diff --git a/replayer/metrics.py b/replayer/metrics.py
index aaf9f90..4fbf841 100644
--- a/replayer/metrics.py
+++ b/replayer/metrics.py
@@ -28,6 +28,12 @@ class RequestMetrics:
     requested_output_tokens: int | None = None
     finish_reason: str | None = None
     error: str | None = None
+    t_dispatch_unix: float | None = None
+    t_first_token_unix: float | None = None
+    t_finish_unix: float | None = None
+    proxy_request_id: str | None = None
+    endpoint_url: str | None = None
+    trace_hash_ids: tuple[int, ...] = ()
 
 
 class IncrementalMetricSink:
diff --git a/replayer/replay.py b/replayer/replay.py
index 6b80723..78e07d1 100644
--- a/replayer/replay.py
+++ b/replayer/replay.py
@@ -125,6 +125,8 @@ async def _dispatch_request(
     }
 
     start = time.perf_counter()
+    t_dispatch_unix = time.time()
+    t_first_token_unix: float | None = None
     ttft_s = None
     n_output = 0
     cached_tokens = 0
@@ -133,7 +135,10 @@ async def _dispatch_request(
     token_times: list[float] = []
     output_token_ids: list[int] = []
 
-    req_headers = {"X-Session-Id": req.session_id}
+    req_headers = {
+        "X-Session-Id": req.session_id,
+        "X-Request-Id": req.request_id,
+    }
 
     async with sem:
         try:
@@ -169,11 +174,13 @@ async def _dispatch_request(
                             if clean_ids:
                                 if ttft_s is None:
                                     ttft_s = now - start
+                                    t_first_token_unix = time.time()
                                 output_token_ids.extend(clean_ids)
                                 token_times.extend([now] * len(clean_ids))
                         elif delta:
                             if ttft_s is None:
                                 ttft_s = now - start
+                                t_first_token_unix = time.time()
                             token_times.append(now)
                         fr = choices[0].get("finish_reason")
                         if fr:
@@ -187,6 +194,7 @@ async def _dispatch_request(
             err = repr(exc)[:300]
 
     end = time.perf_counter()
+    t_finish_unix = time.time()
     e2e = end - start
     if output_token_ids:
         n_output = len(output_token_ids)
@@ -222,6 +230,12 @@ async def _dispatch_request(
             requested_output_tokens=req.output_length,
             finish_reason=finish_reason,
             error=err,
+            t_dispatch_unix=t_dispatch_unix,
+            t_first_token_unix=t_first_token_unix,
+            t_finish_unix=t_finish_unix,
+            proxy_request_id=req.request_id,
+            endpoint_url=endpoint,
+            trace_hash_ids=req.hash_ids,
         ),
         output_token_ids=output_token_ids,
     )
diff --git a/tests/test_metrics_instrumentation.py b/tests/test_metrics_instrumentation.py
new file mode 100644
index 0000000..26062e4
--- /dev/null
+++ b/tests/test_metrics_instrumentation.py
@@ -0,0 +1,75 @@
+"""Tests for A1 replayer instrumentation: new timestamp + join fields."""
+
+from __future__ import annotations
+
+import json
+from dataclasses import asdict
+
+from replayer.metrics import RequestMetrics
+
+
+def _make_metric(**overrides) -> RequestMetrics:
+    base = dict(
+        request_id="sess1:0:42:0",
+        session_id="sess1",
+        turn_id=0,
+        trace_timestamp_s=1.0,
+        input_length=1000,
+        output_length=50,
+        request_type="user",
+        effective_input_length=1000,
+        cached_tokens=512,
+        latency_s=2.5,
+        ttft_s=0.5,
+        tpot_s=0.04,
+    )
+    base.update(overrides)
+    return RequestMetrics(**base)
+
+
+def test_request_metrics_has_new_join_fields():
+    m = _make_metric()
+    fields = asdict(m)
+    for key in (
+        "t_dispatch_unix",
+        "t_first_token_unix",
+        "t_finish_unix",
+        "proxy_request_id",
+        "endpoint_url",
+        "trace_hash_ids",
+    ):
+        assert key in fields, f"missing {key}"
+
+
+def test_request_metrics_defaults_when_unset():
+    m = _make_metric()
+    assert m.t_dispatch_unix is None
+    assert m.t_first_token_unix is None
+    assert m.t_finish_unix is None
+    assert m.proxy_request_id is None
+    assert m.endpoint_url is None
+    assert m.trace_hash_ids == ()
+
+
+def test_request_metrics_round_trips_through_json():
+    m = _make_metric(
+        t_dispatch_unix=1700000000.123,
+        t_first_token_unix=1700000000.5,
+        t_finish_unix=1700000002.0,
+        proxy_request_id="sess1:0:42:0",
+        endpoint_url="http://node:8000",
+        trace_hash_ids=(11, 22, 33),
+    )
+    payload = json.dumps(asdict(m), sort_keys=True)
+    parsed = json.loads(payload)
+    assert parsed["t_dispatch_unix"] == 1700000000.123
+    assert parsed["t_first_token_unix"] == 1700000000.5
+    assert parsed["t_finish_unix"] == 1700000002.0
+    assert parsed["proxy_request_id"] == "sess1:0:42:0"
+    assert parsed["endpoint_url"] == "http://node:8000"
+    assert parsed["trace_hash_ids"] == [11, 22, 33]
+
+
+def test_proxy_request_id_matches_replayer_request_id():
+    m = _make_metric(proxy_request_id="sess1:0:42:0")
+    assert m.proxy_request_id == m.request_id