diff --git a/gateway/platforms/slack.py b/gateway/platforms/slack.py
index 8685b92ed..26184b7eb 100644
--- a/gateway/platforms/slack.py
+++ b/gateway/platforms/slack.py
@@ -100,6 +100,7 @@ class SlackAdapter(BasePlatformAdapter):
         # events, and they carry the user/thread identity needed for stable
         # session + memory scoping.
         self._assistant_threads: Dict[Tuple[str, str], Dict[str, str]] = {}
+        self._ASSISTANT_THREADS_MAX = 5000
 
     async def connect(self) -> bool:
         """Connect to Slack via Socket Mode."""
@@ -826,6 +827,12 @@ class SlackAdapter(BasePlatformAdapter):
         merged.update({k: v for k, v in metadata.items() if v})
         self._assistant_threads[key] = merged
 
+        # Evict oldest entries when the cache exceeds the limit
+        if len(self._assistant_threads) > self._ASSISTANT_THREADS_MAX:
+            excess = len(self._assistant_threads) - self._ASSISTANT_THREADS_MAX // 2
+            for old_key in list(self._assistant_threads)[:excess]:
+                del self._assistant_threads[old_key]
+
         team_id = merged.get("team_id", "")
         if team_id and channel_id:
             self._channel_team[channel_id] = team_id
diff --git a/tests/gateway/test_slack.py b/tests/gateway/test_slack.py
index 0bad0abe5..67c7cce1d 100644
--- a/tests/gateway/test_slack.py
+++ b/tests/gateway/test_slack.py
@@ -927,6 +927,28 @@ class TestAssistantThreadLifecycle:
         assert msg_event.source.thread_id == "171.000"
         assert msg_event.source.user_name == "Tyler"
 
+    def test_assistant_threads_cache_eviction(self, assistant_adapter):
+        """Cache should evict oldest entries when exceeding the size limit."""
+        assistant_adapter._ASSISTANT_THREADS_MAX = 10
+        # Fill to the limit
+        for i in range(10):
+            assistant_adapter._cache_assistant_thread_metadata({
+                "channel_id": f"D{i}",
+                "thread_ts": f"{i}.000",
+                "user_id": f"U{i}",
+            })
+        assert len(assistant_adapter._assistant_threads) == 10
+
+        # Adding one more should trigger eviction (down to max // 2 = 5)
+        assistant_adapter._cache_assistant_thread_metadata({
+            "channel_id": "D999",
+            "thread_ts": "999.000",
+            "user_id": "U999",
+        })
+        assert len(assistant_adapter._assistant_threads) <= 10
+        # The newest entry must survive eviction
+        assert ("D999", "999.000") in assistant_adapter._assistant_threads
+
 
 # ---------------------------------------------------------------------------
 # TestUserNameResolution