Protocol types for 2026-07-28: superset monolith, committed per-version packages, and wire-method maps#2849
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…s, and wire-method maps One public type set (mcp.types) covering every protocol revision through 2026-07-28; two per-version model packages that act as the schema-exact validating layer; and plain (method, version) maps with two-step parse functions. Version gating is data: key absence is the gate. Outbound serialization is the model dump, with the new 2026-07-28 result fields as ordinary serialized defaults. Includes a surface-to-monolith field-parity test that asserts every wire alias in either per-version package is carried by the monolith.
Vendor schema/2025-11-25.json and schema/2026-07-28.json (pinned to spec SHA 6d441518), restore scripts/gen_surface_types.py to regenerate both src/mcp/types/v* packages from them, and run --check in CI so the committed files cannot drift from the schemas.
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… generated-file pyright suppression
- Add validate_client_request/notification, validate_server_result to
types.methods (surface-only siblings of the parse_* functions)
- ServerRunner validates inbound requests/notifications against the
negotiated version's surface schema; custom methods fall through to
the registered params_type as before
- Handler results are validated against the surface schema after dump;
a spec-invalid result is logged and returns INTERNAL_ERROR to the client
- Map JSON-Schema format byte/uri/uri-template to plain str in codegen
(Base64Str and AnyUrl over-assert on annotation-only formats);
regenerate both surface packages
- Fix two test fixtures that built Tool(input_schema={}) without the
spec-required type field
- Document handler-result validation in migration.md
Generated surface types switch to extra="ignore" (with a per-version allow-list for spec-declared open types: Result, _meta, GetTaskPayloadResult, Tool input/output schemas, URLElicitationRequiredError data). The runner now serializes spec-method results by dumping the monolith model and re-dumping through the negotiated version's surface row, so 2026-only fields (resultType, ttlMs, cacheScope) never reach the wire on a pre-2026 session. - Monolith ttl_ms/cache_scope default to None; the SDK does not stamp a caching policy. EmptyResult.result_type stays None (deployed peer servers strict-validate ping responses). - Spec methods absent at the negotiated version reject with METHOD_NOT_FOUND even if a handler is registered; custom methods fall through unchanged. - Version fallback for pre-handshake/stateless is the literal 2025-11-25, not LATEST_PROTOCOL_VERSION. - Add serialize_server_result to types.methods; codegen drift-guard asserts the open-class substitution count. - Docstrings now say method-gating per version, shape per schema era.
Kludex
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I think the best way to approach this is to use VCR infra, and the tests actually update the code.
…uest ServerSession.send_request and Connection.send_request now run the client's response through validate_client_result before parsing into the caller's result_type. KeyError (spec method without a row at the negotiated version, or a non-spec method on Connection) is tolerated and the existing model_validate proceeds; ValidationError propagates as today. Adds validate_client_result to types.methods and refactors parse_client_result to delegate its surface step to it.
…eration
ClientSession now uses parse_server_request/notification for inbound
dispatch (KeyError -> METHOD_NOT_FOUND), validates callback results
against the surface schema before sending (ValidationError ->
INTERNAL_ERROR), and gates server results through validate_server_result
before parsing. Adds a protocol_version property.
Surface codegen: SCHEMA_PATCHES targets ["integer", "number"] instead
of bare "number" so generated types use int | float (pydantic
smart-union preserves int through the sieve; previously coerced 37 ->
37.0). Regenerated both packages.
elicitation.py: Optional[T] fields now emit {"type": ...} with the
field omitted from required (was non-spec anyOf). Gate tightened to
reject list[...] and multi-primitive unions; only T or T | None
accepted. The interaction-suite divergence for nested requestedSchema is
flipped (client now rejects with INVALID_PARAMS).
Documented the elicit gate change in migration.md.
- Runner discriminator uses direction-specific SPEC_CLIENT_METHODS so
server-direction methods (roots/list, sampling/createMessage,
elicitation/create) registered via add_request_handler reach their
handler instead of METHOD_NOT_FOUND
- Elicit gate now validates the rendered schema property-by-property
against PrimitiveSchemaDefinition (restores list[str] + json_schema_extra
multi-select; bare list[str] without enum still rejected; Literal[...]
now supported)
- gen_surface_types.py always invokes datamodel-codegen via uv (the
shutil.which() shortcut defeated the version pin)
- RequestedSchema.properties relaxed to dict[str, Any] in the generated
surfaces (older python-sdk releases emit anyOf for Optional fields);
ElicitResult.content value union gains a null arm (monolith superset
leniency)
- Client _on_notify logs version-absent notifications at debug,
malformed at warning (mirrors runner)
- migration.md: fix the input_schema={} example; document client-side
surface validation; correct the elicit-gate section
- JSONValue: the spec includes null but the ts-to-json render dropped it alongside number; the existing patch restored number only. Now restores both, with a post-process step to strip the redundant forward-ref | None codegen emits in the recursive definitions. - Tasks types: the 12 Tasks capability classes were defined in _types but not re-exported. Now in __all__, with a parity test that every public _types model is exported.
felixweinberger
approved these changes
Jun 16, 2026
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Overview
Three type sets:
mcp.types.v2025_11_25— generated from the 2025-11-25 schema; serves every version through 2025-11-25mcp.types.v2026_07_28— generated from the 2026-07-28 (draft) schemamcp.types(the monolith) — hand-maintained superset of both; this is the public APImcp.types.methodsholds(method, version)→ surface-type maps and the parse/validate/serialize helpers the runner and sessions call.Generation
scripts/gen_surface_types.pyrunsdatamodel-codegenover vendoredschema/*.json, with a few mappings overridden (format: byte/uri/uri-template→str;number→int | float) andextra="ignore"everywhere except a per-version allow-list of spec-declared open types (_meta,Result,GetTaskPayloadResult, tool input/output schemas, etc.). CI regenerates with--checkand fails on drift.Wiring
ServerRunnerandClientSessionconsult the maps:METHOD_NOT_FOUNDeven if a handler is registered; a malformed payload isINVALID_PARAMS(or dropped+logged for notifications). Non-spec methods registered on the server skip the gate.send_requestreturn path, both directions) are surface-validated before the monolith parse; mismatches raise at the call site.resultType,ttlMs,cacheScope) never reach a pre-2026 peer.SUPPORTED_PROTOCOL_VERSIONSis unchanged — 2026-07-28 is modeled, not yet negotiable.Breaking changes
Tool(inputSchema={})— missing"type": "object") now producesINTERNAL_ERROR.PrimitiveSchemaDefinition;Optional[T]is unwrapped, barelist[str](no enum) is rejected.Documented in
docs/migration.md.Types of changes
Checklist