Compose Preview Catalogs

ui_builder_post_comment

Say something on a design — a reply into threadId, or a new thread when it is omitted. Pin a new thread with markId (a stroke on the reference overlay), nodeId (a node in the design), or x/y in frame fractions, so the person reading it can see what you meant. The comment is attributed to your own grant; you cannot post as somebody else.

Input Schema

TableJSON Schema

Name	Required	Description
`x`	No	Pin to a point on the frame, 0..1 across.
`y`	No	Pin to a point on the frame, 0..1 down.
`body`	Yes
`token`	No	A grant token from poll_access, when you cannot set the X-Compose-Preview-Token header yourself — an MCP client fixes its headers at connect time, so this is how a token approved during this session is used in it. Prefer the header where you control it.
`markId`	No	Pin to a markup stroke on the reference.
`nodeId`	No	Pin to a design node.
`designId`	Yes
`threadId`	No	Reply into this thread. Omit to start one.
`agentName`	No	Optional agent/client name shown to design viewers; self-reported.
`agentModel`	No	Optional current model name shown to design viewers; self-reported. Omit when unknown.
`displayName`	No	The name to show beside your actor id.

Output Schema

TableJSON Schema

Name	Required	Description	Default
No arguments

Schema Changelog

Changes observed during successful MCP inspections.

Changed2 schema fields changedOct 4, 2026

addedInput schema / properties / agentModel

Added value: +{
+  "description": "Optional current model name shown to design viewers; self-reported. Omit when unknown.",
+  "maxLength": 80,
+  "type": "string"
+}

addedInput schema / properties / agentName

Added value: +{
+  "description": "Optional agent/client name shown to design viewers; self-reported.",
+  "maxLength": 80,
+  "type": "string"
+}

Changed1 schema field changedSep 30, 2026
- changedOutput schema / (root)
```
Previous value: -nullNew value: +{
+  "type": "object"
+}
```
First observedSep 30, 2026

TDQS

A4/5.0

Behavior3/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations provided, the description carries the full behavioral burden. It usefully discloses an attribution constraint ('attributed to your own grant; you cannot post as somebody else'), but says nothing about required permissions, whether the comment is immediately visible, or failure behavior for an 11-parameter mutation tool.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Three sentences, front-loaded with the core action before pinning options and attribution. Every sentence carries information an agent needs, with no filler.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

An output schema exists, so return values need not be described. The description covers both posting modes, all pinning strategies, and attribution, which is adequate for correct invocation; only permission/error context is missing for a mutation tool with no annotations.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 82%, so the baseline is 3. The description reinforces the meaning of threadId, markId, nodeId, and x/y (frame fractions) and gives a rationale for pinning, but largely restates what the schema already documents and adds no syntax beyond it.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a concrete verb+resource (posting a comment on a design) and immediately distinguishes its two modes: a reply into `threadId` versus a new thread when it is omitted. This clearly separates it from read/management siblings like ui_builder_list_comments, ui_builder_react_to_comment, and ui_builder_resolve_comment_thread.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives explicit conditions for the two invocation modes (supply `threadId` to reply, omit it to start a thread) and explains the three ways to pin a new thread (markId, nodeId, or x/y). It stops short of naming alternative tools or stating when not to use this one, so it is clear context without exclusions.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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