[BUG] Cowork Dispatch: every message fails with 400 "text content blocks must contain non-whitespace text" (reproduces on mobile + desktop)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Summary

In Cowork Dispatch mode, every user message — including a single hello in a brand-new task — is
rejected with HTTP 400 invalid_request_error. The error indicates the payload sent to the Messages API
contains an empty/whitespace-only text content block. Since I, as the user, only typed hello, the empty
block is being injected by Cowork itself (tool-init, session setup, or stored thread state).

Steps to reproduce

1. Open Cowork → Dispatch.
2. Click + New task.
3. Type hello and send.
4. Observe repeated API Error: 400 responses with message messages: text content blocks must contain

non-whitespace text.

Request IDs

• req_011CaAffzATeWP6HzQW3AkBh
• req_011CaAFh6aT9obRVVe85cELK
• req_011CaFtbArnzpQWUEeh3fniw

What I ruled out

• Not browser-specific (reproduces cross-device).
• Not a stale thread (reproduces in a freshly created Dispatch task).
• Not the user message itself (a 5-character hello cannot be the whitespace-only block).
• Not account auth (401/403 would be returned instead).

Workaround

Using regular claude.ai chat and Claude Code CLI works fine — issue is scoped to Cowork Dispatch.

<img width="1610" height="263" alt="Image" src="https://github.com/user-attachments/assets/94f30510-f58a-4e7b-89a9-f82fead144cd" />

What Should Happen?

Expected behavior

The message sends successfully and Dispatch responds.

Error Messages/Logs

**Actual behavior**                                                                                        
                                               
  Every send returns:                         
  API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages: text
  content blocks must contain non-whitespace text","request_id":"..."}}

Steps to Reproduce

Steps-to-Reproduce

1. Open Cowork (https://claude.ai/cowork) in any browser on desktop or mobile.
2. In the left sidebar, click Dispatch.
3. Click + New task to create a fresh Dispatch thread.
4. In the message input at the bottom, type a single word — e.g. hello — and press Enter.
5. Error appears immediately in the thread:

API Error: 400 {"type":"error","error":{"type":"invalid_request_error",
"message":"messages: text content blocks must contain non-whitespace text",
"request_id":"req_011CaAffzATeWP6HzQW3AkBh"}}

1. Every subsequent message in the thread fails with the same 400.
2. Creating another fresh Dispatch task reproduces the same error.

Environment at time of failure:

• Keep awake: ON
• Allow all browser actions: ON
• Computer use: ON
• Code permissions: Accept edits
• No Project attached
• No files attached to the message

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.118

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_