api-linter

A reference for the rules used to check MCP tool, resource, and prompt definitions before build or development checks. MCP, the Model Context Protocol, is a standard way for AI clients to call tools and access connected data.

In plain words
What is it for?
Use it when `bun run lint:mcp` or `bun run devcheck` reports issues such as invalid schemas, inconsistent output formatting, or incorrectly named definitions.
Why use it?
It explains lint errors and warnings in plain terms, including what each rule means, how serious it is, and how to correct the definition.

Skill for Claude CodeCodex

Part of the reference-data-mcp-server plugin — 32 skills, 1 MCP server shipped together

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add skills/cyanheads/reference-data-mcp-server/api-linter
Any agent
npx skills add cyanheads/reference-data-mcp-server --skill api-linter
Clone the repo
git clone --depth 1 https://github.com/cyanheads/reference-data-mcp-server

Made for: Claude Code, Codex.

Or install reference-data-mcp-server, the plugin that ships this one along with the rest of its 32 skills, 1 MCP server.

Per session 86 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 12,808 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin 100% copy Near-identical to another mod in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00086 $0.12808
Opus 5 $0.00043 $0.06404
Sonnet 5 $0.00017 $0.02562
Haiku 4.5 $0.00009 $0.01281

Measured 2d ago against content hash 8f2c3de62558, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

api-linter scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

Origin

This is a copy

100% identical to api-linter — 0 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

skills/api-linter/SKILL.md · 984 lines

How it starts

The opening of the file, as written. The whole thing — 984 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Overview

The linter validates tool, resource, and prompt definitions against the MCP spec and framework conventions. It is build-time only — not invoked at server startup. It runs in two places:

Entry point When On failure
bun run lint:mcp Manual or CI Prints errors + warnings, exits non-zero on errors.
bun run devcheck Pre-commit workflow Wraps lint:mcp alongside typecheck, format, bun audit, bun outdated.

Both surface the same LintReport from validateDefinitions() (exported from @cyanheads/mcp-ts-core/linter). Each diagnostic has a stable rule ID — that's the anchor you land on via the See: skills/api-linter/SKILL.md#<rule> breadcrumb appended to every message.

Severity:

  • error — MUST-level spec violation; blocks devcheck.
  • warning — SHOULD-level or quality issue; logged but devcheck continues.

Imports (if you need to run the linter programmatically):

import { validateDefinitions } from '@cyanheads/mcp-ts-core/linter';
import type { LintReport, LintDiagnostic } from '@cyanheads/mcp-ts-core/linter';

const report = validateDefinitions({ tools, resources, prompts, serverJson, packageJson });
if (!report.passed) process.exit(1);

Rule index

Grouped by family. Jump to any rule ID via its anchor.

Family Rules Section
Definition definition-invalid Definition rules
Format parity format-parity, format-parity-threw, format-parity-walk-failed, format-parity-depth-limit Format parity
Schema schema-is-object, describe-on-fields, schema-serializable, schema-unsatisfiable, header-param-designation Schema rules
Portability schema-format-portability, schema-anyof-needs-type, schema-no-discriminator-keyword, schema-no-defs, schema-root-oneof-portability, schema-dialect-tag Portability rules
Names name-required, name-format, name-unique Name rules
Tools description-required, handler-required, auth-type, auth-scope-format, annotation-type, annotation-coherence, meta-ui-type, meta-ui-resource-uri-required, meta-ui-resource-uri-scheme, app-tool-resource-pairing, canvas-consumer-missing Tool rules
Resources uri-template-required, uri-template-valid, resource-name-not-uri, template-params-align Resource rules
Landing landing-* (23 rules — shape, tagline, logo, links, repo, envExample, connectSnippets, theme) Landing config rules
Prompts generate-required Prompt rules
Handler body prefer-mcp-error-in-handler, prefer-error-factory, preserve-cause-on-rethrow, no-stringify-upstream-error Handler body rules
Error contract (structural) error-contract-type, error-contract-empty, error-contract-entry-type, error-contract-code-type, error-contract-code-unknown, error-contract-code-unknown-error, error-contract-reason-required, error-contract-reason-format, error-contract-reason-unique, error-contract-when-required, error-contract-retryable-type, error-contract-recovery-required, error-contract-recovery-empty, error-contract-recovery-min-words Error contract rules
Error contract (conformance) error-contract-conformance, error-contract-prefer-fail Error contract rules
Enrichment enrichment-type, enrichment-empty, enrichment-field-type, enrichment-output-collision, enrichment-prefer-block, enrichment-trailer-render, enrichment-trailer-orphan, enrichment-trailer-unknown-field, capped-list-no-truncation Enrichment rules
server.json ~40 rules prefixed server-json-* server.json rules

Read the full file on GitHub · 984 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 2d ago First seen · 984 lines · 86 tokens per session scan A 8f2c3de62558

Subscribe to this mod's changes

api-linter is a skill published in the GitHub repository cyanheads/reference-data-mcp-server (1 stars, last pushed 8d ago), licensed Apache-2.0. It adds 86 tokens to every session and 12,808 once invoked, about $0.0004 per session on Opus 5. A static security scan graded it A with 0 findings. It is 100% identical to api-linter, differing in 0 lines, and is treated as a copy.

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