diff --git a/src/agent/director.ts b/src/agent/director.ts
index b0dc8dcd9..6943c4172 100644
--- a/src/agent/director.ts
+++ b/src/agent/director.ts
@@ -475,6 +475,7 @@ class ChatDirectorImpl extends DefaultDirector {
   private readonly askOperatorCalls = new Set<string>();
   private readonly _systemPrompt: string;
   private _toolDefinitions: ToolDefinition[];
+  private _toolSetDigest: string | undefined;
   private inactivityTimeoutMs: number | undefined;
   private totalTimeoutMs: number | undefined;
   // Host-owned live object, attached via setWorkflowCoordinator — never a
@@ -699,9 +700,10 @@ class ChatDirectorImpl extends DefaultDirector {
   }
 
   updateToolDefinitions(toolDefinitions: ToolDefinition[]): void {
-    const before = toolSetDigest(this._toolDefinitions);
+    const before = this._toolSetDigest ?? toolSetDigest(this._toolDefinitions);
     const after = toolSetDigest(toolDefinitions);
     this._toolDefinitions = toolDefinitions;
+    this._toolSetDigest = after;
     if (before === after) return;
     logger.debug`tool-set-changed count=${String(this._toolDefinitions.length)} before=${before} after=${after}`;
   }
diff --git a/src/agent/tool-set-digest.test.ts b/src/agent/tool-set-digest.test.ts
new file mode 100644
index 000000000..cf616c4fa
--- /dev/null
+++ b/src/agent/tool-set-digest.test.ts
@@ -0,0 +1,47 @@
+import { describe, expect, test } from "bun:test";
+import { withMockedModule } from "../../testkit/mock-module.js";
+
+let hashCalls = 0;
+await withMockedModule<typeof import("node:crypto")>(
+  import.meta.resolve("node:crypto"),
+  (real) => ({
+    ...real,
+    createHash: (algorithm: string) => {
+      hashCalls += 1;
+      return real.createHash(algorithm);
+    },
+  }),
+);
+
+const { createChatDirector } = await import("./director.js");
+
+const base = {
+  name: "read_file",
+  description: "read a file",
+  inputSchema: { type: "object" },
+};
+
+describe("toolSetDigest memoization", () => {
+  test("hashes each installed tool set once per director", () => {
+    const director = createChatDirector("system", [], {});
+    const tools = [{ ...base }];
+    const changed = [{ ...base, description: "changed" }];
+
+    director.updateToolDefinitions(tools);
+    hashCalls = 0;
+    director.updateToolDefinitions(changed);
+    expect(hashCalls).toBe(1);
+  });
+
+  test("recomputes when a new tool set arrives", () => {
+    const director = createChatDirector("system", [], {});
+    const tools = [{ ...base }];
+    const changed = [{ ...base, description: "changed" }];
+
+    director.updateToolDefinitions(tools);
+    director.updateToolDefinitions(changed);
+    hashCalls = 0;
+    director.updateToolDefinitions([{ ...base }]);
+    expect(hashCalls).toBe(1);
+  });
+});