refactor: improve error response for non-available models (#298)

scripts/system-check.test.ts

@@ -0,0 +1,42 @@
+import { describe, expect, test } from 'bun:test'
+
+import { formatReachabilityFailureDetail } from './system-check.ts'
+
+describe('formatReachabilityFailureDetail', () => {
+  test('returns generic failure detail for non-codex transport', () => {
+    const detail = formatReachabilityFailureDetail(
+      'https://api.openai.com/v1/models',
+      429,
+      '{"error":"rate_limit"}',
+      {
+        transport: 'chat_completions',
+        requestedModel: 'gpt-4o',
+        resolvedModel: 'gpt-4o',
+      },
+    )
+
+    expect(detail).toBe(
+      'Unexpected status 429 from https://api.openai.com/v1/models. Body: {"error":"rate_limit"}',
+    )
+  })
+
+  test('adds alias/entitlement hint for codex model support 400s', () => {
+    const detail = formatReachabilityFailureDetail(
+      'https://chatgpt.com/backend-api/codex/responses',
+      400,
+      '{"detail":"The \\"gpt-5.3-codex-spark\\" model is not supported when using Codex with a ChatGPT account."}',
+      {
+        transport: 'codex_responses',
+        requestedModel: 'codexspark',
+        resolvedModel: 'gpt-5.3-codex-spark',
+      },
+    )
+
+    expect(detail).toContain(
+      'model alias "codexspark" resolved to "gpt-5.3-codex-spark"',
+    )
+    expect(detail).toContain(
+      'Try "codexplan" or another entitled Codex model.',
+    )
+  })
+})