[Observability] Expose python/rust frontend identity in /server_info (#39993)

Signed-off-by: Sage Ahrac <sagiahrak@gmail.com>

python/sglang/srt/entrypoints/http_server.py

@@ -846,6 +846,7 @@ async def server_info():
             "startup_time": _global_state.tokenizer_manager.startup_time,
             "internal_states": internal_states,
             "version": __version__,
+            "frontend": "python",
             # Structured KV-event publisher descriptor for KV-aware routers.
             # `None` when publishing is disabled or misconfigured; see
             # `runtime_context.describe_kv_events_publisher` for the contract.

rust/sglang-server/src/api_server/common.rs

@@ -173,6 +173,7 @@ fn shape_server_info(msgpack: &[u8], server_args: &ServerArgs) -> Result<Vec<u8>
         "max_context_length": server_args.model_config.context_len,
         "max_total_num_tokens": server_args.max_total_num_tokens,
         "version": server_args.version,
+        "frontend": "rust",
         "internal_states": [serde_json::Value::Object(state_out)],
     });
     serde_json::to_vec(&response).map_err(|e| e.to_string())
@@ -240,5 +241,6 @@ mod tests {
         assert!(state0.get("api_key").is_none());
         // Curated top-level config comes from typed accessors, not the dump.
         assert_eq!(v["model_path"], "/m");
+        assert_eq!(v["frontend"], "rust");
     }
 }

test/registered/unit/entrypoints/test_server_info.py

@@ -523,6 +523,7 @@ class TestServerInfoExistingFieldsPreserved(CustomTestCase):
 
         self.assertIn("internal_states", info)
         self.assertIn("version", info)
+        self.assertEqual(info["frontend"], "python")
 
     def test_kv_events_config_raw_field_still_surfaced(self):
         # The new structured `kv_events` block sits alongside the