diff --git a/bridge/audit_log.py b/bridge/audit_log.py
new file mode 100644
index 0000000..730ae48
--- /dev/null
+++ b/bridge/audit_log.py
@@ -0,0 +1,101 @@
+"""Audit log — structured, human-readable per-event log on disk.
+
+PRD §BR-9: "All events and turn outputs written to a structured,
+human-readable audit log."
+
+Format: JSONL — one JSON object per line, append-only, easy to grep.
+Each entry has a stable `kind` plus event-specific fields:
+
+  {"kind": "state_change", "ts": "...", "session_id": "...",
+   "field": "momentum", "before": 5, "after": 0, "note": "..."}
+  {"kind": "turn_complete", "ts": "...", "turn": 12,
+   "rewards": {...}, "per_city": {...}}
+
+The audit log is independent of the save exchange — saves are
+checkpointed snapshots; the audit log is the running history. They
+serve different audiences (mod readers vs. forensic debugging).
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import threading
+from datetime import datetime
+from pathlib import Path
+from typing import Any, Dict, Optional
+
+
+DEFAULT_AUDIT_FILENAME = "audit.jsonl"
+
+
+class AuditLog:
+    """Append-only JSONL audit log."""
+
+    def __init__(self, path: str | Path):
+        p = Path(path)
+        if p.suffix == "":
+            p.mkdir(parents=True, exist_ok=True)
+            p = p / DEFAULT_AUDIT_FILENAME
+        else:
+            p.parent.mkdir(parents=True, exist_ok=True)
+        self.path: Path = p
+        self._lock = threading.Lock()
+
+    def log(self, kind: str, **fields: Any) -> Dict[str, Any]:
+        """Append one entry. Returns the entry dict for caller convenience."""
+        entry: Dict[str, Any] = {
+            "kind": kind,
+            "ts": datetime.now().astimezone().isoformat(),
+        }
+        # Sanitize unserializable values rather than crashing the daemon.
+        for k, v in fields.items():
+            try:
+                json.dumps(v)
+                entry[k] = v
+            except (TypeError, ValueError):
+                entry[k] = repr(v)
+
+        line = json.dumps(entry) + "\n"
+        with self._lock:
+            with open(self.path, "a", encoding="utf-8") as f:
+                f.write(line)
+        return entry
+
+    def log_state_change(self, change: Any) -> Dict[str, Any]:
+        """Convenience for HookListener.StateChange records."""
+        return self.log(
+            "state_change",
+            session_id=change.session_id,
+            event_type=change.event_type,
+            field=change.field,
+            before=change.before,
+            after=change.after,
+            note=change.note,
+        )
+
+    def log_turn_complete(self, turn: int, rewards: Dict[str, int],
+                           per_city: Optional[Dict[str, Dict[str, int]]] = None) -> Dict[str, Any]:
+        return self.log("turn_complete", turn=turn, rewards=rewards,
+                         per_city=per_city or {})
+
+    def log_session_added(self, session_id: str, name: str, city_id: str = "") -> Dict[str, Any]:
+        return self.log("session_added", session_id=session_id, name=name, city_id=city_id)
+
+    def read_lines(self, limit: Optional[int] = None) -> list[Dict[str, Any]]:
+        """Read entries back (for inspection / tests). Skips malformed lines."""
+        if not self.path.exists():
+            return []
+        out: list[Dict[str, Any]] = []
+        with open(self.path, "r", encoding="utf-8") as f:
+            for line in f:
+                line = line.strip()
+                if not line:
+                    continue
+                try:
+                    out.append(json.loads(line))
+                except json.JSONDecodeError:
+                    continue
+        if limit is not None:
+            return out[-limit:]
+        return out
diff --git a/bridge/bridge_loop.py b/bridge/bridge_loop.py
index 96cefb5..c2eb21a 100644
--- a/bridge/bridge_loop.py
+++ b/bridge/bridge_loop.py
@@ -19,6 +19,7 @@
 from pathlib import Path
 from typing import Any, Callable, Dict, List, Optional, TextIO
 
+from bridge.audit_log import AuditLog
 from bridge.hook_listener import HookListener
 from bridge.save_exchange import SaveExchange
 from bridge.scoring import ScoringEngine
@@ -39,15 +40,25 @@ def __init__(
         scoring_engine: ScoringEngine,
         hook_listener: Optional[HookListener] = None,
         save_exchange: Optional[SaveExchange] = None,
+        audit_log: Optional[AuditLog] = None,
         turn_interval_sec: float = 1.0,
     ):
         self.state = state
         self.scoring = scoring_engine
         self.hook_listener = hook_listener
         self.save_exchange = save_exchange
+        self.audit_log = audit_log
         self.turn_interval_sec = turn_interval_sec
         self.callbacks: Dict[str, Callable] = {}
         self.running = False
+        # Wire the listener's on_change to the audit log if both are present.
+        if self.hook_listener is not None and self.audit_log is not None:
+            existing_callback = self.hook_listener.on_change
+            def _audit_then_existing(change):
+                self.audit_log.log_state_change(change)
+                if existing_callback is not None:
+                    existing_callback(change)
+            self.hook_listener.on_change = _audit_then_existing
 
     # ──────────────────────────────────────────────────────────────────
     # Lifecycle
@@ -107,6 +118,9 @@ async def process_turn(self) -> Optional[Dict[str, Any]]:
         if self.save_exchange is not None:
             self.save_exchange.write_payload(self.state)
 
+        if self.audit_log is not None:
+            self.audit_log.log_turn_complete(self.state.current_turn, rewards, per_city)
+
         self._invoke_callbacks("turn_complete", rewards)
         if per_city:
             self._invoke_callbacks("per_city_rewards", per_city)
diff --git a/bridge/daemon.py b/bridge/daemon.py
index 4b9c7e6..91159d2 100644
--- a/bridge/daemon.py
+++ b/bridge/daemon.py
@@ -17,6 +17,7 @@
 import sys
 from pathlib import Path
 
+from bridge.audit_log import AuditLog
 from bridge.bridge_loop import BridgeLoop
 from bridge.hook_listener import HookListener
 from bridge.save_exchange import SaveExchange
@@ -38,17 +39,21 @@ def parse_args(argv: list[str] | None = None) -> argparse.Namespace:
                         help="Seconds after Stop before WAITING (default: 60)")
     parser.add_argument("--no-hooks", action="store_true",
                         help="Run the turn loop only — don't read hook events")
+    parser.add_argument("--audit-dir", type=Path, default=Path.cwd() / "logs",
+                        help="Directory for audit.jsonl (default: ./logs)")
     return parser.parse_args(argv)
 
 
 def build_components(args: argparse.Namespace):
     save_exchange = SaveExchange(args.saves_dir)
+    audit_log = AuditLog(args.audit_dir)
     state = save_exchange.read_payload() or SessionState()
     scoring = ScoringEngine(SimpleScoringStrategy())
     listener = HookListener(state=state, idle_threshold_sec=args.idle_threshold)
     loop = BridgeLoop(
         state=state, scoring_engine=scoring, hook_listener=listener,
-        save_exchange=save_exchange, turn_interval_sec=args.turn_interval,
+        save_exchange=save_exchange, audit_log=audit_log,
+        turn_interval_sec=args.turn_interval,
     )
 
     def _on_turn(rewards):
diff --git a/bridge/tests/test_audit_log.py b/bridge/tests/test_audit_log.py
new file mode 100644
index 0000000..18c894b
--- /dev/null
+++ b/bridge/tests/test_audit_log.py
@@ -0,0 +1,155 @@
+"""Tests for the audit log (PRD §BR-9)."""
+
+import json
+from datetime import datetime
+
+import pytest
+
+from bridge.audit_log import AuditLog, DEFAULT_AUDIT_FILENAME
+from bridge.bridge_loop import BridgeLoop
+from bridge.hook_listener import HookListener, StateChange
+from bridge.save_exchange import SaveExchange
+from bridge.scoring import ScoringEngine, SimpleScoringStrategy
+from bridge.session_state import SessionState, SessionStatus
+
+
+def test_audit_log_directory_path_appends_default_filename(tmp_path):
+    log = AuditLog(tmp_path / "logs")
+    assert log.path.name == DEFAULT_AUDIT_FILENAME
+    assert log.path.parent == tmp_path / "logs"
+
+
+def test_audit_log_explicit_filename_respected(tmp_path):
+    custom = tmp_path / "logs" / "session.jsonl"
+    log = AuditLog(custom)
+    assert log.path == custom
+
+
+def test_log_writes_jsonl_with_timestamp(tmp_path):
+    log = AuditLog(tmp_path / "logs")
+    log.log("test", foo="bar")
+    lines = log.path.read_text(encoding="utf-8").splitlines()
+    assert len(lines) == 1
+    entry = json.loads(lines[0])
+    assert entry["kind"] == "test"
+    assert entry["foo"] == "bar"
+    assert "ts" in entry  # ISO-8601 timestamp
+
+
+def test_log_appends_not_overwrites(tmp_path):
+    log = AuditLog(tmp_path / "logs")
+    log.log("a", n=1)
+    log.log("b", n=2)
+    lines = log.path.read_text(encoding="utf-8").splitlines()
+    assert len(lines) == 2
+
+
+def test_log_state_change_records_all_fields(tmp_path):
+    log = AuditLog(tmp_path / "logs")
+    change = StateChange(
+        event_type="goal_completed", session_id="s1",
+        field="momentum", before=3, after=4, note="task_completed",
+    )
+    log.log_state_change(change)
+    entries = log.read_lines()
+    assert len(entries) == 1
+    e = entries[0]
+    assert e["kind"] == "state_change"
+    assert e["session_id"] == "s1"
+    assert e["event_type"] == "goal_completed"
+    assert e["field"] == "momentum"
+    assert e["before"] == 3
+    assert e["after"] == 4
+    assert e["note"] == "task_completed"
+
+
+def test_log_turn_complete(tmp_path):
+    log = AuditLog(tmp_path / "logs")
+    log.log_turn_complete(turn=5, rewards={"gold": 100},
+                            per_city={"camelot": {"gold": 100}})
+    entries = log.read_lines()
+    assert entries[0]["kind"] == "turn_complete"
+    assert entries[0]["turn"] == 5
+    assert entries[0]["rewards"]["gold"] == 100
+    assert entries[0]["per_city"]["camelot"]["gold"] == 100
+
+
+def test_read_lines_with_limit(tmp_path):
+    log = AuditLog(tmp_path / "logs")
+    for i in range(20):
+        log.log("entry", n=i)
+    last_5 = log.read_lines(limit=5)
+    assert len(last_5) == 5
+    # Last entry should be n=19
+    assert last_5[-1]["n"] == 19
+
+
+def test_read_lines_skips_malformed(tmp_path):
+    log = AuditLog(tmp_path / "logs")
+    log.log("good", n=1)
+    # Manually append a malformed line.
+    with open(log.path, "a", encoding="utf-8") as f:
+        f.write("not json\n")
+    log.log("good", n=2)
+    entries = log.read_lines()
+    # Only the two valid entries.
+    assert len(entries) == 2
+    assert [e["n"] for e in entries] == [1, 2]
+
+
+def test_unserializable_value_is_repr_safe(tmp_path):
+    log = AuditLog(tmp_path / "logs")
+
+    class NotJsonable:
+        def __repr__(self):
+            return "<weird>"
+
+    log.log("test", obj=NotJsonable())
+    entry = log.read_lines()[0]
+    assert entry["obj"] == "<weird>"
+
+
+# ────────────────────────────────────────────────────────────────────────
+# BridgeLoop integration
+# ────────────────────────────────────────────────────────────────────────
+
+@pytest.mark.asyncio
+async def test_bridge_loop_logs_state_changes_and_turns(tmp_path):
+    state = SessionState(kingdom_name="Camelot")
+    s = state.add_session("Royal Court")
+    s.session_id = "sess-1"
+    s.status = SessionStatus.ACTIVE
+
+    listener = HookListener(state=state)
+    audit = AuditLog(tmp_path / "logs")
+    save = SaveExchange(tmp_path / "saves")
+    loop = BridgeLoop(
+        state=state,
+        scoring_engine=ScoringEngine(SimpleScoringStrategy()),
+        hook_listener=listener,
+        save_exchange=save,
+        audit_log=audit,
+    )
+    # Inject a hook event — listener should call audit via on_change.
+    loop.feed_event({"event_type": "TaskCompleted",
+                     "payload": {"session_id": "sess-1"}})
+    # Then advance a turn.
+    await loop.process_turn()
+
+    entries = audit.read_lines()
+    kinds = [e["kind"] for e in entries]
+    assert "state_change" in kinds   # at least one state change from TaskCompleted
+    assert "turn_complete" in kinds  # the turn completion entry
+    turn_entry = next(e for e in entries if e["kind"] == "turn_complete")
+    assert turn_entry["turn"] == 1
+
+
+@pytest.mark.asyncio
+async def test_bridge_loop_audit_log_optional(tmp_path):
+    """Loop without audit log should still work — backward compat."""
+    state = SessionState()
+    s = state.add_session("X")
+    s.status = SessionStatus.ACTIVE
+    loop = BridgeLoop(state=state, scoring_engine=ScoringEngine(SimpleScoringStrategy()))
+    rewards = await loop.process_turn()
+    assert rewards is not None