fix: MCP proxy resilience — timeouts, parallel discovery, error propagation

- McpdClient: add 30s AbortSignal timeout to all fetch calls (was infinite)
- CLI bridge: return JSON-RPC error on stdout when HTTP fails (was silent)
- Router: parallel tool/resource discovery via Promise.allSettled (was sequential — one slow server blocked all)
- vllm-managed: 60s error cooldown prevents retry-on-every-call when vLLM is broken
- Tests: McpdClient timeout suite (9), parallel discovery, vllm cooldown, bridge error response

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/cli/src/commands/mcp.ts

@@ -132,6 +132,15 @@ export async function runMcpBridge(opts: McpBridgeOptions): Promise<void> {
     const trimmed = line.trim();
     if (!trimmed) continue;
 
+    // Parse request ID for error responses
+    let requestId: unknown = null;
+    try {
+      const parsed = JSON.parse(trimmed) as Record<string, unknown>;
+      requestId = parsed.id ?? null;
+    } catch {
+      // Non-JSON or notification — no id to respond to
+    }
+
     try {
       const result = await postJsonRpc(endpointUrl, trimmed, sessionId, token);
 
@@ -156,7 +165,18 @@ export async function runMcpBridge(opts: McpBridgeOptions): Promise<void> {
         }
       }
     } catch (err) {
-      stderr.write(`MCP bridge error: ${err instanceof Error ? err.message : String(err)}\n`);
+      const errMsg = err instanceof Error ? err.message : String(err);
+      stderr.write(`MCP bridge error: ${errMsg}\n`);
+
+      // Send JSON-RPC error response so the client doesn't hang
+      if (requestId !== null) {
+        const errorResponse = JSON.stringify({
+          jsonrpc: '2.0',
+          id: requestId,
+          error: { code: -32603, message: `Bridge error: ${errMsg}` },
+        });
+        stdout.write(errorResponse + '\n');
+      }
     }
   }
 

src/cli/tests/commands/mcp.test.ts

@@ -347,7 +347,7 @@ describe('MCP STDIO Bridge', () => {
     expect(recorded.filter((r) => r.method === 'DELETE')).toHaveLength(0);
   });
 
-  it('writes errors to stderr, not stdout', async () => {
+  it('writes errors to stderr and sends JSON-RPC error to stdout', async () => {
     recorded.length = 0;
     const stdin = new Readable({ read() {} });
     const { stdout, stdoutChunks, stderr, stderrChunks } = createMockStreams();
@@ -364,8 +364,12 @@ describe('MCP STDIO Bridge', () => {
 
     // Error should be on stderr
     expect(stderrChunks.join('')).toContain('MCP bridge error');
-    // stdout should be empty (no corrupted output)
-    expect(stdoutChunks.join('')).toBe('');
+    // stdout should contain a JSON-RPC error response so the client doesn't hang
+    const out = stdoutChunks.join('');
+    const parsed = JSON.parse(out.trim()) as { id: number; error: { code: number; message: string } };
+    expect(parsed.id).toBe(1);
+    expect(parsed.error.code).toBe(-32603);
+    expect(parsed.error.message).toContain('Bridge error');
   });
 
   it('skips blank lines in stdin', async () => {