feat(frontend): make overload rejection status code configurable

docs/fault-tolerance/request-rejection.md

@@ -210,6 +210,15 @@ Content-Type: application/json
 }
 ```
 
+#### Configuring the rejection status code
+
+The status code returned for overload rejections is configurable via the
+`DYN_HTTP_OVERLOAD_STATUS_CODE` environment variable on the frontend. It
+defaults to `529` ("Site is overloaded"). Operators whose proxies or clients
+only understand `503` Service Unavailable retry semantics can set
+`DYN_HTTP_OVERLOAD_STATUS_CODE=503`. Any valid HTTP status code is accepted; an
+unparseable or out-of-range value falls back to `529`.
+
 ### Client Retry Strategy
 
 Clients should implement exponential backoff when receiving 529 responses:

lib/llm/src/http/service/error.rs

@@ -2,10 +2,22 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use axum::http::StatusCode;
+use dynamo_runtime::config::environment_names::llm as env_llm;
 use thiserror::Error;
 
+/// HTTP status code returned for overload / admission-control rejections.
+///
+/// Defaults to 529 ("Site is overloaded"). Operators can override it via the
+/// `DYN_HTTP_OVERLOAD_STATUS_CODE` env var (e.g. `503` for Service Unavailable
+/// retry semantics). An unset, unparseable, or out-of-range value falls back to
+/// the 529 default.
 pub(crate) fn overload_status_code() -> StatusCode {
-    StatusCode::from_u16(529).expect("529 is a valid HTTP status code")
+    let default = StatusCode::from_u16(529).expect("529 is a valid HTTP status code");
+    std::env::var(env_llm::DYN_HTTP_OVERLOAD_STATUS_CODE)
+        .ok()
+        .and_then(|s| s.trim().parse::<u16>().ok())
+        .and_then(|n| StatusCode::from_u16(n).ok())
+        .unwrap_or(default)
 }
 
 /// Implementation of the Completion Engines served by the HTTP service should

lib/runtime/src/config/environment_names.rs

@@ -297,6 +297,13 @@ pub mod llm {
     pub const DYN_HTTP_GRACEFUL_SHUTDOWN_TIMEOUT_SECS: &str =
         "DYN_HTTP_GRACEFUL_SHUTDOWN_TIMEOUT_SECS";
 
+    /// HTTP status code returned when the frontend rejects a request because
+    /// all workers are overloaded. Defaults to 529 ("Site is overloaded"); set
+    /// to 503 for Service Unavailable retry semantics. Any valid HTTP status
+    /// code (100–999) is accepted; an unparseable or out-of-range value falls
+    /// back to 529.
+    pub const DYN_HTTP_OVERLOAD_STATUS_CODE: &str = "DYN_HTTP_OVERLOAD_STATUS_CODE";
+
     /// Enable LoRA adapter support (set to "true" to enable)
     pub const DYN_LORA_ENABLED: &str = "DYN_LORA_ENABLED";