diff --git a/packages/core/tests/unit/builder.test.ts b/packages/core/tests/unit/builder.test.ts
index 3e904222..5b87c8b1 100644
--- a/packages/core/tests/unit/builder.test.ts
+++ b/packages/core/tests/unit/builder.test.ts
@@ -1,6 +1,7 @@
 import { describe, it, expect, expectTypeOf } from "vitest";
 import { z } from "zod";
 import { tool } from "../../src/index";
+import { isToolBuilder, ensureBuilt } from "../../src/builder/ensure-built";
 import type { ToolDef } from "../../src/types/tools";
 
 describe("Tool Builder", () => {
@@ -230,3 +231,54 @@ describe("Tool Builder", () => {
     );
   });
 });
+
+describe("isToolBuilder", () => {
+  it("returns true for an unbuilt tool builder", () => {
+    const builder = tool("check")
+      .describe("Check")
+      .input(z.object({}))
+      .handle(async () => ({}));
+    expect(isToolBuilder(builder)).toBe(true);
+  });
+
+  it("returns false for a built ToolDef", () => {
+    const def = tool("check")
+      .describe("Check")
+      .input(z.object({}))
+      .handle(async () => ({}))
+      .build();
+    expect(isToolBuilder(def)).toBe(false);
+  });
+
+  it("returns false for primitives", () => {
+    expect(isToolBuilder(null)).toBe(false);
+    expect(isToolBuilder(undefined)).toBe(false);
+    expect(isToolBuilder(42)).toBe(false);
+    expect(isToolBuilder("string")).toBe(false);
+    expect(isToolBuilder({})).toBe(false);
+  });
+});
+
+describe("ensureBuilt", () => {
+  it("builds an unbuilt builder and returns a ToolDef", () => {
+    const builder = tool("myTool")
+      .describe("Greet")
+      .input(z.object({}))
+      .handle(async () => ({}));
+    const def = ensureBuilt(builder, "greetTool");
+    expect(def.description).toBe("Greet");
+    expect(typeof def.handler).toBe("function");
+    expect(def.input).toBeInstanceOf(z.ZodObject);
+  });
+
+  it("returns an already-built ToolDef unchanged (same reference)", () => {
+    const def = tool("existing")
+      .describe("Existing")
+      .input(z.object({}))
+      .handle(async () => ({}))
+      .build();
+    const result = ensureBuilt(def, "shouldBeIgnored");
+    expect(result).toBe(def);
+    expect(result.description).toBe("Existing");
+  });
+});