🧪 OpenAPI Contract Tester

An opinionated, lightweight black-box contract tester against a live API using its OpenAPI specification as the source of truth.

This tool validates OpenAPI quality, generates test cases from schemas, and verifies that real HTTP responses match the contract. This "certifies" that the documentation is complete with descriptions, example, and schema, and that the endpoint behaves as the documentation suggests.

Guiding Principles:

1. A service needs to be documented clearly. (This means schemas, descriptions, and examples)
2. When the examples and schemas are used, it should respond as expected from the documentation.

📚 Read the full documentation

✨ What it does

▶️ Quick Example

pytest --openapi=http://localhost:8000 -v

Console Output:

====================================================================================== test session starts =======================================================================================
platform linux -- Python 3.11.14, pytest-9.0.2, pluggy-1.6.0 -- /usr/local/bin/python3.11
cachedir: .pytest_cache
rootdir: /workspace
plugins: openapi-0.3.0, depends-1.0.1, mock-3.15.1
collected 3 items
created 2 items from openapi examples
created 20 items generated from schema

tests/test_samples/test_sample_math.py::test_sample_addition PASSED                                                                                                                        [  4%]
tests/test_samples/test_sample_math.py::test_sample_multiplication PASSED                                                                                                                  [  8%]
.::test_openapi[POST /email [example-1]] PASSED                                                                                                                                            [ 12%]
.::test_openapi[POST /email [generated-2]] PASSED                                                                                                                                          [ 16%]
.::test_openapi[POST /email [generated-3]] PASSED                                                                                                                                          [ 20%]
.::test_openapi[POST /email [generated-4]] PASSED                                                                                                                                          [ 24%]
.::test_openapi[POST /email [generated-5]] PASSED                                                                                                                                          [ 28%]
.::test_openapi[POST /email [generated-6]] PASSED                                                                                                                                          [ 32%]
.::test_openapi[POST /email [generated-7]] PASSED                                                                                                                                          [ 36%]
.::test_openapi[POST /email [generated-8]] PASSED                                                                                                                                          [ 40%]
.::test_openapi[POST /email [generated-9]] PASSED                                                                                                                                          [ 44%]
.::test_openapi[POST /email [generated-10]] PASSED                                                                                                                                         [ 48%]
.::test_openapi[POST /email [generated-11]] PASSED                                                                                                                                         [ 52%]
.::test_openapi[POST /email_bad [example-1]] FAILED                                                                                                                                        [ 56%]
.::test_openapi[POST /email_bad [generated-2]] FAILED                                                                                                                                      [ 60%]
.::test_openapi[POST /email_bad [generated-3]] FAILED                                                                                                                                      [ 64%]
.::test_openapi[POST /email_bad [generated-4]] FAILED                                                                                                                                      [ 68%]
.::test_openapi[POST /email_bad [generated-5]] FAILED                                                                                                                                      [ 72%]
.::test_openapi[POST /email_bad [generated-6]] FAILED                                                                                                                                      [ 76%]
.::test_openapi[POST /email_bad [generated-7]] FAILED                                                                                                                                      [ 80%]
.::test_openapi[POST /email_bad [generated-8]] FAILED                                                                                                                                      [ 84%]
.::test_openapi[POST /email_bad [generated-9]] FAILED                                                                                                                                      [ 88%]
.::test_openapi[POST /email_bad [generated-10]] FAILED                                                                                                                                     [ 92%]
.::test_openapi[POST /email_bad [generated-11]] FAILED                                                                                                                                     [ 96%]
tests/test_samples/test_sample_math.py::test_sample_string_operations PASSED                                                                                                               [100%]
📝 Full test report saved to: /workspace/tests/report.md
   (Configure output file with: --openapi-markdown-output=<filename>)


============================================================================================ FAILURES ============================================================================================

Detailed Report (report.md):

Test #12 ❌

📋 Test case from OpenAPI example

Endpoint: POST /email_bad

Request Body

{
  "body": "Hi Bob, how are you?",
  "from": "alice@example.com",
  "subject": "Hello",
  "to": "bob@example.com"
}

Expected Response

Status: 201

{
  "body": "Hi Bob, how are you?",
  "from": "alice@example.com",
  "id": "12",
  "subject": "Hello",
  "to": "bob@example.com"
}

Actual Response

Status: 201

{
  "body": "Hi Bob, how are you?",
  "from": "alice@example.com",
  "id": 12,
  "subject": 12345,
  "to": "bob@example.com"
}

❌ Error

Type mismatch for key 'subject': expected str, got int. Expected value: Hello, Actual value: 12345

Each OpenAPI test appears as an individual pytest test item.

✔️ Validates OpenAPI request/response definitions ✔️ Enforces schema field descriptions ✔️ Generates test cases from schemas, checks response codes and types in the response ✔️ Tests the examples ✔️ Tests GET / POST / PUT / DELETE endpoints ✔️ Compares live responses against examples ✔️ Produces a readable test report ✔️ Supports OpenAPI 3.0 and 3.1.x (nullable types, const, $ref siblings, allOf)

▶️ Detailed Example

Install

pip install pytest-openapi

Run

Say that you have a service running at port 8000 on localhost. Then, run:

pytest --openapi=http://localhost:8000

Options

• --openapi=BASE_URL: Run contract tests against the API at the specified base URL
• --openapi-no-strict-example-checking: Use lenient validation for example-based tests
• --openapi-markdown-output=FILENAME: (Optional) Write test results in Markdown format to the specified file
• --openapi-ignore=REGEXP: Completely ignore endpoints whose path matches the given regular expression. Useful to skip known-broken or auth-protected paths.
• -v: Verbose mode - shows full test names
• -vv: Very verbose mode - shows request/response with 50 character truncation
• -vvv: Very very verbose mode - shows full request/response without truncation

Examples:

pytest --openapi=http://localhost:8000 --openapi-ignore=mcp
pytest --openapi=http://localhost:8000 --openapi-ignore=(auth|mcp)
pytest --openapi=http://localhost:8000 --openapi-ignore=(v[0-9]+/auth|mcp)
pytest --openapi=http://localhost:8000 -vv  # Show truncated request/response
pytest --openapi=http://localhost:8000 -vvv  # Show full request/response

Strict vs Lenient Example Checking

By default, pytest-openapi performs strict matching on example-based tests:

• When your OpenAPI spec includes explicit request/response examples, the actual response must match the example values exactly
• This ensures examples accurately reflect real API behavior

However, sometimes examples contain placeholder values (like [1, 2, 3]) that don't match actual responses (like []). Use --openapi-no-strict-example-checking for lenient validation:

pytest --openapi=http://localhost:8000 --openapi-no-strict-example-checking

Lenient mode validates:

• Structure and types match (all expected keys present, correct types)
• But ignores exact values and array lengths

Note: Schema-generated tests always use schema validation (not affected by this flag).

Markdown Output Format

You can optionally generate test reports in Markdown format and save them to a file:

pytest --openapi=http://localhost:8000 --openapi-markdown-output=report.md

Note: If you don't specify --openapi-markdown-output, no markdown file is written. The plugin only outputs to pytest's standard output.

This creates a report.md file with:

• Summary statistics (total, passed, failed tests)
• Formatted code blocks for JSON data
• Clear sections for expected vs actual responses
• Error details in formatted blocks

The markdown report is written independently of stdout output.

Example output: See example_report.md for a sample markdown report.

Server

See here an example server - email-server: tests/test_servers/email_server/server.py

Resulting Tests

tests/test_servers/email_server/email_test_output.txt

Future Plans / TODO

This is a work in progress.

• A check that the example matches the schema
• Ask that 400 responses be in the documentation.
• A check for regexp and email formats.

Issues? Feedback?

Seriously, this is a work-in-progress. If you try it and something does not work as intended, or expect, open a ticket! I may be able to fix quickly, especially if you can provide a minimal example to replicate the issue.

In Consideration

• Use LLM-as-a-judge to assess the error messages and check their spelling.

Contributing

Contributions are welcome!

The only requirement is 🐳 Docker.

Test are containerized, run them using the VS Code task test. If you don't want to use VS Code, the command is docker compose -f ./tests/docker-compose.yaml --project-directory ./tests up --build --abort-on-container-exit --exit-code-from test. Run this before making a PR, please.

There is also a development environment for VS Code, if you need it. On this environment, you can run the task run-mock-server to run one of the mock servers and see the output.

You can add your own mock server, and then add integration tests. Just follow the same pattern as every test to make a call - subprocess.run('pytest', '--openapi=http://your-server:8000.

Please reformat and lint before making a PR. The VS Task is lint, and if you don't want to use VS Code, the command is: docker compose -f ./lint/docker-compose.yaml --project-directory ./lint up --build --abort-on-container-exit --exit-code-from linter. Run this before making a PR, please.

If you add a functionality, please add to the the documentation.

Please submit a pull request or open an issue for any bugs or feature requests.

The moment your PR is merged, you get a dev release. You can then set up the version number to use your changes.

License

MIT License. See LICENSE file for the specific wording.