Document smoke-test diagnostics

docs/integrations/openai-compatible-chat.md

@@ -45,6 +45,45 @@ precedence over the backend default. Only the name reaches prepared metadata;
 the environment value is read just before the provider call and is never added
 to the JSON body.
 
+## Explicit Smoke Diagnostics
+
+Promptkit's explicit smoke diagnostics are separate consumer operations; they
+are not part of ordinary `Run`, the offline examples, or release validation.
+With the built-in client, inference diagnostics use the same configured client
+and `/chat/completions` request as generation. Their content cases are private
+to Promptkit and are not application prompt, schema, or provider protocol
+extensions. The public
+[`Engine.SmokeTest` GoDoc](../../smoke_diagnostics.go) owns request selection,
+report facts, and injected-client behavior.
+
+HTTP-only diagnostics use the configured built-in HTTP client. Reachability is
+one unauthenticated `GET` to the selected diagnostic URL; an observed response
+counts as reachable even when its status is non-2xx. Authentication behavior
+is selected from backend metadata, not inferred from the host. The
+`openrouter_key` adapter makes one `GET` to `/key` relative to the effective
+backend base, unless configuration supplies a full diagnostic endpoint (see
+the [backend format reference](../formats.md#built-in-backends)). It may send
+the available bearer credential, but can dispatch without one. A 401 is
+recognized as rejection. A 200 is recognized as acceptance only when its JSON
+object has a `data` object containing string `label`, Boolean `is_free_tier`,
+and numeric `usage`; unrelated fields and account limits are ignored. This is
+only a minimal identifying subset of OpenRouter's documented
+[current-key response](https://github.com/OpenRouterTeam/docs/blob/main/api_reference/limits.mdx#checking-your-limits),
+not validation of the full account document. Other statuses and unrecognized
+200 responses do not establish acceptance.
+
+Diagnostic HTTP evidence describes the last request observed at Promptkit's
+RoundTripper boundary and any response associated with it. It includes the
+selected destination without URL user information, response status, and
+credential presence when observed. Redirects can change the last observed
+destination or strip credentials. This evidence does not prove that bytes
+reached a peer or reveal dispatch hidden inside a custom transport. Diagnostic
+reports do not retain headers, credentials, provider
+bodies, or generated text. A consumer-injected `LLMClient` can support inference
+diagnostics, but does not establish HTTP status, destination, reachability, or
+upstream credential-supply facts; Promptkit does not fall back to a built-in
+transport for HTTP-only diagnostics.
+
 ## Request Body
 
 The request body always contains `model` and `messages`. The execution