diff --git a/codex-rs/config/src/thread_config.rs b/codex-rs/config/src/thread_config.rs
index 1b3ea8fe87..556619357a 100644
--- a/codex-rs/config/src/thread_config.rs
+++ b/codex-rs/config/src/thread_config.rs
@@ -187,6 +187,34 @@ fn session_thread_config_to_toml(
     }
 
     if !config.model_providers.is_empty() {
+        // Session-scoped providers must not pair a session-provided base_url with
+        // process-owned credentials from auth.json or environment variables.
+        if let Some((provider_id, field)) = config
+            .model_providers
+            .iter()
+            .filter_map(|(id, provider)| {
+                let field = if provider.requires_openai_auth {
+                    Some("requires_openai_auth")
+                } else if provider.env_key.is_some() {
+                    Some("env_key")
+                } else if provider.env_http_headers.is_some() {
+                    Some("env_http_headers")
+                } else {
+                    None
+                };
+                field.map(|field| (id, field))
+            })
+            .min_by(|(left, _), (right, _)| left.cmp(right))
+        {
+            return Err(ThreadConfigLoadError::new(
+                ThreadConfigLoadErrorCode::Parse,
+                /*status_code*/ None,
+                format!(
+                    "session model_providers.{provider_id} cannot set {field}; \
+session-scoped providers must not read process-owned credentials",
+                ),
+            ));
+        }
         let model_providers = TomlValue::try_from(config.model_providers).map_err(|err| {
             ThreadConfigLoadError::new(
                 ThreadConfigLoadErrorCode::Parse,
@@ -294,6 +322,49 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn loader_rejects_session_model_provider_process_credentials() {
+        let mut openai_auth_provider = test_provider("local");
+        openai_auth_provider.requires_openai_auth = true;
+        let mut env_key_provider = test_provider("local");
+        env_key_provider.env_key = Some("OPENAI_API_KEY".to_string());
+        let mut env_header_provider = test_provider("local");
+        env_header_provider.env_http_headers = Some(HashMap::from([(
+            "Authorization".to_string(),
+            "OPENAI_API_KEY".to_string(),
+        )]));
+
+        for (field, provider) in [
+            ("requires_openai_auth", openai_auth_provider),
+            ("env_key", env_key_provider),
+            ("env_http_headers", env_header_provider),
+        ] {
+            let loader = StaticThreadConfigLoader::new(vec![ThreadConfigSource::Session(
+                SessionThreadConfig {
+                    model_provider: Some("local".to_string()),
+                    model_providers: HashMap::from([("local".to_string(), provider)]),
+                    features: BTreeMap::new(),
+                },
+            )]);
+            let err = loader
+                .load_config_layers(ThreadConfigContext::default())
+                .await
+                .expect_err("session-scoped providers must not read process credentials");
+
+            assert_eq!(
+                err,
+                ThreadConfigLoadError::new(
+                    ThreadConfigLoadErrorCode::Parse,
+                    /*status_code*/ None,
+                    format!(
+                        "session model_providers.local cannot set {field}; \
+session-scoped providers must not read process-owned credentials",
+                    ),
+                )
+            );
+        }
+    }
+
     fn test_provider(name: &str) -> ModelProviderInfo {
         ModelProviderInfo {
             name: name.to_string(),
diff --git a/codex-rs/model-provider/src/auth.rs b/codex-rs/model-provider/src/auth.rs
index 3c7f4dbd0f..dabc5e3a41 100644
--- a/codex-rs/model-provider/src/auth.rs
+++ b/codex-rs/model-provider/src/auth.rs
@@ -83,6 +83,12 @@ pub(crate) fn resolve_provider_auth(
         return Ok(Arc::new(auth));
     }
 
+    let auth = if provider.requires_openai_auth {
+        auth
+    } else {
+        None
+    };
+
     Ok(match auth {
         Some(auth) => auth_provider_from_auth(auth),
         None => unauthenticated_auth_provider(),
diff --git a/codex-rs/model-provider/src/provider.rs b/codex-rs/model-provider/src/provider.rs
index 1ef7c22962..d3320dd498 100644
--- a/codex-rs/model-provider/src/provider.rs
+++ b/codex-rs/model-provider/src/provider.rs
@@ -475,6 +475,25 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn custom_non_openai_provider_does_not_use_base_auth_manager() {
+        let provider = create_model_provider(
+            provider_for("http://localhost:1234/v1".to_string()),
+            Some(AuthManager::from_auth_for_testing(CodexAuth::from_api_key(
+                "openai-api-key",
+            ))),
+        );
+
+        assert!(
+            provider
+                .api_auth()
+                .await
+                .expect("auth should resolve")
+                .to_auth_headers()
+                .is_empty()
+        );
+    }
+
     #[test]
     fn amazon_bedrock_provider_returns_bedrock_account_state() {
         let provider = create_model_provider(