perf(agent): serialize each tool schema once per drift

Author: TheGreatAxios

src/agent/dynamic-tool-runner.test.ts

@@ -441,4 +441,51 @@ describe("agent tool set pin", () => {
       changed: ["web_fetch"],
     });
   });
+
+  test("drift serializes each tool schema once when pinned and never when unpinned", () => {
+    const count = 4;
+    const tools = Array.from({ length: count }, (_, index) =>
+      stringTool(`tool_${index}`, "ok"),
+    );
+    const runner = createDynamicToolRunner(tools);
+    const stringify = JSON.stringify;
+    const serializationsDuring = (body: () => void): number => {
+      let serializations = 0;
+      JSON.stringify = ((
+        value: unknown,
+        replacer?: Parameters<typeof JSON.stringify>[1],
+        space?: Parameters<typeof JSON.stringify>[2],
+      ) => {
+        serializations += 1;
+        return stringify(value, replacer, space);
+      }) as typeof JSON.stringify;
+      try {
+        body();
+      } finally {
+        JSON.stringify = stringify;
+      }
+      return serializations;
+    };
+
+    expect(
+      serializationsDuring(() => {
+        expect(runner.agentToolSetDrift()).toEqual({
+          added: [],
+          removed: [],
+          changed: [],
+        });
+      }),
+    ).toBe(0);
+
+    runner.pinAgentToolSet();
+    expect(
+      serializationsDuring(() => {
+        expect(runner.agentToolSetDrift()).toEqual({
+          added: [],
+          removed: [],
+          changed: [],
+        });
+      }),
+    ).toBe(count);
+  });
 });

src/agent/dynamic-tool-runner.ts

@@ -120,10 +120,21 @@ export function createDynamicToolRunner(
 
   const agentToolSetDrift = (): AgentToolSetDrift => {
     if (pinned === undefined) return { added: [], removed: [], changed: [] };
+    // The pin holds the same definition objects the live set walks on an idle
+    // tool set. Remember the shape by identity for this call so each schema is
+    // serialized once, not again inside the changed filter.
+    const shapes = new WeakMap<ToolDefinition, string>();
+    const shapeOf = (definition: ToolDefinition): string => {
+      const cached = shapes.get(definition);
+      if (cached !== undefined) return cached;
+      const shape = definitionShape(definition);
+      shapes.set(definition, shape);
+      return shape;
+    };
     const pinnedShapes = new Map(
       pinned.definitions.map((definition) => [
         definition.name,
-        definitionShape(definition),
+        shapeOf(definition),
       ]),
     );
     const live = currentDefinitions();
@@ -136,7 +147,7 @@ export function createDynamicToolRunner(
       changed: live
         .filter((definition) => {
           const before = pinnedShapes.get(definition.name);
-          return before !== undefined && before !== definitionShape(definition);
+          return before !== undefined && before !== shapeOf(definition);
         })
         .map((definition) => definition.name),
     };