Problem

Anthropic API rejects messages with empty content:

The content field in the Message object at messages.0 is empty.
Add a ContentBlock object to the content field and try again.

This occurs when:

1. Tool-use-only assistant turns are stored with content='' and zero message_parts in the database
2. filterNonFreshAssistantToolCalls() strips all tool_use blocks from a non-fresh assistant message, leaving content: []

Root Cause

assembler.ts assemble() passes rawMessages directly to sanitizeToolUseResultPairing() and then to the API without validating that assistant messages have non-empty content.

While filterNonFreshAssistantToolCalls() does skip messages where content.length === 0 after filtering, this only covers the array case. Messages with falsy string content ('') bypass this check via the !Array.isArray() branch and are passed through unchanged.

Fix

Add an explicit filter in assemble() before the return statement that removes assistant messages with empty content (either [] or falsy string). Only targets assistant role — user messages are left untouched.

const cleaned = rawMessages.filter(
  (m) =>
    !(
      m?.role === 'assistant' &&
      (Array.isArray(m.content) ? m.content.length === 0 : !m.content)
    ),
);

Testing

Verified on production instance with 5,246+ affected messages across 12 conversations. Filter correctly removes empty assistant messages while preserving all valid messages. All agents recovered immediately after deploying this fix.