400: text content blocks must be non-empty on resumed sessions

Bug Report

Version: 2.1.142 (also reproduced on 2.1.128)
Date: 2026-05-15

Description

Since today (~2026-05-15), resuming sessions triggers 400 API errors. This affects multiple unrelated projects.

Two error variants observed:

1. API Error: 400 messages.64.content.3.text: cache_control cannot be set for empty text blocks
2. API Error: 400 messages: text content blocks must be non-empty

Steps to Reproduce

1. Have an existing session with some conversation history
2. Resume the session (Resumed session >)
3. Send any message → 400 error

Root Cause (suspected)

The Anthropic API appears to have tightened validation to reject empty text content blocks (text: ""). Claude Code's conversation history (likely after compaction) contains empty text blocks that were previously tolerated by the API.

Expected Behavior

Claude Code should filter out empty text content blocks before sending requests to the API.

Workaround

Start a new session instead of resuming. Old sessions with existing empty blocks in history will consistently fail.