[MCP Stateless] Slice B-2: Stateless request routing — skip initialize, add _meta inline + server/discover

[Lexus2016]

Parent: #1287 (Slice B decomposition)

This is Slice B-2 of the MCP 2026-07-28 stateless migration. Depends on #1511 (B-1: capability flag + synthesize_capabilities).

Scope (this issue — ≤200 lines)

Add the stateless request path that bypasses the initialize/initialized handshake and routes requests with inline _meta routing context. This is the core behavioral change — the 4 session.initialize() sites and 11 dispatch sites get a flag-gated alternative path.

Specific changes

1. Skip session.initialize() for stateless servers (4 sites): gate each behind if self._stateless_enabled and self._is_stateless_server: — synthesize the session instead of calling the handshake.

   • Line 2426 (stdio transport)
   • Line 2732 (SSE transport)
   • Line 2789 (streamable_http, new API)
   • Line 2821 (streamablehttp, deprecated API)

2. server/discover RPC: add a method that calls the new server/discover RPC (introduced in the finalized spec) to obtain server capabilities without a session. This replaces the information previously obtained from the initialize handshake response.

3. Inline _meta routing: when sending RPCs in stateless mode, embed routing context (Mcp-Method, Mcp-Name) as inline _meta in the request body instead of relying on a pinned session. Cover the critical hot path: call_tool (audit line ~4263).

4. _get_session_id sites (2 sites): in stateless mode, these should return None / skip the session-id header entirely (SEP-2567 removes Mcp-Session-Id).

Call sites (from Slice A audit)

Group	Lines	Count
session.initialize()	2426, 2732, 2789, 2821	4
self.session.* dispatches	~4263 (call_tool) + 10 others	11
_get_session_id	2 sites	2
Total		17

What this issue does NOT do

• Does NOT implement the capability detection itself (that's B-1 / [MCP Stateless] Slice B-1: Capability-detection flag + synthesize_capabilities() #1511)
• Does NOT add comprehensive mock-server tests (that's B-3)
• Does NOT flip the flag default or add migration docs (that's Slice C)

Success criteria

• session.initialize() is skipped for stateless-flagged servers (4 sites gated)
• server/discover RPC call implemented and returns capabilities
• call_tool hot path routes via _meta inline when stateless
• _get_session_id returns None in stateless mode (2 sites)
• Existing stateful path fully functional when flag is OFF
• Fits ≤200-line self-merge cap

Dependencies

• [MCP Stateless] Slice B-1: Capability-detection flag + synthesize_capabilities() #1511 (B-1) must land first — provides the flag + synthesize_capabilities()

Audit reference

Full call-site map: docs/mcp/2026-07-28-stateless-migration-audit.md