diff --git a/src/robotproof_studio/audit.py b/src/robotproof_studio/audit.py
new file mode 100644
index 0000000..b1b0525
--- /dev/null
+++ b/src/robotproof_studio/audit.py
@@ -0,0 +1,64 @@
+from __future__ import annotations
+
+import hashlib
+import json
+from dataclasses import dataclass, field
+from datetime import datetime
+from typing import Any, Dict, List
+
+from .contracts import PlanDelta
+
+
+def _now_iso() -> str:
+    return datetime.utcnow().isoformat() + "Z"
+
+
+@dataclass
+class AuditEntry:
+    index: int
+    timestamp: str
+    delta_id: str
+    payload_hash: str
+    prev_hash: str
+    entry_hash: str
+
+
+class AuditLog:
+    """Simple tamper-evident audit log implemented as a hash chain.
+
+    Each entry includes the previous entry hash so that any modification
+    is detectable by re-checking the chain.
+    """
+
+    def __init__(self) -> None:
+        self.entries: List[AuditEntry] = []
+
+    @staticmethod
+    def _hash(obj: Any) -> str:
+        s = json.dumps(obj, sort_keys=True, separators=(",", ":"))
+        return hashlib.sha256(s.encode("utf-8")).hexdigest()
+
+    def append(self, delta: PlanDelta) -> AuditEntry:
+        # hash the important parts of the delta (id, timestamp, data, proof)
+        payload = {"delta_id": delta.delta_id, "timestamp": delta.timestamp, "data": delta.data, "proof": delta.proof}
+        payload_hash = self._hash(payload)
+
+        prev_hash = self.entries[-1].entry_hash if self.entries else ""
+
+        entry_obj = {"index": len(self.entries), "timestamp": _now_iso(), "delta_id": delta.delta_id, "payload_hash": payload_hash, "prev_hash": prev_hash}
+        entry_hash = self._hash(entry_obj)
+
+        entry = AuditEntry(index=entry_obj["index"], timestamp=entry_obj["timestamp"], delta_id=entry_obj["delta_id"], payload_hash=payload_hash, prev_hash=prev_hash, entry_hash=entry_hash)
+        self.entries.append(entry)
+        return entry
+
+    def verify_chain(self) -> bool:
+        prev_hash = ""
+        for i, e in enumerate(self.entries):
+            # recompute payload hash is not possible here, but we can recompute the entry hash from stored fields
+            entry_obj = {"index": e.index, "timestamp": e.timestamp, "delta_id": e.delta_id, "payload_hash": e.payload_hash, "prev_hash": prev_hash}
+            expected_hash = self._hash(entry_obj)
+            if expected_hash != e.entry_hash:
+                return False
+            prev_hash = e.entry_hash
+        return True
diff --git a/tests/test_audit.py b/tests/test_audit.py
new file mode 100644
index 0000000..6da7503
--- /dev/null
+++ b/tests/test_audit.py
@@ -0,0 +1,27 @@
+from datetime import datetime
+
+from robotproof_studio.contracts import PlanDelta
+from robotproof_studio.audit import AuditLog
+
+
+def make_delta(id_suffix: str) -> PlanDelta:
+    return PlanDelta(delta_id=f"d-{id_suffix}", timestamp=datetime.utcnow().isoformat() + "Z", data={"val": id_suffix})
+
+
+def test_audit_log_chain_and_detection():
+    log = AuditLog()
+
+    d1 = make_delta("1")
+    d1.sign("tester")
+    e1 = log.append(d1)
+
+    d2 = make_delta("2")
+    d2.sign("tester")
+    e2 = log.append(d2)
+
+    # chain should be valid
+    assert log.verify_chain() is True
+
+    # tamper with second entry's delta_id (simulates external tampering)
+    log.entries[1].delta_id = "tampered"
+    assert log.verify_chain() is False