fix: prevent cross-provider model env var leaks and sync Codex detection (#243)

Two provider routing bugs that cause silent wrong-model failures:

1. model.ts: getUserSpecifiedModelSetting() read ANTHROPIC_MODEL ||
   GEMINI_MODEL || OPENAI_MODEL with no provider check. A user
   switching from Anthropic to OpenAI with ANTHROPIC_MODEL still set
   would silently send the Anthropic model name to the OpenAI API.
   Now gates each env var behind the active provider from
   getAPIProvider().

2. providers.ts: isCodexModel() maintained a hardcoded list of 8 model
   names that was missing gpt-5.4-mini and gpt-5.2 from the canonical
   CODEX_ALIAS_MODELS table in providerConfig.ts. This caused a
   split-brain: getAPIProvider() returned 'openai' while
   resolveProviderRequest() selected 'codex_responses' transport.
   Now delegates to the exported isCodexAlias() to keep both detection
   systems in sync.

src/services/api/providerConfig.ts

@@ -201,7 +201,7 @@ function parseModelDescriptor(model: string): ModelDescriptor {
   }
 }
 
-function isCodexAlias(model: string): boolean {
+export function isCodexAlias(model: string): boolean {
   const normalized = model.trim().toLowerCase()
   const base = normalized.split('?', 1)[0] ?? normalized
   return base in CODEX_ALIAS_MODELS

src/utils/model/model.ts

@@ -75,7 +75,15 @@ export function getUserSpecifiedModelSetting(): ModelSetting | undefined {
     specifiedModel = modelOverride
   } else {
     const settings = getSettings_DEPRECATED() || {}
-    specifiedModel = process.env.ANTHROPIC_MODEL || process.env.GEMINI_MODEL || process.env.OPENAI_MODEL || settings.model || undefined
+    // Read the model env var that matches the active provider to prevent
+    // cross-provider leaks (e.g. ANTHROPIC_MODEL sent to the OpenAI API).
+    const provider = getAPIProvider()
+    specifiedModel =
+      (provider === 'gemini' ? process.env.GEMINI_MODEL : undefined) ||
+      (provider === 'openai' || provider === 'gemini' ? process.env.OPENAI_MODEL : undefined) ||
+      (provider === 'firstParty' ? process.env.ANTHROPIC_MODEL : undefined) ||
+      settings.model ||
+      undefined
   }
 
   // Ignore the user-specified model if it's not in the availableModels allowlist.

src/utils/model/providers.ts

@@ -1,4 +1,5 @@
 import type { AnalyticsMetadata_I_VERIFIED_THIS_IS_NOT_CODE_OR_FILEPATHS } from '../../services/analytics/index.js'
+import { isCodexAlias } from '../../services/api/providerConfig.js'
 import { isEnvTruthy } from '../envUtils.js'
 
 export type APIProvider =
@@ -33,17 +34,11 @@ export function usesAnthropicAccountFlow(): boolean {
   return getAPIProvider() === 'firstParty'
 }
 function isCodexModel(): boolean {
-  const model = (process.env.OPENAI_MODEL || '').toLowerCase()
+  const model = (process.env.OPENAI_MODEL || '').trim()
-  return (
+  if (!model) return false
-    model === 'codexplan' ||
+  // Delegate to the canonical alias table in providerConfig to keep
-    model === 'codexspark' ||
+  // the two Codex detection systems (provider type + transport) in sync.
-    model === 'gpt-5.4' ||
+  return isCodexAlias(model)
-    model === 'gpt-5.3-codex' ||
-    model === 'gpt-5.3-codex-spark' ||
-    model === 'gpt-5.2-codex' ||
-    model === 'gpt-5.1-codex-max' ||
-    model === 'gpt-5.1-codex-mini'
-  )
 }
 
 export function getAPIProviderForStatsig(): AnalyticsMetadata_I_VERIFIED_THIS_IS_NOT_CODE_OR_FILEPATHS {