diff --git a/src/agents/__tests__/frameworks/serializer.test.ts b/src/agents/__tests__/frameworks/serializer.test.ts
index 025e77c0..43c8ab6f 100644
--- a/src/agents/__tests__/frameworks/serializer.test.ts
+++ b/src/agents/__tests__/frameworks/serializer.test.ts
@@ -431,4 +431,88 @@ describe("serializeFrameworkAgent", () => {
       expect(workers).toHaveLength(0);
     });
   });
+
+  // ── Google ADK shapes ─────────────────────────────────────
+
+  /** Structurally equivalent to a zod v4 schema (ADK bundles zod v4). */
+  function zodV4Object(
+    shape: Record<string, { type: string; description?: string; optional?: boolean }>,
+  ): Record<string, unknown> {
+    const v4 = (def: Record<string, unknown>, description?: string) => ({
+      _zod: { def },
+      ...(description ? { description } : {}),
+    });
+    const shapeOut: Record<string, unknown> = {};
+    for (const [k, v] of Object.entries(shape)) {
+      const inner = v4({ type: v.type }, v.description);
+      shapeOut[k] = v.optional ? v4({ type: "optional", innerType: inner }) : inner;
+    }
+    return v4({ type: "object", shape: shapeOut });
+  }
+
+  describe("Google ADK AgentTool extraction", () => {
+    it("detects an ADK-shaped AgentTool (plain agent property, no markers)", () => {
+      // Regression: undetected ADK AgentTools degraded into generic tool
+      // entries with no callable — the server scheduled a worker task that
+      // no local worker ever polled, hanging the run forever.
+      const childTool = {
+        name: "search_knowledge_base",
+        description: "Search the KB.",
+        parameters: zodV4Object({ query: { type: "string", description: "The search query" } }),
+        execute: async () => "ok",
+      };
+      const childAgent = {
+        name: "researcher",
+        model: "openai/gpt-4o-mini",
+        instruction: "You research.",
+        tools: [childTool],
+      };
+      const agentTool = { name: "researcher", description: "", agent: childAgent };
+      const mockAgent = {
+        name: "manager",
+        model: "openai/gpt-4o-mini",
+        instruction: "You manage.",
+        tools: [agentTool],
+      };
+
+      const [config, workers] = serializeFrameworkAgent(mockAgent);
+
+      const tools = config.tools as Record<string, unknown>[];
+      expect(tools[0]._type).toBe("AgentTool");
+      expect(tools[0].name).toBe("researcher");
+      expect((tools[0].agent as Record<string, unknown>).name).toBe("researcher");
+      // The child agent's tool is extracted as a registrable worker.
+      expect(workers.some((w) => w.name === "search_knowledge_base" && !!w.func)).toBe(true);
+    });
+  });
+
+  describe("zod v4 schema conversion", () => {
+    it("converts a v4 schema to clean JSON Schema instead of mangled internals", () => {
+      // Regression: zod v4 dropped _def.typeName, so _isZodSchema missed v4
+      // schemas and tools serialized with zod internals as their schema —
+      // the LLM saw a parameterless tool and called it with no arguments.
+      const tool = {
+        name: "skb",
+        description: "Search.",
+        parameters: zodV4Object({
+          query: { type: "string", description: "The search query" },
+          limit: { type: "number", optional: true },
+        }),
+        execute: async () => "ok",
+      };
+      const [, workers] = serializeFrameworkAgent({
+        name: "a",
+        model: "m",
+        instruction: "x",
+        tools: [tool],
+      });
+
+      const schema = workers[0].inputSchema as Record<string, unknown>;
+      expect(schema.type).toBe("object");
+      const props = schema.properties as Record<string, Record<string, unknown>>;
+      expect(props.query).toEqual({ type: "string", description: "The search query" });
+      expect(props.limit).toEqual({ type: "number" });
+      expect(schema.required).toEqual(["query"]);
+    });
+  });
 });
diff --git a/src/agents/frameworks/serializer.ts b/src/agents/frameworks/serializer.ts
index e1739b14..570d6a04 100644
--- a/src/agents/frameworks/serializer.ts
+++ b/src/agents/frameworks/serializer.ts
@@ -276,9 +276,25 @@ function _tryExtractAgentTool(obj: unknown, workers: WorkerInfo[]): Record<strin
   if (typeof obj !== "object" || obj === null) return null;
   const asAny = obj as Record<string, unknown>;
 
-  if (!asAny._is_agent_tool && !asAny._agent_instance) return null;
-
-  const childAgent = asAny._agent_instance;
+  // Two agent-as-tool shapes:
+  //  - OpenAI Agents SDK: marker fields _is_agent_tool / _agent_instance
+  //  - Google ADK AgentTool: a plain `agent` property holding the child
+  //    agent (constructor names are minified, so detect by shape). Without
+  //    this branch an ADK AgentTool degrades into a generic tool entry with
+  //    no callable — the server schedules a worker task that no local
+  //    worker ever polls, and the run hangs forever.
+  const maybeAgent = asAny.agent as Record<string, unknown> | undefined;
+  const adkChild =
+    maybeAgent &&
+    typeof maybeAgent === "object" &&
+    typeof maybeAgent.name === "string" &&
+    ("instruction" in maybeAgent || "model" in maybeAgent)
+      ? maybeAgent
+      : null;
+
+  if (!asAny._is_agent_tool && !asAny._agent_instance && !adkChild) return null;
+
+  const childAgent = asAny._agent_instance ?? adkChild;
   if (!childAgent) return null;
 
   const [childConfig, childWorkers] = serializeFrameworkAgent(childAgent);
@@ -391,19 +407,106 @@ function _getSerializableKeys(obj: object): string[] {
 function _isZodSchema(obj: unknown): boolean {
   if (typeof obj !== "object" || obj === null) return false;
   const asAny = obj as Record<string, unknown>;
-  // Zod schemas have _def property with typeName
-  return (
+  // Zod v3 schemas have _def.typeName; v4 schemas (used by e.g. @google/adk)
+  // dropped typeName and carry a _zod internals object instead. Without the
+  // v4 branch, framework tool schemas fall through to generic property
+  // enumeration and serialize as mangled zod internals — the server then
+  // renders a parameterless tool and the LLM calls it with no arguments.
+  if (
     typeof asAny._def === "object" &&
     asAny._def !== null &&
     typeof (asAny._def as Record<string, unknown>).typeName === "string"
+  ) {
+    return true;
+  }
+  const zodInternals = asAny._zod as Record<string, unknown> | undefined;
+  return (
+    typeof zodInternals === "object" &&
+    zodInternals !== null &&
+    typeof zodInternals.def === "object" &&
+    zodInternals.def !== null
   );
 }
 
+/**
+ * Convert a Zod v4 schema to JSON Schema by walking `_zod.def` structurally.
+ *
+ * The SDK's own zod dependency is v3 (no native z.toJSONSchema), and a v3
+ * converter can't read a v4 schema instance from another package's zod, so
+ * the common shapes are handled here directly.
+ */
+function _zodV4ToJsonSchema(schema: unknown): Record<string, unknown> | null {
+  const internals = (schema as Record<string, unknown> | null)?._zod as
+    | Record<string, unknown>
+    | undefined;
+  const def = internals?.def as Record<string, unknown> | undefined;
+  if (!def || typeof def.type !== "string") return null;
+
+  const description = (schema as Record<string, unknown>).description;
+  const withDesc = (out: Record<string, unknown>): Record<string, unknown> =>
+    typeof description === "string" && description.length > 0
+      ? { ...out, description }
+      : out;
+
+  switch (def.type) {
+    case "object": {
+      const shape = (def.shape ?? {}) as Record<string, unknown>;
+      const properties: Record<string, unknown> = {};
+      const required: string[] = [];
+      for (const [key, child] of Object.entries(shape)) {
+        const childDef = (child as Record<string, unknown>)?._zod as
+          | Record<string, unknown>
+          | undefined;
+        const childType = (childDef?.def as Record<string, unknown> | undefined)?.type;
+        const childSchema = _zodV4ToJsonSchema(child);
+        if (childSchema) properties[key] = childSchema;
+        if (childType !== "optional" && childType !== "default") required.push(key);
+      }
+      const out: Record<string, unknown> = { type: "object", properties };
+      if (required.length > 0) out.required = required;
+      return withDesc(out);
+    }
+    case "optional":
+    case "default":
+    case "nullable": {
+      const inner = _zodV4ToJsonSchema(def.innerType);
+      return inner ? withDesc(inner) : null;
+    }
+    case "array": {
+      const items = _zodV4ToJsonSchema(def.element) ?? {};
+      return withDesc({ type: "array", items });
+    }
+    case "enum": {
+      const entries = def.entries as Record<string, unknown> | undefined;
+      return withDesc({ type: "string", enum: entries ? Object.values(entries) : [] });
+    }
+    case "literal": {
+      const values = def.values as unknown[] | undefined;
+      return withDesc({ enum: values ?? [] });
+    }
+    case "string":
+    case "number":
+    case "boolean":
+      return withDesc({ type: def.type });
+    case "int":
+      return withDesc({ type: "integer" });
+    default:
+      // Unknown v4 type — emit a permissive schema rather than zod internals.
+      return withDesc({});
+  }
+}
+
 /**
  * Convert a Zod schema to JSON Schema.
  * Uses zod-to-json-schema if available, falls back to null.
  */
 function _zodToJsonSchema(zodSchema: unknown): Record<string, unknown> | null {
+  // Zod v4 instance (has _zod internals) — the v3 zod-to-json-schema library
+  // can't read these, so walk the def structure directly.
+  const asAny = zodSchema as Record<string, unknown> | null;
+  if (asAny && typeof asAny._zod === "object" && asAny._zod !== null) {
+    return _zodV4ToJsonSchema(zodSchema);
+  }
   try {
     // Try dynamic import of zod-to-json-schema