ACP mode: task tool cannot launch custom agents since 1.0.89 ("Unsupported native sessions host effect 'custom_agent_prompt'") #5030

Describe the bug

In copilot --acp, the task tool cannot start any custom agent from ~/.copilot/agents. The call fails before the agent runs:

host interaction call failed: Error: Unsupported native sessions host effect 'custom_agent_prompt'

Built-in agents such as explore still work in the same session. The same custom agent works in copilot -p, and in copilot --acp 1.0.88 and earlier.

Affected version

GitHub Copilot CLI 1.0.89 and 1.0.90

Steps to reproduce the behavior

1. Create %USERPROFILE%\.copilot\agents\my-agent.agent.md:

   ---
   name: my-agent
   description: Test agent
   ---
   Reply with exactly: LAUNCHED

2. Start copilot --acp and send initialize, then session/new.
3. Send session/prompt asking the model to call the task tool once with agent_type: "my-agent".
4. The tool_call_update for the task call has status failed and the error above.
5. Repeat step 3 with agent_type: "explore". It completes.
6. Repeat with copilot --acp --prefer-version 1.0.88. my-agent completes.

Expected behavior

The custom agent runs and returns its result, as it does in 1.0.88 and in copilot -p.

Additional context

• Windows 11, x86_64. ACP over stdio from a Node client, no terminal emulator involved.
• Possible cause, an inference from reading the bundled JavaScript and not confirmed: before 1.0.89, ACP sessions routed host effects to runNativeSessionHostEffect, which handles custom_agent_prompt. From 1.0.89, ACP goes through the native sessions server, and its clientSession/hostEffect path calls runServerHostEffect, which only handles server effects and throws for custom_agent_prompt. The version results above are tested.
• Workaround: copilot --acp --prefer-version 1.0.88, when that version is cached.