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Codify MCP tool name requirements and add title #152

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6835f7a
Codify MCP tool name requirements and add title
DavidMulder0 Mar 23, 2026
843d9cb
Update index.bs
DavidMulder0 Mar 23, 2026
b63a48e
Codify MCP tool name requirements and add title
Mar 23, 2026
297f15c
Make title optional
Mar 26, 2026
d7254ba
Add note to title that it should be localized to the user's language …
Mar 26, 2026
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28 changes: 27 additions & 1 deletion index.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -118,6 +118,15 @@ A <dfn>tool definition</dfn> is a [=struct=] with the following [=struct/items=]
:: a [=string=] uniquely identifying a tool registered within a [=model context=]'s [=model
context/tool map=]; it is the same as the [=map/key=] identifying this object.

The [=tool definition/name=] SHOULD:
* Have a [=string/length=] between 1 and 128, inclusive.
* Be considered case-sensitive.
* Only consist of [=ASCII alphanumeric=] [=code points=], U+005F LOW LINE (_), U+002D HYPHEN-MINUS (-), and U+002E FULL STOP (.).

: <dfn>title</dfn>
:: A [=string=] representing a human-readable title of the tool to be used in the user interface
to represent the tool. If not provided, the [=tool definition/name=] is used as the default display value.

: <dfn>description</dfn>
:: a [=string=].

Expand Down Expand Up @@ -206,12 +215,18 @@ The <dfn method for=ModelContext>registerTool(<var>tool</var>)</dfn> method step

1. Let |tool name| be |tool|'s {{ModelContextTool/name}}.

1. Let |tool title| be |tool|'s {{ModelContextTool/title}}.

1. If |tool map|[|tool name|] [=map/exists=], then [=exception/throw=] an {{InvalidStateError}}
{{DOMException}}.

1. If either |tool name| or {{ModelContextTool/description}} is the empty string, then
1. If |tool name| or {{ModelContextTool/description}} is an empty string, then
[=exception/throw=] an {{InvalidStateError}} {{DOMException}}.

1. If |tool name|'s [=string/length=] is greater than 128, or if |tool name| contains a
[=code point=] that is not an [=ASCII alphanumeric=], U+005F (_), U+002D (-), or U+002E (.),
then [=exception/throw=] an {{InvalidStateError}}.

1. Let |stringified input schema| be the empty string.

1. If |tool|'s {{ModelContextTool/inputSchema}} [=map/exists=], then set |stringified input schema|
Expand Down Expand Up @@ -240,6 +255,9 @@ The <dfn method for=ModelContext>registerTool(<var>tool</var>)</dfn> method step
: [=tool definition/name=]
:: |tool name|

: [=tool definition/title=]
:: |tool title|

: [=tool definition/description=]
:: |tool|'s {{ModelContextTool/description}}

Expand Down Expand Up @@ -275,6 +293,7 @@ The {{ModelContextTool}} dictionary describes a tool that can be invoked by [=ag
<xmp class="idl">
dictionary ModelContextTool {
required DOMString name;
DOMString title;
required DOMString description;
object inputSchema;
required ToolExecuteCallback execute;
Expand All @@ -294,6 +313,13 @@ callback ToolExecuteCallback = Promise<any> (object input, ModelContextClient cl
<p>A unique identifier for the tool. This is used by [=agents=] to reference the tool when making tool calls.
</dd>

<dt><code><var ignore>tool</var>["{{ModelContextTool/title}}"]</code></dt>
<dd>
<p>A label for the tool. This is used by the user agent to reference the tool in the user interface.
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@beaufortfrancois beaufortfrancois Mar 25, 2026

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Regarding internationalization: should we add a note advising developers to localize the title based on the user's language settings?

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@domfarolino domfarolino Mar 25, 2026

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Does MCP encourage something similar?

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@beaufortfrancois beaufortfrancois Mar 25, 2026

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There is no explicit rule in the spec that says "titles MUST be localized." BUT since those are not passed to the LLM but only for display purposes to the user, it'd make sense to provide them in the browser user language. Thoughts?

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@DavidMulder0 DavidMulder0 Mar 26, 2026

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For now added a line to the non-normative ModelContextTool Dictionary section that states 'It is recommended that this string be localized to the user’s language.'

<p>It is recommended that this string be localized to the user's
{{NavigatorLanguage/language}}.
</dd>

<dt><code><var ignore>tool</var>["{{ModelContextTool/description}}"]</code></dt>
<dd>
<p>A natural language description of the tool's functionality. This helps [=agents=] understand when and how to use the tool.
Expand Down