can1357/oh-my-pi · error · Error

DAP adapter ${parent.adapter.name} cannot accept child sessi

Error message

DAP adapter ${parent.adapter.name} cannot accept child session connections

What it means

Child debug sessions can only connect to a parent adapter that listens on TCP and exposes a port, because multiple clients attach via the network socket. If the parent adapter uses stdio (pipe) transport or has no recorded port, spawning a child client is impossible and the manager refuses up front.

Source

Thrown at packages/coding-agent/src/dap/session.ts:1198

		for (const session of this.#sessions.values()) {
			if (
				session.status === "terminated" ||
				now - session.lastUsedAt > IDLE_TIMEOUT_MS ||
				!session.client.isAlive()
			) {
				this.#disposeSession(session);
			}
		}
	}

	async #startChildSession(
		parent: DapSession,
		request: "launch" | "attach",
		configuration: Record<string, unknown>,
		timeoutMs: number = 30_000,
	): Promise<void> {
		if (parent.adapter.connectMode !== "tcp" || parent.port === undefined) {
			throw new Error(`DAP adapter ${parent.adapter.name} cannot accept child session connections`);
		}
		const cwd = path.resolve(parent.cwd, typeof configuration.cwd === "string" ? configuration.cwd : ".");
		const client = await DapClient.connect({
			adapter: parent.adapter,
			cwd,
			host: "127.0.0.1",
			port: parent.port,
		});
		const child = this.#registerSession(
			client,
			parent.adapter,
			cwd,
			typeof configuration.program === "string" ? configuration.program : undefined,
			parent.id,
		);
		try {
			child.capabilities = await client.initialize(
				this.#buildInitializeArguments(parent.adapter),

View on GitHub (pinned to 9690622007)

Solutions

Launch the parent adapter in TCP/server mode so it accepts multiple client connections and exposes a port
Ensure the adapter's listen output is parsed so parent.port is set (match 'listening at HOST:PORT')
Use an adapter/adapter-config that supports multi-session (e.g. debugpy multi-session, js-debug server mode)
If the adapter is stdio-only, launch a separate top-level session per debuggee instead of a child session

Example fix

// before
// parent adapter: connectMode 'pipe' (stdio)
await manager.startChildSession(parent, 'launch', { program });
// after
// launch parent with TCP server mode first
const parent = await manager.launch({ adapter: tcpServerAdapter, configuration });
await manager.startChildSession(parent, 'attach', { port: parent.port });

Defensive patterns

Strategy: validation

Validate before calling

if (parent.adapter.connectMode !== 'tcp' || typeof parent.port !== 'number') {
  throw new Error('child sessions require a TCP parent adapter with a known port');
}

Type guard

function supportsChildSessions(parent: DapSession): boolean {
  return parent.adapter.connectMode === 'tcp' && typeof parent.port === 'number';
}

Try / catch

try {
  await manager.startChildSession(parent, 'attach', configuration);
} catch (err) {
  if (String((err as Error).message).includes('cannot accept child session')) {
    // relaunch parent in TCP server mode or start a separate root session
    parent = await manager.launch({ ...config, adapter: tcpAdapter });
    await manager.startChildSession(parent, 'attach', configuration);
  } else { throw err; }
}

Prevention

Choose adapters/configs with TCP server (multi-session) support when child sessions are needed
Assert connectMode === 'tcp' and port !== undefined before any child-session API call
Ensure the adapter's listen output is parsed so port is recorded on the parent session
For stdio-only adapters, launch independent root sessions per debuggee instead

When it happens

Trigger: Calling startChildSession (launch/attach of a child) while parent.adapter.connectMode is 'pipe' (stdio); parent launched over TCP but parent.port is undefined (port never recorded, e.g. port announced but not parsed, or socket-mode launch); child attach attempted on an adapter that only supports one stdio client.

Common situations: Multi-process debugging configured against a stdio adapter that only supports single-session; parent session launched without a known port (adapter assigns ephemeral port not propagated); trying to fork child sessions on adapters lacking multi-session support (needs --server mode).

Related errors

AI-assisted analysis of can1357/oh-my-pi@9690622007 (2026-08-31). Data as JSON: /api/errors/7f6cb64443c39fc7. Report an issue: GitHub.