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tarn_impact

Identify which .tarn.yaml tests are most likely affected by changes to files, endpoints, or OpenAPI operations, with confidence tiers and run hints.

Instructions

Map a change (files / endpoints / openapi ops / git diff) to the .tarn.yaml tests it most likely affects, with confidence tiers and run hints. Read-only: no HTTP and no test execution. Equivalent to: tarn impact --format json.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
cwdNoAbsolute path to the project root. Defaults to the workspace root captured during MCP `initialize`, or the server process's current directory.
diffNoWhen true, run `git diff --name-only HEAD` under `cwd` and feed the result in as changed files.
endpointsNoEndpoints touched by the change. Each entry is either a `METHOD:/path` string or a `{method, path}` object.
filesNoChanged source files as plain strings.
min_confidenceNoDrop matches below this tier before returning.
no_default_excludesNoDisable the default discovery ignore rules (e.g. `.git`, `node_modules`).
openapi_opsNoOpenAPI `operationId`s whose behaviour changed.
pathNoRestrict test discovery to this subpath (file or directory). Relative paths resolve against `cwd`.
Behavior4/5

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

With no annotations, the description carries the burden. It discloses read-only behavior (no HTTP, no test execution) and output characteristics (confidence tiers, run hints). While it lacks details on error handling or edge cases, it adequately describes the core behavior.

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 with two sentences. The first sentence front-loads the purpose and key features, and the second adds essential clarifications (read-only, CLI equivalence). 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?

The description lacks an explicit output schema and only vaguely mentions output (confidence tiers, run hints). It does not cover error behavior or edge cases, leaving some gaps for an agent to guess the return structure.

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 coverage is 100%, so the description adds limited value beyond what the schema already provides. It does not elaborate on parameter usage beyond the schema descriptions.

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 maps a change to the tests it likely affects, with confidence tiers and run hints. It distinguishes itself from sibling tools like tarn_run and tarn_run_agent by noting it is read-only and does not execute tests.

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 implies use for impact analysis without running tests, and mentions the CLI equivalent. However, it does not explicitly state when not to use it or provide direct comparisons to sibling tools for guidance.

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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