Verify the conformance preview tarball; tidy mcp_param_headers typing

Address PR review:
- conformance.yml pinned CONFORMANCE_PKG to a mutable pkg.pr.new URL run via
  npx without verification. Restore the repo's pre-#2974 supply-chain pattern:
  a CONFORMANCE_PKG_SHA256 digest plus a fetch-and-verify step that downloads,
  checks the sha256, and repoints CONFORMANCE_PKG at the verified local tarball
  (no-op for registry specs). Verified the scenario still passes 18/18 via the
  file: tarball.
- mcp_param_headers carried an inline node: Any and a cast. Move the
  arbitrary-JSON path walk into a small _value_at_path helper so the main body
  is clean; the single remaining cast (strict-pyright narrowing of a bare
  Mapping) matches _walk_schema_positions in the same file.

Author: Kludex

.github/workflows/conformance.yml

@@ -21,9 +21,13 @@ env:
   #
   # Temporarily pinned to the pkg.pr.new preview build of conformance#371, which
   # fixes the http-custom-headers fixture's spec-forbidden `number`-typed
-  # x-mcp-header annotations. Repin to the published release that includes #371
-  # once it ships (the preview URL is ephemeral).
+  # x-mcp-header annotations. Because this is a mutable URL (not a registry
+  # spec), CONFORMANCE_PKG_SHA256 pins the tarball and the fetch-and-verify step
+  # below downloads, checks the digest, and repoints CONFORMANCE_PKG at the
+  # verified local copy. Repin to the published release that includes #371 once
+  # it ships, then drop CONFORMANCE_PKG_SHA256 and the fetch-and-verify steps.
   CONFORMANCE_PKG: "https://pkg.pr.new/@modelcontextprotocol/conformance@371"
+  CONFORMANCE_PKG_SHA256: "9d8b25874d55e304b006cbaa066571773582f5828143c53a2b8a6830f203ca1d"
 
 jobs:
   server-conformance:
@@ -39,6 +43,19 @@ jobs:
       - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
         with:
           node-version: 24
+      - name: Fetch and verify conformance harness
+        # Only when CONFORMANCE_PKG is a URL: download, check the recorded
+        # sha256, and re-point CONFORMANCE_PKG at the verified local tarball.
+        # When CONFORMANCE_PKG is a registry spec, this step is a no-op (npm's
+        # own integrity check applies).
+        run: |
+          case "$CONFORMANCE_PKG" in
+            https://*)
+              curl -fsSL "$CONFORMANCE_PKG" -o /tmp/conformance.tgz
+              echo "$CONFORMANCE_PKG_SHA256  /tmp/conformance.tgz" | sha256sum -c -
+              echo "CONFORMANCE_PKG=file:/tmp/conformance.tgz" >> "$GITHUB_ENV"
+              ;;
+          esac
       - run: uv sync --frozen --all-extras --package mcp-everything-server
       - name: Run server conformance (active suite)
         run: >-
@@ -70,6 +87,19 @@ jobs:
       - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
         with:
           node-version: 24
+      - name: Fetch and verify conformance harness
+        # Only when CONFORMANCE_PKG is a URL: download, check the recorded
+        # sha256, and re-point CONFORMANCE_PKG at the verified local tarball.
+        # When CONFORMANCE_PKG is a registry spec, this step is a no-op (npm's
+        # own integrity check applies).
+        run: |
+          case "$CONFORMANCE_PKG" in
+            https://*)
+              curl -fsSL "$CONFORMANCE_PKG" -o /tmp/conformance.tgz
+              echo "$CONFORMANCE_PKG_SHA256  /tmp/conformance.tgz" | sha256sum -c -
+              echo "CONFORMANCE_PKG=file:/tmp/conformance.tgz" >> "$GITHUB_ENV"
+              ;;
+          esac
       - run: uv sync --frozen --all-extras --package mcp
       - name: Run client conformance (all suite)
         # The harness runs all scenarios via unbounded Promise.all; with 40

src/mcp/shared/inbound.py

@@ -241,19 +241,24 @@ def mcp_param_headers(header_map: Mapping[tuple[str, ...], str], arguments: Mapp
     """
     headers: dict[str, str] = {}
     for path, token in header_map.items():
-        node: Any = arguments
-        for key in path:
-            if not isinstance(node, Mapping):
-                node = None
-                break
-            node = cast("Mapping[str, Any]", node).get(key)
-        if node is None:
+        value = _value_at_path(arguments, path)
+        if value is None:
             continue
-        rendered = ("true" if node else "false") if isinstance(node, bool) else str(node)
+        rendered = ("true" if value else "false") if isinstance(value, bool) else str(value)
         headers[f"{MCP_PARAM_HEADER_PREFIX}{token}"] = encode_header_value(rendered)
     return headers
 
 
+def _value_at_path(arguments: Mapping[str, Any], path: tuple[str, ...]) -> Any:
+    """Read the value at a `properties`-key path in `arguments`, or `None` if any step is missing or non-mapping."""
+    node: Any = arguments
+    for key in path:
+        if not isinstance(node, Mapping):
+            return None
+        node = cast("Mapping[str, Any]", node).get(key)
+    return node
+
+
 # INTERNAL_ERROR is deliberately unmapped (→ HTTP 200): the spec assigns no status to
 # -32603, and whether handler-origin errors get 5xx is an open S4 question — see TODO(L66).
 ERROR_CODE_HTTP_STATUS: Final[Mapping[int, int]] = MappingProxyType(