Compose Preview Catalogs

ui_builder_diff_designs

Compare two designs, or two retained revisions of one: nodes added, removed and moved, and properties, modifiers and other fields changed, each with its node id and path (root/slot[index]/node), plus changed document fields (title, environment, state). Works across a design and its fork. b defaults to the current revision of a's design. Values are the protocol's own JSON, ready to send back through ui_builder_apply. Read access to both.

Input Schema

TableJSON Schema

Name	Required	Description
`a`	Yes
`b`	No
`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.
`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
`a`	Yes
`b`	Yes
`nodes`	No
`counts`	Yes
`schema`	Yes
`summary`	Yes
`document`	No
`identical`	Yes
`truncated`	No

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"
+}

AddedOct 1, 2026

TDQS

A4.3/5.0

Behavior4/5

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

With no annotations, the description carries the burden and does well: it declares 'Read access to both', explains that output values are protocol JSON ready to be sent back through ui_builder_apply, and notes `b` defaults to `a`'s current revision. It omits auth/permission requirements and any limits, but the read-only and output-shape disclosures are strong.

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?

Dense but front-loaded: the comparison scope and diff contents come first, with the default and output-consumption notes trailing. Each sentence carries information, though the run-on enumeration of diffed fields is slightly heavy.

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 spelled out, yet the description still conveys the diff's structure and how to feed results back. For a nested-object, 5-param read tool with no annotations, the coverage of scope, defaults, and read semantics is nearly complete.

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

Parameters4/5

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

Schema coverage is 60%, and the description adds meaning beyond the schema by clarifying that `b` defaults to the current revision of `a`'s design and by describing the node id/path format produced for each change. The token/agentName/agentModel parameters are covered by the schema itself, so this is adequate.

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 (compare/diff) and resource (two designs or two retained revisions), and enumerates exactly what the diff reports (nodes added/removed/moved, changed properties, modifiers, document fields). This clearly distinguishes it from siblings like ui_builder_compare_branches and ui_builder_compare_reference.

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?

Explains the comparison modes (two designs, two revisions of one, across a fork) and the default for `b`, giving clear context for when the tool applies. However, it never explicitly excludes or routes against the closely-related compare_branches/compare_reference siblings, leaving that inference to the agent.

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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