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Evaluate Rego with coverage

rego_eval_with_coverage

Evaluate Rego policies with per-line coverage data to verify that tests exercise the rules they target.

Instructions

Evaluate with --coverage and return per-line coverage data. Useful for verifying that tests actually exercise the rules they're meant to.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
inputNoInline input document.
pathsNoPolicy / data file or directory paths. Each must be inside an allowed root.
queryYesRego query to evaluate, e.g. "data.example.allow".
sourceNoInline Rego policy source. Optional: without `source` or `paths` the query runs on its own, which is enough to try a built-in or an expression.
partialNoRun partial evaluation rather than full evaluation.
unknownsNoRefs to treat as unknown during partial evaluation.
inputPathNoPath to a JSON input file. Mutually exclusive with `input`.
v0CompatibleNoRead the policy as Rego v0 (`--v0-compatible`), the syntax OPA used before 1.0: rules without `if`, partial sets as `deny[msg] { ... }`. Needed for a policy that has not been migrated, which OPA 1.x otherwise refuses to load. Where the tool also takes a query, the query is read as v0 too, with the future keywords imported so `in`, `every` and `some x in` still work in it.
strictBuiltinErrorsNoTreat builtin errors as fatal instead of returning undefined.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changedv0.8.0
    • changedInput schema / properties / v0Compatible / description
      Previous value: -"Read the policy as Rego v0 (`--v0-compatible`), the syntax OPA used before 1.0: rules without `if`, partial sets as `deny[msg] { ... }`. Needed for a policy that has not been migrated, which OPA 1.x otherwise refuses to load."New value: +"Read the policy as Rego v0 (`--v0-compatible`), the syntax OPA used before 1.0: rules without `if`, partial sets as `deny[msg] { ... }`. Needed for a policy that has not been migrated, which OPA 1.x otherwise refuses to load. Where the tool also takes a query, the query is read as v0 too, with the future keywords imported so `in`, `every` and `some x in` still work in it."
  2. Changed2 schema fields changedv0.7.0
    • changedInput schema / properties / source / description
      Previous value: -"Inline Rego policy source. Mutually exclusive with `paths`."New value: +"Inline Rego policy source. Optional: without `source` or `paths` the query runs on its own, which is enough to try a built-in or an expression."
    • addedInput schema / properties / v0Compatible
      Added value: +{
+  "description": "Read the policy as Rego v0 (`--v0-compatible`), the syntax OPA used before 1.0: rules without `if`, partial sets as `deny[msg] { ... }`. Needed for a policy that has not been migrated, which OPA 1.x otherwise refuses to load.",
+  "type": "boolean"
+}
  3. Addedv0.1.13
  4. Removedv0.1.5
  5. Addedv0.1.2
  6. Removedv0.1.1
  7. First observedv0.1.0

TDQS

A3.5/5.0
Behavior3/5

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

Annotations give only readOnlyHint=false and openWorldHint=true, so the description carries most of the behavioral burden. It discloses the return content (per-line coverage data), which is genuinely useful without an output schema, but says nothing about cost, performance, or what happens under partial evaluation. Notably readOnlyHint=false for an eval tool is odd, though the description neither confirms nor contradicts it.

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?

Two tight sentences with the distinguishing behavior (coverage) front-loaded and the rationale following. Zero 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?

With nine parameters, sparse annotations, and no output schema, the description is thin for a tool whose headline output is coverage data. It states that per-line coverage is returned but does not characterize that shape or note anything about partial/unknowns interaction, leaving the schema and agent inference to fill sizable gaps.

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 100%, so the schema already documents all nine parameters, including partial/unknowns and v0Compatible in notable detail. The description adds only the implicit `--coverage` flag mapping and no syntax or format guidance beyond the schema, so baseline 3 applies.

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

Purpose4/5

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

States a specific verb+resource: 'Evaluate with `--coverage` and return per-line coverage data.' This clearly separates it from the plain `rego_eval` and from the explain/profile variants. It never names a sibling explicitly, but the coverage framing is distinctive enough for an agent to select it.

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

Usage Guidelines3/5

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

Provides an implied when-to-use: 'verifying that tests actually exercise the rules they're meant to.' That is real context, but there is no when-not guidance and no routing to related siblings like rego_coverage_gaps, rego_test, or rego_eval_with_explain.

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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