From fd008cd8e0b8277e691efd8e69dc96ac8e188d9e Mon Sep 17 00:00:00 2001
From: Kaoru0822 <aisakamimaki@gmail.com>
Date: Tue, 6 Oct 2026 03:58:12 -0700
Subject: [PATCH] fix(mcp): retain shutdown failure diagnostics

---
 src/main/shutdown.ts                | 30 +++++++++++++-
 tests/boundary/mcp/shutdown.test.ts | 61 +++++++++++++++++++++++++++++
 2 files changed, 90 insertions(+), 1 deletion(-)
 create mode 100644 tests/boundary/mcp/shutdown.test.ts

diff --git a/src/main/shutdown.ts b/src/main/shutdown.ts
index f4bf1e4d7..eb6cc229d 100644
--- a/src/main/shutdown.ts
+++ b/src/main/shutdown.ts
@@ -29,7 +29,11 @@ export const createShutdown = (input: {
     return shutdownPromise;
   };
   const requestShutdown = (): void => {
-    shutdown().catch(() => {
+    shutdown().catch((cause: unknown) => {
+      serverLogger.debug(
+        { failure_cause: describeShutdownFailure(cause) },
+        "MCP shutdown rejected",
+      );
       dependencies.setExitCode(1);
       serverLogger.error(MCP_SHUTDOWN_FAILED);
       dependencies.writeStderr(`${MCP_SHUTDOWN_FAILED}\n`);
@@ -38,3 +42,27 @@ export const createShutdown = (input: {
   unregisterShutdown = dependencies.registerShutdown(requestShutdown);
   return { shutdown, request: requestShutdown };
 };
+
+const describeShutdownFailure = (
+  cause: unknown,
+): Readonly<Record<string, string | number | boolean | null>> => {
+  if (cause instanceof Error) {
+    const code = "code" in cause ? cause.code : undefined;
+    return {
+      name: cause.name,
+      message: cause.message,
+      ...(typeof code === "string" ||
+      (typeof code === "number" && Number.isFinite(code))
+        ? { code }
+        : {}),
+    };
+  }
+  if (
+    cause === null ||
+    typeof cause === "string" ||
+    typeof cause === "boolean" ||
+    (typeof cause === "number" && Number.isFinite(cause))
+  )
+    return { type: cause === null ? "null" : typeof cause, value: cause };
+  return { type: typeof cause };
+};
diff --git a/tests/boundary/mcp/shutdown.test.ts b/tests/boundary/mcp/shutdown.test.ts
new file mode 100644
index 000000000..4465d7120
--- /dev/null
+++ b/tests/boundary/mcp/shutdown.test.ts
@@ -0,0 +1,61 @@
+import type { StdioServerHandle } from "@modelcontextprotocol/server/stdio";
+import pino from "pino";
+import { expect, it } from "vitest";
+
+import { createManagedBinarySession } from "../../../src/application/runtime.js";
+import { MCP_SHUTDOWN_FAILED } from "../../../src/main/messages.js";
+import { createShutdown } from "../../../src/main/shutdown.js";
+import type { RuntimeDependencies } from "../../../src/main/types.js";
+
+it("keeps shutdown output stable while logging the rejected cause at debug level", async () => {
+  const logs: string[] = [];
+  const output: string[] = [];
+  const exitCodes: number[] = [];
+  const requests: Array<() => void> = [];
+  const failure = Object.assign(
+    new TypeError("fixture transport close failed"),
+    {
+      code: "ECONNRESET",
+    },
+  );
+  const handle: StdioServerHandle = {
+    close: () => Promise.reject(failure),
+  };
+  const dependencies: RuntimeDependencies = {
+    env: {},
+    serve: () => {
+      throw new Error("unused fixture transport");
+    },
+    writeStderr: (text) => output.push(text),
+    setExitCode: (code) => exitCodes.push(code),
+    registerShutdown: (request) => {
+      requests.push(request);
+      return () => undefined;
+    },
+  };
+  const logger = pino({ level: "debug" }, { write: (line) => logs.push(line) });
+  createShutdown({
+    handle,
+    session: createManagedBinarySession(),
+    unregisterReload: () => undefined,
+    dependencies,
+    serverLogger: logger,
+  });
+
+  requests[0]?.();
+  await new Promise((resolve) => setImmediate(resolve));
+
+  expect(exitCodes).toEqual([1]);
+  expect(output).toEqual([`${MCP_SHUTDOWN_FAILED}\n`]);
+  expect(logs.map((line) => JSON.parse(line))).toContainEqual(
+    expect.objectContaining({
+      level: 20,
+      msg: "MCP shutdown rejected",
+      failure_cause: {
+        name: "TypeError",
+        message: "fixture transport close failed",
+        code: "ECONNRESET",
+      },
+    }),
+  );
+});