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Explain Rego decision

rego_explain_decision

Evaluate a Rego query with full tracing to return a structured trace and per-rule fired/not-fired summary, explaining why a policy decision was denied.

Instructions

Evaluate a Rego query with full tracing and return a structured trace plus per-rule fired/not-fired summary. Use this when you need to answer "why was this denied?" -- the agent reads the structured trace and narrates the cause without re-implementing the trace parser.

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?

Adds genuine context beyond annotations by describing the return shape and the intended agent workflow (read trace, narrate cause, no custom parser). With annotations limited to readOnlyHint=false/openWorldHint=true, it still omits any disclosure of tracing cost, side effects, or why the call is flagged non-read-only.

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?

Two well-formed sentences, front-loaded with what the tool does followed by when to use it. No filler; only lightly redundant restatement of the output in the second clause.

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 9-parameter, no-output-schema tool, the description compensates by naming the return contents (structured trace, per-rule fired/not-fired summary) and the intended usage. Behavioral specifics like tracing performance and the non-read-only flag remain unaddressed, keeping it short of full completeness.

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 9 parameters (v0Compatible, partial, unknowns, etc.). The description adds no format/syntax detail beyond the schema, so the baseline of 3 is appropriate.

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 a Rego query with full tracing) and its distinctive output (structured trace + per-rule fired/not-fired summary), which separates it from plain rego_eval. It does not, however, name the close siblings (rego_eval_with_explain, rego_explain_undefined) that an agent must choose between.

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 a clear motivating scenario ('why was this denied?'), which implies when to reach for it. But it gives no explicit exclusions or alternatives, and with near-identical siblings like rego_eval_with_explain and rego_explain_undefined present, the agent is left to infer the routing.

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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