Anthropic Common Errors

Overview

Every Anthropic API error includes a type field and HTTP status code. Here are the real errors you'll hit and how to fix them.

Error Reference

Instructions

Step 1: authentication_error (401)

{"type":"error","error":{"type":"authentication_error","message":"invalid x-api-key"}}

Cause: API key is missing, malformed, or revoked. Fix:

# Verify key exists and starts with sk-ant-
echo $ANTHROPIC_API_KEY | head -c 10
# Should print: sk-ant-api

# Test directly
curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "claude-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{"model":"claude-sonnet-4-20250514","max_tokens":10,"messages":[{"role":"user","content":"hi"}]}'

---

Step 2: rate_limit_error (429)

{"type":"error","error":{"type":"rate_limit_error","message":"Number of request tokens has exceeded your per-minute rate limit"}}

Cause: Exceeded requests per minute (RPM) or tokens per minute (TPM). Fix:

// The SDK has built-in retries with backoff
const client = new Anthropic({
  maxRetries: 3, // default is 2
});

// Or handle manually using the retry-after header
try {
  const msg = await client.messages.create({ ... });
} catch (err) {
  if (err instanceof Anthropic.RateLimitError) {
    const retryAfter = err.headers?.['retry-after'];
    await sleep(Number(retryAfter) * 1000 || 5000);
    // retry...
  }
}

Rate limit tiers (as of 2025):

Tier	RPM	TPM (input)	TPM (output)
Tier 1 (free)	50	40,000	8,000
Tier 2 ($40+)	1,000	80,000	16,000
Tier 3 ($200+)	2,000	160,000	32,000
Tier 4 ($400+)	4,000	400,000	80,000

---

Step 3: overloaded_error (529)

{"type":"error","error":{"type":"overloaded_error","message":"Overloaded"}}

Cause: Anthropic API is temporarily at capacity. This is NOT a rate limit — it's server load. Fix:

// SDK retries 529s automatically. Increase retries if needed:
const client = new Anthropic({ maxRetries: 5 });

// For critical paths, implement fallback:
try {
  return await client.messages.create({ model: 'claude-sonnet-4-20250514', ... });
} catch (err) {
  if (err instanceof Anthropic.APIError && err.status === 529) {
    // Fall back to a different model or provider
    return await client.messages.create({ model: 'claude-haiku-4-5-20251001', ... });
  }
  throw err;
}

---

Step 4: invalid_request_error (400)

{"type":"error","error":{"type":"invalid_request_error","message":"messages: roles must alternate between \"user\" and \"assistant\", but found multiple \"user\" roles in a row"}}

Common causes:

• Messages don't alternate user/assistant
• max_tokens missing or exceeds model limit
• Image too large (>5MB) or wrong format
• Invalid model ID

Fix: Validate messages before sending:

function validateMessages(messages: Anthropic.MessageParam[]) {
  for (let i = 1; i < messages.length; i++) {
    if (messages[i].role === messages[i - 1].role) {
      throw new Error(`Messages must alternate roles. Index ${i} has same role as ${i-1}`);
    }
  }
  if (messages[0]?.role !== 'user') {
    throw new Error('First message must be from user');
  }
}

---

Step 5: not_found_error (404)

{"type":"error","error":{"type":"not_found_error","message":"model: model_not_found"}}

Cause: Invalid model ID or model not available on your plan. Fix: Use exact model IDs:

• claude-opus-4-20250514
• claude-sonnet-4-20250514
• claude-haiku-4-5-20251001

---

Step 6: Content too long (context window)

{"type":"error","error":{"type":"invalid_request_error","message":"prompt is too long: 204521 tokens > 200000 maximum"}}

Fix:

// Count tokens before sending (use Anthropic's token counting)
const count = await client.messages.countTokens({
  model: 'claude-sonnet-4-20250514',
  messages,
});
console.log(`Input tokens: ${count.input_tokens}`);

if (count.input_tokens > 180000) {
  // Truncate conversation history, keeping system + last N messages
  messages = [messages[0], ...messages.slice(-10)];
}

Quick Diagnostic

# Check API status
curl -s https://status.anthropic.com/api/v2/status.json | jq '.status.description'

# Verify API key works
curl -s https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "claude-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{"model":"claude-sonnet-4-20250514","max_tokens":10,"messages":[{"role":"user","content":"ping"}]}' | jq '.content[0].text'

# Check current usage/limits
# (No API for this — check console.anthropic.com/settings/limits)

Output

• Identified error type and HTTP status code
• Root cause determined from error message
• Applied fix (key rotation, backoff, input validation, model ID correction)
• Verified resolution with successful API call

Error Handling

Error	Cause	Solution
API Error	Check error type and status code	See clade-common-errors

Examples

Each error section above includes the exact JSON error response, cause analysis, and fix code. See Quick Diagnostic section for curl commands to test connectivity.

Resources

• Error Types Reference
• Rate Limits
• Anthropic Status

Next Steps

For deeper debugging, see clade-debug-bundle.

Prerequisites

• Anthropic SDK installed (@claude-ai/sdk or anthropic)
• API credentials configured
• Access to application logs or console output