fix(acp): keep server logs off stdout

src/acp/server.startup.test.ts

@@ -21,6 +21,7 @@ const mockState = vi.hoisted(() => ({
   gatewayAuth: [] as GatewayClientAuth[],
   agentSideConnectionCtor: vi.fn(),
   agentStart: vi.fn(),
+  routeLogsToStderr: vi.fn(),
   resolveGatewayClientBootstrap: vi.fn<ResolveGatewayClientBootstrap>(async (_params) => ({
     url: "ws://127.0.0.1:18789",
     urlSource: "local loopback",
@@ -104,6 +105,10 @@ vi.mock("../infra/is-main.js", () => ({
   isMainModule: () => false,
 }));
 
+vi.mock("../logging/console.js", () => ({
+  routeLogsToStderr: () => mockState.routeLogsToStderr(),
+}));
+
 vi.mock("./translator.js", () => ({
   AcpGatewayAgent: class {
     start(): void {
@@ -166,6 +171,7 @@ describe("serveAcpGateway startup", () => {
     mockState.gatewayAuth.length = 0;
     mockState.agentSideConnectionCtor.mockReset();
     mockState.agentStart.mockReset();
+    mockState.routeLogsToStderr.mockReset();
     mockState.resolveGatewayClientBootstrap.mockReset();
     mockState.resolveGatewayClientBootstrap.mockResolvedValue({
       url: "ws://127.0.0.1:18789",
@@ -192,6 +198,26 @@ describe("serveAcpGateway startup", () => {
     }
   });
 
+  it("routes logs to stderr before loading gateway config", async () => {
+    const { signalHandlers, onceSpy } = captureProcessSignalHandlers();
+
+    try {
+      const servePromise = serveAcpGateway({});
+      await Promise.resolve();
+
+      expect(mockState.routeLogsToStderr).toHaveBeenCalledTimes(1);
+      expect(mockState.routeLogsToStderr.mock.invocationCallOrder[0]).toBeLessThan(
+        mockState.resolveGatewayClientBootstrap.mock.invocationCallOrder[0] ??
+          Number.MAX_SAFE_INTEGER,
+      );
+
+      await emitHelloAndWaitForAgentSideConnection();
+      await stopServeWithSigint(signalHandlers, servePromise);
+    } finally {
+      onceSpy.mockRestore();
+    }
+  });
+
   it("rejects startup when gateway connect fails before hello", async () => {
     const onceSpy = vi
       .spyOn(process, "once")

src/acp/server.ts

@@ -7,12 +7,14 @@ import { resolveGatewayClientBootstrap } from "../gateway/client-bootstrap.js";
 import { GatewayClient } from "../gateway/client.js";
 import { GATEWAY_CLIENT_MODES, GATEWAY_CLIENT_NAMES } from "../gateway/protocol/client-info.js";
 import { isMainModule } from "../infra/is-main.js";
+import { routeLogsToStderr } from "../logging/console.js";
 import { normalizeOptionalString } from "../shared/string-coerce.js";
 import { readSecretFromFile } from "./secret-file.js";
 import { AcpGatewayAgent } from "./translator.js";
 import { normalizeAcpProvenanceMode, type AcpServerOptions } from "./types.js";
 
 export async function serveAcpGateway(opts: AcpServerOptions = {}): Promise<void> {
+  routeLogsToStderr();
   const cfg = loadConfig();
   const bootstrap = await resolveGatewayClientBootstrap({
     config: cfg,