[void-mckenzie]

Closes #813. Production deployments front FreeToken with proxies and dashboards that track per-request stats, and those read prefill/decode rates and cache hits from the response itself; without this block the numbers silently go missing. Every chat completion, completion and Responses response carries a llama.cpp-shaped timings object next to usage (fields and arithmetic from server_slot_stats::to_json), so frontends like llama-swap and Open Web UI show them. On streams it rides the final finish chunk as well as the include_usage chunk, like llama.cpp.

• prompt_n counts only the tokens evaluated after the cached prefix, and decode rates divide by n_gen - 1 (the first token rides the last prompt batch), matching llama.cpp exactly, so cache_n + prompt_n reconstructs the prompt total.
• Spans match feat(server): publish cumulative prefill and decode timing on /v1/stats #553's /v1/stats cumulative timing: prefill = request start to the first output reply, decode = between output replies. The two features report identical spans for the same request.
• cache_n is unconditional (llama.cpp and stats semantics); --enable-cache-report still gates only usage.prompt_tokens_details.cached_tokens.
• Engine and scheduler untouched; capture lives in the ack-consume loops, emission in the adapters.
• Anthropic /v1/messages is left out on purpose: llama.cpp emits no timings there either (to_json_anthropic keeps the strict Anthropic schema, with the cache hit in usage.cache_read_input_tokens), and FreeToken already reports that field under --enable-cache-report. Adding an off-spec key would be looser than llama.cpp itself.

Verified

• pytest tests/server/: 627 passed, 3 new tests (llama.cpp arithmetic, edge shapes, span capture) in the existing accounting file.
• Live serving, unsloth GLM-5.3-Flash UD-IQ4_XS (RTX 5090, ft serve ... --moe-cache-size 800 --max-prefill-length 12288): a 19,008-token cold prefill reports 1727 t/s over 11.0 s (2 chunks); a 1,800-token coding decode reports 16.31 t/s / 61.29 ms per token; a repeated 1,300-token prompt reports cache_n: 1280 and prompt_n: 20, summing to the prompt total. All four surfaces (chat stream + full, completions, Responses) emit the block.

[void-mckenzie]

Works with llama-swap now. When speculative decoding is eventually added, drafted/accepted fields can also be filled in to maintain parity with llama-server stats reporting.