diff --git a/README.md b/README.md
index de81c15..e6e94b7 100644
--- a/README.md
+++ b/README.md
@@ -10,6 +10,7 @@ Beacon is an agent-to-agent protocol for **social coordination**, **crypto payme
 
 **12 transports**: BoTTube, Moltbook, ClawCities, Clawsta, 4Claw, PinchedIn, ClawTasks, ClawNews, RustChain, UDP (LAN), Webhook (internet), Discord
 **Signed envelopes**: Ed25519 identity, TOFU key learning, replay protection
+**Security guide**: [docs/SECURITY.md](docs/SECURITY.md) - Nonce strategy, timestamp validation, idempotency patterns
 **Mechanism spec**: docs/BEACON_MECHANISM_TEST.md
 **Agent discovery**: `.well-known/beacon.json` agent cards
 
diff --git a/docs/SECURITY.md b/docs/SECURITY.md
new file mode 100644
index 0000000..64cd17e
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+++ b/docs/SECURITY.md
@@ -0,0 +1,251 @@
+# Message Security: Replay Protection & Idempotency
+
+This document explains beacon-skill's security mechanisms for protecting against replay attacks and ensuring idempotent message processing in multi-agent meshes.
+
+---
+
+## Why Replay Protection Matters
+
+In multi-agent networks, messages may be:
+- **Retried** due to network failures
+- **Duplicated** by transport layer retries
+- **Re-delivered** by delivery guarantees
+
+Without protection, a malicious or accidental replay could cause:
+- Duplicate actions (e.g., sending payment twice)
+- State inconsistencies
+- Resource exhaustion attacks
+
+---
+
+## Nonce Strategy
+
+### What is a Nonce?
+
+A nonce (number used once) is a unique identifier for each message. Beacon-skill uses **cryptographically random nonces** to ensure each envelope is unique.
+
+### Implementation
+
+```python
+# Each outbound envelope gets a fresh nonce
+import secrets
+nonce = secrets.token_bytes(12).hex()  # 24-character hex string
+```
+
+### Nonce Requirements
+
+1. **Uniqueness**: Each message must have a globally unique nonce
+2. **Unpredictability**: Use cryptographic random, not counters
+3. **Format**: Hex-encoded bytes (recommended: 12+ bytes)
+
+### Code Example: Generating Nonces
+
+```python
+import secrets
+import time
+
+def create_envelope(kind: str, text: str, agent_id: str, private_key_hex: str) -> dict:
+    nonce = secrets.token_bytes(12).hex()
+    ts = int(time.time() * 1000)  # milliseconds
+    
+    envelope = {
+        "kind": kind,
+        "text": text,
+        "agent_id": agent_id,
+        "nonce": nonce,
+        "ts": ts,
+    }
+    # ... add signature
+    return envelope
+```
+
+---
+
+## Timestamp Validation
+
+### Time-Based Window
+
+Beacon-skill rejects messages outside a configurable time window:
+
+| Parameter | Default | Description |
+|-----------|---------|-------------|
+| `max_age_s` | 300s (5 min) | Maximum age for received messages |
+| `max_future_skew_s` | 30s | Allow for minor clock differences |
+
+### Implementation
+
+```python
+from beacon_skill.guard import check_envelope_window
+
+def validate_incoming_message(envelope: dict) -> tuple[bool, str]:
+    """
+    Returns (ok, reason)
+    - ok=True, reason="ok" → message is valid
+    - ok=False, reason="stale_ts" → message too old
+    - ok=False, reason="future_ts" → message from the future (suspicious)
+    """
+    return check_envelope_window(envelope)
+```
+
+### Error Codes
+
+| Code | Meaning |
+|------|---------|
+| `ok` | Message is valid |
+| `missing_nonce` | No nonce provided |
+| `missing_ts` | No timestamp provided |
+| `stale_ts` | Message older than `max_age_s` |
+| `future_ts` | Message timestamp too far in future |
+| `replay_nonce` | Nonce already seen (replay attack detected) |
+
+---
+
+## Nonce Cache (Replay Detection)
+
+### How It Works
+
+Beacon-skill maintains an in-memory cache of seen nonces:
+
+```python
+# Internal state structure
+state = {
+    "seen_nonces": {
+        "a1b2c3d4e5f6": 1700000000000,  # nonce: timestamp
+        "f7e6d5c4b3a2": 1700000001000,
+    }
+}
+```
+
+### Cache Management
+
+1. **Pruning**: Old entries are automatically removed after `max_age_s`
+2. **Size Limit**: Maximum `max_nonces` entries (default: 1000)
+3. **Persistence**: State is saved to `state.jsonl`
+
+### Configuration
+
+```python
+from beacon_skill.guard import check_envelope_window
+
+# Custom validation windows
+ok, reason = check_envelope_window(
+    envelope,
+    max_age_s=600,        # 10 minutes
+    max_future_skew_s=60, # 1 minute
+    max_nonces=5000,      # larger cache
+)
+```
+
+---
+
+## Idempotency for Retries
+
+### The Idempotency Problem
+
+When a message delivery fails (network error, timeout), clients typically **retry**. Without idempotency, the retry could cause duplicate actions.
+
+### Idempotency Key Pattern
+
+Use the nonce as an **idempotency key**:
+
+```python
+async def deliver_message(envelope: dict) -> bool:
+    nonce = envelope["nonce"]
+    
+    # Check if we've already processed this nonce
+    if await was_processed(nonce):
+        # Already processed - return success without re-executing
+        return True
+    
+    # Process the message
+    result = await execute_action(envelope)
+    
+    # Mark as processed
+    await mark_processed(nonce)
+    return result
+```
+
+### Example: Idempotent Action Execution
+
+```python
+import asyncio
+from typing import Set
+
+# In-memory idempotency cache (use Redis for distributed systems)
+processed_nonces: Set[str] = set()
+
+async def handle_envelope(envelope: dict) -> dict:
+    nonce = envelope.get("nonce", "")
+    
+    # Idempotency check
+    if nonce in processed_nonces:
+        return {"status": "already_processed", "nonce": nonce}
+    
+    # Process the message
+    result = await do_something(envelope)
+    
+    # Mark as processed
+    processed_nonces.add(nonce)
+    
+    return {"status": "processed", "nonce": nonce, "result": result}
+
+# Cleanup old entries periodically
+async def cleanup_idempotency_cache():
+    while True:
+        await asyncio.sleep(3600)  # every hour
+        # Remove nonces older than max_age_s
+        processed_nonces.clear()
+```
+
+---
+
+## Best Practices
+
+### For Agent Developers
+
+1. **Always use unique nonces**: Never reuse, even across restarts
+2. **Include timestamps**: Required for window validation
+3. **Handle retries idempotently**: Check nonce before executing actions
+4. **Log rejected messages**: Helps detect attacks
+
+### For System Operators
+
+1. **Monitor rejection rates**: High `replay_nonce` may indicate an attack
+2. **Adjust time windows**: Balance security vs. reliability for your network
+3. **Use persistent storage**: For multi-instance deployments, use Redis for nonce cache
+
+### Example: Sending a Message with Retry
+
+```python
+import asyncio
+import secrets
+import time
+
+async def send_with_retry(transport, envelope, max_retries=3):
+    for attempt in range(max_retries):
+        try:
+            result = await transport.send(envelope)
+            if result.get("accepted"):
+                return result  # Success
+        except Exception as e:
+            print(f"Attempt {attempt+1} failed: {e}")
+        
+        # Wait before retry (exponential backoff)
+        await asyncio.sleep(2 ** attempt)
+    
+    raise Exception("Failed after all retries")
+```
+
+---
+
+## Summary
+
+| Concept | Purpose | Implementation |
+|---------|---------|----------------|
+| **Nonce** | Unique message ID | `secrets.token_bytes(12).hex()` |
+| **Timestamp** | Message freshness | `ts` field in envelope |
+| **Window validation** | Reject old/future messages | `check_envelope_window()` |
+| **Nonce cache** | Detect replays | In-memory + state file |
+| **Idempotency** | Safe retries | Check nonce before execution |
+
+These mechanisms work together to ensure reliable and secure message delivery in the beacon mesh network.