[BUG] Remote control: "Failed to send message" error when sending from phone. Read works but send fails

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?

When using Claude Code remote control on a mobile browser, the session
loads and displays the conversation correctly (read works), but
attempting to send a message from the phone fails with: "Failed to send
message: An unknown error has occurred. Please try again later."

What Should Happen?

Remote control should be fully bidirectional — the phone should be able
to both read the conversation and send messages to the running Claude
Code session.

Error Messages/Logs

Steps to Reproduce

• Claude Code CLI running inside a tmux session on a remote Linux

server

• Remote control session accessed via claude.ai/code/session_... URL on

a mobile browser

• No direct network connection between phone and server — relay goes

through claude.ai

Notes:

• The session displays conversation history correctly, so the

read/stream side of the relay is working

• Only sending (input from phone → relay → Claude Code) is broken
• Error appears immediately on send attempt

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.76 (Claude Code)

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

iTerm2

Additional Information

_No response_