# Experimental MCP Sessions: fast-agent status and next steps

## Goal

Demonstrate an end-to-end **experimental MCP Session v2** flow where:

1. fast-agent detects server session capability (`experimental.session`),
2. fast-agent establishes a session (`session/create`) with a title hint,
3. session cookie metadata (`_meta["mcp/session"]`) is echoed automatically on tool/resource/prompt calls,
4. session status and cookie content are visible in `/mcp` (`mcp_display.py`).

---

## What is now implemented in fast-agent

### 1) Metadata routing fix (required foundation)

- `src/fast_agent/mcp/mcp_aggregator.py`
  - Fixed metadata argument routing in `_execute_on_server()`:
    - `call_tool(...)` now receives `meta=...` (correct for `ClientSession.call_tool`)
    - `read_resource/get_prompt` continue to use `_meta=...` (local adapter path)

- Tests:
  - `tests/unit/fast_agent/mcp/test_mcp_aggregator_metadata_passthrough.py`


### 2) Experimental session client behavior in MCPAgentClientSession

- `src/fast_agent/mcp/mcp_agent_client_session.py`
  - Added experimental session capability/cookie state:
    - `experimental_session_supported`
    - `experimental_session_features`
    - `experimental_session_cookie`
    - `experimental_session_id`
    - `experimental_session_title`
  - On `initialize()`:
    - parses `capabilities.experimental["session"]`
    - if supported and feature includes `create`, sends `session/create`
  - Session create title hint:
    - label: `"{agent_name} · {server_name}"`
    - hints data: `{"title": ...}`
  - Auto metadata echo:
    - merges current session cookie into outbound request metadata for
      `call_tool`, `read_resource`, `get_prompt` (unless caller already supplies `mcp/session`)
  - Cookie updates from responses:
    - updates from `result.meta["mcp/session"]`
    - handles revocation marker (`null`) by clearing cookie

- Tests:
  - `tests/unit/fast_agent/mcp/test_mcp_agent_client_session_sessions.py`


### 3) MCP status model enrichment + `/mcp` display

- `src/fast_agent/mcp/mcp_aggregator.py`
  - `ServerStatus` now includes:
    - `experimental_session_supported`
    - `experimental_session_features`
    - `session_cookie`
    - `session_title`
  - `collect_server_status()` populates these from active MCP sessions.

- `src/fast_agent/ui/mcp_display.py`
  - `/mcp` output now shows:
    - `session`
    - `exp sess` (enabled/not advertised + features + title)
    - `cookie` (compact JSON, truncated)

- Tests:
  - `tests/unit/fast_agent/mcp/test_mcp_aggregator_session_status.py`
  - `tests/unit/fast_agent/ui/test_mcp_display.py`


### Validation status

- `uv run scripts/lint.py --fix` ✅
- `uv run scripts/typecheck.py` ✅
- `ENVIRONMENT_DIR= uv run pytest tests/unit` ✅

---

## What to do next: produce a compatible demo server

This is the remaining work to demonstrate the feature end-to-end.

### A) Build/finish a server compatible with current MCP SDK (1.26)

Use `../mcp-session-lib` (already partially modernized) and provide a server that:

1. Advertises experimental capability:
   - `experimental.session = {"version": 2, "features": ["create", "list", "delete"]}`
2. Implements `session/create` and returns cookie in `_meta["mcp/session"]`
3. Stores title (label/data) in session cookie data
4. Returns updated cookie on each tool response
5. Supports revocation (`_meta["mcp/session"] = null`) for demo completeness

Notes:
- `mcp-session-lib` imports were modernized (`FastMCP`, MCPModel compatibility fallback), but we still need to ensure custom request wiring works cleanly with current SDK runtime.
- If custom low-level handlers are awkward in FastMCP, fallback option is:
  - create session lazily on first tool call,
  - still advertise `create` feature,
  - and keep `session/create` optional for first demo.


### B) Add a reproducible demo scenario in fast-agent

Create example(s) under `examples/experimental/mcp_sessions/`:

1. `session_server.py` (or wrapper) launching the compatible server.
2. `demo_fast_agent_sessions.py` that:
   - creates a `McpAgent`, runtime-attaches server,
   - runs 3-4 calls to show establish/echo/revoke/new behavior,
   - prints `/mcp` status snapshots between turns.

Expected visible outcomes in `/mcp`:
- `exp sess: enabled (create,...) title=...`
- `cookie: {"id":...,"data":{"title":...,...}}`
- after revocation, `cookie: none` then re-established cookie on next call.


### C) Add integration test coverage (post-demo)

Add integration tests in fast-agent for the runtime behavior:

1. Detects experimental session capability on initialize.
2. Auto-sends `session/create` when `create` feature exists.
3. Echoes cookie metadata on subsequent calls.
4. Shows session fields in collected status (`collect_server_status`).

---

## Risks / caveats

1. `session/create` request in fast-agent currently uses internal typed model + cast to `ClientRequest`.
   - Works in unit coverage and runtime experiments, but this is an extension path outside core typed unions.
2. Status `session_id` today is still transport-level (`local`/HTTP session id), while cookie id appears in `cookie`.
   - If desired, we can add a dedicated display field for `cookie id` to remove ambiguity.

---

## Definition of done for demo milestone

- Compatible server runs with current MCP SDK.
- fast-agent attaches and auto-establishes session when experimental session feature is present.
- `/mcp` clearly displays experimental session status and cookie content.
- Demo script is reproducible from repo commands.
