[BUG] Queued message delivered inside WebSearch tool result wrapped in `<system-reminder>`

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 a user message is queued while a WebSearch tool call is in progress, the queued message is delivered to the assistant inside the WebSearch tool result, wrapped in a <system-reminder> block, rather than as a normal user turn between tool calls.

From the assistant's perspective in an affected session, this looks like a prompt injection.
Same thing happens with system reminders like the one on TaskCreate. These can also land in the WebSearch result.

Observed consequences:

• The assistant reads the queued message as part of the fetched web content.
• The assistant flags the queued user content as a possible prompt-injection attempt (since <system-reminder>-shaped instructions arriving from web content are exactly what prompt-injection guidance tells it to ignore).

What Should Happen?

A queued user message should be delivered as a distinct user turn after the in-flight tool call completes, not concatenated into the tool result, and not wrapped in <system-reminder> tags.

Error Messages/Logs

If the message is any form of instruction, Claude warns that the response is likely a prompt injection and flags it as such.

Steps to Reproduce

1. Send a prompt that triggers any WebSearch (any query that the assistant will resolve via web search).
2. While the WebSearch is in progress, queue a follow-up message in the prompt input and submit it.
3. When the tool result returns, the queued message appears inside the tool result, wrapped in a <system-reminder> block, instead of as a separate user turn.

For system reminders, the reminder has to fire during step 2. for the same effect.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.139

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

GNOME Terminal

Additional Information

• First observed ~2 weeks ago.
• Related: #58227. Looks like the same underlying bug, but that issue reports only the case with system reminders. The bug seems to be broader, however.