fix(improvement): handle MCP verifier stdin pipe errors

src/improvement/mcp-serve-verifier.ts

@@ -73,14 +73,16 @@ export function mcpServeVerifier(spec: McpServeSpec): Verifier {
       const failCandidate = (msg: string) =>
         settle(() => resolve({ ok: false, feedback: withStderr(msg) }))
       const setupFault = (err: Error) => settle(() => reject(err))
+      const failStdin = (err: Error) =>
+        failCandidate(`writing to MCP server stdin failed: ${err.message}`)
 
       const send = (msg: Record<string, unknown>): boolean => {
         try {
           child.stdin.write(`${JSON.stringify(msg)}\n`)
           return true
         } catch (err) {
           // EPIPE: the server died mid-handshake — a failed candidate, not a fault.
-          failCandidate(`writing to MCP server stdin failed: ${(err as Error).message}`)
+          failStdin(err as Error)
           return false
         }
       }
@@ -100,6 +102,8 @@ export function mcpServeVerifier(spec: McpServeSpec): Verifier {
         // server crashed on boot); after we settle, our own SIGKILL fires here.
         failCandidate(`MCP server exited (code ${code}, signal ${signal}) before serving`)
       })
+      // Pipe failures normally arrive asynchronously, outside send()'s try/catch.
+      child.stdin.on('error', failStdin)
       child.stderr.on('data', (d) => stderr.push(String(d)))
 
       const rl = createInterface({ input: child.stdout })

tests/mcp-serve-verifier.test.ts

@@ -6,9 +6,21 @@ import { mcpServeVerifier } from '../src/improvement/mcp-serve-verifier'
 
 // A minimal stdio MCP server (newline-delimited JSON-RPC 2.0) used as the
 // "built server" the verifier boots. Variants exercise each outcome.
-function serverScript(variant: 'ok' | 'no-tools' | 'crash' | 'hang'): string {
+function serverScript(variant: 'ok' | 'no-tools' | 'crash' | 'hang' | 'closed-stdin'): string {
   if (variant === 'crash') return 'process.exit(1)\n'
   if (variant === 'hang') return 'setInterval(() => {}, 1000)\n' // never answers
+  if (variant === 'closed-stdin') {
+    return [
+      'import { closeSync } from "node:fs"',
+      'process.stdin.on("error", () => {})',
+      'process.stdin.once("data", (chunk) => {',
+      '  const request = JSON.parse(String(chunk).trim())',
+      '  closeSync(0)',
+      '  process.stdout.write(JSON.stringify({ jsonrpc: "2.0", id: request.id, result: { protocolVersion: "2024-11-05", capabilities: {}, serverInfo: { name: "fake", version: "0" } } }) + "\\n")',
+      '})',
+      'setInterval(() => {}, 1000)',
+    ].join('\n')
+  }
   const tools =
     variant === 'no-tools'
       ? '[]'
@@ -31,7 +43,7 @@ beforeEach(() => {
 })
 afterEach(() => rmSync(dir, { recursive: true, force: true }))
 
-function write(variant: 'ok' | 'no-tools' | 'crash' | 'hang'): string {
+function write(variant: 'ok' | 'no-tools' | 'crash' | 'hang' | 'closed-stdin'): string {
   const path = join(dir, `server-${variant}.mjs`)
   writeFileSync(path, serverScript(variant))
   return path
@@ -57,6 +69,17 @@ describe('mcpServeVerifier — boot-and-probe', () => {
     expect(res.feedback).toMatch(/exited|serving/)
   })
 
+  it('fails when the server closes stdin during the handshake', async () => {
+    const verify = mcpServeVerifier({
+      command: 'node',
+      args: [write('closed-stdin')],
+      timeoutMs: 800,
+    })
+    const res = await verify(dir)
+    expect(res.ok).toBe(false)
+    expect(res.feedback).toContain('writing to MCP server stdin failed')
+  })
+
   it('fails (does not hang) a server that never answers, via timeout', async () => {
     const verify = mcpServeVerifier({ command: 'node', args: [write('hang')], timeoutMs: 800 })
     const res = await verify(dir)