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Mirror the frontend's protocol era on a proxy's backend connection (#4573)

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* Mirror front protocol era onto proxy backend connection

A proxy created from a non-Client target now negotiates, on its backend,
whatever era its front client negotiated, instead of pinning one era.
Explicit create_proxy(mode=...) still overrides. Guards the eager backend
initialize() so an explicit modern pin behind a handshake front no longer
crashes.

* Carry the mirrored proxy era into multi-server config backends

A multi-server MCPConfig target mounts one proxy per configured server on a
composite router, so setting the era on the outer client stopped at the router
and every real backend stayed on its default era. TransportOptions.backend_mode
carries it down, resolved per request alongside the outer mirroring.

The router is also sealed under a policy held on the transport rather than a
fresh per-router ephemeral key, so a guard tool's request_state survives the
router being rebuilt between rounds.
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Jeremiah Lowin 2026-07-20 12:31:02 -04:00 • committed by GitHub
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6
docs/development/v4-notes/change-register.mdx
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@ -380,6 +380,12 @@ A tool can gather client input across rounds on a `2026-07-28` call by returning
*Verify:* `fastmcp_slim/fastmcp/server/context.py` (`input_responses`/`request_state` properties), `fastmcp_slim/fastmcp/server/low_level.py` (`RequestStateBoundary` install), `fastmcp_slim/fastmcp/server/server.py` (`request_state_security` param), `fastmcp_slim/fastmcp/server/mixins/mcp_operations.py` (`_on_call_tool` input-required passthrough + era gate), `fastmcp_slim/fastmcp/tools/base.py` (`InputRequiredToolResult`), `tests/server/test_mrtr_guards.py`.
### Proxy era mirroring — New (behavior)
A proxy is a server on its front and a client on its back, and the two eras have mutually exclusive interaction models on a single session: the handshake era pushes server-initiated requests (sampling/elicitation/roots) that the proxy forwards to its client, while the modern era forbids those and round-trips a guard tool's `InputRequiredResult` as a result instead. A proxy created from a non-Client target with no explicit `mode` now MIRRORS the front connection's negotiated era onto its backend session per request, so the whole chain speaks one era end-to-end — a modern client reaches a modern backend (guard round-trips work), a handshake client reaches a handshake backend (push-forwarding works), and the same proxy serves both without a backend session ever crossing eras. Because the default factory builds a fresh backend client per request and derives its `mode` from the front era at call time, only the metadata-only component caches are shared across eras. An explicit `create_proxy(target, mode=...)` still pins the backend era regardless of the front, overriding mirroring for a backend that only speaks one era; the resulting cross-era feature mismatches surface through the existing era gates. `ProxyInitializeMiddleware` no longer force-calls the handshake-only `client.initialize()` when the backend negotiated the modern era, so an explicit modern pin behind a handshake front no longer crashes on connect. The mirrored era carries through a multi-server `MCPConfig` target as well: that form mounts one proxy per configured server onto a composite router, and `TransportOptions.backend_mode` hands the era down to those mounted legs so every real backend negotiates it, not just the router in front of them. That router is also now sealed under a policy held on the transport rather than a fresh per-router ephemeral key, so a guard tool's `request_state` survives the router being rebuilt between rounds.
*Verify:* `fastmcp_slim/fastmcp/server/providers/proxy.py` (`_mirror_front_era_mode`, the `_create_client_factory` non-Client branch, the era guard in `ProxyInitializeMiddleware.on_initialize`), `fastmcp_slim/fastmcp/client/transports/base.py` (`TransportOptions.backend_mode`), `fastmcp_slim/fastmcp/client/transports/config.py` (`MCPConfigTransport.connect_session` / `_create_proxy`), `fastmcp_slim/fastmcp/server/server.py` (`create_proxy` docstring), `tests/server/test_mrtr_guards.py` (`TestProxyEraMirroring`, `TestMultiServerConfigEraMirroring`).
### The xfail register — Known gap
Roughly forty `xfail` markers across the test tree (concentrated in `tests/server/tasks/`, `tests/client/tasks/`, and `test_protocol_eras.py`) are the built-in beta tracker: each names the SDK gap it waits on. They are enumerated and mapped to sdk-feedback findings on the [Known Gaps](/development/v4-notes/known-gaps) page.