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WebMCP: Implement tool title
Per webmachinelearning/webmcp#152, this CL implements the `title` property for registered tools, consistent with MCP's tool definition [1]. This is done by adding a new `title` property to the `ModelContextTool` dictionary for imperative tools, and a new `tooltitle` attribute for declarative tools. A subsequent spec PR will be made to spec the `getTools()` API and the reflect the fact that `RegisteredTool#title` is always non-undefined, and is the empty string when not provided on registration time, per some discussion we had about the right API shape. [1]: https://modelcontextprotocol.io/specification/2025-11-25/server/tools#tool R=masonf Bug: 489045948 Change-Id: If848ba85ba1b4aeaa39834eca11ba91108e493c0 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7742044 Commit-Queue: Dominic Farolino <dom@chromium.org> Reviewed-by: Mason Freed <masonf@chromium.org> Cr-Commit-Position: refs/heads/main@{#1636305}
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‎webmcp/declarative/toolchange-on-attribute-mutation.https.html‎

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<!DOCTYPE html>
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<meta charset="utf-8">
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<title>WebMCP: toolchange event and getTools() update on form attribute mutation</title>
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<link rel="author" href="mailto:dom@chromium.org">
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<script src="/resources/testharness.js"></script>
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<script src="/resources/testharnessreport.js"></script>
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<script src="../resources/helpers.js"></script>
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<body>
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<form id="f1" toolname="my_tool" tooltitle="My Title" tooldescription="desc">
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<input id="input1" type="text" name="input_name">
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</form>
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<script>
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function waitForToolChange() {
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return new Promise(resolve => {
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navigator.modelContext.addEventListener('toolchange', resolve, { once: true });
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});
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}
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promise_test(async t => {
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const form = document.getElementById('f1');
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// Wait for initial registration to complete.
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await waitForTool('my_tool');
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let tools = await navigator.modelContext.getTools();
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assert_equals(tools.length, 1, "Initially registered tool");
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assert_equals(tools[0].name, 'my_tool');
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assert_equals(tools[0].title, 'My Title');
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assert_equals(tools[0].description, 'desc');
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// Test updating tooltitle attribute
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{
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const changePromise = waitForToolChange();
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form.setAttribute('tooltitle', 'New Title');
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await changePromise;
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let tools = await navigator.modelContext.getTools();
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assert_equals(tools.length, 1, "Still one tool");
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assert_equals(tools[0].title, 'New Title', "Title is updated");
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}
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// Test updating tooldescription attribute
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{
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const changePromise = waitForToolChange();
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form.setAttribute('tooldescription', 'New Description');
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await changePromise;
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let tools = await navigator.modelContext.getTools();
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assert_equals(tools.length, 1);
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assert_equals(tools[0].description, 'New Description', "Description is updated");
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}
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// Test updating toolname attribute
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{
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const changePromise = waitForToolChange();
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form.setAttribute('toolname', 'new_tool_name');
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await changePromise;
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let tools = await navigator.modelContext.getTools();
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assert_equals(tools.length, 1);
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assert_equals(tools[0].name, 'new_tool_name', "Name is updated");
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}
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// Test removing tooltitle attribute (should fall back to empty string)
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{
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const changePromise = waitForToolChange();
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form.removeAttribute('tooltitle');
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await changePromise;
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let tools = await navigator.modelContext.getTools();
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assert_equals(tools.length, 1);
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assert_equals(tools[0].title, '', "Title should fall back to empty string");
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}
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// Test removing tooldescription attribute (should unregister the tool since it's required)
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{
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const changePromise = waitForToolChange();
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form.removeAttribute('tooldescription');
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await changePromise;
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let tools = await navigator.modelContext.getTools();
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assert_equals(tools.length, 0, "Tool should be unregistered when description is removed");
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}
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// Restore the description attribute to re-register the tool
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{
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const changePromise = waitForToolChange();
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form.setAttribute('tooldescription', 'Restored Description');
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await changePromise;
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let tools = await navigator.modelContext.getTools();
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assert_equals(tools.length, 1);
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assert_equals(tools[0].name, 'new_tool_name');
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assert_equals(tools[0].description, 'Restored Description');
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}
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// Test removing toolname attribute (should unregister the tool since it's required)
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{
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const changePromise = waitForToolChange();
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form.removeAttribute('toolname');
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await changePromise;
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let tools = await navigator.modelContext.getTools();
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assert_equals(tools.length, 0, "Tool should be unregistered when name is removed");
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}
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}, "Test that mutating or removing form attributes (toolname, tooldescription, tooltitle) fires toolchange and updates getTools() correctly");
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</script>
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</body>

‎webmcp/imperative/register-tool-title.https.html‎

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<!DOCTYPE html>
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<html>
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<head>
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<title>Tool title is exposed correctly</title>
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<meta name="timeout" content="long">
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<link rel="author" href="mailto:dom@chromium.org">
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<script src="/resources/testharness.js"></script>
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<script src="/resources/testharnessreport.js"></script>
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<script src="/common/get-host-info.sub.js"></script>
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<script src="resources/helpers.js"></script>
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</head>
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<body>
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<script>
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promise_test(async () => {
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navigator.modelContext.registerTool({
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name: 'with-title',
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title: 'My Custom Title',
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description: 'tool with title',
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execute: async () => 'empty'
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});
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const tools = await navigator.modelContext.getTools();
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const tool = tools.find(t => t.name === 'with-title');
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assert_equals(tool.title, 'My Custom Title', 'Title is preserved');
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}, 'Tool title is preserved when provided');
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promise_test(async () => {
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navigator.modelContext.registerTool({
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name: 'without-title',
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description: 'tool without title',
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execute: async () => 'empty'
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});
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const tools = await navigator.modelContext.getTools();
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const tool = tools.find(t => t.name === 'without-title');
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// If not provided upon registration, `title` should be empty string in the
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// dictionary.
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assert_equals(tool.title, '', 'Title is empty string when not provided');
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}, 'Missing tool title in registration is exposed as empty string');
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promise_test(async () => {
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// \uD800 is an unpaired high surrogate.
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navigator.modelContext.registerTool({
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name: 'with-unpaired-surrogate',
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title: 'Title with \uD800 unpaired surrogate',
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description: 'tool with unpaired surrogate in title',
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execute: async () => 'empty'
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});
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const tools = await navigator.modelContext.getTools();
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const tool = tools.find(t => t.name === 'with-unpaired-surrogate');
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// USVString replaces unpaired surrogates with U+FFFD.
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assert_equals(tool.title, 'Title with \uFFFD unpaired surrogate');
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}, 'Tool title with unpaired surrogate is fixed up by USVString');
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</script>
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</body>
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</html>

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