fix(runtime-fallback): make fallback provider selection provider-agnostic (fixes #2303)

src/features/background-agent/fallback-retry-handler.test.ts

@@ -19,6 +19,8 @@ mock.module("../../shared/provider-model-id-transform", () => ({
 
 import { tryFallbackRetry } from "./fallback-retry-handler"
 import { shouldRetryError } from "../../shared/model-error-classifier"
+import { selectFallbackProvider } from "../../shared/model-error-classifier"
+import { readProviderModelsCache } from "../../shared"
 import type { BackgroundTask } from "./types"
 import type { ConcurrencyManager } from "./concurrency"
 
@@ -82,6 +84,8 @@ function createDefaultArgs(taskOverrides: Partial<BackgroundTask> = {}) {
 describe("tryFallbackRetry", () => {
   beforeEach(() => {
     ;(shouldRetryError as any).mockImplementation(() => true)
+    ;(selectFallbackProvider as any).mockImplementation((providers: string[]) => providers[0])
+    ;(readProviderModelsCache as any).mockReturnValue(null)
   })
 
   describe("#given retryable error with fallback chain", () => {
@@ -267,4 +271,24 @@ describe("tryFallbackRetry", () => {
       expect(args.task.attemptCount).toBe(2)
     })
   })
+
+  describe("#given disconnected fallback providers with connected preferred provider", () => {
+    test("keeps fallback entry and selects connected preferred provider", () => {
+      ;(readProviderModelsCache as any).mockReturnValue({ connected: ["provider-a"] })
+      ;(selectFallbackProvider as any).mockImplementation(
+        (_providers: string[], preferredProviderID?: string) => preferredProviderID ?? "provider-b",
+      )
+
+      const args = createDefaultArgs({
+        fallbackChain: [{ model: "fallback-model-1", providers: ["provider-b"], variant: undefined }],
+        model: { providerID: "provider-a", modelID: "original-model" },
+      })
+
+      const result = tryFallbackRetry(args)
+
+      expect(result).toBe(true)
+      expect(args.task.model?.providerID).toBe("provider-a")
+      expect(args.task.model?.modelID).toBe("fallback-model-1")
+    })
+  })
 })

src/features/background-agent/fallback-retry-handler.ts

@@ -35,10 +35,14 @@ export function tryFallbackRetry(args: {
   const providerModelsCache = readProviderModelsCache()
   const connectedProviders = providerModelsCache?.connected ?? readConnectedProvidersCache()
   const connectedSet = connectedProviders ? new Set(connectedProviders.map(p => p.toLowerCase())) : null
+  const preferredProvider = task.model?.providerID?.toLowerCase()
 
   const isReachable = (entry: FallbackEntry): boolean => {
     if (!connectedSet) return true
-    return entry.providers.some((p) => connectedSet.has(p.toLowerCase()))
+    if (entry.providers.some((provider) => connectedSet.has(provider.toLowerCase()))) {
+      return true
+    }
+    return preferredProvider ? connectedSet.has(preferredProvider) : false
   }
 
   let selectedAttemptCount = attemptCount