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Claude models behind OpenAI-compatible proxies get 400 prefill errors #20415
Description
Bug
Claude models routed through non-native SDK paths (@ai-sdk/openai-compatible, GitHub Copilot, Azure AI / LiteLLM) fail with:
[400]: This model does not support assistant message prefill.
The conversation must end with a user message.
This is likely the shared root cause behind #19517 and #13768.
Root Cause
ProviderTransform in transform.ts uses three different conditions to detect Claude models, and they disagree:
| Transform | Condition | Lines |
|---|---|---|
| Empty content stripping | api.npm === "@ai-sdk/anthropic" |
L56 |
| Tool call ID scrubbing | api.id.includes("claude") |
L76 |
| Prompt caching | providerID || api.id || model.id || api.npm |
L282-287 |
When a Claude model uses any SDK other than @ai-sdk/anthropic — proxies, Copilot, Azure AI — caching activates but empty-content stripping does not. Empty assistant messages survive, get cache metadata, and Anthropic rejects them as prefill.
Reproduction
- Configure a Claude model through an OpenAI-compatible proxy with a model name containing "claude"
- Use it in a multi-agent conversation with tool calls
- After several turns, the request fails with the prefill error
Fix
PR #20414 — extracts isClaudeModel() helper used at all three detection sites.
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github-actions commentedon Apr 1, 2026
This issue might be a duplicate of existing issues. Please check:
github-actions commentedon Jul 4, 2026
To stay organized issues are automatically closed after 60 days of no activity. If the issue is still relevant please open a new one.