fix: include stderr in process exceptions

src/CodexAcpServer.ts

@@ -113,6 +113,7 @@ export class CodexAcpServer implements acp.Agent {
     private readonly connection: acp.AgentSideConnection;
     private readonly defaultAuthRequest: CodexAuthRequest | null;
     private readonly getExitCode: () => number | null;
+    private readonly getRecentStderr: () => string;
     private readonly availableCommands: CodexCommands;
     private clientInfo: acp.Implementation | null;
     private terminalOutputMode: TerminalOutputMode;
@@ -130,6 +131,7 @@ export class CodexAcpServer implements acp.Agent {
         codexAcpClient: CodexAcpClient,
         defaultAuthRequest?: CodexAuthRequest,
         getExitCode?: () => number | null,
+        getRecentStderr?: () => string,
     ) {
         this.sessions = new Map();
         this.pendingMcpStartupSessions = new Map();
@@ -142,6 +144,7 @@ export class CodexAcpServer implements acp.Agent {
         this.codexAcpClient = codexAcpClient;
         this.defaultAuthRequest = defaultAuthRequest ?? null;
         this.getExitCode = getExitCode ?? (() => null);
+        this.getRecentStderr = getRecentStderr ?? (() => "");
         this.clientInfo = null;
         this.terminalOutputMode = "terminal_output_delta";
         this.availableCommands = new CodexCommands(
@@ -1458,7 +1461,9 @@ export class CodexAcpServer implements acp.Agent {
                 throw new RequestError(requestErrorCode, `VC++ redistributable should be installed`);
             }
             if (exitCode !== null) {
-                throw new RequestError(requestErrorCode, `Codex process has exited with code ${exitCode}`);
+                const stderr = this.getRecentStderr().trim();
+                const detail = stderr ? `:\n${stderr}` : "";
+                throw new RequestError(requestErrorCode, `Codex process has exited with code ${exitCode}${detail}`);
             }
             throw err;
         }

src/__tests__/CodexACPAgent/process-exit-error.test.ts

@@ -0,0 +1,37 @@
+import { describe, it, expect } from 'vitest';
+import * as fs from 'node:fs';
+import * as os from 'node:os';
+import * as path from 'node:path';
+import { once } from 'node:events';
+import * as acp from '@agentclientprotocol/sdk';
+import { startCodexConnection } from '../../CodexJsonRpcConnection';
+import { CodexAppServerClient } from '../../CodexAppServerClient';
+import { CodexAcpClient } from '../../CodexAcpClient';
+import { CodexAcpServer } from '../../CodexAcpServer';
+import { createMockConnections } from './test-utils';
+
+describe('CodexACPAgent - process exit error', () => {
+    it.skipIf(process.platform === 'win32')('includes the crashed process stderr', async () => {
+        const fakeCodex = path.join(fs.mkdtempSync(path.join(os.tmpdir(), 'codex-bad-')), 'codex');
+        fs.writeFileSync(fakeCodex, "#!/bin/sh\necho 'codex: failed to launch' >&2\nexit 1\n");
+        fs.chmodSync(fakeCodex, 0o755);
+
+        const connection = startCodexConnection(fakeCodex);
+        let stderr = '';
+        connection.process.stderr.on('data', (chunk: Buffer) => { stderr += chunk.toString(); });
+
+        const codexClient = new CodexAcpClient(new CodexAppServerClient(connection.connection));
+        const agent = new CodexAcpServer(
+            createMockConnections().mockAcpConnection,
+            codexClient,
+            undefined,
+            () => connection.process.exitCode,
+            () => stderr,
+        );
+
+        await once(connection.process, 'close'); // process exited and stderr flushed
+
+        await expect(agent.initialize({ protocolVersion: acp.PROTOCOL_VERSION }))
+            .rejects.toThrow("Codex process has exited with code 1:\ncodex: failed to launch");
+    });
+});

src/index.ts

@@ -57,6 +57,13 @@ function startAcpServer() {
     });
 
     const codexConnection = startCodexConnection(codexPath);
+
+    const maxStderrTailChars = 2 * 1024;
+    let stderr = "";
+    codexConnection.process.stderr.addListener("data", (data: Buffer) => {
+        stderr = (stderr + data.toString()).slice(-maxStderrTailChars);
+    });
+
     process.stdin.on("close", (chunk: Buffer) => {
         codexConnection.process.stdin.end();
         // Kill the codex process if it doesn't exit naturally
@@ -73,7 +80,7 @@ function startAcpServer() {
     function createAgent(connection: acp.AgentSideConnection): CodexAcpServer {
         const appServerClient = new CodexAppServerClient(codexConnection.connection);
         const codexClient = new CodexAcpClient(appServerClient, config, modelProvider);
-        return new CodexAcpServer(connection, codexClient, defaultAuthRequest, () => codexConnection.process.exitCode);
+        return new CodexAcpServer(connection, codexClient, defaultAuthRequest, () => codexConnection.process.exitCode, () => stderr);
     }
 
     new acp.AgentSideConnection(createAgent, acpJsonStream);