feat(llm): interrogate a draft provider endpoint for its models

Once a pi-ai route became a declaration rather than a catalog lookup,
adding an OpenAI-compatible gateway meant knowing its model ids up
front. Most such endpoints publish that list at `GET /models`, but no
seam operation could ask: every one is keyed by a registered provider
route, and the provider being added has no route, no stored profile,
and no stored credential — the endpoint and key are values in a form.

Interrogation is therefore keyed by settings namespace, which a
configuration surface already holds from the configurable-provider
directory. `registerModelDiscovery` offers it per namespace,
`discoverModels` asks, and the request carries the draft itself. The
reply is candidates, not a catalog: every field but the id is optional
because most listings disclose nothing else, and adopting one is a
settings write like any other. Nothing here reads or writes settings or
credentials, so `settings.yaml` still decides what a route serves.

`llm.discoverModels` carries the same draft over the wire. Its apiKey is
the third and last payload a secret may ride, and it is never stored,
logged, or echoed; every refusal folds into `model-discovery-failed`,
naming the endpoint asked but never the credential offered.

The pi-ai side is a plain GET for OpenAI-compatible protocols only —
their listing shape is the one gateways, self-hosted servers, and the
official endpoints agree on. Others say so, sending the user to
hand-entry rather than reporting a guessed shape as an empty provider.
The reply is read under a four-megabyte ceiling held on the bytes
actually received, because the endpoint is a URL the user typed.

packages/host/apiproxy/src/api/llm.schema.ts

@@ -6,7 +6,7 @@
 import { z } from 'zod'
 import type { RequestPayload, ResponseValue } from './rpc-map.ts'
 import type { Wire } from './rpc.schema.ts'
-import type { ConfigurableProviderView } from './llm.ts'
+import type { ConfigurableProviderView, DiscoveredModelView } from './llm.ts'
 import { modelCatalogFailureSchema, modelProviderGroupSchema } from './sessions.schema.ts'
 
 /** ConfigurableProviderView row of llm.providers. */
@@ -34,3 +34,27 @@ export const llmModelsValueSchema = z.object({
   groups: z.array(modelProviderGroupSchema),
   failures: z.array(modelCatalogFailureSchema),
 }) satisfies z.ZodType<Wire<ResponseValue<'llm.models'>>>
+
+/** DiscoveredModelView row of llm.discoverModels. */
+export const discoveredModelViewSchema = z.object({
+  id: z.string().min(1),
+  name: z.string().min(1).optional(),
+  contextWindow: z.number().int().positive().optional(),
+  maxTokens: z.number().int().positive().optional(),
+}) satisfies z.ZodType<Wire<DiscoveredModelView>>
+
+/** llm.discoverModels request payload. */
+export const llmDiscoverModelsRequestSchema = z.object({
+  settingsNs: z.string().min(1),
+  baseURL: z.string().min(1),
+  api: z.string().min(1).optional(),
+  // Write-only: the host uses it for this one interrogation and never stores,
+  // logs, or returns it. Kept out of any redacted echo for the same reason
+  // `credentials.set` never reads a value back.
+  apiKey: z.string().min(1).optional(),
+}) satisfies z.ZodType<Wire<RequestPayload<'llm.discoverModels'>>>
+
+/** llm.discoverModels response value. */
+export const llmDiscoverModelsValueSchema = z.object({
+  models: z.array(discoveredModelViewSchema),
+}) satisfies z.ZodType<Wire<ResponseValue<'llm.discoverModels'>>>

packages/host/apiproxy/src/api/llm.ts

@@ -40,4 +40,38 @@ export interface LlmApi {
    * failures ride `failures` without failing the sound groups.
    */
   models(request: RpcRequest<{}>): Promise<RpcResponse<{ groups: ModelProviderGroup[]; failures: ModelCatalogFailure[] }>>
+
+  /**
+   * Interrogate a provider endpoint the configuration surface is still
+   * drafting, and return the models it advertises for the user to adopt.
+   *
+   * The payload is the draft, not a stored route: `settingsNs` selects the
+   * adapter family that knows how to read the listing, and the endpoint,
+   * protocol, and key come from the form. Nothing is written — the reply is
+   * candidates, and only a later `settings.mutate` decides what a route
+   * serves. `apiKey` is therefore accepted here but never stored, logged, or
+   * echoed back; a provider whose key is already stored omits it and the
+   * endpoint answers unauthenticated or refuses.
+   */
+  discoverModels(
+    request: RpcRequest<{
+      settingsNs: string
+      baseURL: string
+      api?: string
+      apiKey?: string
+    }>,
+    signal?: AbortSignal,
+  ): Promise<RpcResponse<{ models: DiscoveredModelView[] }>>
+}
+
+/** Wire view of one model an interrogated endpoint advertises. */
+export interface DiscoveredModelView {
+  /** Model id the endpoint accepts. */
+  id: string
+  /** Human-readable name when the endpoint supplies one. */
+  name?: string
+  /** Maximum combined request and response context, when disclosed. */
+  contextWindow?: number
+  /** Maximum output tokens, when disclosed. */
+  maxTokens?: number
 }

packages/host/apiproxy/src/api/rpc-map.ts

@@ -66,6 +66,7 @@ export interface RpcMethodMap {
   'credentials.unset': CredentialsApi['unset']
   'llm.providers': LlmApi['providers']
   'llm.models': LlmApi['models']
+  'llm.discoverModels': LlmApi['discoverModels']
 }
 
 /** Business request payload of method K (reaches through the RpcRequest narrow form to payload). */

packages/host/apiproxy/src/api/rpc.schema.ts

@@ -55,6 +55,7 @@ export const rpcErrorSchema: z.ZodType<RpcError> = z.discriminatedUnion('code',
   z.object({ code: z.literal('settings-not-exposed'), message: z.string(), details: z.object({ ns: z.string() }) }),
   z.object({ code: z.literal('settings-conflict'), message: z.string(), details: z.object({ ns: z.string(), expected: z.number(), actual: z.number() }) }),
   z.object({ code: z.literal('credential-rejected'), message: z.string(), details: z.object({ ref: z.string() }) }),
+  z.object({ code: z.literal('model-discovery-failed'), message: z.string(), details: z.object({ settingsNs: z.string(), baseURL: z.string() }) }),
   z.object({ code: z.literal('title-invalid'), message: z.string(), details: z.object({ sessionId: z.string() }) }),
   z.object({ code: z.literal('fork-unavailable'), message: z.string(), details: z.object({ sessionId: z.string() }) }),
   z.object({ code: z.literal('subagent-parent-unavailable'), message: z.string(), details: z.object({ parentSessionId: z.string() }) }),

packages/host/apiproxy/src/api/rpc.ts

@@ -70,6 +70,15 @@ export interface RpcErrorDetailsMap {
   'settings-conflict': { ns: string; expected: number; actual: number }
   /** A credential write was refused (read-only shadowing layer or storage failure); the message is the seam's own text. */
   'credential-rejected': { ref: string }
+  /**
+   * Interrogating a draft provider endpoint did not produce a model listing:
+   * no adapter family serves the namespace, the protocol has no listing this
+   * build can read, or the endpoint was unreachable, refused the credential,
+   * or answered with something else. The message is the adapter's own text —
+   * it is what the form shows before falling back to hand-entry — and the
+   * details name the endpoint asked, never the credential offered.
+   */
+  'model-discovery-failed': { settingsNs: string; baseURL: string }
   'title-invalid': { sessionId: SessionId }
   'fork-unavailable': { sessionId: SessionId }
   'subagent-parent-unavailable': { parentSessionId: SessionId }