CODEX_ADK/FRONTEND/.claude-docs/api-contract-workflow.md

API Contract Workflow

Single Source of Truth: Backend OpenAPI Specification

---

Overview

This project uses OpenAPI-driven development where the backend C# API is the authoritative source for API contracts. The frontend Flutter app automatically generates type-safe Dart code from the OpenAPI specification.

---

Architecture

Backend (C#)                    Frontend (Flutter/Dart)
─────────────                   ───────────────────────

Controllers with                api-schema.json
XML docs          ──────────►   (copied from backend)
                                       │
docs/openapi.json                      │
(auto-generated)  ──────────►          │
                                       ▼
                                lib/api/generated/
                                (auto-generated types)
                                       │
                                       ▼
                                lib/api/client.dart
                                (CQRS API client)
                                       │
                                       ▼
                                lib/api/endpoints/
                                (endpoint extensions)

---

Backend Responsibilities

1. XML Documentation

All controllers and DTOs must have complete XML documentation:

/// <summary>Gets paginated users with filtering</summary>
/// <param name="page">Page number (1-based)</param>
/// <response code="200">Returns paginated user list</response>
/// <response code="401">Unauthorized</response>
[HttpGet]
[ProducesResponseType(typeof(PagedResult<UserDto>), 200)]
[ProducesResponseType(401)]
public async Task<IActionResult> GetUsers([FromQuery] int page = 1) { }

2. OpenAPI Export

Backend generates docs/openapi.json:

cd backend
dotnet run --project Codex.Api &
sleep 5
curl https://localhost:7108/swagger/v1/swagger.json > docs/openapi.json
pkill -f "Codex.Api"

3. Schema Distribution

Frontend copies docs/openapi.json to api-schema.json:

cp ../backend/docs/openapi.json ./api-schema.json

---

Frontend Responsibilities

1. Install Dependencies

OpenAPI generator packages are in pubspec.yaml:

dependencies:
  http: ^1.2.2
  json_annotation: ^4.9.0

dev_dependencies:
  build_runner: ^2.4.14
  json_serializable: ^6.9.2
  openapi_generator_annotations: ^5.0.1

2. Code Generation

Generate Dart types from OpenAPI spec:

flutter pub run build_runner build --delete-conflicting-outputs

3. Generated Output

Code is generated to lib/api/generated/:

• ✅ DO NOT EDIT - These files are auto-generated
• ✅ DO NOT COMMIT - Listed in .gitignore
• ✅ REGENERATE on every API schema update

4. Manual Code (Stable)

These files are manually maintained:

• lib/api/client.dart - CQRS client framework
• lib/api/types.dart - Core types (Result, ApiError, pagination)
• lib/api/endpoints/*.dart - Endpoint-specific extensions

---

Workflow: Making API Changes

Backend Developer Flow

1. Update C# code with XML documentation
2. Run API to regenerate Swagger
3. Export OpenAPI spec:

   curl https://localhost:7108/swagger/v1/swagger.json > docs/openapi.json
   

4. Commit docs/openapi.json to git
5. Notify frontend that API contract changed

Frontend Developer Flow

1. Pull latest backend changes
2. Copy schema:

   cp ../backend/docs/openapi.json ./api-schema.json
   

3. Regenerate types:

   flutter pub run build_runner build --delete-conflicting-outputs
   

4. Update endpoint code if needed (new queries/commands)
5. Test with new types

---

Type Safety Guarantees

Strict Typing Rules

All generated code follows project strict typing standards:

• ✅ No dynamic types
• ✅ No any types
• ✅ Explicit type annotations everywhere
• ✅ Null safety enforced

Example: Generated Query

Backend defines:

public record HealthQuery();

Frontend generates:

class HealthQuery {
  const HealthQuery();

  Map<String, Object?> toJson() => {};

  factory HealthQuery.fromJson(Map<String, Object?> json) =>
    const HealthQuery();
}

---

Error Handling

Backend Contract

Backend returns structured errors:

{
  "message": "Validation failed",
  "statusCode": 422,
  "details": "Email is required"
}

Frontend Handling

Client wraps all responses in Result<T>:

final result = await client.executeQuery<UserDto>(
  endpoint: 'users/123',
  query: const GetUserQuery(),
  fromJson: UserDto.fromJson,
);

result.when(
  success: (user) => print('Got user: ${user.name}'),
  error: (error) => print('Error: ${error.message}'),
);

---

File Structure

Console/
├── api-schema.json              # Copied from backend (DO NOT EDIT)
├── build.yaml                   # Code generation config
├── lib/api/
│   ├── client.dart              # CQRS client (manual)
│   ├── types.dart               # Core types (manual)
│   ├── generated/               # Auto-generated (git-ignored)
│   │   └── .gitkeep
│   └── endpoints/
│       └── health_endpoint.dart # Endpoint extensions (manual)
└── .claude-docs/
    └── api-contract-workflow.md # This file

---

Benefits

For Backend

• ✅ Single source of truth (C# code with XML docs)
• ✅ Type-safe APIs enforced by compiler
• ✅ Swagger UI for testing
• ✅ Automatic client generation

For Frontend

• ✅ Type-safe API calls (no runtime errors)
• ✅ Auto-completion in IDE
• ✅ Compile-time validation
• ✅ No manual type definitions
• ✅ Always in sync with backend

For Team

• ✅ Clear contract boundaries
• ✅ Breaking changes caught early
• ✅ No API drift
• ✅ Shared understanding via OpenAPI spec

---

Troubleshooting

"Generated code has type errors"

Solution: Backend may have incomplete XML docs or invalid schema. Ask backend team to validate openapi.json.

"Types don't match backend"

Solution: Regenerate frontend types:

cp ../backend/docs/openapi.json ./api-schema.json
flutter pub run build_runner build --delete-conflicting-outputs

"Build runner fails"

Solution: Clean and rebuild:

flutter clean
flutter pub get
flutter pub run build_runner build --delete-conflicting-outputs

---

References

• OpenAPI Spec: api-schema.json
• Backend Docs: ../backend/docs/ARCHITECTURE.md
• Strict Typing: .claude-docs/strict-typing.md