Openrouter Assistant Prefill Host Rejection
Diagnose and fix "This model does not support assistant message prefill. The conversation must end with a user message." errors when running Anthropic models through OpenRouter (opencode, agent harnesses, any client that sometimes ends a request on an assistant turn). Use when: (1) that exact error kills a run, branded [Azure], [Amazon Bedrock], [Google], or [Anthropic]; (2) the failure is intermittent across otherwise-identical runs; (3) a provider routing pin "fixed" it and it came back. Covers why pins cannot fix it, why intermittent green runs prove nothing, the behavioral probe that actually tests a candidate model, and the model-family change that resolves it.
Security AStatic scan found no risk patternsHow grading works ›
What this skill does
Openrouter Assistant Prefill Host Rejection is a community-contributed Claude Code skill in the threat-hunting sub-category. It ships as a SKILL.md file that Claude Code auto-discovers under ~/.claude/skills/openrouter-assistant-prefill-host-rejection/ and loads when your prompt matches the skill's trigger.
When to invoke it: Use when: (1) that exact error kills a run, branded [Azure], [Amazon Bedrock], [Google], or [Anthropic]; (2) the failure is intermittent across otherwise-identical runs; (3) a provider routing pin "fixed" it and it came back. Covers why pins cannot fix it, why intermittent green runs prove nothing, the behavioral probe that actually tests a candidate model, and the model-family change that resolves it.
Who uses this skill
The Openrouter Assistant Prefill Host Rejection Claude Code skill is built for security engineers, penetration testers, DevSecOps practitioners, and development teams hardening codebases and infrastructure. It's part of ClaudSkills (also referred to as Claude Skills or Claude Code Skills) — the open community-curated registry of 191,000+ SKILL.md files for Anthropic's Claude Code agent and the wider Claude ecosystem (Claude API, Claude Agent SDK).
How to install
Free
Manual install (2 steps)
mkdir -p ~/.claude/skills/openrouter-assistant-prefill-host-rejection
curl -L https://claudskills.com/skills/openrouter-assistant-prefill-host-rejection/SKILL.md \
-o ~/.claude/skills/openrouter-assistant-prefill-host-rejection/SKILL.md
Or just download SKILL.md directly and drop it into ~/.claude/skills/openrouter-assistant-prefill-host-rejection/. Claude Code auto-discovers it on next session.
Skills live at ~/.claude/skills/openrouter-assistant-prefill-host-rejection/SKILL.md on macOS/Linux, or %USERPROFILE%\.claude\skills\openrouter-assistant-prefill-host-rejection\SKILL.md on Windows. See the full install guide for step-by-step instructions.
Telegram
📱 Install from your phone or desktop Telegram
Open @claudskills_bot on Telegram, tap Open Desktop App, and the desktop app installs this skill for you. Or share the bot link with a colleague — they get the same one-tap install. Learn more →
Pro
One-click install via the desktop app
The ClaudSkills desktop app installs any skill directly into ~/.claude/skills/ with one click — no terminal required. Pro starts at $9/mo or $149 lifetime.
Pro
For the full experience including quality scoring and one-click install features for each skill — upgrade to Pro.
Frequently asked questions
How do I install the Openrouter Assistant Prefill Host Rejection Claude Code skill?
Install via the ClaudSkills desktop app (one click) or copy
SKILL.md from the source repository to
~/.claude/skills/openrouter-assistant-prefill-host-rejection/SKILL.md and restart Claude Code. Both flows are detailed at
claudskills.com/install/.
What does the Openrouter Assistant Prefill Host Rejection skill do?
Diagnose and fix "This model does not support assistant message prefill. The conversation must end with a user message." errors when running Anthropic models through OpenRouter (opencode, agent harnesses, any client that sometimes ends a request on an assistant turn). Use when: (1) that exact error kills a run, branded [Azure], [Amazon Bedrock], [Google], or [Anthropic]; (2) the failure is intermittent across otherwise-identical runs; (3) a provider routing pin "fixed" it and it came back. Covers why pins cannot fix it, why intermittent green runs prove nothing, the behavioral probe that actually tests a candidate model, and the model-family change that resolves it.
Is this skill free to install?
Yes. ClaudSkills is an open registry — every skill keeps its source repository's license, and manual install via copy is free. ClaudSkills Pro ($9/mo, $79/yr, or $149 one-time) adds one-click install via the desktop app and a multi-signal Quality Score.
When should I use the Openrouter Assistant Prefill Host Rejection skill?
Use Openrouter Assistant Prefill Host Rejection when your Claude Code task falls under the Security category — specifically in the threat hunting area. Claude Code auto-discovers installed skills and invokes the right one based on the task description, so you can also ask Claude directly (e.g. "use Openrouter Assistant Prefill Host Rejection" or describe the task and let Claude pick). Browse related skills at
/category/security/.
What is a Claude Code skill and how does the Openrouter Assistant Prefill Host Rejection skill fit in?
A Claude Code skill is a
SKILL.md file that lives under
~/.claude/skills/<name>/ and tells the Claude Code CLI agent how to perform a specific task (instructions, prompts, allowed tools). Skills are auto-discovered at session start. Openrouter Assistant Prefill Host Rejection is one of 67,000+ skills indexed in the open ClaudSkills catalog, classified under the Security category. Learn more at
/learn/what-is-a-claude-skill/.
Attribution & license
Cite this skill
If you reference this skill in a blog post, paper, or documentation, you can cite it as:
APA
Claude Code. (2026). Openrouter Assistant Prefill Host Rejection [Claude Code skill]. ClaudSkills. https://claudskills.com/skills/openrouter-assistant-prefill-host-rejection/
BibTeX
@misc{openrouter-assistant-prefill-host-rejection-2026,
author = {Claude Code},
title = {Openrouter Assistant Prefill Host Rejection [Claude Code skill]},
year = {2026},
publisher = {ClaudSkills},
url = {https://claudskills.com/skills/openrouter-assistant-prefill-host-rejection/}
}Embed this skill
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Security scan
Grade A · scanned 2026-08-26 — free static scan against the OWASP Agentic Skills Top 10.
No risk patterns were found in any of the ten OWASP-aligned categories. How grading works ›
- ✓ Prompt injection
- ✓ Data exfiltration
- ✓ Supply chain
- ✓ Reverse shell
- ✓ Credentials
- ✓ Execution
- ✓ Filesystem
- ✓ Persistence
- ✓ Obfuscation
- ✓ Network
Show this grade on your repo (click to copy):
[](https://claudskills.com/skills/openrouter-assistant-prefill-host-rejection/#security)
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