diff --git a/mempalace/palace_graph.py b/mempalace/palace_graph.py
index 71cad89ec..125ec0d4a 100644
--- a/mempalace/palace_graph.py
+++ b/mempalace/palace_graph.py
@@ -18,6 +18,8 @@
 import hashlib
 import json
 import os
+import threading
+import time
 from collections import Counter, defaultdict
 from datetime import datetime, timezone
 
@@ -25,6 +27,23 @@
 from .palace import get_collection as _get_palace_collection
 from .palace import mine_lock
 
+# Module-level graph cache with TTL and write-invalidation.
+# Warm cache serves build_graph() in O(1); invalidate_graph_cache() clears on writes.
+_graph_cache_lock = threading.Lock()
+_graph_cache_nodes = None
+_graph_cache_edges = None
+_graph_cache_time = 0.0
+_GRAPH_CACHE_TTL = 60.0  # seconds — graph changes less often than metadata
+
+
+def invalidate_graph_cache():
+    """Clear the graph cache. Called from mcp_server.py on writes."""
+    global _graph_cache_nodes, _graph_cache_edges, _graph_cache_time
+    with _graph_cache_lock:
+        _graph_cache_nodes = None
+        _graph_cache_edges = None
+        _graph_cache_time = 0.0
+
 
 def _get_collection(config=None):
     config = config or MempalaceConfig()
@@ -42,10 +61,25 @@ def build_graph(col=None, config=None):
     """
     Build the palace graph from ChromaDB metadata.
 
+    Returns cached result if fresh (within TTL). Cache is invalidated
+    on writes via invalidate_graph_cache(). Thread-safe via _graph_cache_lock.
+
+    Note: warm cache ignores ``col`` and ``config`` arguments — this is
+    intentional for the MCP server's single-palace use case. Callers
+    switching collections should call ``invalidate_graph_cache()`` first.
+
     Returns:
         nodes: dict of {room: {wings: set, halls: set, count: int}}
         edges: list of {room, wing_a, wing_b, hall} — one per tunnel crossing
     """
+    global _graph_cache_nodes, _graph_cache_edges, _graph_cache_time
+    now = time.time()
+    # NOTE: warm cache ignores col/config args — intentional for the MCP server's
+    # single-palace use case. Callers switching collections must invalidate first.
+    with _graph_cache_lock:
+        if _graph_cache_nodes is not None and (now - _graph_cache_time) < _GRAPH_CACHE_TTL:
+            return _graph_cache_nodes, _graph_cache_edges
+
     if col is None:
         col = _get_collection(config)
     if not col:
@@ -101,6 +135,14 @@ def build_graph(col=None, config=None):
             "dates": sorted(data["dates"])[-5:] if data["dates"] else [],
         }
 
+    # Only cache non-empty graphs so new data is picked up immediately
+    # when the palace is first populated.
+    if nodes:
+        with _graph_cache_lock:
+            _graph_cache_nodes = nodes
+            _graph_cache_edges = edges
+            _graph_cache_time = time.time()
+
     return nodes, edges
 
 
diff --git a/tests/test_palace_graph.py b/tests/test_palace_graph.py
index ddda2724e..7bc45e04b 100644
--- a/tests/test_palace_graph.py
+++ b/tests/test_palace_graph.py
@@ -30,6 +30,7 @@ def fake_get(limit=1000, offset=0, include=None):
         build_graph,
         find_tunnels,
         graph_stats,
+        invalidate_graph_cache,
         traverse,
     )
 
@@ -38,6 +39,9 @@ def fake_get(limit=1000, offset=0, include=None):
 
 
 class TestBuildGraph:
+    def setup_method(self):
+        invalidate_graph_cache()
+
     def test_empty_collection(self):
         col = _make_fake_collection([])
         nodes, edges = build_graph(col=col)
@@ -114,11 +118,43 @@ def test_dates_capped_at_five(self):
         nodes, _ = build_graph(col=col)
         assert len(nodes["busy"]["dates"]) <= 5
 
+    def test_cache_returns_same_result(self):
+        """Second call within TTL returns cached nodes without re-scanning.
+
+        The cache intentionally ignores col/config args when warm — this is
+        correct for the MCP server's single-palace use case. Callers that
+        switch collections must call invalidate_graph_cache() first.
+        """
+        col = _make_fake_collection(
+            [{"room": "auth", "wing": "wing_code", "hall": "security", "date": "2026-01-01"}]
+        )
+        nodes1, edges1 = build_graph(col=col)
+        # Second call with a *different* collection — should still return cached result
+        col2 = _make_fake_collection([])
+        nodes2, edges2 = build_graph(col=col2)
+        assert nodes1 == nodes2
+        assert edges1 == edges2
+
+    def test_invalidate_clears_cache(self):
+        """invalidate_graph_cache() forces a fresh scan on next call."""
+        col = _make_fake_collection(
+            [{"room": "auth", "wing": "wing_code", "hall": "security", "date": "2026-01-01"}]
+        )
+        build_graph(col=col)
+        invalidate_graph_cache()
+        col_empty = _make_fake_collection([])
+        nodes, edges = build_graph(col=col_empty)
+        assert nodes == {}
+        assert edges == []
+
 
 # --- traverse ---
 
 
 class TestTraverse:
+    def setup_method(self):
+        invalidate_graph_cache()
+
     def _build_col(self):
         return _make_fake_collection(
             [
@@ -156,6 +192,9 @@ def test_traverse_max_hops(self):
 
 
 class TestFindTunnels:
+    def setup_method(self):
+        invalidate_graph_cache()
+
     def _build_tunnel_col(self):
         return _make_fake_collection(
             [
@@ -192,6 +231,9 @@ def test_find_tunnels_both_wings(self):
 
 
 class TestGraphStats:
+    def setup_method(self):
+        invalidate_graph_cache()
+
     def test_empty_graph(self):
         col = _make_fake_collection([])
         stats = graph_stats(col=col)