Mirror x-mcp-header tool arguments into Mcp-Param-* request headers (SEP-2243) (#2990)

.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:

.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

.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.

.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

docs/migration.md

@@ -399,6 +399,10 @@ 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.

src/mcp/client/session.py

@@ -41,6 +41,8 @@ from mcp.shared.inbound import (
     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 _make_handshake_stamp(protocol_version: str) -> Callable[[dict[str, Any], Ca
 
 
 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 _make_modern_stamp(
         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 @@ class ClientSession:
         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 @@ class ClientSession:
                 )
             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 @@ class ClientSession:
     ) -> 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 @@ class ClientSession:
             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 @@ class ClientSession:
             )
         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 @@ class ClientSession:
             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
 

src/mcp/shared/inbound.py

@@ -42,6 +42,7 @@ __all__ = [
     "InboundModernRoute",
     "MCP_METHOD_HEADER",
     "MCP_NAME_HEADER",
+    "MCP_PARAM_HEADER_PREFIX",
     "MCP_PROTOCOL_VERSION_HEADER",
     "NAME_BEARING_METHODS",
     "X_MCP_HEADER_KEY",
@@ -49,6 +50,8 @@ __all__ = [
     "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"
@@ -216,6 +219,56 @@ 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(

tests/interaction/_requirements.py

@@ -3372,6 +3372,19 @@ REQUIREMENTS: dict[str, Requirement] = {
         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=(

tests/interaction/transports/test_hosting_http_modern.py

@@ -38,6 +38,7 @@ from mcp_types import (
 from mcp_types.version import LATEST_MODERN_VERSION
 
 from mcp import MCPError
+from mcp.client.client import Client
 from mcp.client.session import ClientSession
 from mcp.client.streamable_http import streamable_http_client
 from mcp.server import Server, ServerRequestContext
@@ -388,3 +389,164 @@ async def test_pinned_client_stateless_tools_call_round_trips_against_the_modern
     assert len(responses) == len(requests)
     assert all("mcp-session-id" not in r.headers for r in requests)
     assert all("mcp-session-id" not in r.headers for r in responses)
+
+
+_CUSTOM_HEADER_TOOL = Tool(
+    name="run",
+    input_schema={
+        "type": "object",
+        "properties": {
+            "region": {"type": "string", "x-mcp-header": "Region"},
+            "priority": {"type": "integer", "x-mcp-header": "Priority"},
+            "verbose": {"type": "boolean", "x-mcp-header": "Verbose"},
+            "note": {"type": "string", "x-mcp-header": "Note"},
+            "query": {"type": "string"},
+        },
+        "required": ["region"],
+    },
+)
+
+
+def _custom_header_server() -> Server:
+    """A server with one tool whose schema annotates four args with `x-mcp-header` and leaves `query` plain."""
+
+    async def list_tools(ctx: ServerRequestContext, params: PaginatedRequestParams | None) -> ListToolsResult:
+        return ListToolsResult(tools=[_CUSTOM_HEADER_TOOL], ttl_ms=0, cache_scope="public")
+
+    async def call_tool(ctx: ServerRequestContext, params: CallToolRequestParams) -> CallToolResult:
+        return CallToolResult(content=[TextContent(text="ok")])
+
+    return Server("custom-headers", on_list_tools=list_tools, on_call_tool=call_tool)
+
+
+@requirement("client-transport:http:custom-param-headers")
+async def test_modern_client_mirrors_x_mcp_header_args_into_mcp_param_headers() -> None:
+    """A tools/call mirrors the tool's `x-mcp-header` arguments into `Mcp-Param-*` headers.
+
+    After `list_tools` caches the tool's annotations, the client renders each annotated argument into
+    its header per the spec's Value Encoding rules: `region` verbatim, `priority` as a decimal, `verbose`
+    as `false`, and the non-ASCII `note` base64-sentinel-wrapped. The unannotated `query` and the omitted
+    `verbose`-sibling stay out of the headers, and every mirrored value remains in the request body. Asserted
+    at the wire because the client never surfaces the outgoing headers.
+    """
+    requests: list[httpx.Request] = []
+
+    async def on_request(request: httpx.Request) -> None:
+        requests.append(request)
+
+    discover = DiscoverResult(
+        supported_versions=[LATEST_MODERN_VERSION],
+        capabilities=ServerCapabilities(),
+        server_info=Implementation(name="srv", version="0"),
+    )
+    with anyio.fail_after(5):
+        async with (
+            mounted_app(_custom_header_server(), on_request=on_request) as (http, _),
+            Client(
+                streamable_http_client(f"{BASE_URL}/mcp", http_client=http),
+                mode=LATEST_MODERN_VERSION,
+                prior_discover=discover,
+            ) as client,
+        ):
+            await client.list_tools()
+            await client.call_tool("run", {"region": "us-west1", "priority": 42, "verbose": False, "note": "héllo"})
+
+    call = next(r for r in requests if json.loads(r.content)["method"] == "tools/call")
+    assert {k: v for k, v in call.headers.items() if k.startswith("mcp-param-")} == snapshot(
+        {
+            "mcp-param-region": "us-west1",
+            "mcp-param-priority": "42",
+            "mcp-param-verbose": "false",
+            "mcp-param-note": "=?base64?aMOpbGxv?=",
+        }
+    )
+    # Mirroring is additive: the arguments are unchanged in the body.
+    assert json.loads(call.content)["params"]["arguments"] == snapshot(
+        {"region": "us-west1", "priority": 42, "verbose": False, "note": "héllo"}
+    )
+
+
+@requirement("client-transport:http:custom-param-headers")
+async def test_modern_client_emits_no_param_headers_for_an_unlisted_tool() -> None:
+    """A `tools/call` for a tool the client never listed carries no `Mcp-Param-*` headers.
+
+    The spec lets a client that lacks the tool's `inputSchema` send the request without custom headers.
+    The call is made with no prior `list_tools`, so the first `tools/call` POST -- captured before the
+    implicit output-schema `list_tools` runs -- has no cached annotations and emits no `Mcp-Param-*` header.
+    """
+    requests: list[httpx.Request] = []
+
+    async def on_request(request: httpx.Request) -> None:
+        if json.loads(request.content)["method"] == "tools/call":
+            requests.append(request)
+
+    discover = DiscoverResult(
+        supported_versions=[LATEST_MODERN_VERSION],
+        capabilities=ServerCapabilities(),
+        server_info=Implementation(name="srv", version="0"),
+    )
+    with anyio.fail_after(5):
+        async with (
+            mounted_app(_custom_header_server(), on_request=on_request) as (http, _),
+            Client(
+                streamable_http_client(f"{BASE_URL}/mcp", http_client=http),
+                mode=LATEST_MODERN_VERSION,
+                prior_discover=discover,
+            ) as client,
+        ):
+            await client.call_tool("run", {"region": "us-west1"})
+
+    assert not any(k.startswith("mcp-param-") for k in requests[0].headers)
+
+
+@requirement("client-transport:http:custom-param-headers")
+async def test_modern_client_stops_mirroring_after_a_re_list_drops_the_tool() -> None:
+    """A re-list that drops a previously valid tool stops mirroring its `x-mcp-header` args.
+
+    The tool is first listed with a valid annotation (so a call mirrors `Mcp-Param-Region`), then re-listed
+    with an invalid annotation -- the modern client drops it and evicts the cached map, so a later `tools/call`
+    by name carries no `Mcp-Param-*` header. Asserted at the wire, where the eviction is observable.
+    """
+    schema = {"type": "object", "properties": {"a": {"type": "string", "x-mcp-header": "Region"}}}
+    bad_schema = {"type": "object", "properties": {"a": {"type": "string", "x-mcp-header": "bad name"}}}
+    valid = Tool(name="run", input_schema=schema)
+    invalid = Tool(name="run", input_schema=bad_schema)
+    listings = iter([valid, invalid])
+
+    async def list_tools(ctx: ServerRequestContext, params: PaginatedRequestParams | None) -> ListToolsResult:
+        return ListToolsResult(tools=[next(listings)], ttl_ms=0, cache_scope="public")
+
+    async def call_tool(ctx: ServerRequestContext, params: CallToolRequestParams) -> CallToolResult:
+        return CallToolResult(content=[TextContent(text="ok")])
+
+    server = Server("evict", on_list_tools=list_tools, on_call_tool=call_tool)
+
+    tool_calls: list[httpx.Request] = []
+
+    async def on_request(request: httpx.Request) -> None:
+        if json.loads(request.content)["method"] == "tools/call":
+            tool_calls.append(request)
+
+    discover = DiscoverResult(
+        supported_versions=[LATEST_MODERN_VERSION],
+        capabilities=ServerCapabilities(),
+        server_info=Implementation(name="srv", version="0"),
+    )
+    with anyio.fail_after(5):
+        async with (
+            mounted_app(server, on_request=on_request) as (http, _),
+            Client(
+                streamable_http_client(f"{BASE_URL}/mcp", http_client=http),
+                mode=LATEST_MODERN_VERSION,
+                prior_discover=discover,
+            ) as client,
+        ):
+            assert [t.name for t in (await client.list_tools()).tools] == ["run"]
+            await client.call_tool("run", {"a": "x"})
+
+            assert [t.name for t in (await client.list_tools()).tools] == []
+            await client.call_tool("run", {"a": "x"})
+
+    before, after = tool_calls
+    assert before.headers.get("mcp-param-region") == "x"
+    assert not any(k.startswith("mcp-param-") for k in after.headers)

tests/shared/test_inbound.py

@@ -40,6 +40,8 @@ from mcp.shared.inbound import (
     decode_header_value,
     encode_header_value,
     find_invalid_x_mcp_header,
+    mcp_param_headers,
+    x_mcp_header_map,
 )
 
 CLIENT_INFO = {"name": "t", "version": "0"}
@@ -505,3 +507,70 @@ def test_find_invalid_x_mcp_header_reports_dotted_path_for_nested_property() ->
     schema = _schema(outer={"type": "object", "properties": {"r": {"type": "object", "x-mcp-header": "R"}}})
     reason = find_invalid_x_mcp_header(schema)
     assert reason is not None and "'outer.r'" in reason
+
+
+# --- x_mcp_header_map ----------------------------------------------------------
+
+
+def test_x_mcp_header_map_keys_top_level_and_nested_properties_by_path() -> None:
+    """Each annotated property maps to its token under its full `properties` path; unannotated props are absent."""
+    schema = _schema(
+        region={"type": "string", "x-mcp-header": "Region"},
+        query={"type": "string"},
+        outer={"type": "object", "properties": {"inner": {"type": "string", "x-mcp-header": "Inner"}}},
+    )
+    assert x_mcp_header_map(schema) == {("region",): "Region", ("outer", "inner"): "Inner"}
+
+
+@pytest.mark.parametrize("input_schema", [None, "not-a-mapping", {"type": "object"}])
+def test_x_mcp_header_map_empty_for_schemas_without_annotations(input_schema: Any) -> None:
+    assert x_mcp_header_map(input_schema) == {}
+
+
+# --- mcp_param_headers ---------------------------------------------------------
+
+
+def test_mcp_param_headers_renders_primitive_types_per_spec() -> None:
+    """String verbatim, integer as decimal, boolean as lowercase `true`/`false`, header named `Mcp-Param-<token>`."""
+    header_map = {("region",): "Region", ("priority",): "Priority", ("verbose",): "Verbose", ("debug",): "Debug"}
+    arguments = {"region": "us-west1", "priority": 42, "verbose": False, "debug": True}
+    assert mcp_param_headers(header_map, arguments) == {
+        "Mcp-Param-Region": "us-west1",
+        "Mcp-Param-Priority": "42",
+        "Mcp-Param-Verbose": "false",
+        "Mcp-Param-Debug": "true",
+    }
+
+
+@pytest.mark.parametrize(
+    ("value", "encoded"),
+    [
+        pytest.param("us-west1", "us-west1", id="plain-ascii"),
+        pytest.param("Hello, 世界", "=?base64?SGVsbG8sIOS4lueVjA==?=", id="non-ascii"),
+        pytest.param(" padded ", "=?base64?IHBhZGRlZCA=?=", id="edge-whitespace"),
+        pytest.param("line1\nline2", "=?base64?bGluZTEKbGluZTI=?=", id="control-char"),
+        pytest.param("=?base64?literal?=", "=?base64?PT9iYXNlNjQ/bGl0ZXJhbD89?=", id="sentinel-lookalike"),
+    ],
+)
+def test_mcp_param_headers_base64_wraps_header_unsafe_strings(value: str, encoded: str) -> None:
+    """Matches the spec's Value Encoding table: a non-header-safe string is base64-sentinel wrapped."""
+    assert mcp_param_headers({("v",): "Val"}, {"v": value}) == {"Mcp-Param-Val": encoded}
+
+
+def test_mcp_param_headers_omits_absent_or_null_arguments() -> None:
+    """A path that hits a missing key or a `None` value emits no header (spec: omit when no value is present)."""
+    header_map = {("present",): "Present", ("missing",): "Missing", ("nulled",): "Nulled"}
+    assert mcp_param_headers(header_map, {"present": "x", "nulled": None}) == {"Mcp-Param-Present": "x"}
+
+
+def test_mcp_param_headers_reads_nested_argument_path() -> None:
+    """A nested annotated property reads its value at the matching nested `arguments` path."""
+    headers = mcp_param_headers({("outer", "inner"): "Inner"}, {"outer": {"inner": "deep"}})
+    assert headers == {"Mcp-Param-Inner": "deep"}
+
+
+def test_mcp_param_headers_omits_when_nested_path_is_broken() -> None:
+    """A nested path through a non-mapping or missing intermediate node yields no header."""
+    header_map = {("outer", "inner"): "Inner"}
+    assert mcp_param_headers(header_map, {"outer": "not-a-mapping"}) == {}
+    assert mcp_param_headers(header_map, {}) == {}