diff --git a/webmcp/declarative/executeTool-abort.https.html b/webmcp/declarative/executeTool-abort.https.html
index b5b6bb4c55669c..4fcdc6dc1f29bd 100644
--- a/webmcp/declarative/executeTool-abort.https.html
+++ b/webmcp/declarative/executeTool-abort.https.html
@@ -39,7 +39,7 @@
   const tool = tools.find(t => t.name === 'submit_test');
 
   const controller = new AbortController();
-  const executionPromise = document.modelContext.executeTool(tool, '{}', { signal: controller.signal });
+  const executionPromise = document.modelContext.executeTool(tool, {}, { signal: controller.signal });
 
   await toolActivatedPromise;
 
diff --git a/webmcp/declarative/executeTool-respondWith-circular-object.https.html b/webmcp/declarative/executeTool-respondWith-circular-object.https.html
index a7dddbf5305f32..2f6fda2bdfa7d6 100644
--- a/webmcp/declarative/executeTool-respondWith-circular-object.https.html
+++ b/webmcp/declarative/executeTool-respondWith-circular-object.https.html
@@ -27,7 +27,7 @@
   await promise_rejects_dom(
     t,
     'UnknownError',
-    document.modelContext.executeTool(tool, '{}'),
+    document.modelContext.executeTool(tool),
     'executeTool() should reject with UnknownError when respondWith is passed a circular object'
   );
 }, 'Declarative tool executeTool() rejects when respondWith() receives a circular object');
diff --git a/webmcp/declarative/execute_tool_change_event.https.html b/webmcp/declarative/execute_tool_change_event.https.html
index 3cec95b69a3f77..b332cc9242a15e 100644
--- a/webmcp/declarative/execute_tool_change_event.https.html
+++ b/webmcp/declarative/execute_tool_change_event.https.html
@@ -37,16 +37,16 @@
     }
   }
 
-  let inputArgs = JSON.stringify({
+  let inputObject = {
     text: "regular text",
     number: 42,
     checkbox: true,
     radio: "radio2",
     textarea: "some text area content",
     select: "opt2"
-  });
+  };
 
-  await document.modelContext.executeTool(tool, inputArgs);
+  await document.modelContext.executeTool(tool, inputObject);
 
   assert_equals(document.getElementById('myinput').value, 'regular text');
   assert_true(events['myinput'].inputFired, 'text input event should fire');
diff --git a/webmcp/declarative/execute_tool_submit_from_js.https.html b/webmcp/declarative/execute_tool_submit_from_js.https.html
index d626c061859c8d..fa6cb031ccc769 100644
--- a/webmcp/declarative/execute_tool_submit_from_js.https.html
+++ b/webmcp/declarative/execute_tool_submit_from_js.https.html
@@ -24,7 +24,7 @@
   });
 
   // Call executeTool. If it throws, the test fails automatically.
-  await document.modelContext.executeTool(tool, '{}');
+  await document.modelContext.executeTool(tool);
 
   assert_true(submitFired, 'submit event should have fired');
 }, 'executeTool resolves successfully when submit handler calls form.submit()');
diff --git a/webmcp/declarative/form_removal_submit_crash.https.html b/webmcp/declarative/form_removal_submit_crash.https.html
index d6e705782e06b3..4d4b0980d22574 100644
--- a/webmcp/declarative/form_removal_submit_crash.https.html
+++ b/webmcp/declarative/form_removal_submit_crash.https.html
@@ -36,7 +36,7 @@
   // Execute the tool. This asynchronously fires the `submit` event at the form.
   // The tool execution should resolve successfully even if the form is removed
   // from the document during submission.
-  const promise = document.modelContext.executeTool(tool, '{}');
+  const promise = document.modelContext.executeTool(tool);
   await promise;
 
   assert_true(submitFired);
diff --git a/webmcp/declarative/opaque-origin-tools.https.html b/webmcp/declarative/opaque-origin-tools.https.html
index 491a4cd1acf224..ffed048e6006ee 100644
--- a/webmcp/declarative/opaque-origin-tools.https.html
+++ b/webmcp/declarative/opaque-origin-tools.https.html
@@ -26,7 +26,7 @@
   await promise_rejects_dom(
     t,
     'NotSupportedError',
-    document.modelContext.executeTool(tool, '{"param1":"value"}'),
+    document.modelContext.executeTool(tool, {"param1":"value"}),
     'executeTool() must reject with NotSupportedError in opaque origin documents'
   );
 }, 'An opaque origin document can register but not execute its own declarative tools');
@@ -36,7 +36,7 @@
   const tool = tools.find(t => t.name === 'opaque_tool');
 
   let events = [];
-  const p = document.modelContext.executeTool(tool, '{}');
+  const p = document.modelContext.executeTool(tool);
 
   // Since `executeTool()` synchronously rejects the Promise for opaque origins,
   // the microtask to run the catch reaction is already queued before
@@ -49,7 +49,7 @@
   await promise_rejects_dom(
     t,
     'NotSupportedError',
-    document.modelContext.executeTool(tool, '{"param1":"value"}'),
+    document.modelContext.executeTool(tool, {"param1":"value"}),
     'executeTool() must reject with NotSupportedError in opaque origin documents'
   );
 
diff --git a/webmcp/declarative/select-multiple-events.https.html b/webmcp/declarative/select-multiple-events.https.html
index 613aba8cff53a7..0164174760fe68 100644
--- a/webmcp/declarative/select-multiple-events.https.html
+++ b/webmcp/declarative/select-multiple-events.https.html
@@ -37,10 +37,10 @@
     changeEventCount++;
   });
 
-  let inputArgs = JSON.stringify({ fruits: ['apple', 'banana'] });
+  let inputObject = { fruits: ['apple', 'banana'] };
 
   // This should dispatch events.
-  await document.modelContext.executeTool(tool, inputArgs);
+  await document.modelContext.executeTool(tool, inputObject);
 
   assert_true(fruits.options[0].selected);
   assert_false(fruits.options[1].selected);
@@ -50,7 +50,7 @@
   assert_equals(changeEventCount, 1);
 
   // Executing with the same arguments again does not trigger events.
-  await document.modelContext.executeTool(tool, inputArgs);
+  await document.modelContext.executeTool(tool, inputObject);
 
   assert_equals(inputEventCount, 1);
   assert_equals(changeEventCount, 1);
diff --git a/webmcp/declarative/unregister-during-executeTool.https.html b/webmcp/declarative/unregister-during-executeTool.https.html
index ff662f8980a90b..3ed0444ce15a0d 100644
--- a/webmcp/declarative/unregister-during-executeTool.https.html
+++ b/webmcp/declarative/unregister-during-executeTool.https.html
@@ -33,7 +33,7 @@
   const tool = tools.find(t => t.name === 'endless_tool');
   assert_true(!!tool, 'Tool should be registered');
 
-  const execute_promise = document.modelContext.executeTool(tool, '{}');
+  const execute_promise = document.modelContext.executeTool(tool);
 
   // Wait deterministically for the form to submit and call respondWith().
   await submitted_promise;
@@ -79,7 +79,7 @@
   const tool = tools.find(t => t.name === 'endless_tool_attr');
   assert_true(!!tool, 'Tool should be registered');
 
-  const execute_promise = document.modelContext.executeTool(tool, '{}');
+  const execute_promise = document.modelContext.executeTool(tool);
 
   // Wait deterministically for the form to submit and call respondWith().
   await submitted_promise;
diff --git a/webmcp/imperative/cancel-reentrancy-crash.https.html b/webmcp/imperative/cancel-reentrancy-crash.https.html
index 7aac08f4befef6..82da4f25ca84c7 100644
--- a/webmcp/imperative/cancel-reentrancy-crash.https.html
+++ b/webmcp/imperative/cancel-reentrancy-crash.https.html
@@ -44,8 +44,8 @@
   const regA = tools.find(t => t.name === 'toolA');
   const regB = tools.find(t => t.name === 'toolB');
 
-  document.modelContext.executeTool(regA, '{}', { signal: controllerA.signal });
-  document.modelContext.executeTool(regB, '{}');
+  document.modelContext.executeTool(regA, {}, { signal: controllerA.signal });
+  document.modelContext.executeTool(regB);
 
   // Wait for both tool execute callbacks to run and return, ensuring they are
   // inserted into pending_executions_.
diff --git a/webmcp/imperative/detached-frame-executeTool.https.html b/webmcp/imperative/detached-frame-executeTool.https.html
index 6ae14e9fc9e31b..872ba33428f7ce 100644
--- a/webmcp/imperative/detached-frame-executeTool.https.html
+++ b/webmcp/imperative/detached-frame-executeTool.https.html
@@ -40,7 +40,7 @@
   iframe.remove();
 
   // Calling executeTool() should reject with InvalidStateError.
-  await promise_rejects_dom(t, 'InvalidStateError', iDOMException, iframe_modelContext.executeTool(tool, '{}'));
+  await promise_rejects_dom(t, 'InvalidStateError', iDOMException, iframe_modelContext.executeTool(tool));
 }, 'executeTool() throws `InvalidStateError` in detached frame');
 </script>
 </body>
diff --git a/webmcp/imperative/executeTool-abort.https.html b/webmcp/imperative/executeTool-abort.https.html
index c18b523adce64a..43d9f0bba0ede5 100644
--- a/webmcp/imperative/executeTool-abort.https.html
+++ b/webmcp/imperative/executeTool-abort.https.html
@@ -23,7 +23,7 @@
   }, { signal: regController.signal });
 
   const [tool] = await document.modelContext.getTools();
-  const result = await document.modelContext.executeTool(tool, '{}');
+  const result = await document.modelContext.executeTool(tool);
   assert_equals(result, 'success');
   assert_true(received_signal instanceof AbortSignal, 'options.signal should be an AbortSignal instance');
   assert_false(received_signal.aborted, 'signal should not be aborted');
@@ -63,7 +63,7 @@
   const [tool] = await document.modelContext.getTools();
 
   const controller = new AbortController();
-  const executionPromise = document.modelContext.executeTool(tool, '{}', { signal: controller.signal });
+  const executionPromise = document.modelContext.executeTool(tool, {}, { signal: controller.signal });
 
   const { promise: toolCancelPromise, resolve: resolveToolCancel } = Promise.withResolvers();
   window.addEventListener('toolcancel', e => {
@@ -103,7 +103,7 @@
   const [tool] = await document.modelContext.getTools();
   const signal = AbortSignal.abort('already aborted');
   let executionRejectionValue = null;
-  document.modelContext.executeTool(tool, '{}', { signal }).catch(e => executionRejectionValue = e);
+  document.modelContext.executeTool(tool, {}, { signal }).catch(e => executionRejectionValue = e);
 
   // Await a Promise that resolved synchronously, so that we guarantee that in
   // the next microtask, `execute_promise` has already rejected. This asserts
@@ -133,7 +133,7 @@
   const [tool] = await document.modelContext.getTools();
   const controller = new AbortController();
   const executePromise =
-      document.modelContext.executeTool(tool, '{}', { signal: controller.signal });
+      document.modelContext.executeTool(tool, {}, { signal: controller.signal });
 
   // By the time this `abort()` is processed, the tool will have already started
   // running and the abort handler will have already been registered.
@@ -184,8 +184,8 @@
   const c1 = new AbortController();
   const c2 = new AbortController();
 
-  const execPromise1 = document.modelContext.executeTool(tool, '{}', { signal: c1.signal });
-  const execPromise2 = document.modelContext.executeTool(tool, '{}', { signal: c2.signal });
+  const execPromise1 = document.modelContext.executeTool(tool, {}, { signal: c1.signal });
+  const execPromise2 = document.modelContext.executeTool(tool, {}, { signal: c2.signal });
 
   await Promise.all([exec1Started, exec2Started]);
 
diff --git a/webmcp/imperative/executeTool-across-trees.https.html b/webmcp/imperative/executeTool-across-trees.https.html
index a904c5e30bcab5..633e6fe69b5a1e 100644
--- a/webmcp/imperative/executeTool-across-trees.https.html
+++ b/webmcp/imperative/executeTool-across-trees.https.html
@@ -29,7 +29,7 @@
 
   const [tool] = await windowB.document.modelContext.getTools();
   assert_equals(tool.window, windowB);
-  const execute_promise = document.modelContext.executeTool(tool, '{}');
+  const execute_promise = document.modelContext.executeTool(tool);
 
   await promise_rejects_dom(t, 'UnknownError', execute_promise);
 }, 'executeTool() rejects when the tool is hosted in another frame tree');
diff --git a/webmcp/imperative/executeTool-error-window-onerror.https.html b/webmcp/imperative/executeTool-error-window-onerror.https.html
index a29449f2ce3fa7..4bd1282a9c033e 100644
--- a/webmcp/imperative/executeTool-error-window-onerror.https.html
+++ b/webmcp/imperative/executeTool-error-window-onerror.https.html
@@ -33,7 +33,7 @@
 
   // `executeTool()` should reject since the tool execution failed.
   await promise_rejects_dom(t, 'UnknownError',
-      document.modelContext.executeTool(tool, '{}'),
+      document.modelContext.executeTool(tool),
       'executeTool() rejects when the tool execution throws');
 
   assert_false(errorFired, 'window.onerror/error event should not be fired');
@@ -66,7 +66,7 @@
 
   // `executeTool()` should reject since serializing the return value throws a TypeError.
   await promise_rejects_dom(t, 'UnknownError',
-      document.modelContext.executeTool(tool, '{}'),
+      document.modelContext.executeTool(tool),
       'executeTool() rejects when the tool execution returns a circular object');
 
   assert_false(errorFired, 'window.onerror/error event should not be fired');
diff --git a/webmcp/imperative/executeTool-invalid-dictionary.https.html b/webmcp/imperative/executeTool-invalid-dictionary.https.html
index dc43a9426e0d58..713d0af3158dd4 100644
--- a/webmcp/imperative/executeTool-invalid-dictionary.https.html
+++ b/webmcp/imperative/executeTool-invalid-dictionary.https.html
@@ -31,7 +31,7 @@
   await promise_rejects_js(
     t,
     TypeError,
-    document.modelContext.executeTool(invalidTool, '{}'),
+    document.modelContext.executeTool(invalidTool),
     "executeTool() must reject with TypeError if 'origin' member is omitted"
   );
 }, "executeTool() rejects with TypeError when the required 'origin' dictionary member is omitted");
@@ -47,13 +47,32 @@
   const tool = tools.find(t => t.name === "json-test-tool");
   assert_true(!!tool, "Legitimate tool must be successfully discovered");
 
-  await promise_rejects_dom(
+  const circularObject = {};
+  circularObject.self = circularObject;
+  await promise_rejects_js(
     t,
-    "UnknownError",
-    document.modelContext.executeTool(tool, "invalid-json"),
-    "executeTool() must reject with UnknownError for invalid JSON input"
+    TypeError,
+    document.modelContext.executeTool(tool, circularObject),
+    "executeTool() must reject with TypeError for circular JSON input"
+  );
+
+  const undefinedToJSON = {
+    toJSON: () => undefined
+  };
+  await promise_rejects_js(
+    t,
+    TypeError,
+    document.modelContext.executeTool(tool, undefinedToJSON),
+    "executeTool() must reject with TypeError when toJSON returns undefined"
+  );
+
+  await promise_rejects_js(
+    t,
+    TypeError,
+    document.modelContext.executeTool(tool, BigInt(42)),
+    "executeTool() must reject with TypeError for non-serializable types JSON input"
   );
-}, "executeTool() rejects with UnknownError for invalid JSON arguments");
+}, "executeTool() rejects with TypeError for invalid JSON arguments");
 
 promise_test(async t => {
   const dummyTool = {
@@ -66,7 +85,7 @@
   await promise_rejects_dom(
     t,
     "UnknownError",
-    document.modelContext.executeTool(dummyTool, "{}"),
+    document.modelContext.executeTool(dummyTool),
     "executeTool() must reject with UnknownError for non-existent tool object"
   );
 }, "executeTool() rejects with UnknownError when a non-existent tool object is supplied");
diff --git a/webmcp/imperative/executeTool-signal-cross-origin.https.html b/webmcp/imperative/executeTool-signal-cross-origin.https.html
index 655ef4b65ab32e..8425731549a566 100644
--- a/webmcp/imperative/executeTool-signal-cross-origin.https.html
+++ b/webmcp/imperative/executeTool-signal-cross-origin.https.html
@@ -60,7 +60,7 @@
   t.add_cleanup(() => window.removeEventListener('message', message_handler));
 
   const controller = new AbortController();
-  const execute_promise = document.modelContext.executeTool(tool, '{}', { signal: controller.signal });
+  const execute_promise = document.modelContext.executeTool(tool, {}, { signal: controller.signal });
 
   await tool_started;
   controller.abort('parent cross-origin abort reason');
diff --git a/webmcp/imperative/executeTool-target-detachment.https.html b/webmcp/imperative/executeTool-target-detachment.https.html
index 9b93fca8fec1b6..a962346eb6643f 100644
--- a/webmcp/imperative/executeTool-target-detachment.https.html
+++ b/webmcp/imperative/executeTool-target-detachment.https.html
@@ -54,7 +54,7 @@
   await promise_rejects_dom(
     t,
     'InvalidStateError',
-    document.modelContext.executeTool(tool, '{}'),
+    document.modelContext.executeTool(tool),
     'Promise should be rejected with InvalidStateError when target frame is detached before execution'
   );
 }, 'executeTool() rejects when target frame is detached before execution');
@@ -95,7 +95,7 @@
   assert_true(!!tool, 'Tool should be retrieved successfully');
 
   // Execute the tool (hangs forever).
-  const execute_promise = document.modelContext.executeTool(tool, '{}');
+  const execute_promise = document.modelContext.executeTool(tool);
 
   iframe.remove();
 
diff --git a/webmcp/imperative/executeTool-target-navigation.https.html b/webmcp/imperative/executeTool-target-navigation.https.html
index bbbc59f9cbe125..e9916447a3e378 100644
--- a/webmcp/imperative/executeTool-target-navigation.https.html
+++ b/webmcp/imperative/executeTool-target-navigation.https.html
@@ -40,7 +40,7 @@
   // Invoke tool and immediately navigate iframe away.
   const tools = await document.modelContext.getTools();
   const tool = tools.find(t => t.name === 'iframe_tool');
-  const promise = document.modelContext.executeTool(tool, '{}');
+  const promise = document.modelContext.executeTool(tool);
   iframe.src = 'about:blank';
 
   // The promise should be rejected because the target document died.
diff --git a/webmcp/imperative/executeTool-unauthorized-origin.https.html b/webmcp/imperative/executeTool-unauthorized-origin.https.html
index 7c07a14f365065..f0f14ebb5ab4b8 100644
--- a/webmcp/imperative/executeTool-unauthorized-origin.https.html
+++ b/webmcp/imperative/executeTool-unauthorized-origin.https.html
@@ -54,7 +54,7 @@
   };
 
   // Parent attempts to execute the unexposed tool directly. This should reject with UnknownError.
-  const promise = document.modelContext.executeTool(fake_tool, '{}');
+  const promise = document.modelContext.executeTool(fake_tool);
   await promise_rejects_dom(t, 'UnknownError', promise, 'executeTool should reject with UnknownError when parent is not authorized to execute the iframe tool');
 }, 'executeTool() rejects when parent frame attempts unauthorized execution of cross-origin iframe tool');
 </script>
diff --git a/webmcp/imperative/executeTool-unregister-resolution-race.https.html b/webmcp/imperative/executeTool-unregister-resolution-race.https.html
index 85619287676c88..32acc2765eeb7e 100644
--- a/webmcp/imperative/executeTool-unregister-resolution-race.https.html
+++ b/webmcp/imperative/executeTool-unregister-resolution-race.https.html
@@ -30,7 +30,7 @@
   }, {signal: ac.signal});
 
   const [tool] = await document.modelContext.getTools();
-  const execute_promise = document.modelContext.executeTool(tool, '{}');
+  const execute_promise = document.modelContext.executeTool(tool);
 
   const result = await execute_promise;
   assert_equals(result, '{"status":"resolved"}');
diff --git a/webmcp/imperative/exposedTo-cross-origin-child.https.html b/webmcp/imperative/exposedTo-cross-origin-child.https.html
index c2fcda8204bc8b..61f4905fba298b 100644
--- a/webmcp/imperative/exposedTo-cross-origin-child.https.html
+++ b/webmcp/imperative/exposedTo-cross-origin-child.https.html
@@ -109,7 +109,7 @@
   }), 'Tool details should match');
 
   // Parent executes iframe's tool.
-  const result = await document.modelContext.executeTool(tool, '{}');
+  const result = await document.modelContext.executeTool(tool);
   assert_equals(result, 'hello from iframe', 'Parent should get correct result from iframe tool');
 
   // Tell the iframe to unregister its tool, and confirm that `toolchange` fires here.
diff --git a/webmcp/imperative/exposedTo-defaults-same-origin.https.html b/webmcp/imperative/exposedTo-defaults-same-origin.https.html
index db91516725cf13..92bb2465a84e3e 100644
--- a/webmcp/imperative/exposedTo-defaults-same-origin.https.html
+++ b/webmcp/imperative/exposedTo-defaults-same-origin.https.html
@@ -143,7 +143,7 @@
   }), 'Tool details should match');
 
   // Parent executes iframe's tool.
-  const result = await document.modelContext.executeTool(tool, '{}');
+  const result = await document.modelContext.executeTool(tool);
   assert_equals(result, 'hello from iframe', 'Parent should get correct result from iframe tool');
 
   // Unregister tool.
@@ -182,7 +182,7 @@
   }), 'Tool details should match');
 
   // Parent executes iframe's tool.
-  const result = await document.modelContext.executeTool(tool, '{}');
+  const result = await document.modelContext.executeTool(tool);
   assert_equals(result, 'hello from iframe', 'Parent should get correct result from iframe tool');
 
   // Unregister tool.
diff --git a/webmcp/imperative/exposedTo-invalid-origins.https.html b/webmcp/imperative/exposedTo-invalid-origins.https.html
index 80927e046c4e32..42235a94897ff3 100644
--- a/webmcp/imperative/exposedTo-invalid-origins.https.html
+++ b/webmcp/imperative/exposedTo-invalid-origins.https.html
@@ -90,7 +90,7 @@
 
   // 6. Execute the cached RegisteredTool.
   // This must execute callback2 and resolve to 'callback2', not callback3.
-  const result = await document.modelContext.executeTool(tool, '{}');
+  const result = await document.modelContext.executeTool(tool);
   assert_equals(result, 'callback2', 'Executing the cached RegisteredTool must run callback2');
 }, 'Aborting a signal from a rejected registration must not unregister a later valid tool with the same name');
 </script>
diff --git a/webmcp/imperative/object-arguments.https.html b/webmcp/imperative/object-arguments.https.html
index 54247f115f49f3..003068ad95710c 100644
--- a/webmcp/imperative/object-arguments.https.html
+++ b/webmcp/imperative/object-arguments.https.html
@@ -19,30 +19,46 @@
   const [registeredTool] = await document.modelContext.getTools();
 
   // Pass an array. Since arrays are ECMAScript objects, this succeeds.
-  const arrayArgs = `[1, 2, 3]`;
+  const arrayArgs = [1, 2, 3];
   const result = await document.modelContext.executeTool(registeredTool, arrayArgs);
   assert_equals(result, "Success");
   assert_array_equals(executeReceivedArgs, [1, 2, 3]);
 
   // Now pass non-Object JSON primitives as the argument, and observe a failure.
-  const stringArgs = `"hello"`;
-  await promise_rejects_dom(t, "UnknownError",
+  const stringArgs = "hello";
+  await promise_rejects_js(t, TypeError,
       document.modelContext.executeTool(registeredTool, stringArgs),
       "Primitive string should be rejected");
 
-  const numberArgs = `123`;
-  await promise_rejects_dom(t, "UnknownError",
+  const numberArgs = 123;
+  await promise_rejects_js(t, TypeError,
       document.modelContext.executeTool(registeredTool, numberArgs),
       "Primitive number should be rejected");
 
-  const booleanArgs = `true`;
-  await promise_rejects_dom(t, "UnknownError",
+  const booleanArgs = true;
+  await promise_rejects_js(t, TypeError,
       document.modelContext.executeTool(registeredTool, booleanArgs),
       "Primitive boolean should be rejected");
 
-  const nullArgs = `null`;
-  await promise_rejects_dom(t, "UnknownError",
+  const nullArgs = null;
+  await promise_rejects_js(t, TypeError,
       document.modelContext.executeTool(registeredTool, nullArgs),
       "null should be rejected");
+
+  const undefinedArgs = null;
+  await promise_rejects_js(t, TypeError,
+      document.modelContext.executeTool(registeredTool, undefinedArgs),
+      "undefined should be rejected");
+
+  // Pass an object whose toJSON() method returns undefined, and observe
+  // a failure.
+  const nullishObject = { toJSON: () => undefined };
+  await promise_rejects_js(t, TypeError,
+      document.modelContext.executeTool(registeredTool, nullishObject),
+      "Object whose toJSON() returns undefined should be rejected");
+  // Omitting input argument defaults to {}
+  const defaultResult = await document.modelContext.executeTool(registeredTool);
+  assert_equals(defaultResult, "Success");
+  assert_object_equals(executeReceivedArgs, {});
 }, "tool execution requires a JSON Object input argument; and Arrays are Objects");
 </script>
diff --git a/webmcp/imperative/opaque-origin-tools.https.html b/webmcp/imperative/opaque-origin-tools.https.html
index 4c5e29b098a06c..549c0cd1d92416 100644
--- a/webmcp/imperative/opaque-origin-tools.https.html
+++ b/webmcp/imperative/opaque-origin-tools.https.html
@@ -26,7 +26,7 @@
   await promise_rejects_dom(
     t,
     'NotSupportedError',
-    document.modelContext.executeTool(tool, '{}'),
+    document.modelContext.executeTool(tool),
     'executeTool() must reject with NotSupportedError in opaque origin documents'
   );
 }, 'An opaque origin document can register but not execute its own tools');
@@ -49,7 +49,7 @@
   assert_equals(tool.origin, "null", "getTools() gives");
 
   let events = [];
-  const p = document.modelContext.executeTool(tool, '{}');
+  const p = document.modelContext.executeTool(tool);
 
   // Since `executeTool()` synchronously rejects the Promise for opaque origins,
   // the microtask to run the catch reaction is already queued before
@@ -62,7 +62,7 @@
   await promise_rejects_dom(
     t,
     'NotSupportedError',
-    document.modelContext.executeTool(tool, '{"param1":"value"}'),
+    document.modelContext.executeTool(tool, {"param1":"value"}),
     'executeTool() must reject with NotSupportedError in opaque origin documents'
   );
 
@@ -81,7 +81,7 @@
     };
 
     let events = [];
-    const p = document.modelContext.executeTool(fake_tool, '{}');
+    const p = document.modelContext.executeTool(fake_tool);
     p.catch(() => events.push('rejection'));
 
     // Queue another microtask immediately after.
@@ -90,7 +90,7 @@
     await promise_rejects_dom(
       t,
       'NotSupportedError',
-      document.modelContext.executeTool(fake_tool, '{"param1":"value"}'));
+      document.modelContext.executeTool(fake_tool, {"param1":"value"}));
 
     assert_array_equals(events, ['rejection', 'microtask'],
       'returned promise is rejected before custom microtask is queued');
diff --git a/webmcp/imperative/register_tool_invalid_json_schema.https.html b/webmcp/imperative/register_tool_invalid_json_schema.https.html
index 5e353248ac3ebc..3182f028315ce8 100644
--- a/webmcp/imperative/register_tool_invalid_json_schema.https.html
+++ b/webmcp/imperative/register_tool_invalid_json_schema.https.html
@@ -8,7 +8,7 @@
 </head>
 <body>
 <script>
-function test_register_tool_schema_error(inputSchema, expectedError, testName) {
+function test_register_tool_schema_error(inputSchema, testName) {
   promise_test((t) => {
     return promise_rejects_js(
       t,
@@ -19,7 +19,7 @@
         inputSchema,
         execute: () => {},
       }),
-      `Should throw ${expectedError.name} for invalid schema`,
+      'Should throw TypeError for invalid schema',
     );
   }, testName);
 }
diff --git a/webmcp/imperative/resources/document-domain-enabled-iframe.html b/webmcp/imperative/resources/document-domain-enabled-iframe.html
index 8df04353c66e67..217a2ba6aec10e 100644
--- a/webmcp/imperative/resources/document-domain-enabled-iframe.html
+++ b/webmcp/imperative/resources/document-domain-enabled-iframe.html
@@ -23,6 +23,6 @@
     window: window,
     origin: window.location.origin,
   };
-  return promise_rejects_dom(t, 'SecurityError', document.modelContext.executeTool(dummyTool, ''));
+  return promise_rejects_dom(t, 'SecurityError', document.modelContext.executeTool(dummyTool));
 }, 'modelContext.executeTool rejects with SecurityError when document.domain is enabled');
 </script>
diff --git a/webmcp/imperative/resources/iframe-caller-and-target-crash.html b/webmcp/imperative/resources/iframe-caller-and-target-crash.html
index 2f52500addd696..5ceaa1a9b1d0a6 100644
--- a/webmcp/imperative/resources/iframe-caller-and-target-crash.html
+++ b/webmcp/imperative/resources/iframe-caller-and-target-crash.html
@@ -11,7 +11,7 @@
   });
   const tools = await document.modelContext.getTools();
   const tool = tools.find(t => t.name === "test_tool");
-  document.modelContext.executeTool(tool, "{}");
+  document.modelContext.executeTool(tool);
 }
 run();
 </script>
diff --git a/webmcp/imperative/resources/iframe-caller.html b/webmcp/imperative/resources/iframe-caller.html
index 602b23d903c636..f44ebfe031f329 100644
--- a/webmcp/imperative/resources/iframe-caller.html
+++ b/webmcp/imperative/resources/iframe-caller.html
@@ -11,7 +11,7 @@
       const options = e.data.options || {};
       document.modelContext.getTools(options).then(tools => {
         const tool = tools.find(t => t.name === e.data.name);
-        document.modelContext.executeTool(tool, '{}').then(result => {
+        document.modelContext.executeTool(tool).then(result => {
           parent.postMessage({action: 'executeResponse', result: result, success: true}, '*');
         }).catch(err => {
           parent.postMessage({action: 'executeResponse', result: String(err), success: false}, '*');
@@ -22,7 +22,7 @@
       const controller = new AbortController();
       document.modelContext.getTools(options).then(tools => {
         const tool = tools.find(t => t.name === e.data.name);
-        document.modelContext.executeTool(tool, '{}', { signal: controller.signal }).then(result => {
+        document.modelContext.executeTool(tool, {}, { signal: controller.signal }).then(result => {
           parent.postMessage({action: 'executeResponse', result: result, success: true}, '*');
         }).catch(err => {
           parent.postMessage({action: 'executeResponse', result: String(err), success: false}, '*');
diff --git a/webmcp/imperative/resources/iframe-register-tool.html b/webmcp/imperative/resources/iframe-register-tool.html
index f977a6d93f8cf2..759712e1f4eccc 100644
--- a/webmcp/imperative/resources/iframe-register-tool.html
+++ b/webmcp/imperative/resources/iframe-register-tool.html
@@ -91,7 +91,7 @@
       });
     } else if (action === 'execute') {
       const name = e.data.name;
-      const args = e.data.args || '{}';
+      const args = e.data.args || {};
       const options = e.data.options || {};
       document.modelContext.getTools(options).then(tools => {
         const tool = tools.find(t => t.name === name);
@@ -108,7 +108,7 @@
         window: window,
         origin: self.origin
       };
-      document.modelContext.executeTool(fake_tool, '{}').then(result => {
+      document.modelContext.executeTool(fake_tool).then(result => {
         parent.postMessage({action: 'executeFakeToolResponse', result: result, success: true}, '*');
       }).catch(err => {
         parent.postMessage({action: 'executeFakeToolResponse', result: String(err), success: false, error_name: err.name}, '*');
diff --git a/webmcp/imperative/unregister-during-executeTool.https.html b/webmcp/imperative/unregister-during-executeTool.https.html
index c23bf779a7e380..5ee502ef6bd010 100644
--- a/webmcp/imperative/unregister-during-executeTool.https.html
+++ b/webmcp/imperative/unregister-during-executeTool.https.html
@@ -53,7 +53,7 @@
 
   // 2. Run the tool, and wait for the iframe to report back that execution has
   // begun.
-  const executePromise = document.modelContext.executeTool(tool, '{}');
+  const executePromise = document.modelContext.executeTool(tool);
   await toolStartedPromise;
 
   // Before we tell the iframe to unregister the tool, set up a handler to
@@ -103,7 +103,7 @@
   }, { signal: regController.signal });
 
   const [tool] = await document.modelContext.getTools();
-  const executePromise = document.modelContext.executeTool(tool, '{}');
+  const executePromise = document.modelContext.executeTool(tool);
 
   await toolStartedPromise;