Expose backend identity and capacity errors

docs/api.md

@@ -100,13 +100,13 @@ validation contract. The response contains:
   optional `uri`;
 - `validation`: `status`, `mode`, `repair_attempts`, `is_valid`, plus
   optional `errors` and `schema_path`;
-- `metadata`: run, prompt, rendered-prompt, profile, model, input-hash, usage,
+- `metadata`: run, prompt, rendered-prompt, profile, optional backend identity, model, input-hash, usage,
   timing, validation, and repair-attempt metadata; and
 - optional `raw_model_output` when requested.
 
 `metadata.model_params` has `endpoint`, `model`, `temperature`,
 `max_tokens`, `top_p`, and `timeout_seconds`, plus optional
-`service_tier`, `reasoning_effort`, `api_key_env`, and `extra_params`.
+`backend_id`, `service_tier`, `reasoning_effort`, `api_key_env`, and `extra_params`.
 `metadata.usage` always includes `prompt_tokens`, `completion_tokens`,
 `total_tokens`, `cached_tokens`, and `cache_write_tokens`; unavailable
 cache usage is reported as zero.
@@ -115,6 +115,11 @@ When Promptkit resolves a direct or definition-rendered session ID,
 `metadata.session_id` contains that effective result value. It is omitted when
 no effective session ID exists.
 
+`metadata.selected_backend_id` and `metadata.model_params.backend_id` report
+the corresponding Promptkit result fields independently when present. Both are
+omitted for an endpoint-only profile; Scriptorium does not infer backend
+identity from an endpoint.
+
 A validation failure has `validation.status: "failed"`, `is_valid: false`,
 and any available diagnostic errors, while still returning the artifact and
 metadata.
@@ -150,6 +155,7 @@ Messages are concise and do not expose wrapped internal causes.
 | `500` | `validation_runtime_failed` | Schema or validator runtime failure. |
 | `500` | `internal_error` | Unclassified server failure. |
 | `502` | `llm_failed` | Outbound model request failed. |
+| `503` | `capacity_exceeded` | The selected model backend has no admission capacity. No retry timing is supplied. |
 
 ## Retry And Idempotency
 

docs/cli.md

@@ -150,9 +150,12 @@ CLI inputs are file references. HTTP inline inputs are defined by the
 ## Output And Exit Behavior
 
 - `run` writes generated content to stdout, or to `--out` when supplied, and
-  writes a concise summary to stderr.
+  writes a concise summary to stderr. The summary includes `backend=<id>` when
+  Promptkit selected a backend; endpoint-only profiles omit it.
 - `render` writes prepared-run output to stdout, or to `--out` when supplied,
-  without a success summary.
+  without a success summary. Text output includes `selected_backend_id` after
+  `selected_profile_id` when Promptkit selected one; endpoint-only profiles
+  omit it.
 - `serve` writes startup and server errors to stderr.
 
 Exit statuses:
@@ -163,6 +166,10 @@ Exit statuses:
 | `1` | Parse, configuration, loading, rendering, generation, output-write, or other runtime error. |
 | `2` | `run` generated and wrote output, but validation failed. |
 
+A backend admission rejection is a runtime error and prints `run error: model
+backend capacity is exhausted`. The HTTP capacity response is defined in the
+[HTTP API reference](api.md).
+
 ## Workflows And Examples
 
 The [maintained render script](../examples/render-markdown-summary.sh) is a

docs/operations.md

@@ -94,6 +94,14 @@ protect request bodies, HTTP file artifacts, and encoded responses; configure
 them through the [configuration reference](config.md) and rely on the
 [HTTP API reference](api.md) for their response effects.
 
+Configured backend concurrency and queue capacity are enforced per constructed
+Promptkit engine. A `serve` process constructs one engine for its handler, so
+concurrent HTTP requests share that transient admission state. Scriptorium does
+not retain workflow state: capacity is neither durable nor a queue of resumable
+runs. When admission is exhausted, HTTP returns `503 capacity_exceeded` without
+retry timing; callers choose any retry policy that is safe for another model
+call.
+
 Before increasing a limit:
 
 1. measure representative input, generated-output, and optional raw-output

docs/roadmap/implementation.md

@@ -278,6 +278,8 @@ validation ownership.
 
 ## Stage 5: Present Backend Identity And Map Capacity Outcomes
 
+**Completion: Complete.**
+
 Expose Promptkit's selected routing identity and make overload behavior a
 stable application contract.