diff --git a/agent/chat_completion_helpers.py b/agent/chat_completion_helpers.py
index 52d9642010..65641e9a0b 100644
--- a/agent/chat_completion_helpers.py
+++ b/agent/chat_completion_helpers.py
@@ -2884,6 +2884,9 @@ class _StreamingCall(StreamingWaitMonitor):
                 diag["first_chunk_at"] = self.last_chunk_time["t"]
             # Delta-length estimate: ~3x cheaper than repr() per chunk.
             diag["bytes"] = int(diag.get("bytes", 0)) + _estimate_chunk_bytes(chunk)
+            # Relays re-roll the serving provider per request and report it only in the chunk body,
+            # so attribute a drop to the downstream that served it, not to the aggregator (#90216).
+            self.agent._stream_diag_note_serving_provider(diag, chunk)
 
     # ── chat_completions wire ───────────────────────────────────────────
 
@@ -3776,6 +3779,10 @@ class _StreamingCall(StreamingWaitMonitor):
         # Propagate first-chunk timing for the ``post_api_request`` hook.
         if isinstance(self.clients.diag, dict) and self.clients.diag.get("first_chunk_at"):
             self.agent._last_api_first_chunk_at = float(self.clients.diag["first_chunk_at"])
+        # Same per-attempt stash for the downstream that actually served the stream: relays re-roll it
+        # per request, and plugins auditing route compliance cannot get it from anywhere else (#90216).
+        if isinstance(self.clients.diag, dict) and self.clients.diag.get("serving_provider"):
+            self.agent._last_serving_provider = str(self.clients.diag["serving_provider"])
         return self.result["response"]
 
 
diff --git a/agent/stream_diag.py b/agent/stream_diag.py
index cb1b42e25c..66c691f478 100644
--- a/agent/stream_diag.py
+++ b/agent/stream_diag.py
@@ -24,7 +24,35 @@ STREAM_DIAG_HEADERS = (
 
 def stream_diag_init() -> Dict[str, Any]:
     """Fresh per-attempt diagnostic dict; mutated in place by the streaming functions and read by the retry block."""
-    return {"started_at": time.time(), "first_chunk_at": None, "chunks": 0, "bytes": 0, "headers": {}, "http_status": None}
+    return {
+        "started_at": time.time(), "first_chunk_at": None, "chunks": 0, "bytes": 0,
+        "headers": {}, "http_status": None, "serving_provider": None,
+    }
+
+
+def stream_diag_note_serving_provider(diag: Dict[str, Any], chunk: Any) -> None:
+    """Record which downstream provider actually served this attempt, from a delta chunk body.
+
+    On OpenRouter-style relays the provider is re-rolled per request and reported only inside the
+    chunk JSON (``"provider": "Novita"``) — those responses carry no ``x-openrouter-provider``
+    header, just ``cf-ray`` / ``server: cloudflare``, so the header snapshot cannot attribute a
+    mid-stream drop to a downstream. First non-empty value wins: the attempt was served by whatever
+    produced its first chunk, and later chunks must not overwrite that. Best-effort, never raises.
+    """
+    if not isinstance(diag, dict) or diag.get("serving_provider"):
+        return
+    try:
+        value = getattr(chunk, "provider", None)
+        if value is None:  # unknown top-level fields land in pydantic model_extra on the OpenAI SDK
+            extra = getattr(chunk, "model_extra", None)
+            if isinstance(extra, dict):
+                value = extra.get("provider")
+        if value is None and isinstance(chunk, dict):
+            value = chunk.get("provider")
+        if isinstance(value, str) and value.strip():
+            diag["serving_provider"] = value.strip()[:64]  # keep log lines bounded
+    except Exception:
+        pass
 
 
 def stream_diag_capture_response(agent: Any, diag: Dict[str, Any], http_response: Any) -> None:
@@ -71,10 +99,11 @@ def flatten_exception_chain(error: BaseException) -> str:
 
 
 def _diag_fields(diag: Optional[Dict[str, Any]]) -> tuple:
-    """(http_status, bytes, chunks, elapsed, ttfb, upstream) for the retry log line; ``-`` when unknown."""
+    """(http_status, bytes, chunks, elapsed, ttfb, serving_provider, upstream) for the retry log line;
+    ``-`` when unknown."""
     _bytes = _chunks = 0
     _elapsed = 0.0
-    _ttfb = _headers_repr = _http_status = "-"
+    _ttfb = _headers_repr = _http_status = _serving_provider = "-"
     if isinstance(diag, dict):
         try:
             _now = time.time()
@@ -90,9 +119,11 @@ def _diag_fields(diag: Optional[Dict[str, Any]]) -> tuple:
                 _headers_repr = " ".join(f"{k}={v}" for k, v in headers.items())
             if diag.get("http_status") is not None:
                 _http_status = str(diag.get("http_status"))
+            if diag.get("serving_provider"):
+                _serving_provider = str(diag["serving_provider"])
         except Exception:
             pass
-    return _http_status, _bytes, _chunks, _elapsed, _ttfb, _headers_repr
+    return _http_status, _bytes, _chunks, _elapsed, _ttfb, _serving_provider, _headers_repr
 
 
 def log_stream_retry(
@@ -101,7 +132,11 @@ def log_stream_retry(
 ) -> None:
     """Structured WARNING to ``agent.log`` for a transient stream drop + retry, always logged regardless of
     UI verbosity. With *diag*, also records upstream headers, HTTP status, bytes/chunks, elapsed and TTFB on
-    the dying attempt — enough to tell "one CF edge / downstream provider" from "random across runs"."""
+    the dying attempt — enough to tell "one CF edge / downstream provider" from "random across runs".
+
+    The ``serving_provider`` field names the downstream that actually served the attempt (relays re-roll it
+    per request and report it only in the chunk body), so a drop can be attributed to a provider even when
+    the response carried no ``x-openrouter-provider`` header."""
     try:
         try:
             _summary = agent._summarize_api_error(error)
@@ -116,7 +151,8 @@ def log_stream_retry(
 
         logger.warning(
             "Stream %s on attempt %s/%s — retrying. subagent_id=%s depth=%s provider=%s base_url=%s "
-            "error_type=%s error=%s chain=%s http_status=%s bytes=%d chunks=%d elapsed=%.2fs ttfb=%s upstream=[%s]",
+            "error_type=%s error=%s chain=%s http_status=%s bytes=%d chunks=%d elapsed=%.2fs ttfb=%s "
+            "serving_provider=%s upstream=[%s]",
             kind, attempt, max_attempts,
             getattr(agent, "_subagent_id", None) or "-", getattr(agent, "_delegate_depth", 0),
             agent.provider or "-", agent.base_url or "-",
@@ -217,6 +253,7 @@ __all__ = [
     "connect_exhausted_notice",
     "buffer_connect_exhausted_notice",
     "stream_diag_init",
+    "stream_diag_note_serving_provider",
     "stream_diag_capture_response",
     "flatten_exception_chain",
     "log_stream_retry",
diff --git a/agent/turn_api_request.py b/agent/turn_api_request.py
index b00934cc72..5c339f8cef 100644
--- a/agent/turn_api_request.py
+++ b/agent/turn_api_request.py
@@ -104,6 +104,8 @@ def build_api_request(
     agent._reset_stream_delivery_tracking()
     # Per-attempt first-chunk timestamp so a stale value never leaks into post_api_request.
     agent._last_api_first_chunk_at = None
+    # Same for the downstream provider that served the attempt (relays re-roll it per request).
+    agent._last_serving_provider = None
     # api_messages was built for the primary; a fallback (DeepSeek / Kimi / MiMo) may
     # require reasoning_content — re-apply the echo-back pad (idempotent) and re-render
     # the prompt-cache decoration for the current provider.
diff --git a/agent/turn_response_intake.py b/agent/turn_response_intake.py
index 67374daa78..43182ac143 100644
--- a/agent/turn_response_intake.py
+++ b/agent/turn_response_intake.py
@@ -83,6 +83,9 @@ def _fire_post_api_request_hook(
                 # First stream chunk time (epoch s); None if not streamed / no chunk.
                 # TTFB = first_chunk_at - started_at.
                 first_chunk_at=getattr(agent, "_last_api_first_chunk_at", None),
+                # Downstream that actually served the stream ("Novita"): relays re-roll it per
+                # request and report it only in the chunk body, never in a response header.
+                upstream_provider=getattr(agent, "_last_serving_provider", None),
                 finish_reason=finish_reason,
                 message_count=len(api_messages),
                 response_model=getattr(response, "model", None),
diff --git a/hermes_cli/hooks.py b/hermes_cli/hooks.py
index 6c0f3e64ae..68e1a6322b 100644
--- a/hermes_cli/hooks.py
+++ b/hermes_cli/hooks.py
@@ -144,6 +144,7 @@ _DEFAULT_PAYLOADS = {
         "base_url": "https://api.anthropic.com", "api_mode": "anthropic_messages",
         "api_call_count": 1, "api_duration": 1.234,
         "started_at": 1756000000.0, "ended_at": 1756000001.234, "first_chunk_at": 1756000000.512,
+        "upstream_provider": "Novita",
         "finish_reason": "stop", "message_count": 4, "response_model": "claude-sonnet-4-6",
         "usage": {"input_tokens": 2048, "output_tokens": 512},
         "assistant_content_chars": 1200, "assistant_tool_call_count": 0,
diff --git a/run_agent.py b/run_agent.py
index 83f121f968..ed7acb06fe 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -509,6 +509,7 @@ class AIAgent(
         return {"messages": stripped_messages, "items": stripped_items}
 
     _stream_diag_init = _forward_static("agent.stream_diag", "stream_diag_init")
+    _stream_diag_note_serving_provider = _forward_static("agent.stream_diag", "stream_diag_note_serving_provider")
     _stream_diag_capture_response = _forward("agent.stream_diag", "stream_diag_capture_response")
     _flatten_exception_chain = _forward_static("agent.stream_diag", "flatten_exception_chain")
 
diff --git a/tests/agent/test_stream_serving_provider.py b/tests/agent/test_stream_serving_provider.py
new file mode 100644
index 0000000000..338654dbaa
--- /dev/null
+++ b/tests/agent/test_stream_serving_provider.py
@@ -0,0 +1,154 @@
+"""Regression tests for the serving-provider field in stream diagnostics (#90216).
+
+OpenRouter-style relays re-roll the downstream provider per request — three consecutive calls
+for one model came back served by Alibaba, Novita and StreamLake — and report the winner only
+inside the delta chunk bodies (``{"provider": "Novita", ...}``).  Those responses carry no
+``x-openrouter-provider`` header (just ``cf-ray`` / ``server: cloudflare``), so the existing
+header snapshot cannot attribute a mid-stream drop to a provider at all.
+
+Contract under test:
+
+- The first non-empty top-level ``provider`` in a chunk body lands in the per-attempt diag dict
+  as ``serving_provider``; later chunks must not rewrite that attribution.
+- ``log_stream_retry`` names it on the drop line (``serving_provider=-`` when unknown).
+- A successful streamed attempt stashes it on the agent and the ``post_api_request`` hook
+  payload carries it as ``upstream_provider`` — refreshed per attempt, never leaked from a
+  prior call.
+"""
+
+from __future__ import annotations
+
+import logging
+from types import SimpleNamespace
+from unittest.mock import MagicMock, patch
+
+from run_agent import AIAgent
+from tests.agent.test_run_agent import (  # noqa: F401  (_make_tool_defs used by the agent fixture)
+    _make_tool_defs,
+    _mock_response,
+)
+from tests.agent.test_first_chunk_at_hook import (  # noqa: F401  (shared fixture + harness)
+    _make_stream_chunk,
+    _run_with_hooks,
+    agent,
+)
+
+
+def _make_agent() -> AIAgent:
+    """Standalone agent for the diag/log-level tests (no conversation loop involved)."""
+    return AIAgent(
+        api_key="test-key",
+        base_url="https://openrouter.ai/api/v1",
+        quiet_mode=True,
+        skip_context_files=True,
+        skip_memory=True,
+    )
+
+
+def _chunk_with_provider(content=None, finish_reason=None, model=None, provider=None):
+    """Streaming chunk plus the relay's per-chunk ``provider`` field."""
+    chunk = _make_stream_chunk(content=content, finish_reason=finish_reason, model=model)
+    if provider is not None:
+        chunk.provider = provider
+    return chunk
+
+
+# ── Diag dict level ──────────────────────────────────────────────────────
+
+
+def test_diag_records_first_serving_provider_from_chunk_body():
+    agent_ = _make_agent()
+    diag = AIAgent._stream_diag_init()
+    assert diag["serving_provider"] is None
+
+    agent_._stream_diag_note_serving_provider(diag, _chunk_with_provider(content="Hi", provider="Novita"))
+    assert diag["serving_provider"] == "Novita"
+
+    # The attempt was served by whatever produced its first chunk; a per-chunk rewrite would
+    # just track the last one and lose the drop attribution.
+    agent_._stream_diag_note_serving_provider(diag, _chunk_with_provider(content="!", provider="StreamLake"))
+    assert diag["serving_provider"] == "Novita"
+
+
+def test_diag_reads_provider_from_model_extra_and_ignores_chunks_without_one():
+    """The OpenAI SDK parks unknown top-level fields in pydantic ``model_extra``."""
+    agent_ = _make_agent()
+    diag = AIAgent._stream_diag_init()
+
+    agent_._stream_diag_note_serving_provider(diag, _chunk_with_provider(content="plain"))
+    assert diag["serving_provider"] is None
+
+    agent_._stream_diag_note_serving_provider(diag, SimpleNamespace(model_extra={"provider": "Alibaba"}))
+    assert diag["serving_provider"] == "Alibaba"
+
+    # Empty / non-string values are not a serving provider either.
+    other = AIAgent._stream_diag_init()
+    agent_._stream_diag_note_serving_provider(other, SimpleNamespace(provider="   "))
+    assert other["serving_provider"] is None
+
+
+def test_log_stream_retry_names_the_serving_provider(caplog):
+    agent_ = _make_agent()
+    agent_.provider = "openrouter"
+
+    diag = AIAgent._stream_diag_init()
+    diag["serving_provider"] = "Novita"
+
+    with caplog.at_level(logging.WARNING):
+        agent_._log_stream_retry(
+            kind="drop", error=ConnectionError("peer closed"), attempt=2, max_attempts=3,
+            mid_tool_call=False, diag=diag,
+        )
+
+    msg = next(r.getMessage() for r in caplog.records if "Stream drop" in r.getMessage())
+    assert "serving_provider=Novita" in msg
+
+    caplog.clear()
+    unknown = AIAgent._stream_diag_init()
+    with caplog.at_level(logging.WARNING):
+        agent_._log_stream_retry(
+            kind="drop", error=ConnectionError("peer closed"), attempt=2, max_attempts=3,
+            mid_tool_call=False, diag=unknown,
+        )
+    msg = next(r.getMessage() for r in caplog.records if "Stream drop" in r.getMessage())
+    assert "serving_provider=-" in msg
+
+
+# ── Conversation loop / hook payload level ───────────────────────────────
+
+
+class TestServingProviderReachesPostApiRequest:
+    """``upstream_provider`` on the post_api_request payload (plugin route auditing)."""
+
+    @patch("run_agent.AIAgent._create_request_openai_client")
+    @patch("run_agent.AIAgent._close_request_openai_client")
+    def test_streamed_attempt_reports_the_serving_provider(self, _mock_close, mock_create, agent):
+        chunks = [
+            _chunk_with_provider(content="Hello", provider="Novita"),
+            _chunk_with_provider(content=" world", finish_reason="stop", model="test-model", provider="Novita"),
+        ]
+        mock_client = MagicMock()
+        mock_client.chat.completions.create.return_value = iter(chunks)
+        mock_create.return_value = mock_client
+        # A registered stream consumer forces the streaming path even though agent.client is a Mock.
+        agent.stream_delta_callback = lambda _text: None
+
+        result, post = _run_with_hooks(agent)
+
+        assert result["final_response"] == "Hello world"
+        assert len(post) == 1
+        assert post[0]["upstream_provider"] == "Novita"
+        assert agent._last_serving_provider == "Novita"
+
+    def test_non_streamed_attempt_reports_no_upstream_provider(self, agent):
+        """A stale value from an earlier call must not leak into the next payload."""
+        agent._last_serving_provider = "Novita"  # as a prior streamed attempt left it
+        agent.client.chat.completions.create.return_value = _mock_response(
+            content="Done", finish_reason="stop"
+        )
+
+        result, post = _run_with_hooks(agent)
+
+        assert result["final_response"] == "Done"
+        assert len(post) == 1
+        assert post[0]["upstream_provider"] is None