# API Drifter

> Finds where your API drifted from its spec. Open-source CLI and MCP server that runs your OpenAPI spec against a live implementation and tells you which side to fix.

Open source (Apache-2.0). CLI package: `apidrifter` on npm (https://www.npmjs.com/package/apidrifter). Engine package: `@apidrifter/core`.
Repository: https://github.com/apidrifter/apidrifter
Full text of this site in one Markdown file: https://apidrifter.com/llms-full.txt

## Commands

- `npx apidrifter plan openapi.yaml`: turn a spec into a request plan with the values an agent or a person must fill in.
- `npx apidrifter run openapi.yaml --base-url http://localhost:3000`: execute the plan against a live server and verify every response; each finding carries a fix for the spec and a fix for the implementation.
- `npx apidrifter verify openapi.yaml --observations recorded.json`: verify from recorded request/response pairs.
- `npx apidrifter breaking old.yaml new.yaml`: diff two spec versions, exit 1 on breaking changes.
- `npx apidrifter mcp snapshot <url>` and `npx apidrifter mcp diff <snapshot> <url>`: snapshot and diff MCP servers, including tool description changes.
- `npx apidrifter serve`: run the same capabilities as an MCP server over stdio.

## Docs

- [CLI reference](https://apidrifter.com/docs/cli/): Every apidrifter command and flag: plan, run and verify a live API against its OpenAPI spec, diff specs and MCP servers, and serve the engine over MCP.
- [Rules reference](https://apidrifter.com/docs/rules/): Every change rule and verification finding the engine can report, with its stable ID and default severity, so CI config and reports can refer to them.
- [Design](https://apidrifter.com/docs/design/): How API Drifter is built and why: the pure diff engine, polarity-aware classification, the CLI and MCP server, the hosted service and its security model.

## Research

- [We checked 13 public APIs against their own OpenAPI specs. 9 had hard mismatches.](https://apidrifter.com/research/public-api-spec-drift-2026-11/): A read-only survey of public APIs that publish a spec: what their live responses actually return, classified strictly, with every request reproducible. Markdown twin: https://apidrifter.com/research/public-api-spec-drift-2026-11/index.md

Survey data: 9 of 13 surveyed public APIs had hard spec violations on successful responses (171 distinct schema locations). Per-API pages: https://apidrifter.com/research/<post>/<api-id>/ with Markdown twins at .../index.md. Raw JSON: https://github.com/apidrifter/apidrifter/tree/main/tools/survey/results

## Optional

- [Full text](https://apidrifter.com/llms-full.txt): all research posts, per-API summaries and the CLI reference in one file.
- [RSS](https://apidrifter.com/research/rss.xml): new research posts.