HTTP Error Codes Reference

This file documents HTTP error codes returned by the Claude API, their common causes, and how to handle them. For language-specific error handling examples, see the python/ or typescript/ folders.

Error Code Summary

Detailed Error Information

400 Bad Request

Causes:

• Malformed JSON in request body
• Missing required parameters (model, max_tokens, messages)
• Invalid parameter types (e.g., string where integer expected)
• Empty messages array
• Messages not alternating user/assistant

Example error:

{
  "type": "error",
  "error": {
    "type": "invalid_request_error",
    "message": "messages: roles must alternate between \\"user\\" and \\"assistant\\""
  },
  "request_id": "req_011CSHoEeqs5C35K2UUqR7Fy"
}

Fix: Validate request structure before sending. Check that:

• model is a valid model ID
• max_tokens is a positive integer
• messages array is non-empty and alternates correctly

---

401 Unauthorized

Causes:

• Missing x-api-key header or Authorization header
• Invalid API key format
• Revoked or deleted API key

Fix: Ensure ANTHROPIC_API_KEY environment variable is set correctly.

---

403 Forbidden

Causes:

• API key doesn't have access to the requested model
• Organization-level restrictions
• Attempting to access beta features without beta access

Fix: Check your API key permissions in the Console. You may need a different API key or to request access to specific features.

---

404 Not Found

Causes:

• Typo in model ID (e.g., claude-sonnet-4.6 instead of claude-sonnet-4-6)
• Using deprecated model ID
• Invalid API endpoint

Fix: Use exact model IDs from the models documentation. You can use aliases (e.g., {{OPUS_ID}}).

---

413 Request Too Large

Causes:

• Request body exceeds maximum size
• Too many tokens in input
• Image data too large

Fix: Reduce input size — truncate conversation history, compress/resize images, or split large documents into chunks.

---

400 Validation Errors

Some 400 errors are specifically related to parameter validation:

• max_tokens exceeds model's limit
• Invalid temperature value (must be 0.0-1.0)
• budget_tokens >= max_tokens in extended thinking
• Invalid tool definition schema

Model-specific 400s on Opus 4.7:

• temperature, top_p, top_k are removed — sending any of them returns 400. Delete the parameter; see shared/model-migration.md → Per-SDK Syntax Reference.
• thinking: {type: "enabled", budget_tokens: N} is removed — sending it returns 400. Use thinking: {type: "adaptive"} instead.

Common mistake with extended thinking on older models (Opus 4.6 and earlier):

# Wrong: budget_tokens must be < max_tokens
thinking: budget_tokens=10000, max_tokens=1000  → Error!

# Correct
thinking: budget_tokens=10000, max_tokens=16000

---

429 Rate Limited

Causes:

• Exceeded requests per minute (RPM)
• Exceeded tokens per minute (TPM)
• Exceeded tokens per day (TPD)

Headers to check:

• retry-after: Seconds to wait before retrying
• x-ratelimit-limit-*: Your limits
• x-ratelimit-remaining-*: Remaining quota

Fix: The Anthropic SDKs automatically retry 429 and 5xx errors with exponential backoff (default: max_retries=2). For custom retry behavior, see the language-specific error handling examples.

---

500 Internal Server Error

Causes:

• Temporary Anthropic service issue
• Bug in API processing

Fix: Retry with exponential backoff. If persistent, check status.anthropic.com.

---

529 Overloaded

Causes:

• High API demand
• Service capacity reached

Fix: Retry with exponential backoff. Consider using a different model (Haiku is often less loaded), spreading requests over time, or implementing request queuing.

---

Common Mistakes and Fixes

Typed Exceptions in SDKs

Always use the SDK's typed exception classes instead of checking error messages with string matching. Each HTTP error code maps to a specific exception class:

// ✅ Correct: use typed exceptions
try {
  const response = await client.messages.create({...});
} catch (error) {
  if (error instanceof Anthropic.RateLimitError) {
    // Handle rate limiting
  } else if (error instanceof Anthropic.APIError) {
    console.error(`API error ${error.status}:`, error.message);
  }
}

// ❌ Wrong: don't check error messages with string matching
try {
  const response = await client.messages.create({...});
} catch (error) {
  const msg = error instanceof Error ? error.message : String(error);
  if (msg.includes("429") || msg.includes("rate_limit")) { ... }
}

All exception classes extend Anthropic.APIError, which has a status property. Use instanceof checks from most specific to least specific (e.g., check RateLimitError before APIError).