ACP mode does not implement session/close (sessionCapabilities.close never advertised) — ACP clients cannot release sessions

[DDKinger]

Summary

When Copilot CLI runs as an ACP agent (copilot --acp --stdio), it does not
implement the ACP session/close request, and its initialize response does
not advertise the sessionCapabilities.close capability. As a result an ACP
client has no protocol-level way to tell Copilot that a session it created
via session/new is finished and can be torn down.

For a client that multiplexes many sessions over a single long-lived
copilot --acp process (Zed, Windows Terminal, other IDE integrations), every
session ever created stays resident for the lifetime of the Copilot process.
session/cancel only aborts the in-flight turn — it does not release the
session — so there is no clean way to reclaim per-session resources short of
killing the whole agent process.

This is the same class of gap as #3256 (ACP mode does not advertise
session.fork capability — slash /fork exists, ACP method missing): the ACP
schema defines the method and a capability bit, but the Copilot ACP server
neither implements it nor advertises it.

Environment

• GitHub Copilot CLI 1.0.71-0
• Transport: copilot --acp --stdio
• ACP schema reference: agent-client-protocol / agent-client-protocol-schema
  — SessionCapabilities.close: Option<SessionCloseCapabilities> gates
  session/close; the doc states "only available if the agent advertises the
  sessionCapabilities.close capability."

Current behavior (reproduced)

Driving copilot --acp --stdio with a minimal newline-delimited JSON-RPC client:

1. initialize — sessionCapabilities advertises only list, no close:

{
  "loadSession": true,
  "mcpCapabilities": { "http": true, "sse": true },
  "promptCapabilities": { "image": true, "audio": false, "embeddedContext": true },
  "sessionCapabilities": { "list": {} }
}

2. session/new succeeds, returns e.g. sessionId: a95110c0-8f81-4933-94e6-75127090eb45.

3. session/close on that id is rejected as Method not found:

// --> {"jsonrpc":"2.0","id":5,"method":"session/close","params":{"sessionId":"a95110c0-..."}}
// <-- error:
{
  "code": -32601,
  "message": "\"Method not found\": session/close",
  "data": { "method": "session/close" }
}

(Side note observed during the repro: session/list returns on-disk history —
50 rows — and does not contain the just-created live sessionId, so
session/list cannot be used as a substitute to discover/close live in-memory
sessions.)

Expected behavior

• initialize advertises agentCapabilities.sessionCapabilities.close (e.g.
  {}) when the build supports it.
• session/close with { "sessionId": "<id>" } releases the live session
  (frees its in-memory state / handles) and returns a CloseSessionResponse
  ({} result).
• After a successful session/close, using that sessionId again (e.g. a
  second session/close, session/prompt, session/set_mode) is rejected with
  a "session not found"–style error, so clients can confirm the release.

Why this matters

ACP was designed for a client to own the process lifetime while opening and
closing many sessions against it. Without session/close:

• Resource growth: long-lived hosts accumulate one live session per
  session/new for the whole process lifetime; there is no bounded-lifetime
  path. (Related, though not ACP-specific: CLI headless mode: internal state accumulation causes progressive latency degradation across sessions #2755 internal-state accumulation,
  Copilot CLI never prunes ~/.copilot/session-state, causing EMFILE / file-descriptor exhaustion (crashes VS Code Copilot Chat) #3892 session-state never pruned.)
• No graceful teardown signal: clients can only session/cancel (turn
  abort) or kill the process. A UI that opens a session per tab/pane and closes
  it when the tab closes has no way to tell Copilot the session is done.

Reproduction

Minimal repro with any ndjson JSON-RPC driver over copilot --acp --stdio:

1. Send initialize { "protocolVersion": 1, "clientCapabilities": {} } →
   note sessionCapabilities has no close.
2. Send session/new { "cwd": "<abs path>", "mcpServers": [] } → capture
   sessionId.
3. Send session/close { "sessionId": "<id>" } → observe
   -32601 "Method not found": session/close.

References

• ACP mode does not advertise session.fork capability (slash /fork exists; ACP method missing) #3256 — same pattern for session/fork (ACP method + capability missing while
  the slash command exists).
• ACP session/close method and sessionCapabilities.close capability:
  https://agentclientprotocol.com