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Evaluate Rego with execution trace

rego_eval_with_explain

Evaluate a Rego query with --explain=full and return a structured trace alongside the result, enabling agents to see why a rule fired or did not.

Instructions

Evaluate with --explain=full and return a structured trace alongside the result. Use this when an agent needs to see why a rule fired (or didn't) -- the trace is the basis for rego_explain_decision.

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

A4.2/5.0
Behavior4/5

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

Annotations only cover readOnlyHint=false and openWorldHint=true, so the description's disclosure that this runs with full explain mode and returns a trace alongside the result is meaningful added context. It doesn't describe trace verbosity/size or how the trace is structured, but the core behavioral trait is surfaced.

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 tightly written sentences with the core action front-loaded and no filler. The chained-tool note is brief and earns its place as routing context.

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 evaluation tool with no output schema, the description conveys the essential behavior (trace returned with result) and the workflow purpose. It could say more about what the returned trace contains, since no output schema exists to compensate, but it is sufficient to call the tool correctly.

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 all nine parameters (including the `--explain`-relevant ones like `partial`, `unknowns`, `v0Compatible`) are already documented in the schema. The description adds no parameter-level meaning beyond what the schema provides, which is the expected baseline.

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 and resource (evaluate Rego with `--explain=full` returning a structured trace) and names the differentiator versus plain evaluation. It also positions the tool relative to a sibling (`rego_explain_decision`), so an agent can distinguish it without opening any schema.

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?

"Use this when an agent needs to see why a rule fired (or didn't)" is an explicit, actionable trigger condition, and it hints at the downstream relationship with `rego_explain_decision`. It stops short of stating when to prefer plain `rego_eval` over this costlier variant (e.g. when no explanation is needed), so no exclusions are given.

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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