diff --git a/src/mcp/client/auth/extensions/identity_assertion.py b/src/mcp/client/auth/extensions/identity_assertion.py
index 17738a70bb..88c48e401c 100644
--- a/src/mcp/client/auth/extensions/identity_assertion.py
+++ b/src/mcp/client/auth/extensions/identity_assertion.py
@@ -143,7 +143,6 @@ def _build_token_request(self, scope: str | None, assertion: str) -> httpx2.Requ
         data: dict[str, str] = {
             "grant_type": JWT_BEARER_GRANT_TYPE,
             "assertion": assertion,
-            "client_id": self._client.client_id,
             "resource": self._resource,
         }
         if scope:
@@ -155,7 +154,10 @@ def _build_token_request(self, scope: str | None, assertion: str) -> httpx2.Requ
             encoded_secret = quote(self._client.client_secret, safe="")
             credentials = base64.b64encode(f"{encoded_id}:{encoded_secret}".encode()).decode()
             headers["Authorization"] = f"Basic {credentials}"
+            # RFC 6749 section 2.3: client_id must not also appear in the body when it's
+            # already presented via the Authorization header.
         else:
+            data["client_id"] = self._client.client_id
             data["client_secret"] = self._client.client_secret
         return httpx2.Request("POST", self._token_endpoint, data=data, headers=headers)
 
diff --git a/tests/client/auth/extensions/test_identity_assertion.py b/tests/client/auth/extensions/test_identity_assertion.py
index d4c965edc5..5f9cde25cb 100644
--- a/tests/client/auth/extensions/test_identity_assertion.py
+++ b/tests/client/auth/extensions/test_identity_assertion.py
@@ -293,6 +293,8 @@ async def test_token_endpoint_error_surfaces_as_oauth_token_error() -> None:
 
 @pytest.mark.anyio
 async def test_client_secret_basic_sends_basic_header_not_body_secret() -> None:
+    """RFC 6749 §2.3: with Basic auth, neither client_secret nor client_id may also appear
+    in the body -- both are already presented via the Authorization header."""
     requests: list[httpx2.Request] = []
     auth = make_provider(token_endpoint_auth_method="client_secret_basic")
 
@@ -303,10 +305,30 @@ async def test_client_secret_basic_sends_basic_header_not_body_secret() -> None:
 
     [token_req] = [r for r in requests if r.url.path == "/token"]
     assert "client_secret" not in form(token_req)
+    assert "client_id" not in form(token_req)
     decoded = base64.b64decode(token_req.headers["Authorization"].removeprefix("Basic ")).decode()
     assert decoded == "test-client-id:test-client-secret"
 
 
+@pytest.mark.anyio
+async def test_client_secret_post_sends_client_id_and_secret_in_body() -> None:
+    """Sanity check / regression guard for the client_secret_basic fix above: the post method
+    is unaffected and still sends both client_id and client_secret in the body, not a header."""
+    requests: list[httpx2.Request] = []
+    auth = make_provider(token_endpoint_auth_method="client_secret_post")
+
+    async with httpx2.AsyncClient(
+        transport=mock_transport(requests, asm=asm_body(), token=token_body()), auth=auth
+    ) as http:
+        await http.post(f"{RS}/mcp")
+
+    [token_req] = [r for r in requests if r.url.path == "/token"]
+    assert "Authorization" not in token_req.headers
+    body = form(token_req)
+    assert body["client_id"] == "test-client-id"
+    assert body["client_secret"] == "test-client-secret"
+
+
 @pytest.mark.anyio
 async def test_stored_token_is_reused_without_reauthorizing() -> None:
     """A valid stored token is sent on the first request; on success no ASM or /token is fetched."""