Pass http-custom-headers conformance via the #371 fixture-fix preview

The conformance handler now replays the harness-supplied toolCalls verbatim
(including the null and Base64-edge-case values) instead of synthesizing
arguments from the schema, so every per-parameter check is exercised: the
null-omission and Base64-unsafe checks now fire correctly.

Pins CONFORMANCE_PKG to the pkg.pr.new preview of conformance#371, which fixes
the fixture's spec-forbidden number-typed x-mcp-header annotations, and removes
http-custom-headers from both expected-failures baselines (it now passes 18/18
on both the default and 2026-07-28 legs). Repin to the published release once
#371 ships.

Author: Kludex

.github/actions/conformance/client.py

@@ -335,41 +335,32 @@ async def run_http_invalid_tool_headers(server_url: str) -> None:
                 logger.exception(f"call_tool({tool.name!r}) failed")
 
 
-def _stub_args_for_custom_headers(input_schema: dict[str, Any]) -> dict[str, Any]:
-    """Arguments exercising every `x-mcp-header`-annotated property in a tool schema.
-
-    Each annotated property gets a type-appropriate value so the SDK mirrors it into an
-    `Mcp-Param-*` header; required properties without an annotation get a placeholder so
-    the call is well-formed.
-    """
-    by_type: dict[str, Any] = {"string": "us-west1", "integer": 42, "boolean": False, "number": 3.14}
-    properties: dict[str, Any] = input_schema.get("properties", {})
-    arguments: dict[str, Any] = {}
-    for name, schema in properties.items():
-        if "x-mcp-header" in schema:
-            arguments[name] = by_type.get(schema.get("type"), "x")
-    for name in input_schema.get("required", []):
-        arguments.setdefault(name, by_type.get(properties.get(name, {}).get("type"), "x"))
-    return arguments
-
-
 @register("http-custom-headers")
 async def run_http_custom_headers(server_url: str) -> None:
-    """List tools, then call each surfaced tool so its `x-mcp-header` args mirror into headers (SEP-2243).
+    """List tools, then replay the harness's `toolCalls` so x-mcp-header args mirror into headers (SEP-2243).
 
-    A conforming client drops tools with invalid annotations during `list_tools` (e.g. the
-    harness's `number`-typed properties, which the spec forbids), so the loop only calls tools
-    whose annotations are valid; for those, the SDK emits the `Mcp-Param-*` headers the scenario
-    checks. Per-call failures are logged and skipped rather than aborting the run.
+    The scenario supplies the exact arguments to send (including the null/edge-case values that
+    exercise omission and Base64 encoding) via the context `toolCalls`; using them verbatim is
+    what drives every per-parameter check. `list_tools` first so the SDK caches each tool's
+    annotations; a tool the SDK dropped (invalid annotations) is skipped. Per-call failures are
+    logged and skipped rather than aborting the run.
     """
+    tool_calls: list[dict[str, Any]] = []
+    if os.environ.get("MCP_CONFORMANCE_CONTEXT"):
+        tool_calls = get_conformance_context().get("toolCalls", [])
     async with Client(server_url, mode=client_mode()) as client:
         listed = await client.list_tools()
-        logger.debug(f"Surfaced tools: {[t.name for t in listed.tools]}")
-        for tool in listed.tools:
+        surfaced = {tool.name for tool in listed.tools}
+        logger.debug(f"Surfaced tools: {sorted(surfaced)}")
+        for call in tool_calls:
+            name = call["name"]
+            if name not in surfaced:
+                logger.debug(f"skipping {name!r}: not surfaced by list_tools")
+                continue
             try:
-                await client.call_tool(tool.name, _stub_args_for_custom_headers(tool.input_schema))
+                await client.call_tool(name, call.get("arguments") or {})
             except Exception:
-                logger.exception(f"call_tool({tool.name!r}) failed")
+                logger.exception(f"call_tool({name!r}) failed")
 
 
 @register("elicitation-sep1034-client-defaults")

.github/actions/conformance/expected-failures.2026-07-28.yml

@@ -21,13 +21,7 @@
 # milestone.
 
 client:
-  # SEP-2243 (HTTP standardization): the client now mirrors x-mcp-header args into
-  # Mcp-Param-* headers (S8), but the harness fixture annotates `number`-typed
-  # properties, which the spec forbids ("Parameters with type number are not
-  # permitted"). The SDK drops those tools per spec, so the scenario's
-  # ClientSupportsCustomHeaders check (which requires the tool to be called)
-  # cannot pass until the harness fixture is corrected.
-  - http-custom-headers
+  []
   # auth/enterprise-managed-authorization (SEP-990) is in the 2025 baseline but
   # NOT here: the harness skips it as inapplicable at --spec-version 2026-07-28
   # (it is an extension scenario not carried into the 2026 wire), so it is

.github/actions/conformance/expected-failures.yml

@@ -11,15 +11,6 @@
 # on stale entries), so the baseline burns down per milestone.
 
 client:
-  # --- Draft-spec scenarios (in `--suite draft`, also part of `--suite all`) ---
-  # SEP-2243 (HTTP standardization): the client now mirrors x-mcp-header args into
-  # Mcp-Param-* headers (S8), but the harness fixture annotates `number`-typed
-  # properties, which the spec forbids ("Parameters with type number are not
-  # permitted"). The SDK drops those tools per spec, so the scenario's
-  # ClientSupportsCustomHeaders check (which requires the tool to be called)
-  # cannot pass until the harness fixture is corrected.
-  - http-custom-headers
-
   # --- Pre-existing scenarios that fail on checks added after conformance 0.1.15 ---
   # SEP-990 (enterprise-managed authorization extension): no fixture handler /
   # client support for the token-exchange + JWT bearer flow.

.github/workflows/conformance.yml

@@ -18,7 +18,12 @@ env:
   # Use a published version, e.g. @modelcontextprotocol/conformance@0.2.0-alpha.7.
   # Bump deliberately and reconcile both
   # .github/actions/conformance/expected-failures*.yml files in the same change.
-  CONFORMANCE_PKG: "@modelcontextprotocol/conformance@0.2.0-alpha.7"
+  #
+  # 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).
+  CONFORMANCE_PKG: "https://pkg.pr.new/@modelcontextprotocol/conformance@371"
 
 jobs:
   server-conformance: