openclaw - 💡(How to fix) Fix [Bug] Anthropic prefill error surfaces to user after context compaction (unmatched error pattern + missing trailing-assistant guard) [1 comments, 1 participants]

But does not check for or remove a trailing assistant turn. This causes Anthropic to reject the request with the prefill error.

2. Unrecognized error pattern

The SDK's role-ordering error recovery appears to match: But Anthropic returns a different error message for this case: Since this pattern isn't matched, the error bypasses the existing fallback/recovery logic and surfaces directly to the user. 2. The error recovery pattern should also match "does not support assistant message prefill" or "conversation must end with a user message"

Fix Plan

To address the issue, we need to modify the buildSessionContext() function to ensure the rebuilt context never ends with an assistant message after compaction. We also need to update the error recovery pattern to match the new error message.

Step 1: Update buildSessionContext() function

function buildSessionContext(compactionSummary, keptTail) {
  // Rebuild conversation context
  const context = rebuildContext(compactionSummary, keptTail);
  
  // Check if context ends with an assistant message
  if (context.turns[context.turns.length - 1].role === 'assistant') {
    // Remove trailing assistant turn
    context.turns.pop();
    
    // Add a new user turn to ensure context ends with a user message
    context.turns.push({ role: 'user', message: '' });
  }
  
  return context;
}

Step 2: Update error recovery pattern

const errorPatterns = [
  // Existing pattern
  'incorrect role information|roles must alternate',
  // New pattern to match Anthropic error message
  'does not support assistant message prefill|conversation must end with a user message',
];

function handleError(error) {
  const errorMessage = error.message;
  for (const pattern of errorPatterns) {
    if (errorMessage.match(pattern)) {
      // Trigger fallback/recovery logic
      return recoverFromError(error);
    }
  }
  // If no pattern matches, surface error to user
  return errorMessage;
}

Verification

To verify the fix, test the buildSessionContext() function with a sample compaction summary and kept tail that would previously produce a context ending with an assistant message. Ensure the rebuilt context now ends with a user message.

Additionally, test the error recovery pattern with the new Anthropic error message to ensure it triggers the fallback/recovery logic.

Extra Tips

• Make sure to update the OpenClaw SDK version to include these changes.
• Consider adding more test cases to cover different scenarios and ensure the fix is robust.
• Review the plugin team analysis and reference materials to ensure a thorough understanding of the issue and fix.