diff --git a/agent/conversation_loop.py b/agent/conversation_loop.py
index 502fda1c5..e451c43cb 100644
--- a/agent/conversation_loop.py
+++ b/agent/conversation_loop.py
@@ -1941,6 +1941,15 @@ def run_conversation(
                                 canonical_usage = canonical_usage + _ref_usage
                         except Exception as _moa_acct_exc:  # pragma: no cover - defensive
                             logger.debug("MoA reference usage accounting failed: %s", _moa_acct_exc)
+                    # Flush the full-turn MoA trace (references + aggregator I/O)
+                    # to disk when moa.save_traces is on. No-op otherwise and
+                    # for non-MoA clients. Uses the live session_id so traces
+                    # land in the right per-session file.
+                    if _moa_client is not None and hasattr(_moa_client, "consume_and_save_trace"):
+                        try:
+                            _moa_client.consume_and_save_trace(agent.session_id)
+                        except Exception as _moa_trace_exc:  # pragma: no cover - defensive
+                            logger.debug("MoA trace flush failed: %s", _moa_trace_exc)
                     prompt_tokens = canonical_usage.prompt_tokens
                     completion_tokens = canonical_usage.output_tokens
                     total_tokens = canonical_usage.total_tokens
diff --git a/agent/moa_loop.py b/agent/moa_loop.py
index 022aafe7d..149142503 100644
--- a/agent/moa_loop.py
+++ b/agent/moa_loop.py
@@ -28,8 +28,8 @@ _MAX_REFERENCE_WORKERS = 8
 
 
 class _RefAccounting:
-    """Per-reference token usage + estimated cost, carried as the third slot
-    of a reference-output tuple.
+    """Per-reference token usage + estimated cost + full trace, carried as the
+    third slot of a reference-output tuple.
 
     Kept as a tiny object (not a bare CanonicalUsage) because an advisor may
     run on a different model/provider than the aggregator, so its cost MUST be
@@ -37,15 +37,48 @@ class _RefAccounting:
     aggregator's usage and pricing the sum at the aggregator's rate would
     misprice every advisor. ``usage`` feeds accurate token counts;
     ``cost_usd`` feeds accurate cost.
+
+    ``messages`` / ``output`` / ``model`` / ``provider`` / ``temperature``
+    carry the FULL reference input and output for trace persistence (the
+    display ``text`` is a truncated preview and is not enough to audit what an
+    advisor actually saw). They are only populated when tracing is on; they add
+    negligible cost otherwise.
     """
 
-    __slots__ = ("usage", "cost_usd", "cost_status", "cost_source")
+    __slots__ = (
+        "usage",
+        "cost_usd",
+        "cost_status",
+        "cost_source",
+        "messages",
+        "output",
+        "model",
+        "provider",
+        "temperature",
+    )
 
-    def __init__(self, usage: Any, cost_usd: Any = None, cost_status: str | None = None, cost_source: str | None = None):
+    def __init__(
+        self,
+        usage: Any,
+        cost_usd: Any = None,
+        cost_status: str | None = None,
+        cost_source: str | None = None,
+        *,
+        messages: Any = None,
+        output: str | None = None,
+        model: str | None = None,
+        provider: str | None = None,
+        temperature: Any = None,
+    ):
         self.usage = usage
         self.cost_usd = cost_usd
         self.cost_status = cost_status
         self.cost_source = cost_source
+        self.messages = messages
+        self.output = output
+        self.model = model
+        self.provider = provider
+        self.temperature = temperature
 
 # Per-tool-result character budget for the advisory reference view. Tool
 # results can be huge (a full diff, a 5000-line file dump); replaying them
@@ -219,11 +252,29 @@ def _run_reference(
             cost_source = cost.source
         except Exception:  # pragma: no cover - defensive
             pass
-        acct = _RefAccounting(usage, cost_usd, cost_status, cost_source)
-        return label, _extract_text(response) or "(empty response)", acct
+        _output_text = _extract_text(response) or "(empty response)"
+        acct = _RefAccounting(
+            usage,
+            cost_usd,
+            cost_status,
+            cost_source,
+            messages=messages,
+            output=_output_text,
+            model=slot.get("model"),
+            provider=runtime.get("provider") or slot.get("provider"),
+            temperature=temperature,
+        )
+        return label, _output_text, acct
     except Exception as exc:
         logger.warning("MoA reference model %s failed: %s", label, exc)
-        return label, f"[failed: {exc}]", _RefAccounting(CanonicalUsage())
+        return label, f"[failed: {exc}]", _RefAccounting(
+            CanonicalUsage(),
+            messages=[{"role": "system", "content": _REFERENCE_SYSTEM_PROMPT}, *ref_messages],
+            output=f"[failed: {exc}]",
+            model=slot.get("model"),
+            provider=runtime.get("provider") or slot.get("provider"),
+            temperature=temperature,
+        )
 
 
 def _run_references_parallel(
@@ -545,6 +596,10 @@ class MoAChatCompletions:
 
         self._pending_reference_usage: Any = CanonicalUsage()
         self._pending_reference_cost: Any = None
+        # Full-turn trace parts stashed on a cache-MISS create(), awaiting the
+        # caller to stitch in the live session_id + resolved aggregator output
+        # and flush to the trace file (only when moa.save_traces is on).
+        self._pending_trace: Any = None
 
     def consume_reference_usage(self) -> tuple[Any, Any]:
         """Pop pending reference-fan-out usage + cost, resetting both to empty.
@@ -563,6 +618,37 @@ class MoAChatCompletions:
         self._pending_reference_cost = None
         return usage, cost
 
+    def consume_and_save_trace(self, session_id: Any = None) -> None:
+        """Flush the pending full-turn trace to disk, if one is pending.
+
+        No-op when tracing is off (``save_moa_turn`` checks the config), when
+        there is no pending trace (a cache-HIT iteration ran no references), or
+        when the aggregator input was never recorded. Clears the pending trace
+        so a repeat consume cannot double-write. Best-effort — never raises.
+        """
+        pending = self._pending_trace
+        self._pending_trace = None
+        if not pending or "aggregator_input_messages" not in pending:
+            return
+        try:
+            from agent.moa_trace import save_moa_turn
+
+            agg_slot = pending.get("aggregator_slot") or {}
+            save_moa_turn(
+                session_id=session_id,
+                preset_name=pending.get("preset", ""),
+                reference_outputs=pending.get("reference_outputs", []),
+                aggregator_label=pending.get("aggregator_label", ""),
+                aggregator_model=agg_slot.get("model"),
+                aggregator_provider=agg_slot.get("provider"),
+                aggregator_temperature=pending.get("aggregator_temperature"),
+                aggregator_input_messages=pending.get("aggregator_input_messages"),
+                aggregator_output=pending.get("aggregator_output"),
+                aggregator_streamed=bool(pending.get("aggregator_streamed")),
+            )
+        except Exception as exc:  # pragma: no cover - tracing must never break a turn
+            logger.debug("MoA trace flush failed: %s", exc)
+
     def _emit(self, event: str, **kwargs: Any) -> None:
         cb = self.reference_callback
         if cb is None:
@@ -618,6 +704,10 @@ class MoAChatCompletions:
             # advisor spend by the tool-iteration count, so pending is zero.
             self._pending_reference_usage = CanonicalUsage()
             self._pending_reference_cost = None
+            # Likewise no trace on a cache HIT — the full turn was already
+            # traced on the MISS that ran the references. A repeat iteration is
+            # not a new MoA turn.
+            self._pending_trace = None
         else:
             reference_outputs = _run_references_parallel(
                 reference_models,
@@ -645,6 +735,17 @@ class MoAChatCompletions:
                         _ref_cost = (_ref_cost or 0) + _acct.cost_usd
             self._pending_reference_usage = _ref_usage
             self._pending_reference_cost = _ref_cost
+            # Stash the full reference fan-out for trace persistence. The
+            # aggregator input/label are filled in below once agg_messages is
+            # built; the aggregator OUTPUT is stitched in by the caller
+            # (consume_and_save_trace) once the response resolves — the caller
+            # holds the live session_id and the resolved aggregator response.
+            self._pending_trace = {
+                "preset": self.preset_name,
+                "reference_outputs": list(reference_outputs),
+                "aggregator_slot": aggregator,
+                "aggregator_temperature": aggregator_temperature,
+            }
 
             # Surface each reference model's answer to the display BEFORE the
             # aggregator acts — once per turn (only on the iteration that
@@ -694,6 +795,12 @@ class MoAChatCompletions:
             raise RuntimeError("MoA aggregator cannot be another MoA preset")
         agg_kwargs = dict(api_kwargs)
         agg_kwargs["messages"] = agg_messages
+        # Record the exact aggregator INPUT (incl. the injected reference
+        # context) into the pending trace so a trace captures what the
+        # aggregator actually saw, not a reconstruction.
+        if self._pending_trace is not None:
+            self._pending_trace["aggregator_input_messages"] = agg_messages
+            self._pending_trace["aggregator_label"] = _slot_label(aggregator)
         # The aggregator is the acting model. Resolve its slot to the provider's
         # real runtime (base_url/api_key/api_mode) and call it through the same
         # request-building path any model uses — so per-model wire-format
@@ -720,7 +827,7 @@ class MoAChatCompletions:
             # actually governs the aggregator stream, not just call_llm's default.
             if api_kwargs.get("timeout") is not None:
                 stream_kwargs["timeout"] = api_kwargs["timeout"]
-        return call_llm(
+        _agg_response = call_llm(
             task="moa_aggregator",
             messages=agg_messages,
             temperature=aggregator_temperature,
@@ -730,6 +837,22 @@ class MoAChatCompletions:
             **stream_kwargs,
             **_slot_runtime(aggregator),
         )
+        # Non-streaming path (quiet mode / eval / subagents): the aggregator
+        # output is available inline, so capture it into the pending trace now.
+        # Streaming path: the aggregator's raw token stream is returned to the
+        # consumer live and its acting output lands as the turn's assistant
+        # message; the trace marks it streamed and points there.
+        if self._pending_trace is not None:
+            if stream:
+                self._pending_trace["aggregator_streamed"] = True
+                self._pending_trace["aggregator_output"] = None
+            else:
+                self._pending_trace["aggregator_streamed"] = False
+                try:
+                    self._pending_trace["aggregator_output"] = _extract_text(_agg_response)
+                except Exception:  # pragma: no cover - defensive
+                    self._pending_trace["aggregator_output"] = None
+        return _agg_response
 
 
 class MoAClient:
@@ -744,3 +867,10 @@ class MoAClient:
         usage without reaching into ``.chat.completions`` internals.
         """
         return self.chat.completions.consume_reference_usage()
+
+    def consume_and_save_trace(self, session_id: Any = None) -> None:
+        """Flush the pending full-turn MoA trace via the completions facade.
+
+        No-op unless ``moa.save_traces`` is enabled and a turn is pending.
+        """
+        return self.chat.completions.consume_and_save_trace(session_id)
diff --git a/agent/moa_trace.py b/agent/moa_trace.py
new file mode 100644
index 000000000..a18a26df8
--- /dev/null
+++ b/agent/moa_trace.py
@@ -0,0 +1,153 @@
+"""Full MoA turn trace persistence (opt-in via config ``moa.save_traces``).
+
+When enabled, every Mixture-of-Agents turn that actually runs the reference
+fan-out (a cache MISS in ``MoAChatCompletions.create``) appends one JSON line
+to ``<hermes_home>/moa-traces/<session_id>.jsonl``. The record is the TRUE
+FULL turn — the exact messages array each reference model received (system
+prompt + advisory view, not the truncated display preview), each reference's
+full output, and the exact messages array the aggregator received (including
+the injected reference-context guidance block) plus its output when available
+— so a run can be audited end-to-end offline: what every model saw, what every
+model said, and what it cost.
+
+This is a side-channel trace. It is NOT the conversation ``messages`` table and
+never enters message history or replay — MoA references are advisory side-calls
+with their own system prompt, not conversation turns, so persisting them as
+message rows would corrupt role alternation / replay. Traces live in their own
+files, keyed by session id, and are safe to delete.
+
+Cost model note: gated OFF by default. When off, the only overhead is the
+``_traces_enabled()`` config read (cheap) — no file I/O, no serialization.
+"""
+
+from __future__ import annotations
+
+import json
+import logging
+import os
+import time
+from pathlib import Path
+from typing import Any, Optional
+
+from hermes_constants import get_hermes_home
+
+logger = logging.getLogger(__name__)
+
+
+def _traces_enabled_and_dir() -> Optional[Path]:
+    """Return the trace directory if ``moa.save_traces`` is on, else None.
+
+    Reads config lazily per call (config is cheap to load and this only runs on
+    a cache-MISS MoA turn, i.e. once per user turn, not per tool iteration).
+    ``moa.trace_dir`` overrides the default ``<hermes_home>/moa-traces/``.
+    """
+    try:
+        from hermes_cli.config import load_config
+
+        moa_cfg = (load_config() or {}).get("moa") or {}
+    except Exception:  # pragma: no cover - defensive: never break a turn over tracing
+        return None
+    if not moa_cfg.get("save_traces"):
+        return None
+    override = moa_cfg.get("trace_dir")
+    if override:
+        base = Path(os.path.expandvars(os.path.expanduser(str(override))))
+    else:
+        base = get_hermes_home() / "moa-traces"
+    return base
+
+
+def _sanitize_session_id(session_id: Optional[str]) -> str:
+    """Make a session id safe as a filename component."""
+    if not session_id:
+        return "unknown-session"
+    return "".join(c if (c.isalnum() or c in "-_.") else "_" for c in str(session_id))
+
+
+def _slot_trace(acct: Any, label: str) -> dict[str, Any]:
+    """Render one reference's _RefAccounting into a full trace dict.
+
+    Includes the FULL input messages the reference received and its FULL
+    output — not the truncated display preview.
+    """
+    usage = getattr(acct, "usage", None)
+    usage_dict: dict[str, Any] = {}
+    if usage is not None:
+        usage_dict = {
+            "input_tokens": getattr(usage, "input_tokens", 0),
+            "output_tokens": getattr(usage, "output_tokens", 0),
+            "cache_read_tokens": getattr(usage, "cache_read_tokens", 0),
+            "cache_write_tokens": getattr(usage, "cache_write_tokens", 0),
+            "reasoning_tokens": getattr(usage, "reasoning_tokens", 0),
+        }
+    return {
+        "label": label,
+        "model": getattr(acct, "model", None),
+        "provider": getattr(acct, "provider", None),
+        "temperature": getattr(acct, "temperature", None),
+        "input_messages": getattr(acct, "messages", None),
+        "output": getattr(acct, "output", None),
+        "usage": usage_dict,
+        "cost_usd": getattr(acct, "cost_usd", None),
+        "cost_status": getattr(acct, "cost_status", None),
+        "cost_source": getattr(acct, "cost_source", None),
+    }
+
+
+def save_moa_turn(
+    *,
+    session_id: Optional[str],
+    preset_name: str,
+    reference_outputs: list[tuple[str, str, Any]],
+    aggregator_label: str,
+    aggregator_model: Optional[str],
+    aggregator_provider: Optional[str],
+    aggregator_temperature: Any,
+    aggregator_input_messages: Any,
+    aggregator_output: Optional[str],
+    aggregator_streamed: bool,
+) -> None:
+    """Append one full MoA turn record to the session's trace JSONL, if enabled.
+
+    Best-effort: any failure is logged at debug and swallowed — tracing must
+    never break a live turn. Called once per turn on a reference cache MISS.
+
+    ``aggregator_output`` is the aggregator's synthesized text when it was
+    captured inline (non-streaming path — the eval / quiet-mode path). When the
+    aggregator streamed to a live consumer, ``aggregator_streamed`` is True and
+    the output is delivered as the turn's assistant message in the session
+    store instead; the trace records the full aggregator INPUT either way.
+    """
+    base = _traces_enabled_and_dir()
+    if base is None:
+        return
+    try:
+        base.mkdir(parents=True, exist_ok=True)
+        path = base / f"{_sanitize_session_id(session_id)}.jsonl"
+        record = {
+            "ts": time.time(),
+            "session_id": session_id,
+            "preset": preset_name,
+            "references": [
+                _slot_trace(acct, label)
+                for label, _text, acct in reference_outputs
+            ],
+            "aggregator": {
+                "label": aggregator_label,
+                "model": aggregator_model,
+                "provider": aggregator_provider,
+                "temperature": aggregator_temperature,
+                "input_messages": aggregator_input_messages,
+                "output": aggregator_output,
+                "streamed": aggregator_streamed,
+                # When streamed, the aggregator's acting output is persisted as
+                # the turn's assistant message in state.db (see the session
+                # store); it is not duplicated here.
+                "output_location": "assistant_message_in_session_db"
+                if aggregator_streamed else "inline",
+            },
+        }
+        with path.open("a", encoding="utf-8") as f:
+            f.write(json.dumps(record, ensure_ascii=False, default=str) + "\n")
+    except Exception as exc:  # pragma: no cover - tracing must never break a turn
+        logger.debug("MoA trace write failed (session=%s): %s", session_id, exc)
diff --git a/hermes_cli/config.py b/hermes_cli/config.py
index 5b2ad0fd9..b19ef5479 100644
--- a/hermes_cli/config.py
+++ b/hermes_cli/config.py
@@ -2155,6 +2155,14 @@ DEFAULT_CONFIG = {
     "moa": {
         "default_preset": "default",
         "active_preset": "",
+        # When true, every MoA turn that runs the reference fan-out writes the
+        # FULL turn (each reference's exact input messages + output + usage/cost,
+        # and the aggregator's exact input + output) to a JSONL file at
+        # <hermes_home>/moa-traces/<session_id>.jsonl. Off by default — turn it
+        # on to audit / improve MoA behavior from real runs. Set trace_dir to
+        # override the output directory.
+        "save_traces": False,
+        "trace_dir": "",
         "presets": {
             "default": {
                 "reference_models": [
diff --git a/tests/run_agent/test_moa_loop_mode.py b/tests/run_agent/test_moa_loop_mode.py
index 46976c77a..33103c5ff 100644
--- a/tests/run_agent/test_moa_loop_mode.py
+++ b/tests/run_agent/test_moa_loop_mode.py
@@ -884,3 +884,132 @@ def test_canonical_usage_add():
     assert total.cache_read_tokens == 5
     assert total.cache_write_tokens == 3
     assert total.request_count == 2
+
+
+def test_moa_full_trace_written_when_enabled(monkeypatch, tmp_path):
+    """With moa.save_traces on, a full MoA turn is written to JSONL.
+
+    Asserts the record captures each reference's FULL input messages + output
+    and the aggregator's FULL input (incl. injected reference guidance) +
+    output — the true full turn, auditable offline.
+    """
+    import json
+
+    home = tmp_path / ".hermes"
+    home.mkdir()
+    (home / "config.yaml").write_text(
+        """
+moa:
+  save_traces: true
+  default_preset: review
+  presets:
+    review:
+      reference_models:
+        - provider: openrouter
+          model: adv-a
+        - provider: openrouter
+          model: adv-b
+      aggregator:
+        provider: openrouter
+        model: anthropic/claude-opus-4.8
+""".strip(),
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("HERMES_HOME", str(home))
+
+    def fake_call_llm(**kwargs):
+        if kwargs["task"] == "moa_reference":
+            # Echo the model so we can prove per-reference output is captured.
+            model = kwargs.get("model", "?")
+            return _response_with_usage(content=f"advice from {model}", prompt=500, completion=80)
+        return _response("AGGREGATOR FINAL ANSWER")
+
+    monkeypatch.setattr("agent.moa_loop.call_llm", fake_call_llm)
+    monkeypatch.setattr(
+        "agent.moa_loop._slot_runtime",
+        lambda slot: {"provider": "openrouter", "model": slot.get("model")},
+    )
+    monkeypatch.setattr(
+        "agent.usage_pricing.estimate_usage_cost",
+        lambda *a, **k: SimpleNamespace(amount_usd=0.001, status="estimated", source="table"),
+    )
+
+    from agent.moa_loop import MoAChatCompletions
+
+    facade = MoAChatCompletions("review")
+    # Non-streaming create() → aggregator output captured inline.
+    facade.create(messages=[{"role": "user", "content": "please review the plan"}], tools=[])
+    facade.consume_and_save_trace(session_id="sess-xyz")
+
+    trace_file = home / "moa-traces" / "sess-xyz.jsonl"
+    assert trace_file.exists(), "trace file not written"
+    lines = trace_file.read_text(encoding="utf-8").strip().splitlines()
+    assert len(lines) == 1
+    rec = json.loads(lines[0])
+
+    # Turn framing.
+    assert rec["session_id"] == "sess-xyz"
+    assert rec["preset"] == "review"
+
+    # Both references captured, each with FULL input messages + output.
+    assert len(rec["references"]) == 2
+    for ref in rec["references"]:
+        assert ref["model"] in ("adv-a", "adv-b")
+        assert ref["provider"] == "openrouter"
+        # Full input messages present (system advisory prompt + advisory view).
+        assert isinstance(ref["input_messages"], list) and len(ref["input_messages"]) >= 2
+        assert ref["input_messages"][0]["role"] == "system"
+        # Full output present and model-specific.
+        assert ref["output"] == f"advice from {ref['model']}"
+        assert ref["usage"]["input_tokens"] == 500
+        assert ref["cost_usd"] == 0.001
+
+    # Aggregator: full input (with injected reference guidance) + inline output.
+    agg = rec["aggregator"]
+    assert agg["model"] == "anthropic/claude-opus-4.8"
+    assert agg["streamed"] is False
+    assert agg["output"] == "AGGREGATOR FINAL ANSWER"
+    agg_text = json.dumps(agg["input_messages"])
+    assert "Mixture of Agents reference context" in agg_text
+    assert "advice from adv-a" in agg_text and "advice from adv-b" in agg_text
+
+
+def test_moa_trace_not_written_when_disabled(monkeypatch, tmp_path):
+    """Default (save_traces off) writes nothing."""
+    home = tmp_path / ".hermes"
+    home.mkdir()
+    (home / "config.yaml").write_text(
+        """
+moa:
+  default_preset: review
+  presets:
+    review:
+      reference_models:
+        - provider: openrouter
+          model: adv-a
+      aggregator:
+        provider: openrouter
+        model: anthropic/claude-opus-4.8
+""".strip(),
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("HERMES_HOME", str(home))
+
+    def fake_call_llm(**kwargs):
+        if kwargs["task"] == "moa_reference":
+            return _response_with_usage(content="advice")
+        return _response("acted")
+
+    monkeypatch.setattr("agent.moa_loop.call_llm", fake_call_llm)
+    monkeypatch.setattr(
+        "agent.moa_loop._slot_runtime",
+        lambda slot: {"provider": "openrouter", "model": slot.get("model")},
+    )
+
+    from agent.moa_loop import MoAChatCompletions
+
+    facade = MoAChatCompletions("review")
+    facade.create(messages=[{"role": "user", "content": "hi"}], tools=[])
+    facade.consume_and_save_trace(session_id="sess-off")
+
+    assert not (home / "moa-traces").exists()