fix: model selection precedence to prefer provider default over agent model (#36)

Fixes #35. The app was selecting claude-sonnet-4-6 (agent default) instead of gpt-5.4 (provider default), causing send_message failures in CI where only Azure provider is available.

src/lib/model-selection.test.ts

@@ -0,0 +1,50 @@
+import { test } from "node:test"
+import assert from "node:assert/strict"
+import { chooseModelSelection } from "./model-selection.ts"
+
+const providers = [
+  {
+    id: "azure",
+    models: [{ id: "gpt-5.4" }, { id: "gpt-5.2" }],
+  },
+]
+
+test("keeps existing selection when still available", () => {
+  const selected = chooseModelSelection({
+    providers,
+    defaults: { azure: "gpt-5.4" },
+    existing: { providerID: "azure", modelID: "gpt-5.2" },
+    agentModel: { providerID: "azure", modelID: "claude-sonnet-4-6" },
+  })
+  assert.deepEqual(selected, { providerID: "azure", modelID: "gpt-5.2" })
+})
+
+test("prefers provider default over unavailable agent model", () => {
+  const selected = chooseModelSelection({
+    providers,
+    defaults: { azure: "gpt-5.4" },
+    existing: null,
+    agentModel: { providerID: "azure", modelID: "claude-sonnet-4-6" },
+  })
+  assert.deepEqual(selected, { providerID: "azure", modelID: "gpt-5.4" })
+})
+
+test("falls back to first connected provider model when default missing", () => {
+  const selected = chooseModelSelection({
+    providers,
+    defaults: {},
+    existing: null,
+    agentModel: null,
+  })
+  assert.deepEqual(selected, { providerID: "azure", modelID: "gpt-5.4" })
+})
+
+test("returns null when no connected providers", () => {
+  const selected = chooseModelSelection({
+    providers: [],
+    defaults: {},
+    existing: null,
+    agentModel: null,
+  })
+  assert.equal(selected, null)
+})

src/lib/model-selection.ts

@@ -0,0 +1,50 @@
+export interface ProviderModelRef {
+  id: string
+}
+
+export interface ProviderRef {
+  id: string
+  models: ProviderModelRef[]
+}
+
+export interface ModelSelection {
+  providerID: string
+  modelID: string
+}
+
+export function isModelAvailable(
+  providers: ProviderRef[],
+  selection: ModelSelection | null | undefined,
+): selection is ModelSelection {
+  if (!selection) return false
+  const provider = providers.find((p) => p.id === selection.providerID)
+  if (!provider) return false
+  return provider.models.some((m) => m.id === selection.modelID)
+}
+
+export function chooseModelSelection(params: {
+  providers: ProviderRef[]
+  defaults: Record<string, string>
+  existing: ModelSelection | null
+  agentModel: ModelSelection | null
+}): ModelSelection | null {
+  const { providers, defaults, existing, agentModel } = params
+
+  if (isModelAvailable(providers, existing)) return existing
+
+  for (const provider of providers) {
+    const defaultModelID = defaults[provider.id]
+    if (!defaultModelID) continue
+    if (provider.models.some((m) => m.id === defaultModelID)) {
+      return { providerID: provider.id, modelID: defaultModelID }
+    }
+  }
+
+  if (providers.length > 0 && providers[0].models.length > 0) {
+    return { providerID: providers[0].id, modelID: providers[0].models[0].id }
+  }
+
+  if (isModelAvailable(providers, agentModel)) return agentModel
+
+  return null
+}

src/stores/catalog.ts

@@ -1,6 +1,7 @@
 import { create } from "zustand"
 import { useConnections } from "./connections"
 import type { Agent, Command } from "../lib/sdk"
+import { chooseModelSelection } from "../lib/model-selection"
 
 export interface ProviderModel {
   id: string
@@ -92,19 +93,16 @@ export const useCatalog = create<CatalogState>((set, get) => ({
     const current = get().agent
     const agent = current && visible.some((a) => a.name === current) ? current : visible[0]?.name || "build"
 
-    // Default model: use default agent's model, or first connected provider's default model
+    // Default model: keep valid existing selection; otherwise prefer connected
+    // provider defaults, then first connected model; agent model is last fallback.
     const existing = get().model
-    const fallback = (() => {
     const defaultAgent = visible[0]
-      if (defaultAgent?.model) return defaultAgent.model
-      for (const p of providers) {
-        const defaultModelID = defaults[p.id]
-        const match = defaultModelID ? p.models.find((m) => m.id === defaultModelID) : p.models[0]
-        if (match) return { providerID: p.id, modelID: match.id }
-      }
-      return null
-    })()
-    const model = existing || fallback
+    const model = chooseModelSelection({
+      providers,
+      defaults,
+      existing,
+      agentModel: defaultAgent?.model || null,
+    })
 
     set({ agents: visible, commands, providers, defaults, agent, model, loaded: true })
   },