diff --git a/src/agents/sessions/tools/tool-definition-wrapper.test.ts b/src/agents/sessions/tools/tool-definition-wrapper.test.ts
new file mode 100644
index 000000000000..8d0e1bed42ff
--- /dev/null
+++ b/src/agents/sessions/tools/tool-definition-wrapper.test.ts
@@ -0,0 +1,57 @@
+import { describe, expect, it, vi } from "vitest";
+import type { AgentTool } from "../../runtime/index.js";
+import type { ToolDefinition } from "../extensions/types.js";
+import {
+  createToolDefinitionFromAgentTool,
+  wrapToolDefinition,
+} from "./tool-definition-wrapper.js";
+
+describe("tool definition wrapper", () => {
+  it("preserves an unreadable extension tool definition schema without crashing the wrapper", async () => {
+    const execute = vi.fn(async () => ({ content: [] }));
+    const prepareArguments = vi.fn((params: unknown) => params);
+    const definition = {
+      name: "hostile_definition",
+      label: "Hostile Definition",
+      description: "throws while reading parameters",
+      prepareArguments,
+      execute,
+    } as unknown as ToolDefinition;
+    Object.defineProperty(definition, "parameters", {
+      get() {
+        throw new Error("definition parameters exploded");
+      },
+    });
+
+    const tool = wrapToolDefinition(definition, () => ({}) as never);
+
+    expect(() => tool.parameters).toThrow("definition parameters exploded");
+    expect(tool.prepareArguments).toBe(prepareArguments);
+    await tool.execute("call-1", {}, undefined, undefined);
+    expect(execute).toHaveBeenCalledTimes(1);
+  });
+
+  it("preserves an unreadable agent tool schema without crashing definition synthesis", async () => {
+    const execute = vi.fn(async () => ({ content: [] }));
+    const prepareArguments = vi.fn((params: unknown) => params);
+    const tool = {
+      name: "hostile_agent_tool",
+      label: "Hostile Agent Tool",
+      description: "throws while reading parameters",
+      prepareArguments,
+      execute,
+    } as unknown as AgentTool;
+    Object.defineProperty(tool, "parameters", {
+      get() {
+        throw new Error("agent parameters exploded");
+      },
+    });
+
+    const definition = createToolDefinitionFromAgentTool(tool);
+
+    expect(() => definition.parameters).toThrow("agent parameters exploded");
+    expect(definition.prepareArguments).toBe(prepareArguments);
+    await definition.execute("call-1", {}, undefined, undefined, {} as never);
+    expect(execute).toHaveBeenCalledTimes(1);
+  });
+});
diff --git a/src/agents/sessions/tools/tool-definition-wrapper.ts b/src/agents/sessions/tools/tool-definition-wrapper.ts
index ad5f02670ebd..00e0309cce6e 100644
--- a/src/agents/sessions/tools/tool-definition-wrapper.ts
+++ b/src/agents/sessions/tools/tool-definition-wrapper.ts
@@ -2,6 +2,19 @@ import type { TSchema } from "typebox";
 import type { AgentTool } from "../../runtime/index.js";
 import type { ExtensionContext, ToolDefinition } from "../extensions/types.js";
 
+function defineForwardedParameters<TParams extends TSchema>(
+  target: { parameters?: TParams },
+  source: { parameters: TParams },
+): void {
+  Object.defineProperty(target, "parameters", {
+    enumerable: true,
+    configurable: true,
+    get() {
+      return source.parameters;
+    },
+  });
+}
+
 /** Wrap a ToolDefinition into an AgentTool for the core runtime. */
 export function wrapToolDefinition<
   TParams extends TSchema = TSchema,
@@ -11,16 +24,17 @@ export function wrapToolDefinition<
   definition: ToolDefinition<TParams, TDetails, TState>,
   ctxFactory?: () => ExtensionContext,
 ): AgentTool<TParams, TDetails> {
-  return {
+  const tool = {
     name: definition.name,
     label: definition.label,
     description: definition.description,
-    parameters: definition.parameters,
     prepareArguments: definition.prepareArguments,
     executionMode: definition.executionMode,
     execute: (toolCallId, params, signal, onUpdate) =>
       definition.execute(toolCallId, params, signal, onUpdate, ctxFactory?.() as ExtensionContext),
-  };
+  } as AgentTool<TParams, TDetails>;
+  defineForwardedParameters(tool, definition);
+  return tool;
 }
 
 /** Wrap multiple ToolDefinitions into AgentTools for the core runtime. */
@@ -38,14 +52,15 @@ export function wrapToolDefinitions(
  * provides plain AgentTool overrides that do not include prompt metadata or renderers.
  */
 export function createToolDefinitionFromAgentTool(tool: AgentTool): ToolDefinition {
-  return {
+  const definition = {
     name: tool.name,
     label: tool.label,
     description: tool.description,
-    parameters: tool.parameters,
     prepareArguments: tool.prepareArguments,
     executionMode: tool.executionMode,
     execute: async (toolCallId, params, signal, onUpdate) =>
       tool.execute(toolCallId, params, signal, onUpdate),
-  };
+  } as ToolDefinition;
+  defineForwardedParameters(definition, tool);
+  return definition;
 }