diff --git a/packages/plugin/src/tool.ts b/packages/plugin/src/tool.ts
index 9c6daa34d04a..4eb9b12d8929 100644
--- a/packages/plugin/src/tool.ts
+++ b/packages/plugin/src/tool.ts
@@ -47,6 +47,10 @@ export function tool<Args extends z.ZodRawShape>(input: {
   args: Args
   execute(args: z.infer<z.ZodObject<Args>>, context: ToolContext): Promise<ToolResult>
 }) {
+  const invalid = Object.entries(input.args).find(
+    (entry) => typeof entry[1] !== "object" || entry[1] === null || !("_zod" in entry[1]),
+  )
+  if (invalid) throw new TypeError(`Invalid tool argument "${invalid[0]}": args must contain Zod schemas`)
   return input
 }
 tool.schema = z
diff --git a/packages/plugin/test/tool.test.ts b/packages/plugin/test/tool.test.ts
new file mode 100644
index 000000000000..c628fe55fc49
--- /dev/null
+++ b/packages/plugin/test/tool.test.ts
@@ -0,0 +1,27 @@
+import { describe, expect, test } from "bun:test"
+import { tool } from "../src/tool"
+
+describe("tool", () => {
+  test("rejects plain-object argument definitions with a clear error", () => {
+    expect(() =>
+      tool({
+        description: "invalid tool",
+        args: {
+          // @ts-expect-error Verify the runtime diagnostic for JavaScript callers.
+          foo: "string",
+        },
+        execute: async () => "ok",
+      }),
+    ).toThrow('Invalid tool argument "foo": args must contain Zod schemas')
+  })
+
+  test("accepts Zod argument definitions", () => {
+    expect(
+      tool({
+        description: "valid tool",
+        args: { foo: tool.schema.string() },
+        execute: async ({ foo }) => foo,
+      }),
+    ).toBeDefined()
+  })
+})