diff --git a/src/tether/runtime/server.py b/src/tether/runtime/server.py
index 8318b70..499de77 100644
--- a/src/tether/runtime/server.py
+++ b/src/tether/runtime/server.py
@@ -19,10 +19,12 @@
 from __future__ import annotations
 
 import base64
+import contextvars
 import io
 import json
 import logging
 import time
+import uuid
 from pathlib import Path
 from typing import Any
 
@@ -65,6 +67,7 @@ def track_in_flight(*args, **kwargs):
 
 logger = logging.getLogger(__name__)
 _tracer = get_tracer(__name__)
+_request_id_var: contextvars.ContextVar[str] = contextvars.ContextVar("request_id", default="")
 
 try:
     from tether import __version__ as _TETHER_VERSION
@@ -2180,6 +2183,17 @@ async def _heartbeat_loop():
         lifespan=lifespan,
     )
 
+    @app.middleware("http")
+    async def _request_id_middleware(request, call_next):
+        req_id = str(uuid.uuid4())
+        token = _request_id_var.set(req_id)
+        try:
+            response = await call_next(request)
+        finally:
+            _request_id_var.reset(token)
+        response.headers["X-Reflex-Request-ID"] = req_id
+        return response
+
     # Bearer auth dependency (Phase 1 auth-bearer feature).
     # If api_key is set at app-creation time, every protected route requires
     # the caller to pass `Authorization: Bearer <token>` (preferred) OR the
@@ -2274,6 +2288,9 @@ async def act(request: PredictRequest, _auth: None = Depends(_require_api_key)):
             # Non-standard attrs under gen_ai.action.* — proposed for upstream
             # OTel GenAI working group contribution (Phase 2 per spec).
             span.set_attribute("gen_ai.action.embodiment", _emb_label)
+            _req_id = _request_id_var.get()
+            if _req_id:
+                span.set_attribute("tether.request_id", _req_id)
             # chunk_size + denoise_steps are set AFTER predict returns (we don't
             # know them until the result is in hand). See ~line 1590 below.
             span.set_attribute(
diff --git a/tests/test_request_id_header.py b/tests/test_request_id_header.py
new file mode 100644
index 0000000..9df2ad2
--- /dev/null
+++ b/tests/test_request_id_header.py
@@ -0,0 +1,86 @@
+"""Tests for the X-Reflex-Request-ID response header middleware.
+
+Verifies that every HTTP response carries a unique UUID4 in the
+X-Reflex-Request-ID header, and that the value is available to the
+OTel span via the _request_id_var context variable.
+
+Uses FastAPI's TestClient — no model loading, no ONNX runtime.
+"""
+from __future__ import annotations
+
+import contextvars
+import uuid
+
+import pytest
+from fastapi import FastAPI
+from fastapi.responses import JSONResponse
+from fastapi.testclient import TestClient
+
+
+def _build_app() -> tuple[FastAPI, contextvars.ContextVar[str]]:
+    """Minimal FastAPI app with only the request-ID middleware wired in."""
+    request_id_var: contextvars.ContextVar[str] = contextvars.ContextVar(
+        "request_id", default=""
+    )
+
+    app = FastAPI()
+
+    @app.middleware("http")
+    async def _request_id_middleware(request, call_next):
+        req_id = str(uuid.uuid4())
+        token = request_id_var.set(req_id)
+        try:
+            response = await call_next(request)
+        finally:
+            request_id_var.reset(token)
+        response.headers["X-Reflex-Request-ID"] = req_id
+        return response
+
+    @app.get("/health")
+    async def health():
+        return JSONResponse({"status": "ok"})
+
+    @app.post("/act")
+    async def act():
+        return JSONResponse({"actions": []})
+
+    return app, request_id_var
+
+
+@pytest.fixture
+def client():
+    app, _ = _build_app()
+    return TestClient(app)
+
+
+class TestRequestIDHeader:
+    def test_health_response_has_header(self, client):
+        assert "X-Reflex-Request-ID" in client.get("/health").headers
+
+    def test_act_response_has_header(self, client):
+        assert "X-Reflex-Request-ID" in client.post("/act").headers
+
+    def test_header_value_is_valid_uuid(self, client):
+        value = client.get("/health").headers["X-Reflex-Request-ID"]
+        assert str(uuid.UUID(value)) == value  # raises ValueError if malformed
+
+    def test_each_request_gets_a_unique_id(self, client):
+        ids = {client.get("/health").headers["X-Reflex-Request-ID"] for _ in range(5)}
+        assert len(ids) == 5
+
+    def test_request_id_is_accessible_inside_route(self):
+        """Context var holds the same ID that ends up in the response header."""
+        app, request_id_var = _build_app()
+
+        captured: dict[str, str] = {}
+
+        @app.post("/act_span")
+        async def act_span():
+            captured["id"] = request_id_var.get()
+            return JSONResponse({"actions": []})
+
+        c = TestClient(app)
+        response = c.post("/act_span")
+
+        assert captured["id"] != ""
+        assert response.headers["X-Reflex-Request-ID"] == captured["id"]