ui-diff-mcp

start_ui_diff_run

Start a background UI diff run by providing expected and actual image paths. Poll the returned runId to monitor progress, suited for large audits.

Instructions

Starts a UI diff run in the background without holding the MCP request open. Returns a runId to poll with get_ui_diff_run_status. Use this for large or slow audits like Calorix full scans.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "expectedImagePath": "<expectedImagePath>",
      "actualImagePath": "<actualImagePath>"
    },
    "name": "start_ui_diff_run"
  }
}

Input Schema

Name	Required	Default
`mode`	No	free
`label`	No
`projectRoot`	No
`resumeRunId`	No
`actualImagePath`	Yes
`expectedImagePath`	Yes

Output Schema

Name	Required	Description	Default
`runId`	Yes
`status`	Yes
`message`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

First observedv0.1.0Jun 30, 2026

TDQS

A3.6/5.0

Behavior3/5

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

With no annotations provided, the description must disclose all behavioral traits. It correctly states the tool runs in the background and is non-blocking, but omits details such as authentication needs, rate limits, or side effects like creating persistent state.

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?

The description is extremely concise at two sentences, with each sentence providing distinct value: the first states the action and key behavior, the second gives a usage suggestion. No wasted words.

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

Completeness3/5

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

Given the tool's complexity (6 parameters, async) and the presence of an output schema, the description covers the async nature and return value adequately. However, it lacks explanation of parameters, required vs optional, and how this tool differs from synchronous siblings like compare_ui_images.

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

Parameters1/5

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

Schema description coverage is 0%, and the description does not mention any parameters or their roles. The required paths (expectedImagePath, actualImagePath) are not explained, nor is the mode enum or other optional fields. This provides no assistance to the agent beyond the schema itself.

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 clearly states the tool starts a UI diff run asynchronously, returns a runId, and distinguishes from sibling tools like get_ui_diff_run_status and compare_ui_images by emphasizing background execution for large/slow audits.

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?

The description explicitly recommends use for large/slow audits and mentions the workflow of polling with get_ui_diff_run_status. However, it does not explicitly state when not to use this tool or mention alternatives like compare_ui_images for smaller diffs.

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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