Error Documentation — Error Handling in APIs | OneNoughtOne

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System Design (LLD)Error Handling in APIs

Error Handling in APIs

LevelIntermediate

Duration75 mins

TopicError Handling in APIs

4 / 4

Error Documentation

The Missing Chapter in Most API Documentation

Visit nearly any API documentation, and you'll find exhaustive coverage of success paths: endpoints, parameters, request/response schemas, authentication flows. Then look for error documentation. What you'll often find is... almost nothing. Perhaps a generic "Errors" section listing 400, 401, and 500 without context. Maybe a mention that "errors follow a standard format." This documentation gap creates real problems. Developers discover error conditions through production incidents. Integration partners build brittle error handling because they don't know what to expect. Support tickets multiply because consumers can't self-diagnose failures. Comprehensive error documentation is a competitive advantage. It signals maturity, builds trust, and reduces integration friction—yet it remains one of the most neglected aspects of API design.

What You Will Learn

By the end of this page, you will understand what error documentation should include, strategies for organizing error catalogs, techniques for keeping documentation current, and tools and formats that make error documentation discoverable and useful.

Why Error Documentation Matters

Error documentation directly impacts the success of API consumers and the cost of supporting your API. The investment pays dividends across multiple dimensions:

Benefits of Comprehensive Error Documentation

•Faster Integration — Developers can anticipate failure modes and build robust error handling from day one, not discover them through trial and error.
•Reduced Support Load — When consumers can self-diagnose errors, fewer tickets reach your support team. Each documented error code is a deflected ticket.
•Better User Experiences — When integrators know what errors to expect, they can craft appropriate user-facing messages. 'Payment declined (error code 4242)' isn't helpful; documented guidance is.
•Faster Debugging — Documented error codes with causes and solutions dramatically reduce mean time to resolution for production issues.
•Professional Trust — Comprehensive documentation signals a mature, well-engineered API. Enterprise customers evaluate documentation quality when choosing vendors.
•Contractual Clarity — Documented errors become part of the API's contract. Consumers know what to expect; you know what you've promised.

The Support Ticket Multiplier

Each undocumented error generates multiple support interactions: the initial discovery, attempts to reproduce, escalation for investigation, communication of the resolution. A single paragraph of documentation can eliminate hundreds of hours of support over the API's lifetime.

What to Document for Each Error

Effective error documentation answers the questions developers ask when encountering an error. For each error code or condition, include:

Essential Error Documentation Components
Component	Purpose	Example
Error Code	Unique identifier for programmatic handling	`PAYMENT.CARD_DECLINED`
HTTP Status	HTTP status code returned	`402 Payment Required`
Summary	One-line description of the error	Payment method was declined by issuer
Description	Detailed explanation of why this occurs	This error occurs when the card issuer refuses the charge. Common reasons include...
Causes	Specific conditions that trigger this error	Insufficient funds, Card expired, Fraud detection triggered
Resolution Steps	How to fix or work around the error	Try a different card. 2. Contact card issuer. 3. Verify billing details.
Retryable?	Whether retrying might succeed	No - customer action required
Example Response	Actual JSON of the error response	Complete response body example
Related Errors	Similar errors consumers might confuse	See also: `PAYMENT.CARD_EXPIRED`, `PAYMENT.INVALID_CARD`

error-documentation-example.md
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# PAYMENT.CARD_DECLINED
 
**HTTP Status:** 402 Payment Required
 
## Summary
The payment method was declined by the card issuer.
 
## Description
This error occurs when the customer's card issuer refuses to authorize the 
transaction. The issuer does not provide detailed reasons to merchants for 
security reasons, but common causes include insufficient funds, suspected 
fraud, or card restrictions.
 
## Common Causes
 
| Cause | Description |
|-------|-------------|
| Insufficient funds | Card/account lacks funds for this amount |
| Fraud prevention | Issuer's fraud detection blocked the transaction |
| Card restrictions | Card may not be authorized for this merchant type |
| Daily limit | Customer exceeded their daily spending limit |
 
## Resolution Steps
 
1. **Try a different payment method** — Use a different card or payment method
2. **Contact the card issuer** — The customer should call their bank for details
3. **Verify billing details** — Ensure address matches card registration
4. **Try a smaller amount** — May help identify if it's a limit issue
 
## Retryable?
**No** — Retrying with the same card will produce the same result. Customer 
action is required before retry.
 
## Example Response
 
```json
{
  "code": "PAYMENT.CARD_DECLINED",
  "message": "The card ending in 4242 was declined",
  "details": {
    "declineCode": "insufficient_funds",
    "cardLast4": "4242",
    "cardBrand": "visa"
  },
  "requestId": "req_abc123",
  "timestamp": "2024-01-15T14:30:00Z"
}
```
 
## Related Errors
- [PAYMENT.CARD_EXPIRED](/errors/payment-card-expired) — Card past expiration
- [PAYMENT.INVALID_CVV](/errors/payment-invalid-cvv) — CVV verification failed
- [PAYMENT.INSUFFICIENT_FUNDS](/errors/payment-insufficient-funds) — Specific 
  decline code when issuer provides it

Organizing Error Documentation

As your API grows, errors accumulate. Thoughtful organization makes documentation navigable and useful.

Organizational Strategies

•By Category/Domain — Group errors by functional area: Authentication, Payments, Users, Validation. Readers find errors relevant to what they're implementing.
•By HTTP Status — Group by 4xx vs 5xx, or by specific status codes. Useful for developers who know the status but not the specific error.
•By Endpoint — Document errors inline with each endpoint. Readers see all possible responses when implementing that endpoint.
•Alphabetical Error Code List — A comprehensive reference when developers know the error code and need details.
•Searchable Error Catalog — A searchable interface for large APIs with many error codes.

Recommended Approach: Multiple Views The best API documentation provides multiple entry points to error information: Global Error Reference — Complete catalog of all error codes, organized by category Per-Endpoint Errors — Each endpoint lists errors it can return, linked to the global reference Quick Reference Table — One-page table of all codes with one-line descriptions Searchable Interface — Filter/search errors by code, keyword, or category

error-organization-example
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# Per-endpoint error documentation in OpenAPI
paths:
  /orders:
    post:
      summary: Create an order
      operationId: createOrder
      responses:
        '201':
          description: Order created successfully
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Order'
        '400':
          description: Invalid request
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ErrorResponse'
              examples:
                invalid_json:
                  summary: Malformed request body
                  value:
                    code: "REQUEST.INVALID_JSON"
                    message: "Request body is not valid JSON"
        '401':
          $ref: '#/components/responses/Unauthorized'
        '422':
          description: Validation failed
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ValidationErrorResponse'
              examples:
                missing_items:
                  summary: Order has no items
                  value:
                    code: "VALIDATION.ORDER_EMPTY"
                    message: "Order must contain at least one item"
                    details:
                      field: "items"
                invalid_quantity:
                  summary: Invalid item quantity
                  value:
                    code: "VALIDATION.INVALID_QUANTITY"
                    message: "Item quantity must be between 1 and 100"
                    details:
                      field: "items[0].quantity"
                      value: 0
                      constraint:
                        min: 1
                        max: 100
 
# Reusable error responses
components:
  responses:
    Unauthorized:
      description: Authentication required or failed
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/ErrorResponse'
          examples:
            missing_token:
              value:
                code: "AUTH.TOKEN_MISSING"
                message: "Authorization header required"
            invalid_token:
              value:
                code: "AUTH.TOKEN_INVALID"
                message: "Authorization token is invalid"

In-Context Error Documentation

Beyond standalone error pages, errors should be documented where developers encounter them—in endpoint documentation, SDK references, and even in the errors themselves.

In-Context Documentation Patterns

•Endpoint Response Tables — Each endpoint shows all possible response codes with brief descriptions and links to full docs.
•Code Examples with Error Handling — Show how to handle errors in code samples, not just success paths.
•SDK Exception Documentation — In typed SDKs, document exception classes with causes and resolution.
•Error Response Links — Include documentation URLs directly in error response bodies.
•Interactive API Explorers — Show error responses as real examples users can trigger in sandboxes.

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/**
 * Creates a new order in the system.
 * 
 * @param items - Array of items to include in the order
 * @param shippingAddress - Destination address for delivery
 * @returns The created order with generated ID and status
 * 
 * @throws {ValidationError} When items are empty or invalid
 *         - `VALIDATION.ORDER_EMPTY`: No items provided
 *         - `VALIDATION.INVALID_QUANTITY`: Quantity out of range (1-100)
 *         - `VALIDATION.INVALID_SKU`: Unknown product SKU
 * 
 * @throws {AuthenticationError} When authentication fails
 *         - `AUTH.TOKEN_EXPIRED`: Refresh your token and retry
 *         - `AUTH.TOKEN_INVALID`: Re-authenticate required
 * 
 * @throws {PaymentError} When payment processing fails
 *         - `PAYMENT.CARD_DECLINED`: Customer must use different card
 *         - `PAYMENT.INSUFFICIENT_FUNDS`: Customer's card lacks funds
 * 
 * @throws {InventoryError} When items are unavailable
 *         - `INVENTORY.OUT_OF_STOCK`: Reduce quantity or remove item
 *         - `INVENTORY.DISCONTINUED`: Item no longer available
 * 
 * @example
 * // Handling errors appropriately
 * try {
 *     const order = await client.orders.create({
 *         items: [{ sku: 'WIDGET-001', quantity: 2 }],
 *         shippingAddress: address
 *     });
 *     console.log('Order created:', order.id);
 * } catch (err) {
 *     if (err instanceof ValidationError) {
 *         // Show validation messages to user
 *         displayErrors(err.details.errors);
 *     } else if (err instanceof PaymentError) {
 *         // Prompt for different payment method
 *         showPaymentForm({ error: err.message });
 *     } else if (err instanceof InventoryError) {
 *         // Update cart to reflect availability
 *         refreshCart();
 *     } else {
 *         // Unexpected error - log and show generic message
 *         console.error('Order failed:', err);
 *         showGenericError();
 *     }
 * }
 */
async function createOrder(
    items: OrderItem[], 
    shippingAddress: Address
): Promise<Order>;

Keeping Documentation Current

Outdated documentation is often worse than no documentation—it actively misleads consumers. Keeping error documentation current requires systematic processes:

Documentation Maintenance Strategies

•Error as Code — Define errors in code with metadata that generates documentation. When error definitions change, docs change automatically.
•CI Documentation Checks — Verify all error codes returned by the API are documented. Fail builds when undocumented errors appear.
•Schema-Driven Generation — Use OpenAPI or similar schemas as the source of truth. Generate docs from schemas.
•Review Error Changes — PR reviews should check: "Does this change add/modify/remove errors? Is documentation updated?"
•Version Documentation — Track when errors were added, deprecated, or changed. Consumers on older versions need old docs.
•Monitor Production Errors — Track error codes returned in production. Unknown codes indicate documentation gaps.

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// Errors defined as code with full documentation metadata
// This single source generates API responses AND documentation
 
export const Errors = {
    AUTH_TOKEN_EXPIRED: defineError({
        code: "AUTH.TOKEN_EXPIRED",
        httpStatus: 401,
        
        // Documentation fields
        summary: "Authentication token has expired",
        description: `
            Access tokens expire after a configurable period (default: 1 hour).
            When expired, requests fail with this error until a new token is obtained.
        `,
        causes: [
            "Token was issued more than TTL ago",
            "Token was explicitly revoked"
        ],
        resolution: [
            "Use refresh token to obtain new access token",
            "Re-authenticate if refresh token also expired"
        ],
        retryable: true,
        retryAfter: "After obtaining new token",
        
        // Links
        docs: "/errors/auth-token-expired",
        guide: "/guides/authentication#token-refresh",
        
        // Version tracking
        since: "1.0.0",
        deprecated: false,
        
        // Example response
        example: {
            expiredAt: "2024-01-15T14:00:00Z"
        },
        
        // Related errors
        related: ["AUTH.TOKEN_INVALID", "AUTH.TOKEN_MISSING"]
    }),
    
    // ... more errors
};
 
// Generate markdown docs
function generateDocs(): string {
    return Object.values(Errors)
        .map(error => `
## ${error.code}
 
**HTTP Status:** ${error.httpStatus}
 
### Summary
${error.summary}
 
### Description
${error.description}
 
### Causes
${error.causes.map(c => `- ${c}`).join('
')}
 
### Resolution
${error.resolution.map((r, i) => `${i + 1}. ${r}`).join('
')}
 
### Retryable
${error.retryable ? `Yes — ${error.retryAfter}` : 'No'}
        `).join('
---
');
}

Documentation as Tests

Treat documentation completeness as a testable property. Just as you test that endpoints return correct responses, test that every returned error code has corresponding documentation. Automation catches gaps before users do.

Error Documentation Formats and Tools

Different tools and formats serve different documentation needs. Choose based on your existing toolchain and consumer preferences.

Error Documentation Formats
Format/Tool	Best For	Considerations
OpenAPI/Swagger	REST APIs with generated docs	Standard format; broad tooling; can be verbose for complex errors
Markdown/Static Sites	Detailed, prose-heavy documentation	Flexible layout; good for guides and examples; manual maintenance
API Reference Platforms	Unified API documentation	ReadMe, Stoplight, Redocly; search and interactivity built-in
Error Lookup Endpoint	Runtime error discovery	GET /errors/{code} returns full docs; self-service debugging
SDK Documentation	Language-specific integration	Javadoc, TSDoc, etc.; exception classes fully documented
Status Pages	System-wide error status	For operational issues affecting many requests; not for per-request errors

error-lookup-endpoint.ts
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// Error lookup API endpoint - self-service error documentation
import express from 'express';
import { errorCatalog, ErrorDocumentation } from './docs/error-catalog';
 
const app = express();
 
// Initialize error index
const errorIndex = new Map<string, ErrorDocumentation>(
    errorCatalog.map(e => [e.code, e])
);
 
// GET /errors - List all error codes
app.get('/errors', (req, res) => {
    const { category, httpStatus, search } = req.query;
    
    let results = [...errorCatalog];
    
    if (category) {
        results = results.filter(e => e.category === category);
    }
    if (httpStatus) {
        results = results.filter(e => e.httpStatus === parseInt(httpStatus as string));
    }
    if (search) {
        const term = (search as string).toLowerCase();
        results = results.filter(e => 
            e.code.toLowerCase().includes(term) ||
            e.summary.toLowerCase().includes(term) ||
            e.description.toLowerCase().includes(term)
        );
    }
    
    res.json({
        errors: results.map(e => ({
            code: e.code,
            httpStatus: e.httpStatus,
            summary: e.summary,
            category: e.category,
            detailUrl: `/errors/${encodeURIComponent(e.code)}`
        })),
        categories: [...new Set(errorCatalog.map(e => e.category))],
        total: results.length
    });
});
 
// GET /errors/:code - Full documentation for specific error
app.get('/errors/:code', (req, res) => {
    const code = decodeURIComponent(req.params.code);
    const error = errorIndex.get(code);
    
    if (!error) {
        return res.status(404).json({
            code: 'ERROR.NOT_FOUND',
            message: `No documentation found for error code: ${code}`,
            suggestion: 'Use GET /errors to list all known error codes'
        });
    }
    
    res.json({
        ...error,
        links: {
            self: `/errors/${encodeURIComponent(code)}`,
            related: error.relatedErrors.map(
                r => `/errors/${encodeURIComponent(r)}`
            ),
            humanReadable: `https://docs.example.com/errors/${error.code}`
        }
    });
});
 
// Include error lookup in error responses themselves
function formatErrorResponse(error: ApiError, requestId: string) {
    return {
        code: error.code,
        message: error.message,
        details: error.details,
        requestId,
        timestamp: new Date().toISOString(),
        documentation: {
            url: `https://api.example.com/errors/${encodeURIComponent(error.code)}`,
            humanUrl: `https://docs.example.com/errors/${error.code}`
        }
    };
}

Error Changelog and Versioning

Errors evolve over time. New errors are introduced, old errors are deprecated, and error details change. Tracking these changes helps consumers maintain compatibility.

Error Changelog Best Practices

•Document when errors were added — Include the API version that introduced each error. Consumers on older versions won't encounter new errors.
•Document deprecations with alternatives — When retiring an error, explain what replaces it and when it will be removed.
•Track detail field changes — If an error's details structure changes, note what was added/removed.
•Version your error documentation — Consumers on v1 of your API need v1 error docs, not v2.
•Announce breaking error changes — Removing an error code or changing its meaning is breaking. Treat it like any other breaking change.

error-changelog.md
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# Error Changelog
 
## Version 2.3.0 (2024-01-15)
 
### New Errors
 
- **PAYMENT.3DS_REQUIRED** — Returned when payment requires 3D Secure
  authentication. See [3D Secure Guide](/guides/3d-secure) for handling.
  
- **USER.MFA_REQUIRED** — Returned when action requires MFA verification.
  Includes `mfaMethods` in details with available verification options.
 
### Deprecated Errors
 
- **AUTH.SESSION_EXPIRED** — *Deprecated*. Will be removed in v3.0.
  Use `AUTH.TOKEN_EXPIRED` instead. Both are returned during transition period.
 
### Changed
 
- **VALIDATION.FIELD_INVALID** — Now includes `suggestedValue` in details
  when the system can infer a correction (e.g., typo in country code).
 
---
 
## Version 2.2.0 (2023-11-01)
 
### New Errors
 
- **RATE_LIMIT.QUOTA_EXHAUSTED** — Separate from `RATE_LIMIT.EXCEEDED`.
  Returned when monthly quota is exhausted (not rate-limited).
  Includes `quotaResetAt` timestamp.
 
### Breaking Changes
 
- **VALIDATION.REQUIRED_FIELD** renamed to **VALIDATION.FIELD_REQUIRED**
  for consistency. Old code will not work.
 
---
 
## Migration Guide: v2.1 to v2.2
 
### Error Code Rename
 
If your code handles `VALIDATION.REQUIRED_FIELD`, update to 
`VALIDATION.FIELD_REQUIRED`:
 
```javascript
// Before (v2.1)
if (error.code === 'VALIDATION.REQUIRED_FIELD') { ... }
 
// After (v2.2+)
if (error.code === 'VALIDATION.FIELD_REQUIRED') { ... }
```

Error Code Changes Are Breaking

Never rename an error code in a minor or patch version. Consumers programmatically match on codes. A rename is a breaking change requiring a major version bump and migration guidance.

System Design (LLD)Error Handling in APIs

Error Handling in APIs

LevelIntermediate

Duration75 mins

TopicError Handling in APIs

4 / 4

Error Documentation

The Missing Chapter in Most API Documentation

Visit nearly any API documentation, and you'll find exhaustive coverage of success paths: endpoints, parameters, request/response schemas, authentication flows. Then look for error documentation. What you'll often find is... almost nothing. Perhaps a generic "Errors" section listing 400, 401, and 500 without context. Maybe a mention that "errors follow a standard format." This documentation gap creates real problems. Developers discover error conditions through production incidents. Integration partners build brittle error handling because they don't know what to expect. Support tickets multiply because consumers can't self-diagnose failures. Comprehensive error documentation is a competitive advantage. It signals maturity, builds trust, and reduces integration friction—yet it remains one of the most neglected aspects of API design.

What You Will Learn

By the end of this page, you will understand what error documentation should include, strategies for organizing error catalogs, techniques for keeping documentation current, and tools and formats that make error documentation discoverable and useful.

Why Error Documentation Matters

Error documentation directly impacts the success of API consumers and the cost of supporting your API. The investment pays dividends across multiple dimensions:

Benefits of Comprehensive Error Documentation

•Faster Integration — Developers can anticipate failure modes and build robust error handling from day one, not discover them through trial and error.
•Reduced Support Load — When consumers can self-diagnose errors, fewer tickets reach your support team. Each documented error code is a deflected ticket.
•Better User Experiences — When integrators know what errors to expect, they can craft appropriate user-facing messages. 'Payment declined (error code 4242)' isn't helpful; documented guidance is.
•Faster Debugging — Documented error codes with causes and solutions dramatically reduce mean time to resolution for production issues.
•Professional Trust — Comprehensive documentation signals a mature, well-engineered API. Enterprise customers evaluate documentation quality when choosing vendors.
•Contractual Clarity — Documented errors become part of the API's contract. Consumers know what to expect; you know what you've promised.

The Support Ticket Multiplier

Each undocumented error generates multiple support interactions: the initial discovery, attempts to reproduce, escalation for investigation, communication of the resolution. A single paragraph of documentation can eliminate hundreds of hours of support over the API's lifetime.

What to Document for Each Error

Effective error documentation answers the questions developers ask when encountering an error. For each error code or condition, include:

Essential Error Documentation Components
Component	Purpose	Example
Error Code	Unique identifier for programmatic handling	`PAYMENT.CARD_DECLINED`
HTTP Status	HTTP status code returned	`402 Payment Required`
Summary	One-line description of the error	Payment method was declined by issuer
Description	Detailed explanation of why this occurs	This error occurs when the card issuer refuses the charge. Common reasons include...
Causes	Specific conditions that trigger this error	Insufficient funds, Card expired, Fraud detection triggered
Resolution Steps	How to fix or work around the error	Try a different card. 2. Contact card issuer. 3. Verify billing details.
Retryable?	Whether retrying might succeed	No - customer action required
Example Response	Actual JSON of the error response	Complete response body example
Related Errors	Similar errors consumers might confuse	See also: `PAYMENT.CARD_EXPIRED`, `PAYMENT.INVALID_CARD`

error-documentation-example.md
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# PAYMENT.CARD_DECLINED
 
**HTTP Status:** 402 Payment Required
 
## Summary
The payment method was declined by the card issuer.
 
## Description
This error occurs when the customer's card issuer refuses to authorize the 
transaction. The issuer does not provide detailed reasons to merchants for 
security reasons, but common causes include insufficient funds, suspected 
fraud, or card restrictions.
 
## Common Causes
 
| Cause | Description |
|-------|-------------|
| Insufficient funds | Card/account lacks funds for this amount |
| Fraud prevention | Issuer's fraud detection blocked the transaction |
| Card restrictions | Card may not be authorized for this merchant type |
| Daily limit | Customer exceeded their daily spending limit |
 
## Resolution Steps
 
1. **Try a different payment method** — Use a different card or payment method
2. **Contact the card issuer** — The customer should call their bank for details
3. **Verify billing details** — Ensure address matches card registration
4. **Try a smaller amount** — May help identify if it's a limit issue
 
## Retryable?
**No** — Retrying with the same card will produce the same result. Customer 
action is required before retry.
 
## Example Response
 
```json
{
  "code": "PAYMENT.CARD_DECLINED",
  "message": "The card ending in 4242 was declined",
  "details": {
    "declineCode": "insufficient_funds",
    "cardLast4": "4242",
    "cardBrand": "visa"
  },
  "requestId": "req_abc123",
  "timestamp": "2024-01-15T14:30:00Z"
}
```
 
## Related Errors
- [PAYMENT.CARD_EXPIRED](/errors/payment-card-expired) — Card past expiration
- [PAYMENT.INVALID_CVV](/errors/payment-invalid-cvv) — CVV verification failed
- [PAYMENT.INSUFFICIENT_FUNDS](/errors/payment-insufficient-funds) — Specific 
  decline code when issuer provides it

Organizing Error Documentation

As your API grows, errors accumulate. Thoughtful organization makes documentation navigable and useful.

Organizational Strategies

•By Category/Domain — Group errors by functional area: Authentication, Payments, Users, Validation. Readers find errors relevant to what they're implementing.
•By HTTP Status — Group by 4xx vs 5xx, or by specific status codes. Useful for developers who know the status but not the specific error.
•By Endpoint — Document errors inline with each endpoint. Readers see all possible responses when implementing that endpoint.
•Alphabetical Error Code List — A comprehensive reference when developers know the error code and need details.
•Searchable Error Catalog — A searchable interface for large APIs with many error codes.

Recommended Approach: Multiple Views The best API documentation provides multiple entry points to error information: Global Error Reference — Complete catalog of all error codes, organized by category Per-Endpoint Errors — Each endpoint lists errors it can return, linked to the global reference Quick Reference Table — One-page table of all codes with one-line descriptions Searchable Interface — Filter/search errors by code, keyword, or category

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# Per-endpoint error documentation in OpenAPI
paths:
  /orders:
    post:
      summary: Create an order
      operationId: createOrder
      responses:
        '201':
          description: Order created successfully
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Order'
        '400':
          description: Invalid request
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ErrorResponse'
              examples:
                invalid_json:
                  summary: Malformed request body
                  value:
                    code: "REQUEST.INVALID_JSON"
                    message: "Request body is not valid JSON"
        '401':
          $ref: '#/components/responses/Unauthorized'
        '422':
          description: Validation failed
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ValidationErrorResponse'
              examples:
                missing_items:
                  summary: Order has no items
                  value:
                    code: "VALIDATION.ORDER_EMPTY"
                    message: "Order must contain at least one item"
                    details:
                      field: "items"
                invalid_quantity:
                  summary: Invalid item quantity
                  value:
                    code: "VALIDATION.INVALID_QUANTITY"
                    message: "Item quantity must be between 1 and 100"
                    details:
                      field: "items[0].quantity"
                      value: 0
                      constraint:
                        min: 1
                        max: 100
 
# Reusable error responses
components:
  responses:
    Unauthorized:
      description: Authentication required or failed
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/ErrorResponse'
          examples:
            missing_token:
              value:
                code: "AUTH.TOKEN_MISSING"
                message: "Authorization header required"
            invalid_token:
              value:
                code: "AUTH.TOKEN_INVALID"
                message: "Authorization token is invalid"

In-Context Error Documentation

Beyond standalone error pages, errors should be documented where developers encounter them—in endpoint documentation, SDK references, and even in the errors themselves.

In-Context Documentation Patterns

•Endpoint Response Tables — Each endpoint shows all possible response codes with brief descriptions and links to full docs.
•Code Examples with Error Handling — Show how to handle errors in code samples, not just success paths.
•SDK Exception Documentation — In typed SDKs, document exception classes with causes and resolution.
•Error Response Links — Include documentation URLs directly in error response bodies.
•Interactive API Explorers — Show error responses as real examples users can trigger in sandboxes.

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/**
 * Creates a new order in the system.
 * 
 * @param items - Array of items to include in the order
 * @param shippingAddress - Destination address for delivery
 * @returns The created order with generated ID and status
 * 
 * @throws {ValidationError} When items are empty or invalid
 *         - `VALIDATION.ORDER_EMPTY`: No items provided
 *         - `VALIDATION.INVALID_QUANTITY`: Quantity out of range (1-100)
 *         - `VALIDATION.INVALID_SKU`: Unknown product SKU
 * 
 * @throws {AuthenticationError} When authentication fails
 *         - `AUTH.TOKEN_EXPIRED`: Refresh your token and retry
 *         - `AUTH.TOKEN_INVALID`: Re-authenticate required
 * 
 * @throws {PaymentError} When payment processing fails
 *         - `PAYMENT.CARD_DECLINED`: Customer must use different card
 *         - `PAYMENT.INSUFFICIENT_FUNDS`: Customer's card lacks funds
 * 
 * @throws {InventoryError} When items are unavailable
 *         - `INVENTORY.OUT_OF_STOCK`: Reduce quantity or remove item
 *         - `INVENTORY.DISCONTINUED`: Item no longer available
 * 
 * @example
 * // Handling errors appropriately
 * try {
 *     const order = await client.orders.create({
 *         items: [{ sku: 'WIDGET-001', quantity: 2 }],
 *         shippingAddress: address
 *     });
 *     console.log('Order created:', order.id);
 * } catch (err) {
 *     if (err instanceof ValidationError) {
 *         // Show validation messages to user
 *         displayErrors(err.details.errors);
 *     } else if (err instanceof PaymentError) {
 *         // Prompt for different payment method
 *         showPaymentForm({ error: err.message });
 *     } else if (err instanceof InventoryError) {
 *         // Update cart to reflect availability
 *         refreshCart();
 *     } else {
 *         // Unexpected error - log and show generic message
 *         console.error('Order failed:', err);
 *         showGenericError();
 *     }
 * }
 */
async function createOrder(
    items: OrderItem[], 
    shippingAddress: Address
): Promise<Order>;

Keeping Documentation Current

Outdated documentation is often worse than no documentation—it actively misleads consumers. Keeping error documentation current requires systematic processes:

Documentation Maintenance Strategies

•Error as Code — Define errors in code with metadata that generates documentation. When error definitions change, docs change automatically.
•CI Documentation Checks — Verify all error codes returned by the API are documented. Fail builds when undocumented errors appear.
•Schema-Driven Generation — Use OpenAPI or similar schemas as the source of truth. Generate docs from schemas.
•Review Error Changes — PR reviews should check: "Does this change add/modify/remove errors? Is documentation updated?"
•Version Documentation — Track when errors were added, deprecated, or changed. Consumers on older versions need old docs.
•Monitor Production Errors — Track error codes returned in production. Unknown codes indicate documentation gaps.

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// Errors defined as code with full documentation metadata
// This single source generates API responses AND documentation
 
export const Errors = {
    AUTH_TOKEN_EXPIRED: defineError({
        code: "AUTH.TOKEN_EXPIRED",
        httpStatus: 401,
        
        // Documentation fields
        summary: "Authentication token has expired",
        description: `
            Access tokens expire after a configurable period (default: 1 hour).
            When expired, requests fail with this error until a new token is obtained.
        `,
        causes: [
            "Token was issued more than TTL ago",
            "Token was explicitly revoked"
        ],
        resolution: [
            "Use refresh token to obtain new access token",
            "Re-authenticate if refresh token also expired"
        ],
        retryable: true,
        retryAfter: "After obtaining new token",
        
        // Links
        docs: "/errors/auth-token-expired",
        guide: "/guides/authentication#token-refresh",
        
        // Version tracking
        since: "1.0.0",
        deprecated: false,
        
        // Example response
        example: {
            expiredAt: "2024-01-15T14:00:00Z"
        },
        
        // Related errors
        related: ["AUTH.TOKEN_INVALID", "AUTH.TOKEN_MISSING"]
    }),
    
    // ... more errors
};
 
// Generate markdown docs
function generateDocs(): string {
    return Object.values(Errors)
        .map(error => `
## ${error.code}
 
**HTTP Status:** ${error.httpStatus}
 
### Summary
${error.summary}
 
### Description
${error.description}
 
### Causes
${error.causes.map(c => `- ${c}`).join('
')}
 
### Resolution
${error.resolution.map((r, i) => `${i + 1}. ${r}`).join('
')}
 
### Retryable
${error.retryable ? `Yes — ${error.retryAfter}` : 'No'}
        `).join('
---
');
}

Documentation as Tests

Treat documentation completeness as a testable property. Just as you test that endpoints return correct responses, test that every returned error code has corresponding documentation. Automation catches gaps before users do.

Error Documentation Formats and Tools

Different tools and formats serve different documentation needs. Choose based on your existing toolchain and consumer preferences.

Error Documentation Formats
Format/Tool	Best For	Considerations
OpenAPI/Swagger	REST APIs with generated docs	Standard format; broad tooling; can be verbose for complex errors
Markdown/Static Sites	Detailed, prose-heavy documentation	Flexible layout; good for guides and examples; manual maintenance
API Reference Platforms	Unified API documentation	ReadMe, Stoplight, Redocly; search and interactivity built-in
Error Lookup Endpoint	Runtime error discovery	GET /errors/{code} returns full docs; self-service debugging
SDK Documentation	Language-specific integration	Javadoc, TSDoc, etc.; exception classes fully documented
Status Pages	System-wide error status	For operational issues affecting many requests; not for per-request errors

error-lookup-endpoint.ts
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// Error lookup API endpoint - self-service error documentation
import express from 'express';
import { errorCatalog, ErrorDocumentation } from './docs/error-catalog';
 
const app = express();
 
// Initialize error index
const errorIndex = new Map<string, ErrorDocumentation>(
    errorCatalog.map(e => [e.code, e])
);
 
// GET /errors - List all error codes
app.get('/errors', (req, res) => {
    const { category, httpStatus, search } = req.query;
    
    let results = [...errorCatalog];
    
    if (category) {
        results = results.filter(e => e.category === category);
    }
    if (httpStatus) {
        results = results.filter(e => e.httpStatus === parseInt(httpStatus as string));
    }
    if (search) {
        const term = (search as string).toLowerCase();
        results = results.filter(e => 
            e.code.toLowerCase().includes(term) ||
            e.summary.toLowerCase().includes(term) ||
            e.description.toLowerCase().includes(term)
        );
    }
    
    res.json({
        errors: results.map(e => ({
            code: e.code,
            httpStatus: e.httpStatus,
            summary: e.summary,
            category: e.category,
            detailUrl: `/errors/${encodeURIComponent(e.code)}`
        })),
        categories: [...new Set(errorCatalog.map(e => e.category))],
        total: results.length
    });
});
 
// GET /errors/:code - Full documentation for specific error
app.get('/errors/:code', (req, res) => {
    const code = decodeURIComponent(req.params.code);
    const error = errorIndex.get(code);
    
    if (!error) {
        return res.status(404).json({
            code: 'ERROR.NOT_FOUND',
            message: `No documentation found for error code: ${code}`,
            suggestion: 'Use GET /errors to list all known error codes'
        });
    }
    
    res.json({
        ...error,
        links: {
            self: `/errors/${encodeURIComponent(code)}`,
            related: error.relatedErrors.map(
                r => `/errors/${encodeURIComponent(r)}`
            ),
            humanReadable: `https://docs.example.com/errors/${error.code}`
        }
    });
});
 
// Include error lookup in error responses themselves
function formatErrorResponse(error: ApiError, requestId: string) {
    return {
        code: error.code,
        message: error.message,
        details: error.details,
        requestId,
        timestamp: new Date().toISOString(),
        documentation: {
            url: `https://api.example.com/errors/${encodeURIComponent(error.code)}`,
            humanUrl: `https://docs.example.com/errors/${error.code}`
        }
    };
}

Error Changelog and Versioning

Errors evolve over time. New errors are introduced, old errors are deprecated, and error details change. Tracking these changes helps consumers maintain compatibility.

Error Changelog Best Practices

•Document when errors were added — Include the API version that introduced each error. Consumers on older versions won't encounter new errors.
•Document deprecations with alternatives — When retiring an error, explain what replaces it and when it will be removed.
•Track detail field changes — If an error's details structure changes, note what was added/removed.
•Version your error documentation — Consumers on v1 of your API need v1 error docs, not v2.
•Announce breaking error changes — Removing an error code or changing its meaning is breaking. Treat it like any other breaking change.

error-changelog.md
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# Error Changelog
 
## Version 2.3.0 (2024-01-15)
 
### New Errors
 
- **PAYMENT.3DS_REQUIRED** — Returned when payment requires 3D Secure
  authentication. See [3D Secure Guide](/guides/3d-secure) for handling.
  
- **USER.MFA_REQUIRED** — Returned when action requires MFA verification.
  Includes `mfaMethods` in details with available verification options.
 
### Deprecated Errors
 
- **AUTH.SESSION_EXPIRED** — *Deprecated*. Will be removed in v3.0.
  Use `AUTH.TOKEN_EXPIRED` instead. Both are returned during transition period.
 
### Changed
 
- **VALIDATION.FIELD_INVALID** — Now includes `suggestedValue` in details
  when the system can infer a correction (e.g., typo in country code).
 
---
 
## Version 2.2.0 (2023-11-01)
 
### New Errors
 
- **RATE_LIMIT.QUOTA_EXHAUSTED** — Separate from `RATE_LIMIT.EXCEEDED`.
  Returned when monthly quota is exhausted (not rate-limited).
  Includes `quotaResetAt` timestamp.
 
### Breaking Changes
 
- **VALIDATION.REQUIRED_FIELD** renamed to **VALIDATION.FIELD_REQUIRED**
  for consistency. Old code will not work.
 
---
 
## Migration Guide: v2.1 to v2.2
 
### Error Code Rename
 
If your code handles `VALIDATION.REQUIRED_FIELD`, update to 
`VALIDATION.FIELD_REQUIRED`:
 
```javascript
// Before (v2.1)
if (error.code === 'VALIDATION.REQUIRED_FIELD') { ... }
 
// After (v2.2+)
if (error.code === 'VALIDATION.FIELD_REQUIRED') { ... }
```

Error Code Changes Are Breaking

Never rename an error code in a minor or patch version. Consumers programmatically match on codes. A rename is a breaking change requiring a major version bump and migration guidance.

Error Documentation

The Missing Chapter in Most API Documentation

Why Error Documentation Matters

What to Document for Each Error

Organizing Error Documentation

In-Context Error Documentation

Keeping Documentation Current

Error Documentation Formats and Tools

Error Changelog and Versioning

There’s more to understand.

Error Documentation

The Missing Chapter in Most API Documentation

Why Error Documentation Matters

What to Document for Each Error

Organizing Error Documentation

In-Context Error Documentation

Keeping Documentation Current

Error Documentation Formats and Tools

Error Changelog and Versioning

There’s more to understand.