Forward allowlisted x-litellm-api-key on A2A hops

_forwarding_headers stripped every x-litellm-* name, so an admin
extra_headers entry for x-litellm-api-key never reached the agent.
Only mint reserved identity headers (user/team/trace id).

litellm/proxy/agent_endpoints/a2a_endpoints.py

@@ -172,6 +172,18 @@ async def _resolve_backend_auth_header(
     return await resolve_a2a_hop_auth_header(litellm_params, custom_llm_provider)
 
 
+# Identity / hop headers LiteLLM mints itself. Drop client-supplied copies (any
+# casing) so they cannot sit beside the authenticated values. Other x-litellm-*
+# names, including an admin-allowlisted x-litellm-api-key, still pass through.
+_RESERVED_X_LITELLM_HEADERS: Final = frozenset(
+    {
+        "x-litellm-user-id",
+        "x-litellm-team-id",
+        "x-litellm-trace-id",
+    }
+)
+
+
 def _forwarding_headers(
     caller_identity: Mapping[str, str],
     request_data: Mapping[str, object],
@@ -180,10 +192,11 @@ def _forwarding_headers(
 ) -> dict[str, str] | None:
     backend_auth: Final = tuple(backend_auth_header.items()) if backend_auth_header else ()
     minted_names: Final = frozenset(name.lower() for name, _ in backend_auth)
+    blocked: Final = _RESERVED_X_LITELLM_HEADERS | minted_names
     passthrough: Final = tuple(
         (name, value)
         for name, value in (agent_extra_headers.items() if agent_extra_headers else ())
-        if not name.lower().startswith("x-litellm-") and name.lower() not in minted_names
+        if name.lower() not in blocked
     )
     trace_id: Final = request_data.get("litellm_trace_id")
     trace: Final = (("X-LiteLLM-Trace-Id", str(trace_id)),) if trace_id else ()

tests/test_litellm/proxy/agent_endpoints/test_a2a_endpoints.py

@@ -2684,3 +2684,48 @@ def test_forwarding_headers_minted_bearer_replaces_a_forwarded_authorization_of_
     )
 
     assert merged == {"X-Custom": "kept", "Authorization": "Bearer minted-token"}
+
+
+def test_forwarding_headers_keeps_allowlisted_x_litellm_api_key():
+    """Admin-configured extra_headers may include x-litellm-api-key. That name is not a minted
+    caller-identity header, so it must reach the backend agent (#43450)."""
+    from litellm.proxy.agent_endpoints.a2a_endpoints import _forwarding_headers
+
+    merged = _forwarding_headers(
+        caller_identity={"X-LiteLLM-User-Id": "u1", "X-LiteLLM-Team-Id": "t1"},
+        request_data={},
+        agent_extra_headers={
+            "authorization": "Bearer client",
+            "x-litellm-api-key": "sk-forward-me",
+            "x-litellm-user-id": "attacker",
+        },
+        backend_auth_header=None,
+    )
+
+    assert merged is not None
+    assert merged.get("x-litellm-api-key") == "sk-forward-me"
+    assert merged.get("authorization") == "Bearer client"
+    assert merged.get("X-LiteLLM-User-Id") == "u1"
+    assert merged.get("X-LiteLLM-Team-Id") == "t1"
+    # Spoofed lowercase identity must not sit beside the minted header.
+    assert "x-litellm-user-id" not in merged
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("method", ["message/send", "message/stream"])
+async def test_message_methods_forward_allowlisted_x_litellm_api_key(method: str):
+    agent = _make_agent_mock()
+    agent.extra_headers = ["authorization", "x-litellm-api-key"]
+    mock_request = _make_request_mock(method, _HELLO_MESSAGE_PARAMS)
+    mock_request.headers = {
+        "authorization": "Bearer client-key",
+        "x-litellm-api-key": "sk-allowlisted",
+    }
+    user_api_key_dict = UserAPIKeyAuth(api_key="sk-test", user_id="u1", team_id="t1")
+
+    captured = await _invoke_message_method(method, mock_request, user_api_key_dict, agent=agent)
+
+    forwarded = captured.agent_extra_headers or {}
+    assert forwarded.get("authorization") == "Bearer client-key"
+    assert forwarded.get("x-litellm-api-key") == "sk-allowlisted"
+    assert forwarded.get("X-LiteLLM-Team-Id") == "t1"