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opa-mcp-server

by OrygnsCode
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Get OPA policy by ID

opa_get_policy
Read-onlyIdempotent

Retrieve a policy's Rego source from OPA by ID, with optional AST inclusion.

Instructions

Fetch a single policy by ID from the running OPA server. Returns the Rego source; the parsed AST is omitted unless asked for, since it is roughly forty times the size of the source it came from. Use rego_parse_ast on the source when an AST is what's wanted.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
idYesPolicy ID, e.g. "rbac" or "policies/auth/main".
includeAstNoInclude OPA's parsed AST alongside the source. Off by default.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changedv0.5.0
    • addedInput schema / properties / includeAst
      Added value: +{
+  "description": "Include OPA's parsed AST alongside the source. Off by default.",
+  "type": "boolean"
+}
  2. Addedv0.1.13
  3. Removedv0.1.5
  4. Addedv0.1.2
  5. Removedv0.1.1
  6. First observedv0.1.0

TDQS

A4.7/5.0
Behavior5/5

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

Annotations already declare readOnly/idempotent/non-destructive, so the bar for added context is lower. The description adds real behavior: the tool returns Rego source, omits the AST by default, explains the size tradeoff, and notes the includeAst alternative. No contradiction.

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?

Three sentences, all substantive, with the main purpose in the first clause. No filler or duplication of schema fields.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a simple one-required-parameter read tool, the description covers what the caller gets (Rego source), the optional behavior (includeAst), and the alternative for AST. Annotations cover safety and idempotency, and schema covers parameters, so nothing essential is missing.

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 parameters are fully documented; the description adds little beyond what the schema already provides. The mention that AST is omitted 'unless asked for' aligns with includeAst's schema description, so no additional compensation is needed.

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?

Description opens with a specific verb+resource: 'Fetch a single policy by ID from the running OPA server.' It clearly scopes to one policy, distinguishes from list/put/delete siblings, and differentiates from rego_parse_ast by stating this returns Rego source and AST is optional.

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

Usage Guidelines5/5

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

Explicitly states that the AST is omitted unless asked and directs the agent to use `rego_parse_ast` when an AST is wanted, giving a concrete when-not. It also implies the primary use case—getting the Rego source for one policy—without ambiguity.

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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