diff --git a/README.md b/README.md
index ab4701bf..32a73bdc 100644
--- a/README.md
+++ b/README.md
@@ -97,12 +97,19 @@ export OPENAI_API_KEY="your-api-key-here"
 > - deepseek
 > - xai
 > - groq
+> - any other provider that is compatible with the OpenAI API
 >
 > If you use a provider other than OpenAI, you will need to set the API key for the provider in the config file or in the environment variable as:
 >
 > ```shell
 > export <provider>_API_KEY="your-api-key-here"
 > ```
+>
+> If you use a provider not listed above, you must also set the base URL for the provider:
+>
+> ```shell
+> export <provider>_BASE_URL="https://your-provider-api-base-url"
+> ```
 
 </details>
 <br />
diff --git a/codex-cli/src/utils/config.ts b/codex-cli/src/utils/config.ts
index 1df2e197..7085b6fc 100644
--- a/codex-cli/src/utils/config.ts
+++ b/codex-cli/src/utils/config.ts
@@ -76,6 +76,12 @@ export function getApiKey(provider: string = "openai"): string | undefined {
     return process.env[providerInfo.envKey];
   }
 
+  // Checking `PROVIDER_API_KEY feels more intuitive with a custom provider.
+  const customApiKey = process.env[`${provider.toUpperCase()}_API_KEY`];
+  if (customApiKey) {
+    return customApiKey;
+  }
+
   // If the provider not found in the providers list and `OPENAI_API_KEY` is set, use it
   if (OPENAI_API_KEY !== "") {
     return OPENAI_API_KEY;