Pantheon/docs/reference/http-api.md

Pantheon — HTTP API Reference

The server speaks HTTP/1.1 over TCP. Every response carries Access-Control-Allow-Origin: * and Connection: close. No authentication. All bodies are JSON unless noted (SSE for streaming).

---

Routes

Method	Path	Handler	Audience
GET	/	health	client / operator
OPTIONS	*	CORS preflight	client
GET	/v1/models	list models	client
POST	/v1/chat/completions	chat completion	client
GET	/admin/models	list live model instances	operator
POST	/admin/models/load	load a model	operator
POST	/admin/models/unload	unload a model	operator
GET	/admin/dies	die status	operator
POST	/admin/session/export	export a session's state as a blob	operator
POST	/session/restore	restore a session from a blob	client
POST	/session/release	free a session's slot	client
GET	/session/estimate	warm-restore vs re-prefill time estimate	client
GET	/session/watch	hold a session alive while connected	client

Any other path → 404 {"error":{"message":"unknown route","type":"not_found","code":null}}.

---

GET /

Health check. Always 200.

Response:

{"service":"pantheon","status":"ok","backend":"cuda",
 "build":"5d818bfd1c9a","accelerator":true,"dies_visible":8}

Field	Type	Meaning
backend	string	The accelerator backend this binary was COMPILED with: cuda, tt, metal, cpu. Compile-time constant, not a probe.
build	string	Commit this binary was built from (-dirty when the tree was not clean), or unknown.
accelerator	bool	False means every forward pass in this process runs on the host CPU, whatever the flags say.
dies_visible	int	Dies the die manager holds (pn_dm_count()). 0 on a CUDA build means no driver / no card / empty CUDA_VISIBLE_DEVICES.

The cheapest route on the server is also the one that answers "what am I running?" — see GET /v1/models for why that question needed an endpoint at all.

---

OPTIONS *

CORS preflight. Always 200 with {} body.

---

GET /v1/models

Serve's own endpoint (openai_chat.c:oai_handle_models, serve_command.c:dispatch): same-name replica slots (multi-die LB copies) are deduplicated — clients see one logical id per served name. The response does not include live or free_slots.

Response — 200:

{
  "object": "list",
  "backend": "cuda",
  "build": "5d818bfd1c9a",
  "data": [
    {
      "id": "my-model",
      "object": "model",
      "created": 0,
      "owned_by": "pantheon",
      "max_sessions": 4,
      "max_concurrent": 4,
      "context_window": 32000,
      "tool_choice_required_with_thinking": true,
      "graphs": {"live": 30, "total": 40, "failed": 0,
                 "ineligible": 10, "pending": 0},
      "attention": {"ondie": 10, "host": 0, "total": 10}
    }
  ]
}

Field	Type	Meaning
backend	string	Top-level. The accelerator backend this binary was COMPILED with: cuda, tt, metal or cpu. Compile-time constant, not a probe.
build	string	Top-level. The commit this binary was built from (-dirty suffixed when the tree was not clean), or unknown.
id	string	The routing key for model on a completion
max_sessions	int	RESIDENCY: how many conversations may hold KV at once
max_concurrent	int	CONCURRENCY: how many requests this model ACTUALLY decodes at once — the served value (bs_get_load capacity), not the configured cap. See below.
context_window	int	Per-session context window in tokens — see below. Omitted when unknown.
tool_choice_required_with_thinking	bool	true when tool_choice: "required" may be sent with thinking on: the think block is unconstrained and the turn ends in a parsed tool call (the think-gated grammar models ORNITH_JSON and QWEN_XML; false for MiniCPM's attribute dialect, which no grammar models, and for every model when PANTHEON_TOOL_GRAMMAR=0).
graphs	object	CUDA-graph residency — see below. Omitted when the model has no graph slots.
attention	object	Attention-path residency — see below. Omitted until the forward has reached an attention layer.

backend / build — what this binary actually is

Top-level on the response, not per-model, because it is a property of the process: the accelerator backend it was compiled with (cuda, tt, metal, cpu) and the commit it came from.

Compile-time constant, deliberately — not a runtime probe. A serve whose binary has no CUDA compiled in cannot use a GPU whatever the driver reports, and the completion text is identical either way, so nothing downstream can detect it from the output.

The same values are on GET / (plus dies_visible), and out of band from pantheon --backend, which prints one lowercase token for a deploy script to assert against.

max_concurrent — the SERVED number, not the configured cap

max_concurrent on /v1/models is what the model actually decodes at once (bs_get_load's capacity), which is not always what --max-concurrent asked for. It reads 1 whenever there is no batch scheduler — no CUDA compiled in, QWEN35_BATCH_SLOTS off, bs_create failed — and it drops below the cap when the memory-pressure ladder clamps the slot count.

Same argument as context_window below: a client sizes its in-flight requests from this number, so advertising a cap the serve cannot honour (e.g. 2 with no scheduler, which turns every second concurrent chat into a 429) is worse than advertising nothing.

Operators still get the configured value: /admin/models carries both, as max_concurrent (served) and max_concurrent_configured (asked for). Seeing them differ is the signal.

context_window — the advertised window

context_window is the window a client may actually plan a prompt against, in tokens. It is the effective window (pn_effective_ctx_window): the operator's --ctx-cap after the reactive memory ladder's runtime clamp — not the configured cap, and not the model's theoretical maximum.

That distinction is the whole point of the field. Publishing the configured cap while admission enforces a smaller one hands the client a prefill the serve then rejects — on a K80 that rejection lands after minutes of cold-prefill TTFT, and it lands mid-task.

Two properties clients depend on:

• It is the same number a completion reports as usage.kv_cap. The model list is the client's INITIAL plan; kv_cap is the LIVE truth on every turn. A client that reads both needs no polling and no extra endpoint to notice a mid-session clamp.
• It is additive and omitted when unknown. A plain OpenAI-compatible client ignores the extra key; a client that reads it never sees a 0 it has to special-case. An absent field means undetected; a client should fall back to a conservative window rather than guess.

Field name: Pantheon emits context_window. Clients that detect the window from other servers' spellings (context_length, max_model_len, n_ctx, ...) need to accept this one.

graphs — CUDA-graph residency

Each layer of the hybrid stack can hold a captured CUDA graph that collapses its ~13-30 kernel launches into one replay. Across 40 layers those graphs are worth about 1.8 ms/step. A capture that fails falls back to the direct kernel body and produces correct output — so losing them costs only speed, silently. This object is the only surface that reports it.

Key	Meaning
live	Slots holding a captured graph right now
total	Slots the forward has actually reached (see below)
failed	Attempted and did not stick — the only value that is a fault
ineligible	Gated out; this model never captures. Not a fault.
pending	Eligible, warming up, no attempt yet
gave_up	Subset of failed whose bounded retry budget is spent (omitted when 0)

ineligible is normal. The captured path requires the grouped MoE kernel, which requires Q3_K expert tensors. A Q5_K_M model reports live 0 / total 40 / ineligible 40 / failed 0 — correct behaviour, zero exposure, and also zero of the +4%. Read failed, not live/total, to decide whether something is wrong.

total counts only exercised slots. A serve decoding batched never touches its single-token graph slots, so those are not reported as missing graphs.

A failed capture is retried on a bounded backoff and only stops after the budget is spent, so a transient failed on one read that is live on the next is the system self-healing, not a flapping metric.

attention — on-die vs host-softmax residency

The same argument as graphs, only much sharper. Each full-attention layer normally runs the on-die flash kernels. When it cannot it falls back to host-side causal GQA softmax on the serve host's CPU — the exact wall this engine was built to escape (it was 87.5% of an 8k prefill; moving attention on-die took that prefill from 26 minutes to 3.82). The fallback returns the same tokens, logs nothing per step, and on the depth bench costs 13.02 -> 1.22 tok/s (K=2, depth 4096) — and it worsens with context, because host softmax is O(context) per layer per token. Without this object a user simply waits, and nothing anywhere says why.

Key	Meaning
ondie	Attention layers running the on-die flash kernels right now.
host	Layers on the host-softmax fallback — any non-zero value is a fault
total	Layers the forward has actually reached (same rule as graphs.total)
gave_up	Layers whose KV-ring allocation spent its retry budget (omitted when 0)
impossible	Subset of gave_up: the rings cannot fit this die at all (omitted when 0)

ondie + host == total. Unlike graphs, there is no benign state here: host > 0 always means this model is serving degraded, and the reasons histogram on /admin/models says why. The two common causes are a per-slot KV-ring allocation that did not fit (lower --ctx-cap or --max-sessions) and a session that has run past the KV window, which wraps the ring.

A ring allocation that fails is retried on a bounded backoff, so a host reading that clears on the next poll is the system self-healing. gave_up is the number that should page someone.

No error cases (returns an empty data array when nothing is loaded).

Router's /v1/models (registry.c:reg_models_json) is a different shape: it aggregates across live serve instances and adds live (count of live instances serving that id) and free_slots (summed across live replicas). Do not conflate the two.

The router also emits context_window, aggregated the only honest way: the minimum across that model_id's live replicas (a dispatch may land on any of them), and omitted entirely if any live replica has not reported one. Serve instances report their window on both /registry/register and every /registry/heartbeat, because the ladder can move it at runtime.

---

POST /v1/chat/completions

OpenAI-compatible chat completion. Supports streaming (SSE) and non-streaming. Includes reasoning (<think>…</think> split) and tool-call support.

Request headers

Header	Required	Effect
Content-Type: application/json	no	Not checked; the body is parsed as JSON regardless.
X-Session-Id: <sid>	no	Prefix-cache session key. A body key (session_id, prompt_cache_key, promptCacheKey) takes precedence. opencode sends this header on every request.

Request body

{
  "model": "my-model",
  "messages": [
    {"role": "system", "content": "..."},
    {"role": "user",   "content": "hello"}
  ],
  "temperature":  0.7,
  "top_p":        0.95,
  "top_k":        40,
  "max_tokens":   1024,
  "stop":         ["<|im_end|>"],
  "stream":       false,
  "tools":        [...],
  "tool_choice":  "auto",
  "session_id":   "abc123"
}

Field	Type	Default	Notes
model	string	—	Routing key. With one loaded model, any value (or absent) routes to it. With multiple models, must match a served name exactly; unknown → 404.
messages	array	required	Non-empty array of {role, content}. Roles: system, user, assistant, tool; a missing role is treated as user. content must be a string — any other type (e.g. an array of content parts) is read as empty.
messages[].reasoning_content	string	unset	Assistant turns only. The reasoning this serve returned for that turn; replayed into the prompt so a session can resume from its prefix cache.
messages[].tool_calls	array	unset	Assistant turns only. Rendered back into the prompt in the model's tool dialect.
temperature	float	0.7	<= 0 is greedy argmax.
top_p	float	0.95	Nucleus sampling.
top_k	int	40	Top-K sampling.
max_tokens	int	1024	Max completion tokens. 0 or negative → 512. Clamped to 1048576.
max_completion_tokens	int	1024	Alias; max_tokens wins if both are present.
stop	string or array	unset	Stop strings.
stream	bool	false	true → SSE (text/event-stream).
stream_options.include_usage	bool	false	Streaming only: send a terminal usage chunk before [DONE].
seed	number	unset	Makes sampling reproducible. With n > 1, choice i uses seed + i.
presence_penalty	float	0	Clamped to [-2, 2].
frequency_penalty	float	0	Clamped to [-2, 2].
logit_bias	object	unset	{"<token id>": bias}; bias clamped to [-100, 100]. Non-numeric keys or values are skipped.
logprobs	bool	false	Return per-token log-probabilities (choices[].logprobs.content[], with token, logprob, bytes).
top_logprobs	int	0	Alternatives per token; clamped to [0, 20].
n	int	1	Number of completions. Each is a full separate decode (only the first uses the session's prefix cache). Capped at 8; a larger n returns 8 and sets n_status: "capped_to_8".
response_format	object	{"type":"text"}	json_object constrains the answer to valid JSON. json_schema gets the same JSON constraint but the schema's shape is not enforced. See response_format_status below.
enable_thinking	bool	unset	Think-style templates: true opens a think block, false prefills a closed one. Absent → the PANTHEON_NO_THINK env decides.
reasoning_budget	int	per-model	Force </think> after N reasoning tokens. Overrides the model's --reasoning-budget; 0 or negative disables it. max_reasoning_tokens is an alias (reasoning_budget wins).
tools	array	unset	OpenAI-format tool definitions. Rendered into a system turn in the model's tool dialect (below). Ignored when tool_choice is "none".
tool_choice	string or object	"auto"	"none" disables tools. "required" forces a call: the grammar is armed by default (PANTHEON_TOOL_GRAMMAR=0 opts out). With thinking on the think block is unconstrained and the call is compelled from </think> (models advertising tool_choice_required_with_thinking). {"type":"function","function":{"name":"X"}} is enforced only when PANTHEON_TOOL_GRAMMAR=1; otherwise it behaves like "auto".
session_id	string	unset	Prefix-cache session key (overrides X-Session-Id). Enables delta prefill on later turns of the same session.
prompt_cache_key / promptCacheKey	string	unset	Aliases for session_id (OpenAI's name; opencode's camelCase body field). Order: session_id, prompt_cache_key, promptCacheKey, then the X-Session-Id header.

Tool calling

Tools are rendered into a system turn (merged with a leading system message) in the dialect the routed model emits. The dialect is chosen from the model name and architecture (tool_calling.c:tc_dialect_for_model), not from the GGUF chat template:

Dialect	Used for	Call syntax
Qwen3-Coder XML	default	<tool_call><function=NAME><parameter=KEY>VALUE</parameter></function></tool_call>
MiniCPM XML	Llama-arch models whose template adds BOS (MiniCPM5)	<function name="NAME"><param name="KEY">VALUE</param>…
JSON	served names starting ornith	fenced or bare {"function"|"name": NAME, "arguments": {…}}

The model's output is parsed into OpenAI tool_calls, with arguments typed from each tool's JSON schema. PANTHEON_TOOL_GRAMMAR decides which requests are grammar-constrained:

Value	Constrained requests
unset (default)	tool_choice: "required" only — after reasoning, the output is one well-formed call to an offered tool, arguments typed and required keys present. Everything else decodes unconstrained.
1	every request with tools: "auto" masks once a call opener appears; "required" and a named function compel a call.
0	none — "required" is unconstrained too.

On the Tenstorrent serve a "required" turn keeps speculative decoding: drafts are checked against the grammar and the output equals non-speculative constrained decoding. On a "required" turn (every dialect, single-user and batched) an end token is refused before the call opens and, inside the call, allowed only where the call can still be closed; the serve then appends the closing text. Any other call the model started but did not finish is left out of tool_calls and reported in tool_call_error (code, optional function, parameter, message) on the choice.

tools and response_format cannot both constrain one decode: when the request carries tools, tools win and response_format_status is "ignored_tools_present".

Non-streaming response — 200

{
  "id": "chatcmpl-a1b2c3d4",
  "object": "chat.completion",
  "created": 1720000000,
  "model": "my-model",
  "choices": [{
    "index": 0,
    "message": {
      "role": "assistant",
      "content": "Hello!",
      "reasoning_content": "...",
      "tool_calls": [...]
    },
    "finish_reason": "stop"
  }],
  "usage": {
    "prompt_tokens": 12,
    "completion_tokens": 7,
    "total_tokens": 19,
    "prompt_tokens_details": {"cached_tokens": 0},
    "kv_used": 12,
    "kv_cap": 32000
  }
}

• reasoning_content — the text inside a <think>…</think> block; present only when non-empty. Only think-style chat templates with thinking on produce it. Non-think templates (plain ChatML such as SmolLM2, Mistral, Phi-3) return the whole answer as content.
• tool_calls — present when a call was parsed; finish_reason is then "tool_calls". content is null for a pure tool call.
• finish_reason — stop, length, tool_calls, or repetition_loop (the decode was stopped because the model was looping; outranks tool_calls).
• logprobs — on the choice, when requested.
• prompt_tokens_details.cached_tokens — prompt tokens reused from the prefix cache rather than computed. Always present (0 on a cold turn).
• kv_used — this session's KV occupancy in tokens after this turn.
• kv_cap — the effective per-session context window, the same number GET /v1/models advertises as context_window, re-read every turn. If the memory ladder moves the window mid-session, the next completion carries the new value. Treat the model list as the initial plan and this as authoritative.
• session_conflict: true (inside usage) — two conversations are sharing one session id, so neither can resume from cache.
• response_format_status (top-level, when the request sent response_format) — applied, schema_downgraded, ignored_tools_present, ignored_unknown_type, or not_enforced (a constraint was armed but the output did not close a complete JSON object).
• n_status (top-level) — why fewer choices came back than n asked for: capped_to_8, partial_slot_unavailable, partial_generation_failed.
• usage covers the whole request: the prompt is counted once, completion tokens are summed across choices.

A prompt longer than the effective window is refused with context_over_budget (below); nothing is silently dropped. The context_capped field the writer supports is never set by the current generation path.

Streaming response — 200 (SSE)

Content-Type: text/event-stream. Each chunk is:

data: {"id":"chatcmpl-...", "object":"chat.completion.chunk", "created":...,
       "model":"...", "choices":[{"index":0,"delta":{...},"finish_reason":null}]}

Delta fields emitted in order:

1. {"role":"assistant"} — first chunk of each choice.
2. {"reasoning_content":"<fragment>"} — inside a think block (think-style templates only).
3. {"content":"<fragment>"} — answer text. logprobs rides the chunk carrying those tokens.
4. Tool-call chunks (when a call was parsed):
   • Opening: {"tool_calls":[{"index":0,"id":"...","type":"function","function":{"name":"...","arguments":""}}]}
   • Arguments: {"tool_calls":[{"index":0,"function":{"arguments":"..."}}]}
5. Closing chunk: {"delta":{},"finish_reason":"..."}. Carries tool_call_error on the choice and response_format_status top-level when applicable.
6. With stream_options.include_usage: a chunk with "choices":[] and usage.
7. Terminal sentinel: data: [DONE]

With n > 1 the choices stream one after another, told apart by choices[].index. A client that disconnects aborts the turn and frees its slot.

Errors detected after the stream opened cannot change the status code: context_over_budget is sent as a terminal event data: {"error":{"message":"...","type":"context_over_budget","code":null,"kv_used":N,"kv_cap":M}} and the stream ends; other generation failures close the choice with finish_reason: "stop".

Error responses

Status	type	Condition
400	invalid_request_error	Empty body, malformed JSON, body not an object, messages missing or empty
404	model_not_found	Several models loaded and model matches none
422	context_over_budget	Prompt exceeds the effective per-session window. Not transient: shorten the conversation and retry. Non-streaming: kv_used/kv_cap are in the message text; streaming: they are fields of the error object.
429	server_busy	Single-user path: a generation is already running on this model. Batched path: all slots busy and the queue full, or 30 s queue wait timed out. Retry-After when the scheduler supplies one.
429	model_at_session_capacity	A new session_id while every session slot (max_sessions) holds a live session. Retry-After: 30; the message estimates the re-prefill time.
503	no_models_loaded	No model loaded
503	server_memory_constrained	Host memory pressure: admission refused the prompt, or the queue is full under critical pressure
503	server_shutting_down	Server is draining for shutdown. Retry-After: 1 when refused at the route.
500	server_error	Generation failed / out of memory

Source: openai_chat.c:oai_handle_chat, serve_generation.c:sg_run_completion, serve_command.c:dispatch.

---

POST /admin/models/load

Operator-only. Load a model at runtime (Phase 7). Serialized against other uploads via reg->upload_lock (one GPU upload in flight at a time).

Request body

{
  "path":             "/models/my-model.gguf",
  "name":             "my-model",
  "dies":             [0, 1],
  "quantize":         false,
  "ctx_cap":          32768,
  "max_sessions":     4,
  "max_concurrent":   2,
  "reasoning_budget": 1024
}

Field	Type	Required	Notes
path	string	yes	GGUF file path
name	string	no	Served id (defaults to file basename)
dies	int array	no	Device ordinals (same CUDA_VISIBLE_DEVICES-relative semantics as --dies). Max PN_MODEL_MAX_DIES=8.
quantize	bool	no	INT8-quantize at load time
ctx_cap	int	no	Per-session context cap in tokens; a prompt past it is refused with context_over_budget. 0/absent = engine default. Mirrors --ctx-cap.
max_sessions	int	no	Residency — how many named sessions may hold KV at once. Over the cap a new session id gets 429 model_at_session_capacity. 0/absent = engine default. Mirrors --max-sessions.
max_concurrent	int	no	Concurrency — how many requests may decode simultaneously (batch-scheduler slots). Surplus queues, then 429/503. Distinct from max_sessions. 0/absent = the resolved max_sessions. Mirrors --max-concurrent.
reasoning_budget	int	no	Force </think> after N tokens of unclosed reasoning. 0/absent = unbounded. Mirrors --reasoning-budget.

Every per-model option available on the serve command line is settable here, so a model added at runtime ends up configured identically to one launched with the process.

Response — 200

{
  "id": "my-model",
  "object": "model",
  "path": "/models/my-model.gguf",
  "dies": [0, 1],
  "vram_bytes": 12345678,
  "max_sessions": 4,
  "max_concurrent": 4,
  "context_window": 32000,
  "context_window_configured": 32000,
  "graphs": {
    "live": 30, "total": 40, "failed": 0, "gave_up": 0,
    "ineligible": 10, "pending": 0,
    "captures": 30, "capture_failures": 0,
    "batched_attn": {"live": 10, "total": 10, "failed": 0, "ineligible": 0, "pending": 0},
    "batched_ssm":  {"live": 20, "total": 30, "failed": 0, "ineligible": 10, "pending": 0}
  },
  "attention": {
    "ondie": 10, "host": 0, "total": 10,
    "gave_up": 0, "impossible": 0,
    "host_engagements": 0, "ring_alloc_failures": 0
  }
}

The operator surface carries both context numbers, because an operator's question is not the client's: context_window is what sessions actually get right now, context_window_configured is what --ctx-cap asked for. They are equal unless the memory ladder is clamping this instant — and seeing them differ is exactly the signal that memory pressure is costing context. (/v1/models advertises only the effective one; a client has no use for a window it cannot have.) Both are omitted when unknown.

Falls back to {"status":"loaded"} if the row cannot be found in the snapshot (race).

Error responses

Status	type	Condition
400	invalid_request_error	Missing path, malformed JSON, too many dies
400	load_failed	dies named on a serve with no accelerator backend compiled in, or with no dies visible. Not retryable — the fix is a different binary (pantheon --backend) or an omitted dies.
409	load_failed	Model already loaded / already unloading
507	load_failed	Admission refused (VRAM or host), die binding conflict, registry full
404	load_failed	File not found
500	load_failed	Other load failure

Source: model_admin.c:ma_handle_load.

---

POST /admin/models/unload

Operator-only. Unload a model by served name. Drains any in-flight generation before freeing.

Request body

{"name": "my-model"}

Field	Type	Required	Notes
name	string	yes	Served model id to unload

Response — 200

{"unloaded": "my-model"}

Error responses

Status	type	Condition
400	invalid_request_error	Missing name, malformed JSON
404	unload_failed	Name not loaded
409	unload_failed	Already unloading
500	unload_failed	Other failure

Source: model_admin.c:ma_handle_unload.

---

GET /admin/models

Operator-only. Every live instance, one row each. Unlike /v1/models these are not deduplicated by name: two instances of one model on different dies are two rows, because to an operator they are two different things.

The row is the same one POST /admin/models/load returns — this route exists so its operator-only fields can be read at any time rather than only as a side effect of loading a model.

Response — 200

{
  "object": "list",
  "data": [
    {
      "id": "my-model",
      "object": "model",
      "path": "/models/my-model.gguf",
      "dies": [2, 3],
      "vram_bytes": 12345678,
      "max_sessions": 4,
      "max_concurrent": 4,
      "max_concurrent_configured": 4,
      "context_window": 32000,
      "context_window_configured": 32000,
      "graphs": { "live": 30, "total": 40, "failed": 0, "gave_up": 0,
                  "ineligible": 10, "pending": 0,
                  "captures": 30, "capture_failures": 0,
                  "batched_attn": {"live": 10, "total": 10, "failed": 0,
                                   "ineligible": 0, "pending": 0},
                  "batched_ssm": {"live": 20, "total": 30, "failed": 0,
                                  "ineligible": 10, "pending": 0} },
      "attention": { "ondie": 8, "host": 2, "total": 10,
                     "gave_up": 2, "impossible": 0,
                     "host_engagements": 2, "ring_alloc_failures": 10,
                     "reasons": {"ring_alloc_failed": 2} }
    }
  ]
}

The operator row carries both concurrency numbers: max_concurrent is what the model serves right now (bs_get_load capacity, the same value /v1/models advertises) and max_concurrent_configured is what --max-concurrent asked for. They differ when there is no batch scheduler at all (no CUDA compiled in, QWEN35_BATCH_SLOTS off, bs_create failed) or when the memory-pressure ladder clamped the slot count — the same split, for the same reason, as context_window vs context_window_configured.

The operator graphs object extends the /v1/models one with gave_up, the lifetime captures / capture_failures counters, and a per-class split: batched_attn and batched_ssm (the multi-user decode path), single_ssm (the one-user per-layer path) and segment (the per-die full-step graph). A class the model has never exercised is omitted rather than reported as an empty deficit. Field meanings: see graphs under GET /v1/models.

The operator attention object adds gave_up / impossible, the lifetime host_engagements / ring_alloc_failures counters, and reasons — a histogram, present only when host > 0, naming why each layer is off-die (ring_alloc_failed, dec_scratch_alloc_failed, kv_geometry_or_ring_wrap, no_slot_decode_kernel, no_flash_kernels, no_batch_scratch, flash_slots_off). Field meanings: see attention under GET /v1/models.

No error cases (returns an empty data array when nothing is loaded).

Source: model_admin.c:ma_handle_models.

---

GET /admin/dies

Operator-only. Snapshot of all probed accelerator dies, their VRAM budget/used, and which model occupies them.

Response — 200

{
  "object": "list",
  "n_dies": 8,
  "data": [
    {
      "die": 0,
      "card": 0,
      "model": "my-model",
      "n_models": 1,
      "vram_budget": 12884901888,
      "vram_used": 11674869760
    }
  ]
}

Field	Type	Notes
die	int	Die manager ordinal (0-indexed across all probed dies)
card	int	Physical card index (pn_dm_card_of(die))
model	string	Name of the model occupying this die, or empty string
n_models	int	Number of models on this die (typically 0 or 1)
vram_budget	int (bytes)	Die's total VRAM minus PN_DIE_RESERVE_BYTES
vram_used	int (bytes)	Bytes allocated by the model manager

Source: model_admin.c:ma_handle_dies.

---

Session routes

A session is a live prefix-cache / decode slot keyed by session_id. In each route below the session id comes from the body's session_id, else the X-Session-Id header (export reads the body only). name selects the model; absent → the default model. Unknown model → 404 model_not_found.

POST /admin/session/export

Operator. Serialize a live session's state so it can be restored later.

Request: {"name": "my-model", "session_id": "abc123"} (session_id required).

Response — 200:

{"session_id":"abc123","weights_version":123,"n_tok":4096,
 "blob_bytes":31457280,"blob_b64":"<base64>","tokens":[...]}

tokens is the committed history the blob covers; it must be sent back on restore.

Status	type	Condition
400	invalid_request_error	Empty/malformed body, missing session_id
400	session_export_unsupported	The model's architecture has no session snapshot format
400	session_caching_disabled	The model has no session cache
404	session_not_found	No live session with that id
429	session_busy	A turn is in flight on that session; retry after it completes
500	server_error	Serialize / out-of-memory failure

Source: model_admin.c:ma_handle_session_export.

POST /session/restore

Install an exported session into a live slot.

Request:

{"name":"my-model","session_id":"abc123","blob":"<blob_b64 from export>","tokens":[...]}

session_id, blob (base64) and tokens are required. The blob must come from the same model architecture and the same weights.

Response — 200: {"restored":true,"session_id":"abc123","n_tok":4096}

Status	type	Condition
400	invalid_request_error	Empty/malformed body, missing session_id/blob/tokens
400	session_caching_disabled	The model has no session cache
400	bad_session_blob	Not base64, corrupt, wrong architecture, or does not fit this model
409	weights_version_mismatch	Blob captured under different weights — replay the conversation instead
429	model_at_session_capacity	No free session slot
500	server_error	Restore failed

Source: openai_chat.c:oai_handle_session_restore.

POST /session/release

Free a session's slot now instead of waiting for the idle timeout (12 h default, PN_SESSION_IDLE_TTL_MS).

Request: {"name": "my-model", "session_id": "abc123"}

Response — 200: {"released":true,"session_id":"abc123"}. Idempotent: an unknown or already-freed session also returns 200.

Errors: 400 invalid_request_error (empty/malformed body, no session id), 404 model_not_found.

Source: openai_chat.c:oai_handle_session_release.

GET /session/estimate

GET /session/estimate?tokens=N[&name=M][&blob_bytes=B][&upload_bps=R]

Arithmetic estimate of restoring a session from a blob vs re-prefilling it. tokens (> 0) is required. Without blob_bytes, the blob is estimated at 16 KiB per token; without upload_bps (bytes/s), 12.5 MB/s is assumed.

Response — 200:

{"model":"my-model","tokens":4096,"prefill_tok_per_s":17.00,
 "cold_reprefill_s":240.94,"warm_restore_s":5.391,"blob_bytes_est":67108864}

prefill_tok_per_s is the serve's measured prefill rate (a fixed fallback before one is measured). An unknown name is not an error; model is then "".

Errors: 400 invalid_request_error when tokens is missing or not positive.

Source: openai_chat.c:oai_handle_session_estimate.

GET /session/watch

GET /session/watch?session_id=S[&name=M] (or X-Session-Id).

Liveness hold. On success the server answers 200 with Content-Type: text/event-stream, a : watch established comment, and keeps the connection open. While it is held the session is never reaped for idleness; when it closes, the session is released after a grace period (45 s default, PN_SESSION_WATCH_GRACE_MS) unless a new watch for it arrives first.

Status	type	Condition
400	invalid_request_error	No session id
400	session_caching_disabled	The model has no session cache
404	model_not_found	Unknown model
503	watch_unavailable	Watch manager not running, or its table is full

Source: serve_command.c:handle_session_watch, session_watch.c.

---

Error body format

All error responses use:

{"error":{"message":"<text>","type":"<code>","code":null}}

The code field is always null.

---

Concurrency and connection model

• One detached pthread per accepted TCP connection. accept() never blocks behind a generation.
• On pthread_create failure the connection is handled inline (no request is dropped).
• SIGPIPE is ignored so a client disconnect mid-stream cannot kill the server.
• Generation serialization:
  • Legacy path (sched == NULL): gen_lock trylock per model. Second concurrent request to the same model → immediate 429.
  • Batched path (QWEN35_BATCH_SLOTS=1): the C3 scheduler owns concurrency. Up to 8 concurrent users; overflow waits in a per-model FIFO queue (default depth 32, BS_QUEUE_DEPTH). 30-second queue wait timeout.