diff --git a/docs/installation.md b/docs/installation.md
index fd4d74ed..07f05535 100644
--- a/docs/installation.md
+++ b/docs/installation.md
@@ -329,6 +329,8 @@ The MCP server indexes each requested content selection on first use and caches
 claude mcp add semble -s user -- uvx --from "semble[mcp]" semble --content all
 ```
 
+Indexes stay in memory for the lifetime of the server. To free memory while idle, set `SEMBLE_MCP_IDLE_TIMEOUT` to a number of seconds; indexes unused for that long are dropped from memory and reloaded from the disk cache on the next search.
+
 ### Instructions (AGENTS.md / CLAUDE.md)
 
 Add the snippet below to your `AGENTS.md` or `CLAUDE.md` so your agent knows when and how to call the semble CLI:
diff --git a/src/semble/mcp.py b/src/semble/mcp.py
index f03f7435..13e35704 100644
--- a/src/semble/mcp.py
+++ b/src/semble/mcp.py
@@ -3,6 +3,7 @@
 import asyncio
 import json
 import logging
+import os
 import time
 from collections import OrderedDict
 from collections.abc import Sequence
@@ -27,6 +28,9 @@
 )
 
 _CACHE_MAX_SIZE = 10  # Max number of cached indexes to keep in memory
+_CACHE_IDLE_TIMEOUT = float(
+    os.environ.get("SEMBLE_MCP_IDLE_TIMEOUT", 0)
+)  # Idle seconds before dropping an index (0 = never)
 _MIN_REVALIDATE_FACTOR = 3  # Don't recheck staleness sooner than this many times the last build's duration
 ContentSelection = Literal["code", "docs", "config", "all"]
 _CacheKey = tuple[str, tuple[ContentType, ...]]
@@ -198,6 +202,7 @@ class _IndexCache:
 
     def __init__(self) -> None:
         """Initialise an empty cache."""
+        self._idle_timers: dict[_CacheKey, asyncio.TimerHandle] = {}
         self._model_path: str | None = None
         self._model_error: BaseException | None = None
         self._model_ready = asyncio.Event()
@@ -263,6 +268,13 @@ def evict(self, cache_key: _CacheKey) -> None:
         """Evict one exact index variant from memory."""
         self._tasks.pop(cache_key, None)
         self._revalidate_after.pop(cache_key, None)
+        if (timer := self._idle_timers.pop(cache_key, None)) is not None:
+            timer.cancel()
+
+    def _evict_idle(self, cache_key: _CacheKey) -> None:
+        """Drop an entry that has not been accessed within the idle timeout; the disk cache is kept."""
+        self.evict(cache_key)
+        self._merged = None  # may hold a reference to the evicted index
 
     async def _evict_if_stale(self, cache_key: _CacheKey) -> None:
         """Evict a cached local-path entry whose on-disk cache no longer matches its files.
@@ -305,13 +317,12 @@ async def get(
             # Re-check after the await: another caller may have populated the entry.
             if cache_key not in self._tasks:
                 if len(self._tasks) >= _CACHE_MAX_SIZE:
-                    evicted_key, _ = self._tasks.popitem(last=False)
-                    self._revalidate_after.pop(evicted_key, None)
+                    self.evict(next(iter(self._tasks)))
                 self._tasks[cache_key] = asyncio.create_task(self._build_tracked(source, ref, model_path, cache_key))
         self._tasks.move_to_end(cache_key)
         task = self._tasks[cache_key]
         try:
-            return await asyncio.shield(task)
+            index = await asyncio.shield(task)
         except asyncio.CancelledError:  # pragma: no cover
             if task.done():
                 self.evict(cache_key)
@@ -321,3 +332,11 @@ async def get(
             if self._tasks.get(cache_key) is task:
                 self.evict(cache_key)
             raise
+        # Start the idle timer only once the index is ready, so slow builds are not evicted mid-build.
+        if _CACHE_IDLE_TIMEOUT > 0 and self._tasks.get(cache_key) is task:
+            if timer := self._idle_timers.get(cache_key):
+                timer.cancel()
+            self._idle_timers[cache_key] = asyncio.get_running_loop().call_later(
+                _CACHE_IDLE_TIMEOUT, self._evict_idle, cache_key
+            )
+        return index
diff --git a/tests/test_mcp.py b/tests/test_mcp.py
index 22acf230..faa81f7d 100644
--- a/tests/test_mcp.py
+++ b/tests/test_mcp.py
@@ -523,6 +523,33 @@ async def test_index_cache_lru_eviction(cache: _IndexCache, tmp_path: Path) -> N
     assert len(cache._tasks) == _CACHE_MAX_SIZE
 
 
+@pytest.mark.anyio
+async def test_index_cache_idle_timeout_eviction(cache: _IndexCache, tmp_path: Path) -> None:
+    """Entries are dropped once unused for the idle timeout, which starts after the build and resets on access."""
+
+    def slow_build(*args: Any, **kwargs: Any) -> MagicMock:
+        time.sleep(0.15)  # longer than the timeout
+        return MagicMock()
+
+    key = cache._compute_cache_key(str(tmp_path))
+    with (
+        patch("semble.mcp._CACHE_IDLE_TIMEOUT", 0.1),
+        patch("semble.mcp.SembleIndex.from_path", side_effect=slow_build),
+        patch("semble.mcp.get_validated_cache", return_value=MagicMock()),
+    ):
+        await cache.get(str(tmp_path))
+        assert key in cache._tasks
+        first_timer = cache._idle_timers[key]
+        cache._merged = ([], MagicMock())
+        await cache.get(str(tmp_path))
+        assert first_timer.cancelled()
+        assert key in cache._tasks
+        await asyncio.sleep(0.3)
+    assert key not in cache._tasks
+    assert key not in cache._idle_timers
+    assert cache._merged is None
+
+
 def test_cache_evict(cache: _IndexCache, tmp_path: Path) -> None:
     """evict() removes an existing exact cache entry."""
     key = cache._compute_cache_key(str(tmp_path))