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Check Rego against a JSON Schema

rego_check_schema
Read-onlyIdempotent

Checks whether every input.* field referenced by a Rego policy exists in the supplied JSON Schema, reporting type-error diagnostics for inconsistencies.

Instructions

Validate that a Rego policy's input.* field references are consistent with a JSON Schema using opa check --schema. Every field the policy reads from input must exist in the schema; mismatches surface as rego_type_error diagnostics with file/line locations. Returns { valid: true, errors: [] } when all references match the schema, or { valid: false, errors: [...] } with structured diagnostics when they do not. Accepts the schema inline (pass the schema output of rego_infer_input_schema directly as inlineSchema) or as a path to a JSON Schema file on disk, or to a schema directory when the policy declares schemas: annotations (schemaPath). Provide source for inline Rego or paths for file/directory checking.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
pathsNoFilesystem paths to policy files or directories to validate. Each path must be inside an allowed root (OPA_MCP_ALLOWED_PATHS). Mutually exclusive with `source`.
sourceNoInline Rego source to validate against the schema. Mutually exclusive with `paths`.
strictNoEnable strict mode -- also fail on unused variables, deprecated builtins, and other non-fatal issues in addition to schema violations.
schemaPathNoPath to a JSON Schema file on disk to use for `input` validation, or to a schema directory when the policy carries `# METADATA` / `schemas:` annotations naming files in it (opa reads a directory only through those). Must be inside an allowed root (OPA_MCP_ALLOWED_PATHS). Mutually exclusive with `inlineSchema`.
inlineSchemaNoJSON Schema (draft-07) object describing the expected shape of the `input` document. Mutually exclusive with `schemaPath`. Accepts the `schema` field from `rego_infer_input_schema` output directly.
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.

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. Changed1 schema field changedv0.7.0
    • 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. Changed1 schema field changedv0.6.0
    • changedInput schema / properties / schemaPath / description
      Previous value: -"Path to a JSON Schema file on disk to use for `input` validation. Must be inside an allowed root (OPA_MCP_ALLOWED_PATHS). Mutually exclusive with `inlineSchema`."New value: +"Path to a JSON Schema file on disk to use for `input` validation, or to a schema directory when the policy carries `# METADATA` / `schemas:` annotations naming files in it (opa reads a directory only through those). Must be inside an allowed root (OPA_MCP_ALLOWED_PATHS). Mutually exclusive with `inlineSchema`."
  4. Added

TDQS

A4.3/5.0
Behavior4/5

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

Annotations already cover the safety profile (readOnly/idempotent/non-destructive), and the description adds real value: it names the failure mode (rego_type_error diagnostics with file/line), and gives the exact return shape `{valid, errors}`. It doesn't discuss rate limits or path-root rejection errors beyond what the schema already says.

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?

Dense but well ordered: purpose first, then diagnostic/return behavior, then the schema-input modes, then source-vs-paths. Every sentence carries information, though the final sentence slightly stacks multiple mode descriptions.

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 6-parameter, nested-object tool with no output schema, the description supplies the return contract and mode semantics the agent needs. Minor gaps remain around ordering when both none of source/paths are supplied, but overall it is complete enough to invoke correctly.

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

Parameters4/5

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

Schema coverage is 100%, so the baseline is 3, but the description adds integration meaning beyond the schema: it tells the agent it can pass `rego_infer_input_schema`'s `schema` output straight through as `inlineSchema`, and clarifies the directory-vs-file semantics of `schemaPath`.

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+resource+mechanism: validate a Rego policy's `input.*` references against a JSON Schema via `opa check --schema`. This clearly distinguishes it from siblings like rego_check, rego_lint, and rego_infer_input_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?

Explains the two schema-input modes (inline via rego_infer_input_schema output, or path/directory) and the source-vs-paths choice, giving clear usage context. It does not, however, explicitly state when to prefer this over rego_check or rego_verify, so no full when-not framing.

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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