modelcontextprotocol
diff --git a/.github/actions/conformance/client.py b/.github/actions/conformance/client.py
@@ -335,6 +335,34 @@ async def run_http_invalid_tool_headers(server_url: str) -> None:
                 logger.exception(f"call_tool({tool.name!r}) failed")
 
 
+@register("http-custom-headers")
+async def run_http_custom_headers(server_url: str) -> None:
+    """List tools, then replay the harness's `toolCalls` so x-mcp-header args mirror into headers (SEP-2243).
+
+    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()
+        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(name, call.get("arguments") or {})
+            except Exception:
+                logger.exception(f"call_tool({name!r}) failed")
+
+
 @register("elicitation-sep1034-client-defaults")
 async def run_elicitation_defaults(server_url: str) -> None:
     """Connect with elicitation callback that applies schema defaults."""
@@ -526,8 +554,6 @@ def main() -> None:
         elif scenario.startswith("auth/"):
             asyncio.run(run_auth_code_client(server_url))
         else:
-            # Unhandled scenarios:
-            #  - http-custom-headers (SEP-2243 / S8: Mcp-Param-* emission)
             print(f"Unknown scenario: {scenario}", file=sys.stderr)
             sys.exit(1)
     else:

diff --git a/.github/actions/conformance/expected-failures.2026-07-28.yml b/.github/actions/conformance/expected-failures.2026-07-28.yml
@@ -21,9 +21,7 @@
 # milestone.
 
 client:
-  # SEP-2243 (HTTP standardization): no client Mcp-Param-* support yet — needs the
-  # tool-schema-cache vs per-call tool_definition design (S8).
-  - 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

diff --git a/.github/actions/conformance/expected-failures.yml b/.github/actions/conformance/expected-failures.yml
@@ -11,11 +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): no client Mcp-Param-* support yet — needs the
-  # tool-schema-cache vs per-call tool_definition design (S8).
-  - 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.

diff --git a/.github/workflows/conformance.yml b/.github/workflows/conformance.yml
@@ -18,7 +18,16 @@ 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. 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:
@@ -34,6 +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: >-
@@ -65,6 +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

diff --git a/docs/migration.md b/docs/migration.md
@@ -398,6 +398,10 @@ For an in-process `Client(server)` (where `server` is a `Server` or `MCPServer`
 
 For protocol 2026-07-28, a `tools/call` request may return an `InputRequiredResult` asking the client to supply additional input and retry. By default `call_tool` (on `ClientSession`, `Client`, and `ClientSessionGroup`) still returns `CallToolResult` and raises `RuntimeError` if the server requests input. Pass `allow_input_required=True` to receive the `InputRequiredResult` instead, then retry with `input_responses=` / `request_state=`.
 
+### `call_tool` mirrors `x-mcp-header` arguments into `Mcp-Param-*` headers (SEP-2243)
+
+For protocol 2026-07-28 over Streamable HTTP, a tool's input-schema property may carry an `x-mcp-header` annotation. When a tool the client has listed is called, each annotated argument is mirrored into an `Mcp-Param-<name>` request header (string verbatim, integer as decimal, boolean as `true`/`false`, base64-sentinel-wrapped when not header-safe; `null`/absent arguments are omitted). The argument is also left in the request body. `list_tools` caches a tool's annotations, so list a tool before calling it to enable mirroring; a tool the client never listed emits no `Mcp-Param-*` headers. Other transports ignore the annotation.
+
 ### `McpError` renamed to `MCPError`
 
 The `McpError` exception class has been renamed to `MCPError` for consistent naming with the MCP acronym style used throughout the SDK.

diff --git a/src/mcp/client/session.py b/src/mcp/client/session.py
@@ -41,6 +41,8 @@
     NAME_BEARING_METHODS,
     encode_header_value,
     find_invalid_x_mcp_header,
+    mcp_param_headers,
+    x_mcp_header_map,
 )
 from mcp.shared.jsonrpc_dispatcher import JSONRPCDispatcher
 from mcp.shared.message import ClientMessageMetadata, SessionMessage
@@ -68,7 +70,10 @@ def stamp(data: dict[str, Any], opts: CallOptions) -> None:
 
 
 def _make_modern_stamp(
-    protocol_version: str, client_info: dict[str, Any], capabilities: dict[str, Any]
+    protocol_version: str,
+    client_info: dict[str, Any],
+    capabilities: dict[str, Any],
+    resolve_param_headers: Callable[[str, Mapping[str, Any]], dict[str, str]],
 ) -> Callable[[dict[str, Any], CallOptions], None]:
     def stamp(data: dict[str, Any], opts: CallOptions) -> None:
         params = data.setdefault("params", {})
@@ -83,6 +88,8 @@ def stamp(data: dict[str, Any], opts: CallOptions) -> None:
         name_key = NAME_BEARING_METHODS.get(data["method"])
         if name_key is not None and isinstance(name := params.get(name_key), str):
             headers[MCP_NAME_HEADER] = encode_header_value(name)
+        if data["method"] == "tools/call" and isinstance(name := params.get("name"), str):
+            headers.update(resolve_param_headers(name, params.get("arguments") or {}))
 
     return stamp
 
@@ -215,6 +222,7 @@ def __init__(
         self._logging_callback = logging_callback or _default_logging_callback
         self._message_handler = message_handler or _default_message_handler
         self._tool_output_schemas: dict[str, dict[str, Any] | None] = {}
+        self._x_mcp_header_maps: dict[str, dict[tuple[str, ...], str]] = {}
         self._initialize_result: types.InitializeResult | None = None
         self._discover_result: types.DiscoverResult | None = None
         self._negotiated_version: str | None = None
@@ -393,7 +401,7 @@ def adopt(self, result: types.InitializeResult | types.DiscoverResult) -> None:
                 )
             client_info = self._client_info.model_dump(by_alias=True, mode="json", exclude_none=True)
             capabilities = self._build_capabilities().model_dump(by_alias=True, mode="json", exclude_none=True)
-            self._stamp = _make_modern_stamp(mutual[-1], client_info, capabilities)
+            self._stamp = _make_modern_stamp(mutual[-1], client_info, capabilities, self._resolve_param_headers)
             self._discover_result = result
             self._initialize_result = None
             self._negotiated_version = mutual[-1]
@@ -646,6 +654,11 @@ async def call_tool(
     ) -> types.CallToolResult | types.InputRequiredResult:
         """Send a tools/call request with optional progress callback support.
 
+        On a modern (2026-07-28) connection, arguments annotated with `x-mcp-header`
+        in the tool's input schema are mirrored into `Mcp-Param-*` request headers.
+        The annotations are read from the tool's last `list_tools` entry, so list
+        the tool before calling it to enable header emission.
+
         Args:
             input_responses: Responses to a prior `InputRequiredResult.input_requests`.
             request_state: Opaque state echoed from a prior `InputRequiredResult`.
@@ -657,7 +670,6 @@ async def call_tool(
             RuntimeError: If the server returns an `InputRequiredResult` and
                 ``allow_input_required`` is ``False``.
         """
-
         result = await self.send_request(
             types.CallToolRequest(
                 params=types.CallToolRequestParams(
@@ -683,6 +695,13 @@ async def call_tool(
             )
         return result
 
+    def _resolve_param_headers(self, name: str, arguments: Mapping[str, Any]) -> dict[str, str]:
+        """`Mcp-Param-*` headers for a `tools/call`, or empty when the tool was never listed."""
+        header_map = self._x_mcp_header_maps.get(name)
+        if header_map is None:
+            return {}
+        return mcp_param_headers(header_map, arguments)
+
     async def _validate_tool_result(self, name: str, result: types.CallToolResult) -> None:
         """Validate the structured content of a tool result against its output schema."""
         if name not in self._tool_output_schemas:
@@ -767,7 +786,12 @@ async def list_tools(self, *, params: types.PaginatedRequestParams | None = None
             for tool in result.tools:
                 if (reason := find_invalid_x_mcp_header(tool.input_schema)) is not None:
                     logger.warning("dropping tool %r: invalid x-mcp-header (%s)", tool.name, reason)
+                    # Evict any map cached from a prior valid listing so a stale entry can't
+                    # mirror headers for a tool this listing dropped.
+                    self._x_mcp_header_maps.pop(tool.name, None)
                     continue
+                # Cache the arg→header map so a later tools/call mirrors it into Mcp-Param-* headers.
+                self._x_mcp_header_maps[tool.name] = x_mcp_header_map(tool.input_schema)
                 kept.append(tool)
             result.tools = kept
 

diff --git a/src/mcp/shared/inbound.py b/src/mcp/shared/inbound.py
@@ -42,13 +42,16 @@
     "InboundModernRoute",
     "MCP_METHOD_HEADER",
     "MCP_NAME_HEADER",
+    "MCP_PARAM_HEADER_PREFIX",
     "MCP_PROTOCOL_VERSION_HEADER",
     "NAME_BEARING_METHODS",
     "X_MCP_HEADER_KEY",
     "classify_inbound_request",
     "decode_header_value",
     "encode_header_value",
     "find_invalid_x_mcp_header",
+    "mcp_param_headers",
+    "x_mcp_header_map",
 ]
 
 MCP_PROTOCOL_VERSION_HEADER: Final = "mcp-protocol-version"
@@ -206,6 +209,56 @@ def find_invalid_x_mcp_header(input_schema: Any) -> str | None:
     return None
 
 
+MCP_PARAM_HEADER_PREFIX: Final = "Mcp-Param-"
+"""Prefix the `x-mcp-header` token is joined to, forming the per-parameter HTTP header name."""
+
+
+def x_mcp_header_map(input_schema: Any) -> dict[tuple[str, ...], str]:
+    """Map each property carrying a valid `x-mcp-header` to its annotation token, keyed by property path.
+
+    The key is the chain of `properties` keys from the schema root to the
+    annotated property; a top-level property has a one-element path, a nested
+    one a longer path. Call only on a schema that
+    :func:`find_invalid_x_mcp_header` accepts; an invalid schema yields an
+    undefined subset.
+    """
+    mapping: dict[tuple[str, ...], str] = {}
+    for path, schema in _walk_schema_positions(input_schema):
+        if path and isinstance(header := schema.get(X_MCP_HEADER_KEY), str):
+            mapping[path] = header
+    return mapping
+
+
+def mcp_param_headers(header_map: Mapping[tuple[str, ...], str], arguments: Mapping[str, Any]) -> dict[str, str]:
+    """Build the `Mcp-Param-*` headers a `tools/call` mirrors from its arguments.
+
+    For each `(path, token)` in `header_map`, read the value at that property
+    path in `arguments` and, when it is present and not `None`, emit
+    `Mcp-Param-<token>` carrying it: `bool` as `true`/`false`, other scalars via
+    `str`, each passed through :func:`encode_header_value` so a non-token value
+    is base64-wrapped. A path that hits a missing key or a non-mapping node is
+    skipped, matching the spec's "omit the header when no value is present".
+    """
+    headers: dict[str, str] = {}
+    for path, token in header_map.items():
+        value = _value_at_path(arguments, path)
+        if value is None:
+            continue
+        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(

diff --git a/tests/interaction/_requirements.py b/tests/interaction/_requirements.py
@@ -3372,6 +3372,19 @@ def __post_init__(self) -> None:
         transports=("streamable-http",),
         note="Only observable over streamable HTTP: headers are derived from the body envelope at the transport seam.",
     ),
+    "client-transport:http:custom-param-headers": Requirement(
+        source=f"{SPEC_2026_BASE_URL}/basic/transports/streamable-http#custom-headers-from-tool-parameters",
+        behavior=(
+            "On a tools/call, a client mirrors each argument annotated with x-mcp-header in the tool's "
+            "inputSchema into an Mcp-Param-<name> header -- string as-is, integer as decimal, boolean as "
+            "true/false, base64-sentinel-wrapped when not header-safe -- omitting null or absent arguments and "
+            "never mirroring unannotated parameters. The schema is taken from the tool's last list_tools entry; "
+            "a tool the client never listed emits no Mcp-Param-* headers."
+        ),
+        added_in="2026-07-28",
+        transports=("streamable-http",),
+        note="Only observable over streamable HTTP: headers are derived from the cached tool schema at the seam.",
+    ),
     "client-transport:http:stateless-ignores-session-id": Requirement(
         source=f"{SPEC_2026_BASE_URL}/basic/transports#stateless-request-headers",
         behavior=(