Add multi-version protocol types: two wire-shape surface packages

One superset type set in mcp.types plus two hand-validated model
packages: mcp.types.v2025_11_25, whose models serve every protocol
version up to and including 2025-11-25 (the schemas through that
revision evolve strictly additively; a structural coverage test proves
the newest one covers them all), and mcp.types.v2026_07_28 for the new
protocol revision.

Serialization is additive-only: nothing is ever removed from a model's
dump at any version. Sessions at 2025-11-25 and earlier emit the plain
dump byte for byte; on 2026-07-28 sessions the wire boundary injects
the protocol-required fields the caller left unset (resultType, the
ttlMs/cacheScope don't-cache defaults, the reserved protocolVersion
_meta entry) and validates the result through the 2026-07-28 models as
a check before emitting the injected dump unchanged.

Author: maxisbey

docs/migration.md

@@ -1166,11 +1166,18 @@ In practice, replace direct `ServerSession` use with `Server.run(read_stream, wr
 
 `BaseSession` is still used by `ClientSession`, which never relied on these members. `RequestResponder.respond()` is unchanged.
 
-### Experimental Tasks support removed
+### Experimental Tasks support removed (types restored, types-only)
 
-Tasks (SEP-1686) have been removed from the MCP specification and are no longer part of this SDK. The `mcp.client.experimental`, `mcp.server.experimental`, `mcp.shared.experimental`, and `mcp.server.lowlevel.experimental` modules have been removed, along with all `Task*` types, the `tasks` capability fields, `Tool.execution`, and the `experimental` properties on `ClientSession`, `ServerSession`, `Server`, and `ServerRequestContext`.
+Tasks (SEP-1686) runtime support has been removed from this SDK. The `mcp.client.experimental`, `mcp.server.experimental`, `mcp.shared.experimental`, and `mcp.server.lowlevel.experimental` modules are gone, along with the `experimental` properties on `ClientSession`, `ServerSession`, `Server`, and `ServerRequestContext`.
 
-Tasks are expected to return as a separate MCP extension in a future release.
+The 2025-11-25 protocol *types* are back in `mcp.types` so that 2025-11-25 task payloads can still be modeled: the `Task*` types, the `tasks` capability subtrees, `Tool.execution`, and the `task` field on the four task-augmentable params classes. They are types-only definitions:
+
+- Attributes are snake_case (`task_id`, `created_at`), aliased to the camelCase wire names.
+- Timestamps are plain `str` values, not `datetime`.
+- `GetTaskPayloadResult` follows the default extra-field policy (`ignore`), so it retains only `_meta`; validate a tasks/result payload into the original request's result type instead.
+- None of the task methods is a member of the request/notification unions, and `add_request_handler` does not dispatch them.
+
+Tasks runtime support is expected to return as a separate MCP extension in a future release.
 
 ## Deprecations
 
@@ -1214,6 +1221,10 @@ If you relied on extra fields round-tripping through MCP types, move that data i
 
 ## New Features
 
+### Newer protocol fields are modeled and retained
+
+`mcp.types` now models the 2025-11-25 and 2026-07-28 protocol fields and types (for example `resultType`, `ttlMs`/`cacheScope` on the cacheable results, and `inputResponses`/`requestState` on retried requests). Inbound payloads carrying these keys used to lose them to the unknown-field policy on re-dump; they now parse into typed fields and survive a user-level round-trip. All of the new fields are optional with `None` defaults, so dumps of values that do not set them are unchanged.
+
 ### `streamable_http_app()` available on lowlevel Server
 
 The `streamable_http_app()` method is now available directly on the lowlevel `Server` class, not just `MCPServer`. This allows using the streamable HTTP transport without the MCPServer wrapper.

pyproject.toml

@@ -143,6 +143,19 @@ venv = ".venv"
 # those private functions instead of testing the private functions directly. It makes it easier to maintain the code source
 # and refactor code that is not public.
 executionEnvironments = [
+    # Generated spec-oracle modules carry two emission shapes pyright rejects
+    # but pydantic handles correctly, and no semantics-preserving spelling
+    # avoids either: (1) the schema's named-error defs require `data` while the
+    # base JSON-RPC error object has it optional - the required-field override
+    # is right on the wire (the requiredness audit in
+    # scripts/update_spec_types.py pins it) but trips the dataclass-transform
+    # rule (reportGeneralTypeIssues); (2) the draft schema's recursive
+    # JSONValue/JSONObject alias group must be emitted as a whole-string
+    # TypeAliasType value for lazy parsing, which pyright flags at its use
+    # sites (reportInvalidTypeForm, reportUnknownVariableType).
+    { root = "tests/spec_oracles", extraPaths = [
+        ".",
+    ], reportUnusedFunction = false, reportPrivateUsage = false, reportGeneralTypeIssues = false, reportInvalidTypeForm = false, reportUnknownVariableType = false },
     { root = "tests", extraPaths = [
         ".",
     ], reportUnusedFunction = false, reportPrivateUsage = false },
@@ -177,6 +190,9 @@ max-complexity = 24 # Default is 10
 
 [tool.ruff.lint.per-file-ignores]
 "__init__.py" = ["F401"]
+# Generated spec-oracle modules: docstrings come verbatim from the spec schema
+# and ruff format does not wrap long docstring lines.
+"tests/spec_oracles/v*.py" = ["E501"]
 "tests/server/mcpserver/test_func_metadata.py" = ["E501"]
 "tests/shared/test_progress_notifications.py" = ["PLW0603"]