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fix(github): drop trailing assistant prefill for Copilot chat - #5802
Conversation
GitHub Copilot's /chat/completions endpoint rejects a conversation that ends with an assistant message: "This model does not support assistant message prefill. The conversation must end with a user message." (HTTP 400). Anthropic clients such as newest Claude Desktop send a trailing assistant turn as a prefill seed — the Anthropic API honors it, but Copilot does not. Add a self-contained GithubExecutor.dropTrailingAssistantPrefill() that strips trailing assistant message(s) before dispatch, called at the end of transformRequest right after the existing sanitizeChatCompletionsMessage map. Scoped to the GitHub executor only — the shared translator/contextManager and other providers that do honor prefill (e.g. Mistral's dedicated Set) are untouched. Never empties the array (an assistant-only conversation keeps its last message); no-op (same array reference) when the conversation already ends with a user/tool message. Two pre-existing executor-github.test.ts fixtures incidentally ended in a bare trailing assistant message while testing unrelated behavior (reasoning-field stripping, tool_calls pass-through); extended them with a trailing user/tool turn so they keep exercising their original intent instead of tripping the new prefill guard. Regression guard: tests/unit/executor-github-prefill-sanitize.test.ts (8 cases — single/multiple trailing assistant, user/tool no-op with reference identity, assistant-only never-empty, null/empty safety, and two end-to-end transformRequest checks). Port of decolua/9router#2143. Co-authored-by: Manuel <baslr@users.noreply.github.com> Inspired-by: decolua/9router#2143
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Code Review
This pull request introduces a mechanism to drop trailing assistant messages (prefill seeds) from conversations sent to the GitHub Copilot executor, preventing HTTP 400 errors. It adds a helper method dropTrailingAssistantPrefill and corresponding unit tests, while updating existing tests to prevent unexpected message stripping. The reviewer suggested making the role check case-insensitive to handle potential variations in role casing (such as Assistant or ASSISTANT) robustly.
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| dropTrailingAssistantPrefill(messages: any): any { | ||
| if (!Array.isArray(messages) || messages.length === 0) return messages; | ||
| let end = messages.length; | ||
| while (end > 1 && messages[end - 1]?.role === "assistant") end--; |
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The check for the assistant role is case-sensitive (messages[end - 1]?.role === "assistant"). However, some clients may send roles in mixed-case or uppercase (e.g., "Assistant" or "ASSISTANT"). To ensure robustness and prevent unexpected HTTP 400 errors from GitHub Copilot, the role check should be case-insensitive, similar to how it is handled in transformRequest.
| while (end > 1 && messages[end - 1]?.role === "assistant") end--; | |
| while (end > 1 && typeof messages[end - 1]?.role === "string" && messages[end - 1].role.toLowerCase() === "assistant") end--; |
…r-2143-github-prefill
…GitHub Copilot prefill bullet
…r-2143-github-prefill
…r-inference#5802) Integrated into release/v3.8.43
Summary
GitHub Copilot's
/chat/completionsrejects (400) a request whose last message is an assistant prefill. This strips trailing assistant message(s) before dispatching to GitHub Copilot, scoped to the GitHub executor only (real Anthropic/OpenAI upstreams honor prefill, so the shared translator is untouched).Changes
open-sse/executors/github.ts— newdropTrailingAssistantPrefill(messages)method, called at the end oftransformRequest(right aftersanitizeChatCompletionsMessage). Returns the same array reference when the conversation already ends inuser/tool(no-op); never empties an assistant-only array; null/empty safe.executor-github.test.tsfixtures that incidentally ended in a bare trailing assistant were given a realistic trailinguser/toolturn so they keep testing their original intent (response_format injection, tool_calls pass-through) instead of tripping the new guard — no assertions removed or weakened.Attribution
Thanks to @baslr for the original implementation.
Testing
TDD (Hard Rule #18): new
tests/unit/executor-github-prefill-sanitize.test.ts(8 cases) fails before the method exists (7/8 red), passes after (8/8). Fullexecutor-github.test.ts+ related GitHub-executor suites green (25/25).typecheck:core,eslint,check:any-budget:t11clean.