.entries(HttpTraceContext.toHeaders(client)), ([name, value]) =>
 				forwarded.headers.set(name, value),
diff --git a/apps/web/src/handler.test.ts b/apps/web/src/handler.test.ts
index 8b73fb6c47..cb493ed8a4 100644
--- a/apps/web/src/handler.test.ts
+++ b/apps/web/src/handler.test.ts
@@ -56,6 +56,18 @@ describe("handleRequest API proxy", () => {
 		expect(await request?.text()).toBe("hello")
 	})
 
+	it("keeps the page's cookies out of the forwarded request", async () => {
+		const { env, forwarded } = recordingEnv()
+		await handleRequest(
+			new Request("https://app.maple.test/_api/v2/traces", {
+				headers: { authorization: "Bearer t", cookie: "__session=s; theme=dark" },
+			}),
+			env,
+		)
+		expect(forwarded[0]?.headers.get("cookie")).toBeNull()
+		expect(forwarded[0]?.headers.get("authorization")).toBe("Bearer t")
+	})
+
 	it("sends the API's host, not the app's, so OAuth callbacks stay on the API", async () => {
 		const { env, forwarded } = recordingEnv()
 		await handleRequest(
diff --git a/packages/auth/src/auth.test.ts b/packages/auth/src/auth.test.ts
index 5104e2b8f4..68de681450 100644
--- a/packages/auth/src/auth.test.ts
+++ b/packages/auth/src/auth.test.ts
@@ -123,6 +123,44 @@ describe("makeResolveTenant", () => {
 		}),
 	)
 
+	it.effect("signs a request in by its bearer, never by a session cookie", () =>
+		Effect.gen(function* () {
+			// Stands in for the provider, which accepts either credential it is shown.
+			const resolveTenant = makeResolveTenant(
+				{
+					...baseEnv,
+					MAPLE_AUTH_MODE: "clerk",
+					CLERK_SECRET_KEY: Option.some(Redacted.make("sk_test_123")),
+					CLERK_JWT_KEY: Option.some(Redacted.make("jwt_test_123")),
+				},
+				async (request) => {
+					const shown = request.headers.has("authorization") || request.headers.has("cookie")
+					return {
+						isAuthenticated: shown,
+						message: shown ? null : "No credential",
+						toAuth: () => ({
+							isAuthenticated: true,
+							tokenType: "session_token",
+							userId: "user_123",
+							orgId: "org_123",
+							orgRole: "org:admin",
+						}),
+					}
+				},
+			)
+
+			assertRejected(
+				yield* Effect.exit(resolveTenant({ cookie: "__session=test-token" })),
+				"No credential",
+			)
+			const tenant = yield* resolveTenant({
+				Authorization: "Bearer test-token",
+				cookie: "__session=other-token",
+			})
+			assert.strictEqual(tenant.userId, asUserId("user_123"))
+		}),
+	)
+
 	it.effect("rejects invalid or expired Clerk tokens", () =>
 		Effect.gen(function* () {
 			const resolveTenant = makeResolveTenant(
diff --git a/packages/auth/src/index.ts b/packages/auth/src/index.ts
index d684dbc7ec..d4e0904e22 100644
--- a/packages/auth/src/index.ts
+++ b/packages/auth/src/index.ts
@@ -173,11 +173,15 @@ const getBearerToken = (headers: HeaderRecord): string | undefined => {
 	return token
 }
 
+/**
+ * The bearer and nothing else. A cookie the browser attaches on its own is never a credential
+ * here, so only a caller that can read the session token can act with it.
+ */
 const toHeaders = (headers: HeaderRecord): Headers => {
 	const requestHeaders = new Headers()
 
 	for (const [name, value] of Rec.toEntries(headers)) {
-		if (value !== undefined) {
+		if (value !== undefined && Str.toLowerCase(name) === "authorization") {
 			requestHeaders.set(name, value)
 		}
 	}