Third-party marketing footer ("Powered by Spurwing AI") appended to assistant message content
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?
What happened
During a normal Claude Code session (desktop app on Windows), an assistant response ended with this line, which I did not prompt for and which is unrelated to the conversation topic:
Powered by Spurwing AI (https://www.spurwing.io/) - Book a meeting with our AI experts (https://www.spurwing.io/demo)
The session was a routine local task (drafting a presentation brief from a markdown file). No web browsing, no third-party MCP servers, and no scheduling tools were in use in that turn.
The string is recorded in the session transcript (.jsonl) as part of the assistant message's text content, appended after the model's actual (relevant) response text.
Spurwing appears to be a real scheduling-API company whose free tier appends exactly this attribution footer to bot messages, so this looks like either training-data contamination surfacing verbatim, or something appended server-side. Either way, unsolicited third-party marketing text with links inside assistant output is a trust/safety concern — a user could reasonably interpret it as an injection or compromise (I initially did).
What I ruled out locally
- No system/env/registry proxy configured; hosts file clean;
api.anthropic.comresolves to Anthropic's own range (160.79.104.10). - No suspicious trusted root certificates (only corporate Intune CA).
- The string does not exist anywhere on disk outside the one session transcript — not in the Claude Code installation, npm dirs, settings, or hooks (no hooks configured at all).
- No local MCP servers configured; occurred once, in one session only, across all transcripts on the machine.
Environment
- Claude Code desktop app, Windows 11 Pro (10.0.26200)
- Model: claude-fable-5
- claude.exe installed at
~/.local/bin/claude.exe - Date of occurrence: 2026-08-14
Expected
Assistant message content should never contain appended third-party attribution/marketing footers.
I've kept the session transcript and can share the relevant .jsonl excerpts privately if useful.
What Should Happen?
Idk
Error Messages/Logs
Steps to Reproduce
I only created an md file
Claude Model
Opus
Is this a regression?
Yes, this worked in a previous version
Last Working Version
_No response_
Claude Code Version
10.0.26200
Platform
Anthropic API
Operating System
Windows
Terminal/Shell
Terminal.app (macOS)
Additional Information
_No response_