diff --git a/agent/model_metadata.py b/agent/model_metadata.py
index 3626e8db0c9..b60ab1eaedc 100644
--- a/agent/model_metadata.py
+++ b/agent/model_metadata.py
@@ -2445,7 +2445,13 @@ def estimate_request_tokens_rough(
 
 # NOTE: tool schemas can be large. Avoid repeated `str(tools)` conversions,
 # which are CPU-heavy and can stall GUI event loops under GIL pressure.
+#
+# Keyed by ``id(tools)``. A long-lived gateway/desktop backend builds many
+# transient tool lists over its lifetime, so the cache is bounded and evicts
+# oldest-first (insertion-ordered dict) once it exceeds the cap. The cap is
+# generous relative to how rarely toolsets are rebuilt within a process.
 _TOOLS_TOKENS_CACHE: dict[int, Tuple[int, str, str, int]] = {}
+_TOOLS_TOKENS_CACHE_MAX = 256
 
 
 def _tool_name_for_cache(tool: Any) -> str:
@@ -2505,5 +2511,10 @@ def _estimate_tools_tokens_rough(tools: List[Dict[str, Any]]) -> int:
             total_chars += len(str(params))
 
     tokens = (total_chars + 3) // 4
+    # Bound the cache: drop the oldest entry when the cap is exceeded so a
+    # long-running process can't accumulate an unbounded number of stale
+    # ``id(tools)`` entries (id values are recycled after GC anyway).
+    if len(_TOOLS_TOKENS_CACHE) >= _TOOLS_TOKENS_CACHE_MAX:
+        _TOOLS_TOKENS_CACHE.pop(next(iter(_TOOLS_TOKENS_CACHE)), None)
     _TOOLS_TOKENS_CACHE[key] = (n, first, last, tokens)
     return tokens
diff --git a/tests/agent/test_model_metadata.py b/tests/agent/test_model_metadata.py
index 47a95575555..ecaf3909ea2 100644
--- a/tests/agent/test_model_metadata.py
+++ b/tests/agent/test_model_metadata.py
@@ -142,6 +142,32 @@ class TestEstimateRequestTokensRough:
             estimate_request_tokens_rough(messages, system_prompt="x" * 8, tools=tools)
             assert dumps.call_count == 1
 
+    def test_tools_cache_is_bounded(self):
+        # A long-lived process builds many transient tool lists; the cache must
+        # not grow without bound. Feed more distinct lists than the cap and
+        # confirm the cache never exceeds it.
+        import agent.model_metadata as mm
+
+        mm._TOOLS_TOKENS_CACHE.clear()
+        cap = mm._TOOLS_TOKENS_CACHE_MAX
+        # Keep references so ids are not recycled mid-loop, forcing distinct keys.
+        held = []
+        for i in range(cap + 50):
+            tools = [
+                {
+                    "type": "function",
+                    "function": {
+                        "name": f"tool_{i}",
+                        "description": "d",
+                        "parameters": {"type": "object"},
+                    },
+                }
+            ]
+            held.append(tools)
+            mm._estimate_tools_tokens_rough(tools)
+            assert len(mm._TOOLS_TOKENS_CACHE) <= cap
+        assert len(mm._TOOLS_TOKENS_CACHE) == cap
+
 
 # =========================================================================
 # Default context lengths