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Address review: drop tasks from modern capability table; note the reentrancy blocker too

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Jeremiah Lowin 2026-07-20 20:28:23 -04:00
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docs/development/v4-notes/feature-program.mdx
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@ -98,7 +98,7 @@ The migration already routed `initialize` interception through the SDK's `Server
`fastmcp.Client` now defaults to `mode="auto"` (#4572): it probes `server/discover`, falls back to the classic handshake, and answers multi-round-trip `input_required` requests through its existing handlers. The same PR surfaced `extensions=` and `result_claims=` (SEP-2133). The client also dropped its forked protocol helpers — extension folding, the evicting message handler, discover synthesis — in favor of the SDK's own (#4574).
The decision here was **compose, not wrap** (D16): rebuild `fastmcp.Client` on the SDK's high-level `mcp.Client` rather than wrapping `mcp.ClientSession`. The parts that compose cleanly have shipped. The rest is **blocked upstream**: `mcp.Client` constructs its `ClientSession` at a single hardcoded site with no injection hook, while FastMCP's `session_class` is load-bearing (`ProxyClient` substitutes a session that skips result validation so a backend's schema violation surfaces at the end client rather than becoming a proxy error). A `session_factory=` hook on `mcp.Client` — the same shape as the `notification_bindings=` parameter added earlier — would unblock the full rebuild.
The decision here was **compose, not wrap** (D16): rebuild `fastmcp.Client` on the SDK's high-level `mcp.Client` rather than wrapping `mcp.ClientSession`. The parts that compose cleanly have shipped. The rest is **blocked upstream on two counts**. First, `mcp.Client` constructs its `ClientSession` at a single hardcoded site with no injection hook, while FastMCP's `session_class` is load-bearing (`ProxyClient` substitutes a session that skips result validation so a backend's schema violation surfaces at the end client rather than becoming a proxy error) — a `session_factory=` hook on `mcp.Client`, the same shape as the `notification_bindings=` parameter added earlier, would solve this. Second, `mcp.Client.__aenter__` refuses reentry, but FastMCP's client is deliberately reentrant (its refcounted context manager exists to fix a proxy session-reuse deadlock), so the rebuild also needs the SDK client to tolerate reentrant entry. Both must land upstream before the full rebuild is possible; `session_factory=` alone is necessary but not sufficient.
This workstream also owns the server-side statelessness design holes — `ctx.session_id` / `set_state` round-tripping and stateful-proxy affinity — since they turn on the same "what is a session without a session?" question. See [Statelessness on 2026-07-28](/development/v4-notes/known-gaps#statelessness-on-2026-07-28) for the full accounting.