docs: add discover to CLI docstring

8 tests covering:
- Basic discovery returns entries
- Count limits
- Topic filtering (case-insensitive)
- Empty archive handling
- Vitality boost via touch

app.js

@@ -7,6 +7,7 @@ import { SpatialMemory } from './nexus/components/spatial-memory.js';
 import { MemoryBirth } from './nexus/components/memory-birth.js';
 import { MemoryOptimizer } from './nexus/components/memory-optimizer.js';
 import { MemoryInspect } from './nexus/components/memory-inspect.js';
+import { MemoryPulse } from './nexus/components/memory-pulse.js';
 
 // ═══════════════════════════════════════════
 // NEXUS v1.1 — Portal System Update
@@ -715,6 +716,7 @@ async function init() {
   MemoryBirth.wrapSpatialMemory(SpatialMemory);
   SpatialMemory.setCamera(camera);
   MemoryInspect.init({ onNavigate: _navigateToMemory });
+  MemoryPulse.init(SpatialMemory);
   updateLoad(90);
 
   loadSession();
@@ -1945,6 +1947,7 @@ function setupControls() {
         const entry = SpatialMemory.getMemoryFromMesh(hits[0].object);
         if (entry) {
           SpatialMemory.highlightMemory(entry.data.id);
+          MemoryPulse.triggerPulse(entry.data.id);
           const regionDef = SpatialMemory.REGIONS[entry.region] || SpatialMemory.REGIONS.working;
           MemoryInspect.show(entry.data, regionDef);
         }
@@ -2924,6 +2927,7 @@ function gameLoop() {
   if (typeof animateMemoryOrbs === 'function') {
     SpatialMemory.update(delta);
     MemoryBirth.update(delta);
+    MemoryPulse.update();
     animateMemoryOrbs(delta);
   }
 

nexus/components/memory-pulse.js

@@ -0,0 +1,160 @@
+// ═══════════════════════════════════════════════════
+// PROJECT MNEMOSYNE — MEMORY PULSE
+// ═══════════════════════════════════════════════════
+//
+// BFS wave animation triggered on crystal click.
+// When a memory crystal is clicked, a visual pulse
+// radiates through the connection graph — illuminating
+// linked memories hop-by-hop with a glow that rises
+// sharply and then fades.
+//
+// Usage:
+//   MemoryPulse.init(SpatialMemory);
+//   MemoryPulse.triggerPulse(memId);
+//   MemoryPulse.update();  // called each frame
+// ═══════════════════════════════════════════════════
+
+const MemoryPulse = (() => {
+
+  let _sm = null;
+
+  // [{mesh, startTime, delay, duration, peakIntensity, baseIntensity}]
+  const _activeEffects = [];
+
+  // ── Config ───────────────────────────────────────
+  const HOP_DELAY_MS   = 180;   // ms between hops
+  const PULSE_DURATION = 650;   // ms for glow rise + fade per node
+  const PEAK_INTENSITY = 5.5;   // emissiveIntensity at pulse peak
+  const MAX_HOPS       = 8;     // BFS depth limit
+
+  // ── Helpers ──────────────────────────────────────
+
+  // Build memId -> mesh from SpatialMemory public API
+  function _buildMeshMap() {
+    const map = {};
+    const meshes = _sm.getCrystalMeshes();
+    for (const mesh of meshes) {
+      const entry = _sm.getMemoryFromMesh(mesh);
+      if (entry) map[entry.data.id] = mesh;
+    }
+    return map;
+  }
+
+  // Build bidirectional adjacency graph from memory connection data
+  function _buildGraph() {
+    const graph = {};
+    const memories = _sm.getAllMemories();
+    for (const mem of memories) {
+      if (!graph[mem.id]) graph[mem.id] = [];
+      if (mem.connections) {
+        for (const targetId of mem.connections) {
+          graph[mem.id].push(targetId);
+          if (!graph[targetId]) graph[targetId] = [];
+          graph[targetId].push(mem.id);
+        }
+      }
+    }
+    return graph;
+  }
+
+  // ── Public API ───────────────────────────────────
+
+  function init(spatialMemory) {
+    _sm = spatialMemory;
+  }
+
+  /**
+   * Trigger a BFS pulse wave originating from memId.
+   * Each hop level illuminates after HOP_DELAY_MS * hop ms.
+   * @param {string} memId - ID of the clicked memory crystal
+   */
+  function triggerPulse(memId) {
+    if (!_sm) return;
+
+    const meshMap = _buildMeshMap();
+    const graph   = _buildGraph();
+
+    if (!meshMap[memId]) return;
+
+    // Cancel any existing effects on the same meshes (avoids stacking)
+    _activeEffects.length = 0;
+
+    // BFS
+    const visited = new Set([memId]);
+    const queue   = [{ id: memId, hop: 0 }];
+    const now     = performance.now();
+    const scheduled = [];
+
+    while (queue.length > 0) {
+      const { id, hop } = queue.shift();
+      if (hop > MAX_HOPS) continue;
+
+      const mesh = meshMap[id];
+      if (mesh) {
+        const strength      = mesh.userData.strength || 0.7;
+        const baseIntensity = 1.0 + Math.sin(mesh.userData.pulse || 0) * 0.5 * strength;
+
+        scheduled.push({
+          mesh,
+          startTime:     now,
+          delay:         hop * HOP_DELAY_MS,
+          duration:      PULSE_DURATION,
+          peakIntensity: PEAK_INTENSITY,
+          baseIntensity: Math.max(0.5, baseIntensity)
+        });
+      }
+
+      for (const neighborId of (graph[id] || [])) {
+        if (!visited.has(neighborId)) {
+          visited.add(neighborId);
+          queue.push({ id: neighborId, hop: hop + 1 });
+        }
+      }
+    }
+
+    for (const effect of scheduled) {
+      _activeEffects.push(effect);
+    }
+
+    console.info('[MemoryPulse] Pulse triggered from', memId, '—', scheduled.length, 'nodes in wave');
+  }
+
+  /**
+   * Advance all active pulse animations. Call once per frame.
+   */
+  function update() {
+    if (_activeEffects.length === 0) return;
+
+    const now = performance.now();
+
+    for (let i = _activeEffects.length - 1; i >= 0; i--) {
+      const e = _activeEffects[i];
+      const elapsed = now - e.startTime - e.delay;
+
+      if (elapsed < 0) continue;  // waiting for its hop delay
+
+      if (elapsed >= e.duration) {
+        // Animation complete — restore base intensity
+        if (e.mesh.material) {
+          e.mesh.material.emissiveIntensity = e.baseIntensity;
+        }
+        _activeEffects.splice(i, 1);
+        continue;
+      }
+
+      // t: 0 → 1 over duration
+      const t    = elapsed / e.duration;
+      // sin curve over [0, π]: smooth rise then fall
+      const glow = Math.sin(t * Math.PI);
+
+      if (e.mesh.material) {
+        e.mesh.material.emissiveIntensity =
+          e.baseIntensity + glow * (e.peakIntensity - e.baseIntensity);
+      }
+    }
+  }
+
+  return { init, triggerPulse, update };
+})();
+
+export { MemoryPulse };

nexus/mnemosyne/FEATURES.yaml

@@ -67,7 +67,7 @@ modules:
   cli:
     status: shipped
     files: [cli.py]
-    description: CLI interface — stats, search, ingest, link, topics, remove, export, clusters, hubs, bridges, rebuild, tag/untag/retag, timeline, neighbors, consolidate
+    description: CLI interface — stats, search, ingest, link, topics, remove, export, clusters, hubs, bridges, rebuild, tag/untag/retag, timeline, neighbors, consolidate, path, touch, decay, vitality, fading, vibrant
 
   tests:
     status: shipped
@@ -163,12 +163,15 @@ planned:
       - "#TBD"  # Will be filled when PR is created
 
   memory_pulse:
-    status: planned
+    status: shipped
+    files: [nexus/components/memory-pulse.js]
     description: >
       Visual pulse wave radiates through connection graph when
       a crystal is clicked, illuminating linked memories by BFS
-      hop distance. Was attempted in PR #1226 — needs rebasing.
+      hop distance.
     priority: medium
+    merged_prs:
+      - "#1263"
 
   embedding_backend:
     status: shipped
@@ -181,6 +184,19 @@ planned:
     merged_prs:
       - "#TBD"  # Will be filled when PR is created
 
+
+  memory_path:
+    status: shipped
+    files: [archive.py, cli.py, tests/test_path.py]
+    description: >
+      BFS shortest path between two memories through the connection graph.
+      Answers "how is memory X related to memory Y?" by finding the chain
+      of connections. Includes path_explanation for human-readable output.
+      CLI command: mnemosyne path <start_id> <end_id>
+    priority: medium
+    merged_prs:
+      - "#TBD"
+
   memory_consolidation:
     status: shipped
     files: [archive.py, cli.py, tests/test_consolidation.py]

nexus/mnemosyne/archive.py

@@ -1059,6 +1059,221 @@ class MnemosyneArchive:
 
         return merges
 
+
+    def shortest_path(self, start_id: str, end_id: str) -> list[str] | None:
+        """Find shortest path between two entries through the connection graph.
+
+        Returns list of entry IDs from start to end (inclusive), or None if
+        no path exists. Uses BFS for unweighted shortest path.
+        """
+        if start_id == end_id:
+            return [start_id] if start_id in self._entries else None
+        if start_id not in self._entries or end_id not in self._entries:
+            return None
+
+        adj = self._build_adjacency()
+        visited = {start_id}
+        queue = [(start_id, [start_id])]
+
+        while queue:
+            current, path = queue.pop(0)
+            for neighbor in adj.get(current, []):
+                if neighbor == end_id:
+                    return path + [neighbor]
+                if neighbor not in visited:
+                    visited.add(neighbor)
+                    queue.append((neighbor, path + [neighbor]))
+
+        return None
+
+    def path_explanation(self, path: list[str]) -> list[dict]:
+        """Convert a path of entry IDs into human-readable step descriptions.
+
+        Returns list of dicts with 'id', 'title', and 'topics' for each step.
+        """
+        steps = []
+        for entry_id in path:
+            entry = self._entries.get(entry_id)
+            if entry:
+                steps.append({
+                    "id": entry.id,
+                    "title": entry.title,
+                    "topics": entry.topics,
+                    "content_preview": entry.content[:120] + "..." if len(entry.content) > 120 else entry.content,
+                })
+            else:
+                steps.append({"id": entry_id, "title": "[unknown]", "topics": []})
+        return steps
+
+    # ─── Snapshot / Backup ────────────────────────────────────
+
+    def _snapshot_dir(self) -> Path:
+        """Return (and create) the snapshots directory next to the archive."""
+        d = self.path.parent / "snapshots"
+        d.mkdir(parents=True, exist_ok=True)
+        return d
+
+    @staticmethod
+    def _snapshot_filename(timestamp: str, label: str) -> str:
+        """Build a deterministic snapshot filename."""
+        safe_label = "".join(c if c.isalnum() or c in "-_" else "_" for c in label) if label else "snapshot"
+        return f"{timestamp}_{safe_label}.json"
+
+    def snapshot_create(self, label: str = "") -> dict:
+        """Serialize the current archive state to a timestamped snapshot file.
+
+        Args:
+            label: Human-readable label for the snapshot (optional).
+
+        Returns:
+            Dict with keys: snapshot_id, label, created_at, entry_count, path
+        """
+        now = datetime.now(timezone.utc)
+        timestamp = now.strftime("%Y%m%d_%H%M%S")
+        filename = self._snapshot_filename(timestamp, label)
+        snapshot_id = filename[:-5]  # strip .json
+        snap_path = self._snapshot_dir() / filename
+
+        payload = {
+            "snapshot_id": snapshot_id,
+            "label": label,
+            "created_at": now.isoformat(),
+            "entry_count": len(self._entries),
+            "archive_path": str(self.path),
+            "entries": [e.to_dict() for e in self._entries.values()],
+        }
+        with open(snap_path, "w") as f:
+            json.dump(payload, f, indent=2)
+
+        return {
+            "snapshot_id": snapshot_id,
+            "label": label,
+            "created_at": payload["created_at"],
+            "entry_count": payload["entry_count"],
+            "path": str(snap_path),
+        }
+
+    def snapshot_list(self) -> list[dict]:
+        """List available snapshots, newest first.
+
+        Returns:
+            List of dicts with keys: snapshot_id, label, created_at, entry_count, path
+        """
+        snap_dir = self._snapshot_dir()
+        snapshots = []
+        for snap_path in sorted(snap_dir.glob("*.json"), reverse=True):
+            try:
+                with open(snap_path) as f:
+                    data = json.load(f)
+                snapshots.append({
+                    "snapshot_id": data.get("snapshot_id", snap_path.stem),
+                    "label": data.get("label", ""),
+                    "created_at": data.get("created_at", ""),
+                    "entry_count": data.get("entry_count", len(data.get("entries", []))),
+                    "path": str(snap_path),
+                })
+            except (json.JSONDecodeError, OSError):
+                continue
+        return snapshots
+
+    def snapshot_restore(self, snapshot_id: str) -> dict:
+        """Restore the archive from a snapshot, replacing all current entries.
+
+        Args:
+            snapshot_id: The snapshot_id returned by snapshot_create / snapshot_list.
+
+        Returns:
+            Dict with keys: snapshot_id, restored_count, previous_count
+
+        Raises:
+            FileNotFoundError: If no snapshot with that ID exists.
+        """
+        snap_dir = self._snapshot_dir()
+        snap_path = snap_dir / f"{snapshot_id}.json"
+        if not snap_path.exists():
+            raise FileNotFoundError(f"Snapshot not found: {snapshot_id}")
+
+        with open(snap_path) as f:
+            data = json.load(f)
+
+        previous_count = len(self._entries)
+        self._entries = {}
+        for entry_data in data.get("entries", []):
+            entry = ArchiveEntry.from_dict(entry_data)
+            self._entries[entry.id] = entry
+
+        self._save()
+        return {
+            "snapshot_id": snapshot_id,
+            "restored_count": len(self._entries),
+            "previous_count": previous_count,
+        }
+
+    def snapshot_diff(self, snapshot_id: str) -> dict:
+        """Compare a snapshot against the current archive state.
+
+        Args:
+            snapshot_id: The snapshot_id to compare against current state.
+
+        Returns:
+            Dict with keys:
+              - snapshot_id: str
+              - added: list of {id, title} — in current, not in snapshot
+              - removed: list of {id, title} — in snapshot, not in current
+              - modified: list of {id, title, snapshot_hash, current_hash}
+              - unchanged: int — count of identical entries
+
+        Raises:
+            FileNotFoundError: If no snapshot with that ID exists.
+        """
+        snap_dir = self._snapshot_dir()
+        snap_path = snap_dir / f"{snapshot_id}.json"
+        if not snap_path.exists():
+            raise FileNotFoundError(f"Snapshot not found: {snapshot_id}")
+
+        with open(snap_path) as f:
+            data = json.load(f)
+
+        snap_entries: dict[str, dict] = {}
+        for entry_data in data.get("entries", []):
+            snap_entries[entry_data["id"]] = entry_data
+
+        current_ids = set(self._entries.keys())
+        snap_ids = set(snap_entries.keys())
+
+        added = []
+        for eid in current_ids - snap_ids:
+            e = self._entries[eid]
+            added.append({"id": e.id, "title": e.title})
+
+        removed = []
+        for eid in snap_ids - current_ids:
+            snap_e = snap_entries[eid]
+            removed.append({"id": snap_e["id"], "title": snap_e.get("title", "")})
+
+        modified = []
+        unchanged = 0
+        for eid in current_ids & snap_ids:
+            current_hash = self._entries[eid].content_hash
+            snap_hash = snap_entries[eid].get("content_hash")
+            if current_hash != snap_hash:
+                modified.append({
+                    "id": eid,
+                    "title": self._entries[eid].title,
+                    "snapshot_hash": snap_hash,
+                    "current_hash": current_hash,
+                })
+            else:
+                unchanged += 1
+
+        return {
+            "snapshot_id": snapshot_id,
+            "added": sorted(added, key=lambda x: x["title"]),
+            "removed": sorted(removed, key=lambda x: x["title"]),
+            "modified": sorted(modified, key=lambda x: x["title"]),
+            "unchanged": unchanged,
+        }
+
     def rebuild_links(self, threshold: Optional[float] = None) -> int:
         """Recompute all links from scratch.
 
@@ -1093,3 +1308,88 @@ class MnemosyneArchive:
 
         self._save()
         return total_links
+    # ─── Discovery ──────────────────────────────────────────────
+
+    def discover(
+        self,
+        count: int = 5,
+        prefer_fading: bool = True,
+        topic: Optional[str] = None,
+    ) -> list[dict]:
+        """Serendipitous entry discovery — surface forgotten knowledge.
+
+        Selects entries probabilistically, weighting toward fading (low vitality)
+        entries when prefer_fading=True, or toward vibrant entries when False.
+        Optionally filter by topic.
+
+        Touches selected entries to boost their vitality, preventing the same
+        entries from being repeatedly surfaced.
+
+        Args:
+            count: Number of entries to discover.
+            prefer_fading: If True, weight toward neglected entries. If False,
+                           weight toward vibrant entries.
+            topic: Optional topic filter — only discover entries with this tag.
+
+        Returns:
+            List of dicts with keys: entry_id, title, content_preview, topics,
+            vitality, age_days, last_accessed
+        """
+        import random
+
+        candidates = list(self._entries.values())
+
+        # Filter by topic if specified
+        if topic:
+            topic_lower = topic.lower()
+            candidates = [
+                e for e in candidates
+                if topic_lower in [t.lower() for t in e.topics]
+            ]
+
+        if not candidates:
+            return []
+
+        # Compute vitality for each candidate
+        scored = []
+        for entry in candidates:
+            v = self._compute_vitality(entry)
+            scored.append((entry, v))
+
+        # Build selection weights
+        if prefer_fading:
+            # Lower vitality = higher weight. Invert and normalize.
+            weights = [max(0.01, 1.0 - v) for _, v in scored]
+        else:
+            # Higher vitality = higher weight
+            weights = [max(0.01, v) for _, v in scored]
+
+        # Sample without replacement
+        k = min(count, len(scored))
+        selected_indices = random.choices(range(len(scored)), weights=weights, k=k)
+        # Deduplicate while preserving order
+        seen = set()
+        unique_indices = []
+        for idx in selected_indices:
+            if idx not in seen:
+                seen.add(idx)
+                unique_indices.append(idx)
+
+        results = []
+        for idx in unique_indices:
+            entry, v = scored[idx]
+            # Touch to boost vitality
+            self.touch(entry.id)
+            created = self._parse_dt(entry.created_at)
+            age_days = (datetime.now(timezone.utc) - created).days
+            results.append({
+                "entry_id": entry.id,
+                "title": entry.title,
+                "content_preview": entry.content[:200] + "..." if len(entry.content) > 200 else entry.content,
+                "topics": entry.topics,
+                "vitality": round(v, 4),
+                "age_days": age_days,
+                "last_accessed": entry.last_accessed,
+            })
+
+        return results

nexus/mnemosyne/cli.py

@@ -4,7 +4,11 @@ Provides: mnemosyne ingest, mnemosyne search, mnemosyne link, mnemosyne stats,
           mnemosyne topics, mnemosyne remove, mnemosyne export,
           mnemosyne clusters, mnemosyne hubs, mnemosyne bridges, mnemosyne rebuild,
           mnemosyne tag, mnemosyne untag, mnemosyne retag,
-          mnemosyne timeline, mnemosyne neighbors
+          mnemosyne timeline, mnemosyne neighbors, mnemosyne path,
+          mnemosyne touch, mnemosyne decay, mnemosyne vitality,
+          mnemosyne fading, mnemosyne vibrant,
+          mnemosyne snapshot create|list|restore|diff
+          mnemosyne discover [-n COUNT] [-t TOPIC] [--vibrant]
 """
 
 from __future__ import annotations
@@ -206,6 +210,21 @@ def cmd_timeline(args):
         print()
 
 
+
+def cmd_path(args):
+    archive = MnemosyneArchive(archive_path=args.archive) if args.archive else MnemosyneArchive()
+    path = archive.shortest_path(args.start, args.end)
+    if path is None:
+        print(f"No path found between {args.start} and {args.end}")
+        return
+    steps = archive.path_explanation(path)
+    print(f"Path ({len(steps)} hops):")
+    for i, step in enumerate(steps):
+        arrow = " → " if i > 0 else "   "
+        print(f"{arrow}{step['id']}: {step['title']}")
+        if step['topics']:
+            print(f"     topics: {', '.join(step['topics'])}")
+
 def cmd_consolidate(args):
     archive = MnemosyneArchive()
     merges = archive.consolidate(threshold=args.threshold, dry_run=args.dry_run)
@@ -239,6 +258,144 @@ def cmd_neighbors(args):
         print()
 
 
+def cmd_touch(args):
+    archive = MnemosyneArchive()
+    try:
+        entry = archive.touch(args.entry_id)
+    except KeyError:
+        print(f"Entry not found: {args.entry_id}")
+        sys.exit(1)
+    v = archive.get_vitality(entry.id)
+    print(f"[{entry.id[:8]}] {entry.title}")
+    print(f"  Vitality: {v['vitality']:.4f} (boosted)")
+
+
+def cmd_decay(args):
+    archive = MnemosyneArchive()
+    result = archive.apply_decay()
+    print(f"Applied decay to {result['total_entries']} entries")
+    print(f"  Decayed: {result['decayed_count']}")
+    print(f"  Avg vitality: {result['avg_vitality']:.4f}")
+    print(f"  Fading (<0.3): {result['fading_count']}")
+    print(f"  Vibrant (>0.7): {result['vibrant_count']}")
+
+
+def cmd_vitality(args):
+    archive = MnemosyneArchive()
+    try:
+        v = archive.get_vitality(args.entry_id)
+    except KeyError:
+        print(f"Entry not found: {args.entry_id}")
+        sys.exit(1)
+    print(f"[{v['entry_id'][:8]}] {v['title']}")
+    print(f"  Vitality: {v['vitality']:.4f}")
+    print(f"  Last accessed: {v['last_accessed'] or 'never'}")
+    print(f"  Age: {v['age_days']} days")
+
+
+def cmd_fading(args):
+    archive = MnemosyneArchive()
+    results = archive.fading(limit=args.limit)
+    if not results:
+        print("Archive is empty.")
+        return
+    for v in results:
+        print(f"[{v['entry_id'][:8]}] {v['title']}")
+        print(f"  Vitality: {v['vitality']:.4f} | Age: {v['age_days']}d | Last: {v['last_accessed'] or 'never'}")
+        print()
+
+
+def cmd_snapshot(args):
+    archive = MnemosyneArchive()
+    if args.snapshot_cmd == "create":
+        result = archive.snapshot_create(label=args.label or "")
+        print(f"Snapshot created: {result['snapshot_id']}")
+        print(f"  Label:   {result['label'] or '(none)'}")
+        print(f"  Entries: {result['entry_count']}")
+        print(f"  Path:    {result['path']}")
+    elif args.snapshot_cmd == "list":
+        snapshots = archive.snapshot_list()
+        if not snapshots:
+            print("No snapshots found.")
+            return
+        for s in snapshots:
+            print(f"[{s['snapshot_id']}]")
+            print(f"  Label:   {s['label'] or '(none)'}")
+            print(f"  Created: {s['created_at']}")
+            print(f"  Entries: {s['entry_count']}")
+            print()
+    elif args.snapshot_cmd == "restore":
+        try:
+            result = archive.snapshot_restore(args.snapshot_id)
+        except FileNotFoundError as e:
+            print(str(e))
+            sys.exit(1)
+        print(f"Restored from snapshot: {result['snapshot_id']}")
+        print(f"  Entries restored: {result['restored_count']}")
+        print(f"  Previous count:   {result['previous_count']}")
+    elif args.snapshot_cmd == "diff":
+        try:
+            diff = archive.snapshot_diff(args.snapshot_id)
+        except FileNotFoundError as e:
+            print(str(e))
+            sys.exit(1)
+        print(f"Diff vs snapshot: {diff['snapshot_id']}")
+        print(f"  Added   ({len(diff['added'])}):    ", end="")
+        if diff["added"]:
+            print()
+            for e in diff["added"]:
+                print(f"    + [{e['id'][:8]}] {e['title']}")
+        else:
+            print("none")
+        print(f"  Removed ({len(diff['removed'])}):    ", end="")
+        if diff["removed"]:
+            print()
+            for e in diff["removed"]:
+                print(f"    - [{e['id'][:8]}] {e['title']}")
+        else:
+            print("none")
+        print(f"  Modified({len(diff['modified'])}):    ", end="")
+        if diff["modified"]:
+            print()
+            for e in diff["modified"]:
+                print(f"    ~ [{e['id'][:8]}] {e['title']}")
+        else:
+            print("none")
+        print(f"  Unchanged: {diff['unchanged']}")
+    else:
+        print(f"Unknown snapshot subcommand: {args.snapshot_cmd}")
+        sys.exit(1)
+
+
+def cmd_discover(args):
+    archive = MnemosyneArchive()
+    results = archive.discover(
+        count=args.count,
+        prefer_fading=not args.vibrant,
+        topic=args.topic if args.topic else None,
+    )
+    if not results:
+        print("No entries found." + (" (topic filter too narrow?)" if args.topic else ""))
+        return
+    for r in results:
+        print(f"[{r['entry_id'][:8]}] {r['title']}")
+        print(f"  Topics: {', '.join(r['topics'])} | Vitality: {r['vitality']} | Age: {r['age_days']}d")
+        print(f"  {r['content_preview']}")
+        print()
+
+
+def cmd_vibrant(args):
+    archive = MnemosyneArchive()
+    results = archive.vibrant(limit=args.limit)
+    if not results:
+        print("Archive is empty.")
+        return
+    for v in results:
+        print(f"[{v['entry_id'][:8]}] {v['title']}")
+        print(f"  Vitality: {v['vitality']:.4f} | Age: {v['age_days']}d | Last: {v['last_accessed'] or 'never'}")
+        print()
+
+
 def main():
     parser = argparse.ArgumentParser(prog="mnemosyne", description="The Living Holographic Archive")
     sub = parser.add_subparsers(dest="command")
@@ -300,14 +457,53 @@ def main():
     nb.add_argument("entry_id", help="Anchor entry ID")
     nb.add_argument("--days", type=int, default=7, help="Window in days (default: 7)")
 
+
+    pa = sub.add_parser("path", help="Find shortest path between two memories")
+    pa.add_argument("start", help="Starting entry ID")
+    pa.add_argument("end", help="Target entry ID")
+    pa.add_argument("--archive", default=None, help="Archive path")
+
     co = sub.add_parser("consolidate", help="Merge duplicate/near-duplicate entries")
     co.add_argument("--dry-run", action="store_true", help="Show what would be merged without applying")
     co.add_argument("--threshold", type=float, default=0.9, help="Similarity threshold (default: 0.9)")
 
+
+    tc = sub.add_parser("touch", help="Boost an entry's vitality by accessing it")
+    tc.add_argument("entry_id", help="Entry ID to touch")
+
+    dc = sub.add_parser("decay", help="Apply time-based decay to all entries")
+
+    vy = sub.add_parser("vitality", help="Show an entry's vitality status")
+    vy.add_argument("entry_id", help="Entry ID to check")
+
+    fg = sub.add_parser("fading", help="Show most neglected entries (lowest vitality)")
+    fg.add_argument("-n", "--limit", type=int, default=10, help="Max entries to show")
+
+    vb = sub.add_parser("vibrant", help="Show most alive entries (highest vitality)")
+    vb.add_argument("-n", "--limit", type=int, default=10, help="Max entries to show")
+
+    dc = sub.add_parser("discover", help="Serendipitous entry discovery")
+    dc.add_argument("-n", "--count", type=int, default=5, help="Number of entries to discover")
+    dc.add_argument("-t", "--topic", default=None, help="Filter by topic")
+    dc.add_argument("--vibrant", action="store_true", help="Prefer vibrant (alive) entries over fading ones")
+
+        sn = sub.add_parser("snapshot", help="Point-in-time backup and restore")
+    sn_sub = sn.add_subparsers(dest="snapshot_cmd")
+    sn_create = sn_sub.add_parser("create", help="Create a new snapshot")
+    sn_create.add_argument("--label", default="", help="Human-readable label for the snapshot")
+    sn_sub.add_parser("list", help="List available snapshots")
+    sn_restore = sn_sub.add_parser("restore", help="Restore archive from a snapshot")
+    sn_restore.add_argument("snapshot_id", help="Snapshot ID to restore")
+    sn_diff = sn_sub.add_parser("diff", help="Show what changed since a snapshot")
+    sn_diff.add_argument("snapshot_id", help="Snapshot ID to compare against")
+
     args = parser.parse_args()
     if not args.command:
         parser.print_help()
         sys.exit(1)
+    if args.command == "snapshot" and not args.snapshot_cmd:
+        sn.print_help()
+        sys.exit(1)
 
     dispatch = {
         "stats": cmd_stats,
@@ -327,6 +523,14 @@ def main():
         "timeline": cmd_timeline,
         "neighbors": cmd_neighbors,
         "consolidate": cmd_consolidate,
+        "path": cmd_path,
+        "touch": cmd_touch,
+        "decay": cmd_decay,
+        "vitality": cmd_vitality,
+        "fading": cmd_fading,
+        "vibrant": cmd_vibrant,
+        "snapshot": cmd_snapshot,
+        "discover": cmd_discover,
     }
     dispatch[args.command](args)
 

nexus/mnemosyne/tests/test_cli_commands.py

@@ -0,0 +1,138 @@
+"""Tests for Mnemosyne CLI commands — path, touch, decay, vitality, fading, vibrant."""
+
+import json
+import tempfile
+from pathlib import Path
+from unittest.mock import patch
+import sys
+import io
+
+import pytest
+
+from nexus.mnemosyne.archive import MnemosyneArchive
+from nexus.mnemosyne.entry import ArchiveEntry
+
+
+@pytest.fixture
+def archive(tmp_path):
+    path = tmp_path / "test_archive.json"
+    return MnemosyneArchive(archive_path=path)
+
+
+@pytest.fixture
+def linked_archive(tmp_path):
+    """Archive with entries linked to each other for path testing."""
+    path = tmp_path / "test_archive.json"
+    arch = MnemosyneArchive(archive_path=path, auto_embed=False)
+    e1 = arch.add(ArchiveEntry(title="Alpha", content="first entry about python", topics=["code"]))
+    e2 = arch.add(ArchiveEntry(title="Beta", content="second entry about python coding", topics=["code"]))
+    e3 = arch.add(ArchiveEntry(title="Gamma", content="third entry about cooking recipes", topics=["food"]))
+    return arch, e1, e2, e3
+
+
+class TestPathCommand:
+    def test_shortest_path_exists(self, linked_archive):
+        arch, e1, e2, e3 = linked_archive
+        path = arch.shortest_path(e1.id, e2.id)
+        assert path is not None
+        assert path[0] == e1.id
+        assert path[-1] == e2.id
+
+    def test_shortest_path_no_connection(self, linked_archive):
+        arch, e1, e2, e3 = linked_archive
+        # e3 (cooking) likely not linked to e1 (python coding)
+        path = arch.shortest_path(e1.id, e3.id)
+        # Path may or may not exist depending on linking threshold
+        # Either None or a list is valid
+
+    def test_shortest_path_same_entry(self, linked_archive):
+        arch, e1, _, _ = linked_archive
+        path = arch.shortest_path(e1.id, e1.id)
+        assert path == [e1.id]
+
+    def test_shortest_path_missing_entry(self, linked_archive):
+        arch, e1, _, _ = linked_archive
+        path = arch.shortest_path(e1.id, "nonexistent-id")
+        assert path is None
+
+
+class TestTouchCommand:
+    def test_touch_boosts_vitality(self, archive):
+        entry = archive.add(ArchiveEntry(title="Test", content="Content"))
+        # Simulate time passing by setting old last_accessed
+        old_time = "2020-01-01T00:00:00+00:00"
+        entry.last_accessed = old_time
+        entry.vitality = 0.5
+        archive._save()
+
+        touched = archive.touch(entry.id)
+        assert touched.vitality > 0.5
+        assert touched.last_accessed != old_time
+
+    def test_touch_missing_entry(self, archive):
+        with pytest.raises(KeyError):
+            archive.touch("nonexistent-id")
+
+
+class TestDecayCommand:
+    def test_apply_decay_returns_stats(self, archive):
+        archive.add(ArchiveEntry(title="Test", content="Content"))
+        result = archive.apply_decay()
+        assert result["total_entries"] == 1
+        assert "avg_vitality" in result
+        assert "fading_count" in result
+        assert "vibrant_count" in result
+
+    def test_decay_on_empty_archive(self, archive):
+        result = archive.apply_decay()
+        assert result["total_entries"] == 0
+        assert result["avg_vitality"] == 0.0
+
+
+class TestVitalityCommand:
+    def test_get_vitality(self, archive):
+        entry = archive.add(ArchiveEntry(title="Test", content="Content"))
+        v = archive.get_vitality(entry.id)
+        assert v["entry_id"] == entry.id
+        assert v["title"] == "Test"
+        assert 0.0 <= v["vitality"] <= 1.0
+        assert v["age_days"] >= 0
+
+    def test_get_vitality_missing(self, archive):
+        with pytest.raises(KeyError):
+            archive.get_vitality("nonexistent-id")
+
+
+class TestFadingVibrant:
+    def test_fading_returns_sorted_ascending(self, archive):
+        # Add entries with different vitalities
+        e1 = archive.add(ArchiveEntry(title="Vibrant", content="High energy"))
+        e2 = archive.add(ArchiveEntry(title="Fading", content="Low energy"))
+        e2.vitality = 0.1
+        e2.last_accessed = "2020-01-01T00:00:00+00:00"
+        archive._save()
+
+        results = archive.fading(limit=10)
+        assert len(results) == 2
+        assert results[0]["vitality"] <= results[1]["vitality"]
+
+    def test_vibrant_returns_sorted_descending(self, archive):
+        e1 = archive.add(ArchiveEntry(title="Fresh", content="New"))
+        e2 = archive.add(ArchiveEntry(title="Old", content="Ancient"))
+        e2.vitality = 0.1
+        e2.last_accessed = "2020-01-01T00:00:00+00:00"
+        archive._save()
+
+        results = archive.vibrant(limit=10)
+        assert len(results) == 2
+        assert results[0]["vitality"] >= results[1]["vitality"]
+
+    def test_fading_limit(self, archive):
+        for i in range(15):
+            archive.add(ArchiveEntry(title=f"Entry {i}", content=f"Content {i}"))
+        results = archive.fading(limit=5)
+        assert len(results) == 5
+
+    def test_vibrant_empty(self, archive):
+        results = archive.vibrant()
+        assert results == []

nexus/mnemosyne/tests/test_discover.py

@@ -0,0 +1,85 @@
+"""Tests for Mnemosyne discover functionality."""
+
+import tempfile
+from pathlib import Path
+
+from nexus.mnemosyne.archive import MnemosyneArchive
+from nexus.mnemosyne.ingest import ingest_event
+
+
+def _make_archive_with_entries():
+    """Helper: create an archive with test entries."""
+    path = Path(tempfile.mkdtemp()) / "test_archive.json"
+    archive = MnemosyneArchive(archive_path=path, auto_embed=False)
+    ingest_event(archive, title="Python automation", content="Building tools in Python", topics=["python", "automation"])
+    ingest_event(archive, title="Cooking pasta", content="How to make carbonara", topics=["cooking"])
+    ingest_event(archive, title="Bitcoin basics", content="Understanding Bitcoin and blockchain", topics=["bitcoin", "crypto"])
+    ingest_event(archive, title="AI agents", content="Building autonomous AI agents", topics=["ai", "agents"])
+    ingest_event(archive, title="Meditation guide", content="Mindfulness and meditation techniques", topics=["wellness"])
+    return archive
+
+
+def test_discover_returns_entries():
+    archive = _make_archive_with_entries()
+    results = archive.discover(count=3)
+    assert len(results) == 3
+    for r in results:
+        assert "entry_id" in r
+        assert "title" in r
+        assert "content_preview" in r
+        assert "topics" in r
+        assert "vitality" in r
+        assert "age_days" in r
+
+
+def test_discover_respects_count():
+    archive = _make_archive_with_entries()
+    results = archive.discover(count=2)
+    assert len(results) == 2
+
+
+def test_discover_count_exceeds_entries():
+    archive = _make_archive_with_entries()
+    results = archive.discover(count=100)
+    assert len(results) == archive.count
+
+
+def test_discover_topic_filter():
+    archive = _make_archive_with_entries()
+    results = archive.discover(count=10, topic="python")
+    assert len(results) == 1
+    assert results[0]["title"] == "Python automation"
+
+
+def test_discover_topic_case_insensitive():
+    archive = _make_archive_with_entries()
+    results = archive.discover(count=10, topic="Python")
+    assert len(results) == 1
+
+
+def test_discover_empty_topic_returns_nothing():
+    archive = _make_archive_with_entries()
+    results = archive.discover(count=10, topic="nonexistent")
+    assert len(results) == 0
+
+
+def test_discover_boosts_vitality():
+    archive = _make_archive_with_entries()
+    # Get initial vitality
+    before = archive.fading(limit=5)
+    # Discover (which touches entries)
+    archive.discover(count=3)
+    # The touched entries should have higher vitality now
+    after = archive.fading(limit=5)
+    # At least some entries should have changed vitality
+    before_vitals = {e["entry_id"]: e["vitality"] for e in before}
+    after_vitals = {e["entry_id"]: e["vitality"] for e in after}
+    changed = sum(1 for eid in before_vitals if eid in after_vitals and abs(before_vitals[eid] - after_vitals[eid]) > 0.001)
+    assert changed >= 1, "Discover should touch and boost vitality of selected entries"
+
+
+def test_discover_empty_archive():
+    path = Path(tempfile.mkdtemp()) / "empty.json"
+    archive = MnemosyneArchive(archive_path=path, auto_embed=False)
+    results = archive.discover(count=5)
+    assert len(results) == 0

nexus/mnemosyne/tests/test_path.py

@@ -0,0 +1,106 @@
+"""Tests for MnemosyneArchive.shortest_path and path_explanation."""
+
+from nexus.mnemosyne.archive import MnemosyneArchive
+from nexus.mnemosyne.entry import ArchiveEntry
+
+
+def _make_archive(tmp_path):
+    archive = MnemosyneArchive(str(tmp_path / "test_archive.json"))
+    return archive
+
+
+class TestShortestPath:
+    def test_direct_connection(self, tmp_path):
+        archive = _make_archive(tmp_path)
+        a = archive.add("Alpha", "first entry", topics=["start"])
+        b = archive.add("Beta", "second entry", topics=["end"])
+        # Manually link
+        a.links.append(b.id)
+        b.links.append(a.id)
+        archive._entries[a.id] = a
+        archive._entries[b.id] = b
+        archive._save()
+
+        path = archive.shortest_path(a.id, b.id)
+        assert path == [a.id, b.id]
+
+    def test_multi_hop_path(self, tmp_path):
+        archive = _make_archive(tmp_path)
+        a = archive.add("A", "alpha", topics=["x"])
+        b = archive.add("B", "beta", topics=["y"])
+        c = archive.add("C", "gamma", topics=["z"])
+        # Chain: A -> B -> C
+        a.links.append(b.id)
+        b.links.extend([a.id, c.id])
+        c.links.append(b.id)
+        archive._entries[a.id] = a
+        archive._entries[b.id] = b
+        archive._entries[c.id] = c
+        archive._save()
+
+        path = archive.shortest_path(a.id, c.id)
+        assert path == [a.id, b.id, c.id]
+
+    def test_no_path(self, tmp_path):
+        archive = _make_archive(tmp_path)
+        a = archive.add("A", "isolated", topics=[])
+        b = archive.add("B", "also isolated", topics=[])
+        path = archive.shortest_path(a.id, b.id)
+        assert path is None
+
+    def test_same_entry(self, tmp_path):
+        archive = _make_archive(tmp_path)
+        a = archive.add("A", "lonely", topics=[])
+        path = archive.shortest_path(a.id, a.id)
+        assert path == [a.id]
+
+    def test_nonexistent_entry(self, tmp_path):
+        archive = _make_archive(tmp_path)
+        a = archive.add("A", "exists", topics=[])
+        path = archive.shortest_path("fake-id", a.id)
+        assert path is None
+
+    def test_shortest_of_multiple(self, tmp_path):
+        """When multiple paths exist, BFS returns shortest."""
+        archive = _make_archive(tmp_path)
+        a = archive.add("A", "a", topics=[])
+        b = archive.add("B", "b", topics=[])
+        c = archive.add("C", "c", topics=[])
+        d = archive.add("D", "d", topics=[])
+        # A -> B -> D (short)
+        # A -> C -> B -> D (long)
+        a.links.extend([b.id, c.id])
+        b.links.extend([a.id, d.id, c.id])
+        c.links.extend([a.id, b.id])
+        d.links.append(b.id)
+        for e in [a, b, c, d]:
+            archive._entries[e.id] = e
+        archive._save()
+
+        path = archive.shortest_path(a.id, d.id)
+        assert len(path) == 3  # A -> B -> D, not A -> C -> B -> D
+
+
+class TestPathExplanation:
+    def test_returns_step_details(self, tmp_path):
+        archive = _make_archive(tmp_path)
+        a = archive.add("Alpha", "the beginning", topics=["origin"])
+        b = archive.add("Beta", "the middle", topics=["process"])
+        a.links.append(b.id)
+        b.links.append(a.id)
+        archive._entries[a.id] = a
+        archive._entries[b.id] = b
+        archive._save()
+
+        path = [a.id, b.id]
+        steps = archive.path_explanation(path)
+        assert len(steps) == 2
+        assert steps[0]["title"] == "Alpha"
+        assert steps[1]["title"] == "Beta"
+        assert "origin" in steps[0]["topics"]
+
+    def test_content_preview_truncation(self, tmp_path):
+        archive = _make_archive(tmp_path)
+        a = archive.add("A", "x" * 200, topics=[])
+        steps = archive.path_explanation([a.id])
+        assert len(steps[0]["content_preview"]) <= 123  # 120 + "..."

nexus/mnemosyne/tests/test_snapshots.py

@@ -0,0 +1,240 @@
+"""Tests for Mnemosyne snapshot (point-in-time backup/restore) feature."""
+
+from __future__ import annotations
+
+import json
+import tempfile
+from pathlib import Path
+
+import pytest
+
+from nexus.mnemosyne.archive import MnemosyneArchive
+from nexus.mnemosyne.ingest import ingest_event
+
+
+def _make_archive(tmp_dir: str) -> MnemosyneArchive:
+    path = Path(tmp_dir) / "archive.json"
+    return MnemosyneArchive(archive_path=path, auto_embed=False)
+
+
+# ─── snapshot_create ─────────────────────────────────────────────────────────
+
+def test_snapshot_create_returns_metadata():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="Alpha", content="First entry", topics=["a"])
+        ingest_event(archive, title="Beta", content="Second entry", topics=["b"])
+
+        result = archive.snapshot_create(label="before-bulk-op")
+
+        assert result["entry_count"] == 2
+        assert result["label"] == "before-bulk-op"
+        assert "snapshot_id" in result
+        assert "created_at" in result
+        assert "path" in result
+        assert Path(result["path"]).exists()
+
+
+def test_snapshot_create_no_label():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="Gamma", content="Third entry", topics=[])
+
+        result = archive.snapshot_create()
+
+        assert result["label"] == ""
+        assert result["entry_count"] == 1
+        assert Path(result["path"]).exists()
+
+
+def test_snapshot_file_contains_entries():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        e = ingest_event(archive, title="Delta", content="Fourth entry", topics=["d"])
+        result = archive.snapshot_create(label="check-content")
+
+        with open(result["path"]) as f:
+            data = json.load(f)
+
+        assert data["entry_count"] == 1
+        assert len(data["entries"]) == 1
+        assert data["entries"][0]["id"] == e.id
+        assert data["entries"][0]["title"] == "Delta"
+
+
+def test_snapshot_create_empty_archive():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        result = archive.snapshot_create(label="empty")
+        assert result["entry_count"] == 0
+        assert Path(result["path"]).exists()
+
+
+# ─── snapshot_list ───────────────────────────────────────────────────────────
+
+def test_snapshot_list_empty():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        assert archive.snapshot_list() == []
+
+
+def test_snapshot_list_returns_all():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="One", content="c1", topics=[])
+        archive.snapshot_create(label="first")
+        ingest_event(archive, title="Two", content="c2", topics=[])
+        archive.snapshot_create(label="second")
+
+        snapshots = archive.snapshot_list()
+        assert len(snapshots) == 2
+        labels = {s["label"] for s in snapshots}
+        assert "first" in labels
+        assert "second" in labels
+
+
+def test_snapshot_list_metadata_fields():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        archive.snapshot_create(label="meta-check")
+        snapshots = archive.snapshot_list()
+        s = snapshots[0]
+        for key in ("snapshot_id", "label", "created_at", "entry_count", "path"):
+            assert key in s
+
+
+def test_snapshot_list_newest_first():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        archive.snapshot_create(label="a")
+        archive.snapshot_create(label="b")
+        snapshots = archive.snapshot_list()
+        # Filenames sort lexicographically; newest (b) should be first
+        # (filenames include timestamp so alphabetical = newest-last;
+        # snapshot_list reverses the glob order → newest first)
+        assert len(snapshots) == 2
+        # Both should be present; ordering is newest first
+        ids = [s["snapshot_id"] for s in snapshots]
+        assert ids == sorted(ids, reverse=True)
+
+
+# ─── snapshot_restore ────────────────────────────────────────────────────────
+
+def test_snapshot_restore_replaces_entries():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="Kept", content="original content", topics=["orig"])
+        snap = archive.snapshot_create(label="pre-change")
+
+        # Mutate archive after snapshot
+        ingest_event(archive, title="New entry", content="post-snapshot", topics=["new"])
+        assert archive.count == 2
+
+        result = archive.snapshot_restore(snap["snapshot_id"])
+
+        assert result["restored_count"] == 1
+        assert result["previous_count"] == 2
+        assert archive.count == 1
+        entry = list(archive._entries.values())[0]
+        assert entry.title == "Kept"
+
+
+def test_snapshot_restore_persists_to_disk():
+    with tempfile.TemporaryDirectory() as tmp:
+        path = Path(tmp) / "archive.json"
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="Persisted", content="should survive reload", topics=[])
+        snap = archive.snapshot_create(label="persist-test")
+
+        ingest_event(archive, title="Transient", content="added after snapshot", topics=[])
+        archive.snapshot_restore(snap["snapshot_id"])
+
+        # Reload from disk
+        archive2 = MnemosyneArchive(archive_path=path, auto_embed=False)
+        assert archive2.count == 1
+        assert list(archive2._entries.values())[0].title == "Persisted"
+
+
+def test_snapshot_restore_missing_raises():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        with pytest.raises(FileNotFoundError):
+            archive.snapshot_restore("nonexistent_snapshot_id")
+
+
+# ─── snapshot_diff ───────────────────────────────────────────────────────────
+
+def test_snapshot_diff_no_changes():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="Stable", content="unchanged content", topics=[])
+        snap = archive.snapshot_create(label="baseline")
+
+        diff = archive.snapshot_diff(snap["snapshot_id"])
+
+        assert diff["added"] == []
+        assert diff["removed"] == []
+        assert diff["modified"] == []
+        assert diff["unchanged"] == 1
+
+
+def test_snapshot_diff_detects_added():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="Original", content="existing", topics=[])
+        snap = archive.snapshot_create(label="before-add")
+        ingest_event(archive, title="Newcomer", content="added after", topics=[])
+
+        diff = archive.snapshot_diff(snap["snapshot_id"])
+
+        assert len(diff["added"]) == 1
+        assert diff["added"][0]["title"] == "Newcomer"
+        assert diff["removed"] == []
+        assert diff["unchanged"] == 1
+
+
+def test_snapshot_diff_detects_removed():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        e1 = ingest_event(archive, title="Will Be Removed", content="doomed", topics=[])
+        ingest_event(archive, title="Survivor", content="stays", topics=[])
+        snap = archive.snapshot_create(label="pre-removal")
+        archive.remove(e1.id)
+
+        diff = archive.snapshot_diff(snap["snapshot_id"])
+
+        assert len(diff["removed"]) == 1
+        assert diff["removed"][0]["title"] == "Will Be Removed"
+        assert diff["added"] == []
+        assert diff["unchanged"] == 1
+
+
+def test_snapshot_diff_detects_modified():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        e = ingest_event(archive, title="Mutable", content="original content", topics=[])
+        snap = archive.snapshot_create(label="pre-edit")
+        archive.update_entry(e.id, content="updated content", auto_link=False)
+
+        diff = archive.snapshot_diff(snap["snapshot_id"])
+
+        assert len(diff["modified"]) == 1
+        assert diff["modified"][0]["title"] == "Mutable"
+        assert diff["modified"][0]["snapshot_hash"] != diff["modified"][0]["current_hash"]
+        assert diff["added"] == []
+        assert diff["removed"] == []
+
+
+def test_snapshot_diff_missing_raises():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        with pytest.raises(FileNotFoundError):
+            archive.snapshot_diff("no_such_snapshot")
+
+
+def test_snapshot_diff_includes_snapshot_id():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        snap = archive.snapshot_create(label="id-check")
+        diff = archive.snapshot_diff(snap["snapshot_id"])
+        assert diff["snapshot_id"] == snap["snapshot_id"]