The replay failure returns HTTP 400 before generation, so the next diagnostic point must be the chat-template conversion boundary rather than HiCache or scheduler paths. This adds a failure-only summary that records message roles, content shapes, empty indices, tool metadata, bootstrap room, and template mode without logging raw user content.
Constraint: Production replay traffic can contain large/private messages, so diagnostics must avoid full content dumps.
Rejected: Enable full request logging | too noisy and exposes complete prompt payloads.
Rejected: Normalize empty content immediately | would hide whether the malformed message is user, assistant, tool, or template-derived.
Confidence: high
Scope-risk: narrow
Directive: Keep this log failure-only; do not move it to the hot path or print raw content.
Tested: python -m py_compile python/sglang/srt/entrypoints/openai/serving_chat.py
Tested: remote docker py_compile after scp to g0034:/mnt/beegfs/cjy/sglang-dev
Not-tested: Full replay reproduction; requires user-driven traffic after service restart.
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