Compose Preview Catalogs

ui_builder_render_native

Compile a design and render it with real Compose on this host, rather than in the browser's Wasm canvas — the way to see what a design looks like on Android. Returns the first frame, the token the live frame stream is opened with, and the design node ids the render is tagged with, so catalog_get_preview_data can report each node's bounds and a client can put selectable regions over the image. This host has a native render lane. The reply is not an McpResponseEnvelopeV1: the released contract defines no request type for a native render.

Input Schema

TableJSON Schema

Name	Required	Description
`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.
`designId`	Yes
`revision`	No	A past revision. Omit for the current 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.

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

A4.1/5.0

Behavior4/5

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

With no annotations, the description carries the load well: it discloses a host precondition (native render lane required), an auth path (handling tokens you cannot set as headers), and a significant protocol anomaly (the reply is not an McpResponseEnvelopeV1). It does not discuss failure modes, resource cost of compiling, or whether the token must be pre-approved, so it is strong but not exhaustive.

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

Conciseness4/5

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

Front-loaded with the core action and the native-vs-Wasm distinction, and each sentence carries new information (return shape, downstream consumer, protocol caveat). The long em-dash clauses make it denser than necessary but nothing is padding.

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?

For a 5-param tool with no annotations but an output schema, the description supplies the missing behavioral context: auth token provenance, host precondition, and the fact that the response is not an envelope. Given the output schema covers return values, this is near-complete, with only error/edge behavior left unstated.

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 80%, so the schema already documents token, revision, agentName, and agentModel. The description largely restates the token concept and otherwise adds no syntax, defaults, or format guidance for parameters, so the baseline 3 applies.

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?

States a specific verb+resource (compile a design and render it) and pins the distinguishing axis against siblings: native Compose on the host rather than the browser's Wasm canvas. The Android framing and the mention of the native render lane let an agent tell this apart from catalog_render_preview or ui_builder_render_design_matrix without opening a schema.

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?

Clearly states the condition that selects this tool (you want to see the design as it appears on Android, via the host's native lane, not the Wasm canvas) and names catalog_get_preview_data as a downstream consumer for node bounds. It stops short of explicitly naming the render siblings to avoid, so it is not a full when/when-not routing statement.

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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