Test that request _meta is camelCase on the wire and snake_case to a handler

A caller's `_meta.progressToken` is carried over the wire under its camelCase
JSON alias but deserialized back to the snake_case field `progress_token`, so
a server handler reading `params.meta` finds `progress_token`. Pin both forms
of the same params object for `skills/list` and `resources/directory/read`,
and expand the client round-trip test's comment to spell out the distinction.

Author: vijay

tests/client/test_skills.py

@@ -293,7 +293,9 @@ async def handler(ctx: ServerRequestContext[Any, Any], params: ListSkillsParams)
     async with Client(server) as client:
         params = ListSkillsParams.model_validate({"_meta": {"progressToken": "t"}})
         await list_skills(client.session, params)
-    # The transport enriches `_meta` with its own keys; what matters is the caller's token
+    # `_meta` crosses the wire camelCase (`progressToken`), but the server deserializes it back
+    # through the meta model, which exposes the known field snake_case as `progress_token`; the
+    # transport also enriches `_meta` with its own keys. What matters is the caller's token
     # reaching the server on both the first page and the cursor-following second one.
     assert len(seen_meta) == 2
     assert all(m is not None and m.get("progress_token") == "t" for m in seen_meta)

tests/shared/test_skills.py

@@ -5,12 +5,15 @@
 """
 
 import hashlib
+from typing import Any
 
 import pytest
 from mcp_types import Resource
 
 from mcp.shared.skills import (
+    ListSkillsParams,
     ListSkillsResult,
+    ReadDirectoryParams,
     ReadDirectoryResult,
     Skill,
     SkillResource,
@@ -286,6 +289,26 @@ def test_validate_list_result_accepts_an_empty_listing() -> None:
     validate_list_result(ListSkillsResult(skills=[]))
 
 
+@pytest.mark.parametrize(
+    ("params_type", "payload"),
+    [
+        (ListSkillsParams, {"_meta": {"progressToken": "t"}}),
+        (ReadDirectoryParams, {"uri": "skill://pdf/templates", "_meta": {"progressToken": "t"}}),
+    ],
+)
+def test_request_meta_is_camelcase_on_the_wire_but_snakecase_to_a_handler(
+    params_type: type[ListSkillsParams] | type[ReadDirectoryParams], payload: dict[str, Any]
+) -> None:
+    """A caller's `_meta.progressToken` rides the wire camelCase (its JSON alias) yet is read back
+    snake_case as `progress_token` once deserialized - two spellings, one value. This is what lets
+    `list_skills`/`read_directory` forward a caller's `_meta` to every page while a server handler
+    still finds `progress_token` in `params.meta`; both spellings working is the point."""
+    params = params_type.model_validate(payload)
+    wire = params.model_dump(by_alias=True, mode="json", exclude_none=True)
+    assert wire["_meta"] == {"progressToken": "t"}  # camelCase over the wire
+    assert params.meta is not None and params.meta.get("progress_token") == "t"  # snake_case to a handler
+
+
 @pytest.mark.parametrize("uri", ["skill://pdf/templates/", "not-a-uri"])
 def test_parse_directory_uri_rejects_malformed_uris(uri: str) -> None:
     with pytest.raises(ValueError, match="directory URI"):