Say on the Header parameters page that Client re-lists and resends

One bullet under "Mark an argument", with a test that calls the
tutorial's tool without listing it first.

Author: maxisbey

docs/advanced/header-parameters.md

@@ -13,6 +13,7 @@ The mark is one extra key in the argument's JSON Schema. On `MCPServer`, `Field`
 ```
 
 * Over Streamable HTTP on `2026-07-28`, a client that has listed the tool sends `Mcp-Param-Region` alongside the body, and the server rejects a call where the two disagree.
+* A client that hasn't listed the tool yet sends no header, and the call is rejected. This SDK's `Client` then lists the tools and resends the call once, so listing first only saves a round trip.
 * Every other connection ignores the annotation.
 
 Your function doesn't change: `region` still arrives as an argument.

tests/docs_src/test_header_parameters.py

@@ -63,6 +63,31 @@ async def test_a_call_whose_header_and_body_disagree_is_rejected() -> None:
     assert tampered.json()["error"]["code"] == HEADER_MISMATCH
 
 
+async def test_a_client_that_has_not_listed_the_tool_is_rejected_then_lists_and_resends_once() -> None:
+    """tutorial001: the first call has no header and is a 400; after one `tools/list` it is resent with the header."""
+    app = tutorial001.mcp.streamable_http_app()
+    exchanges: list[tuple[str, str | None, int]] = []
+
+    async def record(response: httpx2.Response) -> None:
+        sent = response.request.headers
+        exchanges.append((sent["mcp-method"], sent.get("mcp-param-region"), response.status_code))
+
+    async with (
+        app.router.lifespan_context(app),
+        httpx2.ASGITransport(app) as transport,
+        httpx2.AsyncClient(transport=transport, event_hooks={"response": [record]}) as http,
+        Client(streamable_http_client(URL, http_client=http)) as client,
+    ):
+        result = await client.call_tool("check_stock", ARGUMENTS)
+    assert result.structured_content == {"result": "Dune: 3 copies in eu."}
+    assert exchanges == [
+        ("server/discover", None, 200),
+        ("tools/call", None, 400),
+        ("tools/list", None, 200),
+        ("tools/call", "eu", 200),
+    ]
+
+
 async def test_a_legacy_http_connection_ignores_the_annotation() -> None:
     """tutorial001: before 2026-07-28 the same call succeeds and carries no `Mcp-Param-*` header."""
     async with check_stock_over_http(tutorial001.mcp.streamable_http_app(), mode="legacy") as (_, call):