Metadata-Version: 2.5
Name: fastapi-contract
Version: 0.2.0
Summary: Catch FastAPI breaking changes that OpenAPI diff can't see.
Project-URL: Repository, https://github.com/Ryota-Di/fastapi-contract
Project-URL: Issues, https://github.com/Ryota-Di/fastapi-contract/issues
License:                                  Apache License
                                   Version 2.0, January 2004
                                http://www.apache.org/licenses/
        
           TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
        
           1. Definitions.
        
              "License" shall mean the terms and conditions for use, reproduction,
              and distribution as defined by Sections 1 through 9 of this document.
        
              "Licensor" shall mean the copyright owner or entity authorized by
              the copyright owner that is granting the License.
        
              "Legal Entity" shall mean the union of the acting entity and all
              other entities that control, are controlled by, or are under common
              control with that entity. For the purposes of this definition,
              "control" means (i) the power, direct or indirect, to cause the
              direction or management of such entity, whether by contract or
              otherwise, or (ii) ownership of fifty percent (50%) or more of the
              outstanding shares, or (iii) beneficial ownership of such entity.
        
              "You" (or "Your") shall mean an individual or Legal Entity
              exercising permissions granted by this License.
        
              "Source" form shall mean the preferred form for making modifications,
              including but not limited to software source code, documentation
              source, and configuration files.
        
              "Object" form shall mean any form resulting from mechanical
              transformation or translation of a Source form, including but
              not limited to compiled object code, generated documentation,
              and conversions to other media types.
        
              "Work" shall mean the work of authorship, whether in Source or
              Object form, made available under the License, as indicated by a
              copyright notice that is included in or attached to the work
              (an example is provided in the Appendix below).
        
              "Derivative Works" shall mean any work, whether in Source or Object
              form, that is based on (or derived from) the Work and for which the
              editorial revisions, annotations, elaborations, or other modifications
              represent, as a whole, an original work of authorship. For the purposes
              of this License, Derivative Works shall not include works that remain
              separable from, or merely link (or bind by name) to the interfaces of,
              the Work and Derivative Works thereof.
        
              "Contribution" shall mean any work of authorship, including
              the original version of the Work and any modifications or additions
              to that Work or Derivative Works thereof, that is intentionally
              submitted to Licensor for inclusion in the Work by the copyright owner
              or by an individual or Legal Entity authorized to submit on behalf of
              the copyright owner. For the purposes of this definition, "submitted"
              means any form of electronic, verbal, or written communication sent
              to the Licensor or its representatives, including but not limited to
              communication on electronic mailing lists, source code control systems,
              and issue tracking systems that are managed by, or on behalf of, the
              Licensor for the purpose of discussing and improving the Work, but
              excluding communication that is conspicuously marked or otherwise
              designated in writing by the copyright owner as "Not a Contribution."
        
              "Contributor" shall mean Licensor and any individual or Legal Entity
              on behalf of whom a Contribution has been received by Licensor and
              subsequently incorporated within the Work.
        
           2. Grant of Copyright License. Subject to the terms and conditions of
              this License, each Contributor hereby grants to You a perpetual,
              worldwide, non-exclusive, no-charge, royalty-free, irrevocable
              copyright license to reproduce, prepare Derivative Works of,
              publicly display, publicly perform, sublicense, and distribute the
              Work and such Derivative Works in Source or Object form.
        
           3. Grant of Patent License. Subject to the terms and conditions of
              this License, each Contributor hereby grants to You a perpetual,
              worldwide, non-exclusive, no-charge, royalty-free, irrevocable
              (except as stated in this section) patent license to make, have made,
              use, offer to sell, sell, import, and otherwise transfer the Work,
              where such license applies only to those patent claims licensable
              by such Contributor that are necessarily infringed by their
              Contribution(s) alone or by combination of their Contribution(s)
              with the Work to which such Contribution(s) was submitted. If You
              institute patent litigation against any entity (including a
              cross-claim or counterclaim in a lawsuit) alleging that the Work
              or a Contribution incorporated within the Work constitutes direct
              or contributory patent infringement, then any patent licenses
              granted to You under this License for that Work shall terminate
              as of the date such litigation is filed.
        
           4. Redistribution. You may reproduce and distribute copies of the
              Work or Derivative Works thereof in any medium, with or without
              modifications, and in Source or Object form, provided that You
              meet the following conditions:
        
              (a) You must give any other recipients of the Work or
                  Derivative Works a copy of this License; and
        
              (b) You must cause any modified files to carry prominent notices
                  stating that You changed the files; and
        
              (c) You must retain, in the Source form of any Derivative Works
                  that You distribute, all copyright, patent, trademark, and
                  attribution notices from the Source form of the Work,
                  excluding those notices that do not pertain to any part of
                  the Derivative Works; and
        
              (d) If the Work includes a "NOTICE" text file as part of its
                  distribution, then any Derivative Works that You distribute must
                  include a readable copy of the attribution notices contained
                  within such NOTICE file, excluding those notices that do not
                  pertain to any part of the Derivative Works, in at least one
                  of the following places: within a NOTICE text file distributed
                  as part of the Derivative Works; within the Source form or
                  documentation, if provided along with the Derivative Works; or,
                  within a display generated by the Derivative Works, if and
                  wherever such third-party notices normally appear. The contents
                  of the NOTICE file are for informational purposes only and
                  do not modify the License. You may add Your own attribution
                  notices within Derivative Works that You distribute, alongside
                  or as an addendum to the NOTICE text from the Work, provided
                  that such additional attribution notices cannot be construed
                  as modifying the License.
        
              You may add Your own copyright statement to Your modifications and
              may provide additional or different license terms and conditions
              for use, reproduction, or distribution of Your modifications, or
              for any such Derivative Works as a whole, provided Your use,
              reproduction, and distribution of the Work otherwise complies with
              the conditions stated in this License.
        
           5. Submission of Contributions. Unless You explicitly state otherwise,
              any Contribution intentionally submitted for inclusion in the Work
              by You to the Licensor shall be under the terms and conditions of
              this License, without any additional terms or conditions.
              Notwithstanding the above, nothing herein shall supersede or modify
              the terms of any separate license agreement you may have executed
              with Licensor regarding such Contributions.
        
           6. Trademarks. This License does not grant permission to use the trade
              names, trademarks, service marks, or product names of the Licensor,
              except as required for reasonable and customary use in describing the
              origin of the Work and reproducing the content of the NOTICE file.
        
           7. Disclaimer of Warranty. Unless required by applicable law or
              agreed to in writing, Licensor provides the Work (and each
              Contributor provides its Contributions) on an "AS IS" BASIS,
              WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
              implied, including, without limitation, any warranties or conditions
              of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A
              PARTICULAR PURPOSE. You are solely responsible for determining the
              appropriateness of using or redistributing the Work and assume any
              risks associated with Your exercise of permissions under this License.
        
           8. Limitation of Liability. In no event and under no legal theory,
              whether in tort (including negligence), contract, or otherwise,
              unless required by applicable law (such as deliberate and grossly
              negligent acts) or agreed to in writing, shall any Contributor be
              liable to You for damages, including any direct, indirect, special,
              incidental, or consequential damages of any character arising as a
              result of this License or out of the use or inability to use the
              Work (including but not limited to damages for loss of goodwill,
              work stoppage, computer failure or malfunction, or any and all
              other commercial damages or losses), even if such Contributor
              has been advised of the possibility of such damages.
        
           9. Accepting Warranty or Additional Liability. While redistributing
              the Work or Derivative Works thereof, You may choose to offer,
              and charge a fee for, acceptance of support, warranty, indemnity,
              or other liability obligations and/or rights consistent with this
              License. However, in accepting such obligations, You may act only
              on Your own behalf and on Your sole responsibility, not on behalf
              of any other Contributor, and only if You agree to indemnify,
              defend, and hold each Contributor harmless for any liability
              incurred by, or claims asserted against, such Contributor by reason
              of your accepting any such warranty or additional liability.
        
           END OF TERMS AND CONDITIONS
        
           APPENDIX: How to apply the Apache License to your work.
        
              To apply the Apache License to your work, attach the following
              boilerplate notice, with the fields enclosed by brackets "[]"
              replaced with your own identifying information. (Don't include
              the brackets!)  The text should be enclosed in the appropriate
              comment syntax for the file format. We also recommend that a
              file or class name and description of purpose be included on the
              same "printed page" as the copyright notice for easier
              identification within third-party archives.
        
           Copyright [yyyy] [name of copyright owner]
        
           Licensed under the Apache License, Version 2.0 (the "License");
           you may not use this file except in compliance with the License.
           You may obtain a copy of the License at
        
               http://www.apache.org/licenses/LICENSE-2.0
        
           Unless required by applicable law or agreed to in writing, software
           distributed under the License is distributed on an "AS IS" BASIS,
           WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
           See the License for the specific language governing permissions and
           limitations under the License.
License-File: LICENSE
Requires-Python: >=3.11
Requires-Dist: annotated-types<1,>=0.7
Requires-Dist: fastapi==0.141.1
Requires-Dist: pydantic==2.13.5
Description-Content-Type: text/markdown

# fastapi-contract

Catch FastAPI breaking changes that OpenAPI diff can't see.

`fastapi-contract` compares your FastAPI application's declared runtime contracts
with a saved baseline. It complements OpenAPI diff: a response key can disappear,
input can stop reaching a field, or a hidden parameter can become required while
the documented schema stays the same. SAFE means no incompatibility was detected
in the supported contract surface; it is not a guarantee of full API compatibility.

## Installation and supported versions

Requires **Python 3.11+**, **FastAPI 0.141.1**, and **Pydantic 2.13.5**.
Install into the application's dependency environment:

```bash
pip install fastapi-contract
```

For a source checkout, use `pip install .`. FastAPI and Pydantic are pinned to the
exact supported versions. Starlette follows FastAPI's dependency; use the same
locked environment for baseline generation and checking. Baselines record exact
Python, FastAPI, Starlette and Pydantic versions, including the Python patch
version. Mismatches cause ERROR. CI verifies Python 3.11 and 3.12.

## Quickstart

Create `app/__init__.py` and `app/main.py` in your project:

```python
from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()


class User(BaseModel):
    id: int
    email: str


@app.get("/users", response_model=User)
def users():
    return {"id": 1, "email": "alice@example.com"}
```

Save the accepted application contract, then check proposed changes against it:

```bash
fastapi-contract snapshot app.main:app -o fastapi-contract-baseline.json
fastapi-contract check app.main:app --against fastapi-contract-baseline.json
```

New snapshots use schema v2 and include response, body binding, and parameter
facts. An unchanged app prints:

```text
SAFE: No incompatibility detected in the supported contract surface.
```

For example, add `response_model_exclude={"email"}` to the route decorator. The
OpenAPI response schema still describes `email`, but runtime output loses it:

```text
BREAKING
response-wire-surface GET /users: Response wire key "email" is no longer produced; key remains documented by OpenAPI.
```

The command exits 1. No server, endpoint, startup, or lifespan needs to run.
App loading does execute module-level code; load trusted applications only. An
invalid baseline is rejected before the target application is imported.

## Supported checks

| Owner | Supported contract change |
| --- | --- |
| `response-wire-surface` | Loss of an effective projected top-level response wire key, including `response_model_by_alias` blind spots. Ordinary documented key losses are delegated to OpenAPI diff. |
| `request-body-binding` | Loss of a supported top-level JSON BaseModel input key's binding to a matched documented input slot. A field falling back to its default can be BREAKING even when HTTP stays 200. |
| `hidden-parameter-binding` | Loss of a hidden Query, Header or Cookie wire binding. |
| `hidden-parameter-requirement` | An existing, matched hidden parameter changes from optional to required. New required parameter additions are outside this release. |

Parameter identity uses location and the resolved wire name. Header names are
canonicalized to ASCII lowercase; Query and Cookie names are case-sensitive.
Direct declarations and statically resolved dependencies are supported. Moving
a parameter between them or renaming Python fields/arguments with stable wire
bindings does not itself cause a finding.

Enum/nested sibling value types and field-local body validators do not automatically
block unrelated supported keys. Relevant unresolved selectors, alias forms, model
validators, parameter containers or dependency overrides can produce REVIEW for
the affected candidate. Findings retain independent uncertainty alongside confirmed
losses; BREAKING takes precedence over REVIEW without hiding those findings.

## Results and CI

| Result | Stream | Exit code |
| --- | --- | --- |
| SAFE | stdout | 0 |
| BREAKING | stdout | 1 |
| REVIEW | stdout | 1 |
| ERROR | stderr | 2 |

Successful `snapshot` exits 0. Invalid arguments, invalid snapshots, environment
mismatch, app loading, file and analysis failures exit 2. Informational migration
notices go to stderr and do not affect results or exit codes. Output has no color
or TTY-dependent formatting; consumer wire names use JSON string escaping.

Commit the baseline alongside the application. With locked uv dependencies:

```yaml
- run: uv sync --locked
- run: >-
    uv run --locked fastapi-contract check app.main:app
    --against fastapi-contract-baseline.json
```

Keep the exact Python patch version and lockfile consistent with baseline creation.
Do not refresh baselines during ordinary CI checks. For an intentional contract
change, review consumer impact and commit the accepted new baseline for review.

## Upgrading a schema-v1 baseline

Existing schema-v1 baselines continue working and run **legacy checks only**.
The result retains legacy FAPI001 meaning and output. A valid v1 check prints this
notice exactly once to stderr, before loading the app:

```text
Baseline uses snapshot schema v1.
Running legacy checks only.
Regenerate the baseline to enable current contract checks.
```

V1 does not contain enough information to recover current response facts for
models it marked UNSUPPORTED, body binding, hidden parameter binding, or hidden
requiredness. Legacy REVIEW limitations remain; a v1 SAFE result means only the
legacy checks passed.

To enable current checks, check out the application revision whose contract you
accept as the baseline, use its matching environment, and regenerate:

```bash
fastapi-contract snapshot app.main:app \
  -o fastapi-contract-baseline.json
```

Review and commit the result. Regeneration creates a **new accepted baseline**;
it does not reconstruct historical facts. Generating from an already changed app
accepts that app as the baseline. There is no automatic migration or `migrate`
command. Explicit `--schema-version 1` remains available, and `check` always selects
its lane from the saved baseline version. Explicit `--schema-version 2` also works.

## Known limitations

This release does not add conditional response omission (`exclude_none`,
`exclude_unset`, `exclude_defaults`), nested projection, full AliasChoices/AliasPath
support, request coercion/strictness/validation-constraint checks, missing
Content-Type behavior, route/slash resolution, hidden operation availability,
Path parameter rules, or new required parameter additions. Arbitrary serializer,
dependency, and user-code behavior is not analyzed. Relevant unsupported structures
may require review; unrelated unsupported details do not imply every key is unknown.

Extraction checks declarations, not execution of arbitrary endpoint code. Routes
registered only during startup/lifespan are not captured. There is no Git baseline
loader, environment reconstruction, or automatic baseline refresh.

## Development

```bash
uv sync --locked --group dev
uv run --locked pytest
uv run --locked ruff check .
uv run --locked ruff format --check .
uv run --locked mypy src
uv build --build-constraints tools/release/build-constraints.txt --require-hashes
```

See the [release checklist](docs/release-checklist.md),
[release notes](docs/releases/v0.2.0.md), [architecture](docs/architecture.md),
and [testing strategy](docs/testing-strategy.md).

## License

Apache License 2.0. See [LICENSE](LICENSE).
