Omit empty _meta on outbound JSON-RPC requests

src/mcp/shared/jsonrpc_dispatcher.py

@@ -359,7 +359,6 @@ async def send_raw_request(
         if on_progress is not None:
             # The request id doubles as the progress token, so `_pending[token]` finds `on_progress` directly.
             out_meta["progressToken"] = request_id
-        out_params["_meta"] = out_meta
 
         # buffer=1: a close signal can arrive before the waiter parks in receive();
         # a WouldBlock later just means the waiter already has its one outcome.
@@ -386,8 +385,13 @@ async def send_raw_request(
                 kind=SpanKind.CLIENT,
                 attributes={"mcp.method.name": method, "jsonrpc.request.id": str(request_id)},
             ):
-                # SEP-414: inject W3C trace context; `_meta` stays on the wire even with a no-op tracer.
+                # SEP-414: inject W3C trace context. Omit `_meta` when it is still
+                # empty after injection: some servers reject `_meta: {}`.
                 inject_trace_context(out_meta)
+                if out_meta:
+                    out_params["_meta"] = out_meta
+                else:
+                    out_params.pop("_meta", None)
                 msg = JSONRPCRequest(jsonrpc="2.0", id=request_id, method=method, params=out_params)
                 # Surface a pre-existing cancellation while the request provably
                 # never started; past this point a cancelled write counts as issued.

tests/shared/test_jsonrpc_dispatcher.py

@@ -109,6 +109,42 @@ async def call(method: str) -> None:
     assert results == {"first": {"m": "first"}, "second": {"m": "second"}}
 
 
+@pytest.mark.anyio
+async def test_send_raw_request_omits_empty_meta():
+    """Strict servers reject `_meta: {}`. Leave the field off when it is empty."""
+    seen: dict[str, Mapping[str, Any] | None] = {}
+
+    async def server_on_request(ctx: DCtx, method: str, params: Mapping[str, Any] | None) -> dict[str, Any]:
+        seen["params"] = params
+        return {"ok": True}
+
+    async with running_pair(jsonrpc_pair, server_on_request=server_on_request) as (client, *_):
+        await client.send_raw_request("ping", None)
+
+    params = seen["params"]
+    if params is not None:
+        assert "_meta" not in params
+
+
+@pytest.mark.anyio
+async def test_send_raw_request_keeps_progress_token_meta():
+    seen: dict[str, Mapping[str, Any] | None] = {}
+
+    async def server_on_request(ctx: DCtx, method: str, params: Mapping[str, Any] | None) -> dict[str, Any]:
+        seen["params"] = params
+        return {"ok": True}
+
+    async def on_progress(*_args: Any, **_kwargs: Any) -> None:
+        return None
+
+    async with running_pair(jsonrpc_pair, server_on_request=server_on_request) as (client, *_):
+        await client.send_raw_request("ping", None, {"on_progress": on_progress})
+
+    params = seen["params"]
+    assert params is not None
+    assert "progressToken" in params["_meta"]
+
+
 @pytest.mark.anyio
 async def test_handler_raising_exception_sends_code_zero_with_str_message():
     """Matches the existing server's `_handle_request`: code=0, message=str(e)."""