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+# The serve protocols — engine ⇄ server wire format
+
+The engine speaks two line-oriented protocols over stdin/stdout. Both are plain text
+plus byte-counted payload frames; every outbound line is written with a trailing
+`fflush`, one line per write. On Windows both ends of the pipe are switched to binary
+mode at startup — the CRT's CRLF translation otherwise corrupts the sentinels and
+stalls byte-counted reads (#195).
+
+| protocol | entry | selected by | used by |
+|---|---|---|---|
+| **mux** (continuous batching, up to 16 KV slots) | `run_serve_mux` | `SERVE_BATCH=1` | `openai_server.py`, `coli web` |
+| **legacy** (single slot, interactive) | `run_serve` | `SERVE=1` (without `SERVE_BATCH`) | `coli chat` |
+
+This document is the reference for the **mux** protocol; the legacy protocol is
+summarized at the end. Line formats below are quoted from the emitting `printf`s in
+`glm.c` — if this document and the code disagree, the code wins and this file needs a PR.
+
+## Startup handshake (engine → server)
+
+```
+\x01\x01READY\x01\x01
+STAT 0 0.00 0.0 <rss_gb>
+HWINFO <cores> <ram_total_gb> <ram_avail_gb> <ngpu> <vram_total_gb> <cpu_name>|<gpu_name>
+TIERS <vram_experts> <ram_experts> <disk_experts> <vram_gb> <ram_gb>
+EMAP <rows> <cols> <hex>
+```
+
+The server must not send requests before `READY`. `HWINFO`/`TIERS`/`EMAP` are
+telemetry (see below) and may grow — **servers must ignore line kinds they do not
+recognize**; that is the protocol's forward-compatibility rule.
+
+## Requests (server → engine)
+
+```
+SUBMIT <id> <slot> <bytes> <max_tokens> <temperature> <top_p>\n<payload>\n
+CANCEL <id>\n
+```
+
+- `id` — non-zero u64, unique among in-flight requests.
+- `slot` — KV slot index, `0 … KV_SLOTS-1` (`KV_SLOTS` env, 1–16, default 1). A slot
+  holds one conversation's KV; the engine matches the tokenized payload against the
+  slot's history and reuses the common prefix (truncate-and-extend), so stateless
+  HTTP turns keep their cache.
+- `bytes` — exact byte length of `payload` (UTF-8, may contain newlines). The engine
+  reads exactly that many bytes after the header line, then one trailing `\n`.
+- `payload` — the fully rendered prompt (the server owns the chat template).
+- EOF on stdin = graceful shutdown: in-flight requests finish first.
+
+Prefill is serial; decode is continuously batched — every active slot contributes
+one row per forward.
+
+## Responses (engine → server)
+
+Per request, in order:
+
+```
+DATA <id> <n>\n<n bytes of UTF-8>\n        # a decoded token's text; repeated
+TOPK <id> 5 <logprob> <hextext> ... ×5     # candidates for the sampled token (SERVE_TOPK=1)
+HITS <rows> <cols> <hex>                   # ~every 6 tokens: routed-expert bitmap since last HITS
+REPIN <layer> <eid> <old_tier> <gpu>       # live re-pin swap events, as they happen
+...
+DONE <id> STAT <emitted> <tok_s> <hit_pct> <rss_gb> <prompt_tokens> <length_limited>
+```
+
+Errors replace the stream: `ERROR <id> <CODE>` with codes `BAD_FRAME`, `BAD_REQUEST`,
+`SLOT_BUSY`, `DUPLICATE_ID`, `EMPTY_PROMPT`, `NOT_FOUND` (CANCEL of unknown id),
+`CANCELLED`. A `CANCEL` is acknowledged by `ERROR <id> CANCELLED` after the slot's KV
+is persisted.
+
+Immediately before each `DONE` the engine emits a telemetry block for the finished
+turn: `HWINFO`, `PERF`, `ENTROPY`, `GPUS`, `TIERS`, `EMAP`, `HITS` (formats below).
+`.coli_usage` is persisted at every turn end, not only at exit.
+
+## Telemetry lines
+
+| line | format | meaning |
+|---|---|---|
+| `TIERS` | `TIERS <vram> <ram> <disk> <vram_gb> <ram_gb>` | expert count per tier + resident bytes |
+| `HWINFO` | `HWINFO <cores> <ram_total> <ram_avail> <ngpu> <vram_total> <cpu>\|<gpu>` | host snapshot (GBs are floats) |
+| `EMAP` | `EMAP <rows> <cols> <hex>` | one byte per expert, row-major over `rows×cols` (sparse layers +MTP × experts): `byte = (tier<<6) \| heat` — 2-bit tier (0 disk / 1 RAM / 2 VRAM), 6-bit log₂-bucketed usage heat |
+| `HITS` | `HITS <rows> <cols> <hex>` | 1 bit per expert, experts routed since the previous `HITS` |
+| `PERF` | `PERF <id> <dt> <t_edisk> <t_ewait> <t_emm> <t_attn> <t_kvb> <t_head>` | this turn's PROFILO deltas, seconds |
+| `ENTROPY` | `ENTROPY <h0> <h1> …` | per-sparse-layer routing entropy of the turn, bits |
+| `GPUS` | `GPUS <n> (<used_gb> <total_gb> <experts>)×n` | per-device VRAM + resident expert count (CUDA builds) |
+| `TOPK` | `TOPK <id> 5 (<logprob> <hextext>)×5` | token text hex-encoded so the line stays line-shaped |
+| `REPIN` | `REPIN <layer> <eid> <old_tier> <gpu>` | one line per hot-store swap (`REPIN=n` mode) |
+
+All telemetry is advisory: servers render what they know and skip the rest.
+
+## HTTP surface (`openai_server.py`)
+
+- `POST /v1/chat/completions` — OpenAI-compatible; streaming responses emit one extra
+  SSE frame `data: {"colibri": {stats, perf, topk, entropy, gpus, repin}}` immediately
+  before `data: [DONE]`; non-streaming responses attach the same object as a
+  `"colibri"` field.
+- `GET /experts` — the latest `EMAP`/`HITS` state: `{rows, cols, map, hits, seq,
+  gpus, entropy, repin}`.
+- `GET /*` — static hosting of `web/dist` (SPA fallback, path-traversal-safe), plus
+  `experts.json` if published there (the measured expert atlas, #175/#218).
+
+## Legacy protocol (`run_serve`, `coli chat`)
+
+Interactive lines are prompts; control frames: `\x02RESET` (clear history),
+`\x02MORE` (continue an NGEN-truncated answer), and
+`\x02PROMPT <bytes> <max_tokens> <temperature> <top_p> [kv_slot]\n<prompt>\n`
+(the pre-mux API mode). Responses are raw text terminated by `\x01\x01END\x01\x01`
+plus a `STAT` line. New integrations should use the mux protocol.