fix(anthropic): replace whitespace-only trailing user message with non-whitespace character#6215
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…n-whitespace character inject_trailing_user_message appends a dummy user turn when the chat context ends on an assistant message, to satisfy Claude 4.6+ models that reject prefilling. The placeholder text was a single ASCII space, which Anthropic's Messages API rejects with HTTP 400 (messages: text content blocks must contain non-whitespace text). Replace the space with a period so the structural intent of the dummy turn is preserved while satisfying the API's non-whitespace requirement. Fixes livekit#5254 Fixes livekit#6213
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longcw
approved these changes
Jun 25, 2026
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Fixes #5254
Fixes #6213
to_chat_ctxin_provider_format/anthropic.pyappends a trailing user turn wheninject_trailing_user_message=Trueand the chat context ends on an assistant message. This path is taken for models matched by_NO_PREFILL_PATTERNS(currentlyclaude-sonnet-4-6andclaude-opus-4-6) to satisfy their no-prefill constraint.The placeholder text was a single ASCII space (
" "), which Anthropic's Messages API rejects:This fails every LLM call that goes through the no-prefill branch when the context ends on an assistant turn — common in sequential
generate_reply()calls, agent handoffs, and silence re-engagement flows.The fix replaces the single space with a period. The structural intent (forcing a user turn) is preserved; the one-character change is the minimal-blast-radius fix that satisfies the API constraint.