`copilot --acp` does not work with `--agent`

[Coruscant11]

Describe the bug

Hello! First let me thank you for your hard work. Here is below my described issue. I used copilot a bit to help on having a proper reproducer command and a potential hint. Thanks a lot guys!

---

I created a custom agent in ~/.copilot/agents/test.agent.md

---
name: test
description: A minimal test custom agent
tools: []
---
You are the test custom agent. Always respond by saying: "Hello from test custom agent!"

When I launch copilot in "human mode" with copilot --agent test it works.

● Selected custom agent: test
▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄
 ❯ hello                                                                                                                                            
▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
● Hello from test custom agent!

However when I try to use use ACP, with copilot --acp --agent test. It ALWAYS use the default Copilot agent. I can only use my custom agent as subagent, for example by saying "use test agent for <...>".

It seems like when using --acp, the --agent option is totally ignored.

Affected version

GitHub Copilot CLI 1.0.65

Steps to reproduce the behavior

How to reproduce

1. Create a minimal custom agent file in ~/.copilot/agents/test.agent.md:

---
name: test
description: A minimal test custom agent
tools: []
---
You are the test custom agent. Always respond by saying: "Hello from test custom agent!"

2. Run this single command in your terminal to initialize a session and print the active agent configuration:

(echo '{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"protocolVersion":1,"clientCapabilities":{},"clientInfo":{"name":"test"}}}'; echo '{"jsonrpc":"2.0","id":1,"method":"session/new","params":{"cwd":"'"$(pwd)"'","mcpServers":[]}}'; sleep 6) | copilot --acp --agent test --yolo | grep -o '"id":"agent"[^}]*"currentValue":"[^"]*"'

Expected behaviour

The active agent configuration option should reflect the command-line override ("test"):

"id":"agent","name":"Agent","currentValue":"test"

Actual behaviour

The active agent configuration is empty, falling back to standard Copilot:

"id":"agent","name":"Agent","currentValue":""

Expected behavior

Copilot should be launched in ACP with my custom agent as main agent. I want my session to have only the tools and context I have defined in my subagent, not with the general copilot gent. I don't want my custom agent to be available just as a tool.

Expected behaviour

The active agent configuration option should reflect the command-line override ("test"):

"id":"agent","name":"Agent","currentValue":"test"

Additional context

• Operating System: Linux (Kernel v24.16.0 or similar)
• CPU Architecture: x86_64 / x64 (installed as @github/copilot-linux-x64)
• Terminal Emulator: Ghostty (term/ghostty)
• Shell: Bash (v5.x)
• Copilot Version: v1.0.65
  copilot-acp-agent-bug.log

---

Analysis by Copilot himself

• When starting in --acp server mode, the CLI action handler calls startServerMode({...}, authManager). However, the parsed value of --agent (t.agent / agentName) is completely omitted from the configuration object passed to startServerMode.
• There is no way for the ACP client to specify the agent in the session/new params, and the ACP server does not implement the session/set_config_option method, making it impossible to change the active agent after session creation.

An analysis of the bundled index.js reveals exactly where the flow breaks:

1. CLI Flag Parser:

The parser registers the --agent option correctly:
.option("--agent ", "Specify a custom agent to use")

2. Server Mode Activation Block:

When --acp is supplied, the CLI invokes startServerMode like so:

let { startServerMode: ee } = await Promise.resolve().then(() => (tIn(), Sto));
await ee({
  port: de,
  stdio: $,
  logDir: G,
  logLevel: b,
  settings: p,
  shutdownService: d,
  featureFlags: r
}, a)

Notice that t.agent (the custom agent command-line argument) is completely omitted here! The starting server is never supplied with the --agent option context.

3. ACP Protocol Limitations:

• The JSON-RPC request for session/new (Ny.SESSION_CREATE) does not include any parameter to define or activate an agent dynamically.
• While the ACP protocol schema defines a session/set_config_option method, the registerMethods routing mechanism on the server lacks an implementation of setSessionConfigOption (it is defined in the type definitions/client adapter OSe but is completely absent from the actual server class router).

Suggestion for fix:

Pass the command-line override into startServerMode configuration parameters and use it to define the initial/default currentValue of the "agent" configuration option during session setup.

[Coruscant11]

it would be really great if Copilot ACP could support session/set_config_option
https://agentclientprotocol.com/protocol/v1/session-config-options