FLEET: resumeSession() with customAgents errors with malformed timeout message "[object Object]ms"

[darthmolen]

Summary

Calling client.resumeSession(sessionId, { customAgents: [...] }) on a live session produces a timeout error with a malformed message: "Timeout after [object Object]ms". This reveals two bugs: (1) resumeSession with customAgents fails, and (2) the timeout value is being stringified via implicit .toString() instead of extracting the numeric value.

Steps to Reproduce

import { CopilotClient, approveAll } from '@github/copilot-sdk';

const client = new CopilotClient();
await client.start();

const session = await client.createSession({ onPermissionRequest: approveAll });
const sessionId = session.id;
await session.destroy();

try {
    const resumed = await client.resumeSession(sessionId, {
        onPermissionRequest: approveAll,
        customAgents: [{
            name: 'post-creation-agent',
            displayName: 'Post Creation Agent',
            description: 'Added during resume',
            prompt: 'You are a post-creation agent.',
            tools: ['view'],
            infer: false,
        }],
    });
} catch (err) {
    console.error(err.message);
    // → "Timeout after [object Object]ms waiting for session.idle"
}

Observed Behaviour

Spike data (2026-03-17):

{
  "resumeSucceeded": false,
  "resumeError": "Timeout after [object Object]ms waiting for session.idle",
  "answer": "NO/ERROR"
}

Bug 1: resumeSession with customAgents fails

The TypeScript type ResumeSessionConfig accepts customAgents, but the CLI backend appears to reject or ignore it and the session never reaches idle. Either:

• customAgents is not supported on resume (the type should exclude it), or
• There is a backend defect handling customAgents in session.resume

Bug 2: Malformed timeout error message

"Timeout after [object Object]ms" is a string formatting bug. The timeout config value (likely { ms: number } or similar) is being coerced to string via .toString() instead of extracting the numeric value.

Expected: "Timeout after 30000ms waiting for session.idle"

Workaround

Only set customAgents at initial createSession time. Do not attempt to add agents via resumeSession:

// ✅ Works
const session = await client.createSession({
    onPermissionRequest: approveAll,
    customAgents: [{ name: 'my-agent', ... }],
});

// ❌ Errors with malformed timeout
const session = await client.resumeSession(id, {
    customAgents: [{ name: 'my-agent', ... }],
});

Environment

• SDK: @github/copilot-sdk v0.1.32
• CLI: latest
• Node: v24.13.1