API Docs — OpenAPI Enrichment & Reference

Core Principles

• Enrich OpenAPI spec with summaries, descriptions, and examples
• Generate Markdown API reference from endpoint metadata
• Microservice mode: gateway API docs with gRPC-to-REST mapping notes
• Generic mode: standard REST API reference
• Detect missing documentation and suggest improvements

Key Patterns

OpenAPI Enrichment via Attributes

[HttpGet]
[EndpointSummary("List orders with filtering and pagination")]
[EndpointDescription("Returns a paginated list of orders. Supports search by customer name and status filtering.")]
[ProducesResponseType(typeof(Paginated<OrderSummaryResponse>), 200)]
[ProducesResponseType(typeof(ProblemDetails), 400)]
public async Task<ActionResult<Paginated<OrderSummaryResponse>>> GetAll(
    [Description("Page number (1-based)")] [FromQuery] int page = 1,
    [Description("Items per page (max 100)")] [FromQuery] int pageSize = 20,
    [Description("Search by customer name")] [FromQuery] string? search = null)

Document Transformer for Global Info

options.AddDocumentTransformer((document, context, ct) =>
{
    document.Info = new OpenApiInfo
    {
        Title = "{Company} {Domain} API",
        Version = "v1",
        Description = """
            REST API for the {Domain} service.

            ## Authentication
            All endpoints require a Bearer JWT token.

            ## Pagination
            List endpoints return `Paginated<T>` with `items`, `totalCount`, `page`, `pageSize`.

            ## Error Handling
            Errors return RFC 9457 ProblemDetails.
            """,
    };
    return Task.CompletedTask;
});

Markdown API Reference Template

# {Domain} API Reference

## Authentication
All endpoints require a Bearer JWT token in the Authorization header.

## Endpoints

### Orders

#### List Orders
`GET /api/v1/orders`

| Parameter | Type | Required | Description |
|---|---|---|---|
| page | int | No | Page number (default: 1) |
| pageSize | int | No | Items per page (default: 20, max: 100) |
| search | string | No | Search by customer name |

**Response** `200 OK`
```json
{
  "items": [{ "id": "...", "customerName": "...", "total": 100.00 }],
  "totalCount": 42,
  "page": 1,
  "pageSize": 20
}

Create Order

POST /api/v1/orders

Request Body

{
  "customerName": "string",
  "total": 100.00,
  "items": [{ "productId": "guid", "quantity": 1, "unitPrice": 50.00 }]
}

Response 201 Created

### Scanning for Missing Docs

```bash
# Find endpoints missing summaries
grep -rn "HttpGet\|HttpPost\|HttpPut\|HttpDelete" --include="*.cs" src/ | \
  while read line; do
    file=$(echo $line | cut -d: -f1)
    grep -q "EndpointSummary\|Summary\|WithSummary" "$file" || echo "Missing docs: $line"
  done

# Find controllers missing ProducesResponseType
grep -rn "\[Http" --include="*.cs" src/Controllers/ | \
  while read line; do
    file=$(echo $line | cut -d: -f1)
    grep -q "ProducesResponseType" "$file" || echo "Missing response types: $line"
  done

Anti-Patterns

Anti-Pattern	Correct Approach
No endpoint descriptions	Add EndpointSummary and Description
Missing response type annotations	Add ProducesResponseType for each status
No error response documentation	Document ProblemDetails responses
Outdated API reference	Generate from OpenAPI spec, not manually

Detect Existing Patterns

# Find OpenAPI configuration
grep -r "AddOpenApi\|MapOpenApi" --include="*.cs" src/

# Find endpoint documentation
grep -r "EndpointSummary\|WithSummary\|ProducesResponseType" --include="*.cs" src/

# Find existing API docs
find . -name "*api*" -path "*/docs/*" -name "*.md"

Adding to Existing Project

1. Scan existing endpoints for missing OpenAPI metadata
2. Add summaries and descriptions to undocumented endpoints
3. Add ProducesResponseType for all possible status codes
4. Generate Markdown reference from OpenAPI spec
5. Keep docs in sync by generating from code, not writing manually