🦀🐚 Beavuck Hermit

Hermit is an OpenAPI mock server. Pass it the path to a spec file, and it starts serving lightning-fast schema-accurate responses out of the box. No stubs to write, sensible defaults, configurable.

📊 Status

📗 Use cases

Hermit is useful whenever you need an API to be available but don't want to run the real backend:

• Frontend development -- build and iterate against real HTTP endpoints without depending on a live backend
• Integration and E2E testing -- run tests in CI against a predictable, schema-accurate server with no database or external services
• Contract validation -- verify that your OpenAPI spec produces the shapes your consumers actually expect

🔍 How it works

sequenceDiagram
    participant Client
    participant Hermit
    participant Spec as OpenAPI Spec
    Note over Hermit, Spec: Startup -- once (specs loaded in parallel)
    Hermit ->> Spec: Read & parse YAML (one thread per spec file)
    Hermit ->> Hermit: Resolve $refs, flatten allOf/oneOf
    Hermit ->> Hermit: Pre-generate mock responses for all routes
    Note over Client, Hermit: Runtime -- per request
    Client ->> Hermit: GET /projects/{projectId}
    Hermit -->> Client: 200 {"id": "abc", "name": "echo", ...}
    Client ->> Hermit: POST /projects {"name": "my project"}
    Hermit ->> Hermit: Merge request body into mock response
    Hermit -->> Client: 201 {"id": "abc", "name": "my project", ...}

Loading

All schema work (ref resolution, composition, value generation) happens once at startup. Requests are served from an in-memory map with no I/O.

👓 Request body echo

For POST, PUT, and PATCH requests, Hermit merges the fields you send into the mock response. This means your code sees its own writes reflected back -- the most useful behavior for frontend development:

# Start Hermit
hermit --specs my-api.openapi.yml other-api.openapi.yml

# Create a project -- the response reflects the name you sent
curl -s -X POST http://localhost:8532/projects \
  -H 'Content-Type: application/json' \
  -d '{"name": "Acme Redesign", "status": "active"}' | jq .
# {
#   "id": "foxtrot",
#   "name": "Acme Redesign",
#   "status": "active",
#   ...
# }

🧬 Polymorphic responses

When a POST endpoint uses oneOf with a discriminator, Hermit inspects the request body to pick the right response shape:

# Creating a "feature" task returns a FeatureTask shape
curl -s -X POST http://localhost:8532/projects/abc/tasks \
  -H 'Content-Type: application/json' \
  -d '{"type": "feature", "storyPoints": 8}' | jq .type
# "feature"

# Creating a "bug" task returns a BugTask shape
curl -s -X POST http://localhost:8532/projects/abc/tasks \
  -H 'Content-Type: application/json' \
  -d '{"type": "bug", "severity": "high"}' | jq .type
# "bug"

📄 Response generation

Response field values are resolved in priority order:

Priority	Source	When
1	Request body fields	POST, PUT, PATCH -- caller's values win, except for readOnly fields
2	example in schema	Field has an explicit example value (if --use-examples)
3	default in schema	Field has a default value and no example (if --use-examples)
4	Random value	Fallback -- random word, number, boolean, or format-aware value (UUID, date-time, …)

Fields marked readOnly: true in the schema are never overridden by request body values. Fields marked writeOnly: true are excluded from generated responses entirely.

🛠️ Install

See install.md.

📦 🦀 Install and run from Cargo

See cargo.md.

🐳 Pull from Docker Hub

See docker.md.

🚀 Quick start

Run Hermit against your spec (replace the path with your actual spec file):

hermit --specs ~/Documents/dev/hermit/specs_assets/taskflow.openapi.yml

You can point to a directory containing multiple specs to load them all at once -- Hermit merges their routes and loads them in parallel:

hermit --specs-dir ~/Documents/dev/hermit/specs_assets

To see what arguments are available, run:

hermit --help

Stop the server with Ctrl+C, or if it's running in the background:

kill $(lsof -ti :8532)

Replacing the port number at need.

⚙️ Options and environment variables

See options.md.

🔒 Privacy

See PRIVACY.md.

📜 License

See UNLICENSE.

---

And now some dev stuff

---

🛞 Build and run

Load multiple specs in one command:

cargo run --release -- --specs-dir specs_assets

The server listens on port 8532 by default. Override with --port.

To see what arguments are available, run:

hermit --help

🔧 Development setup

Install just:

cargo install just

Install dev tools:

just setup

In RustRover, go to Settings > Version Control > Commit > Advanced commit checks and choose the pre-commit configuration to run these checks automatically on commit.

Explore justfile for available commands.

✅ API tests

Requires the server to be running. Run with Bruno:

cd api_tests/hermit_api_tests && npx --yes @usebruno/cli run --env hermit_env --reporter-html test_report.html --reporter-json test_report.json

🐳 Docker

See docker.dev.md.

🏗️ Architecture

See architecture.dev.md.

📖 More docs

See docs/dev