fix(sglang): enable memory saver before snapshot launch

.github/filters.yaml

@@ -286,7 +286,11 @@ snapshot_vllm:
   - 'deploy/operator/internal/dynamo/backend_vllm.go'
 
 snapshot_sglang:
+  - 'components/src/dynamo/sglang/args.py'
   - 'components/src/dynamo/sglang/snapshot.py'
+  # The SGLang runtime tag controls the engine version exercised by this suite,
+  # so engine bumps must trigger it. Deliberately omit broad pyproject.toml.
+  - 'container/context.yaml'
   - 'deploy/operator/internal/dynamo/backend_sglang.go'
 
 snapshot_trtllm:

.github/scripts/test-filters.js

@@ -280,6 +280,16 @@ const testCases = [
     },
     desc: 'sglang snapshot.py gates only sglang checkpoint tests'
   },
+  {
+    file: 'container/context.yaml',
+    expect: {
+      snapshot: false,
+      snapshot_vllm: false,
+      snapshot_sglang: true,
+      snapshot_trtllm: false,
+    },
+    desc: 'runtime tag changes gate the SGLang checkpoint tests'
+  },
   {
     file: 'components/src/dynamo/trtllm/snapshot.py',
     expect: {

components/src/dynamo/sglang/args.py

@@ -549,8 +549,13 @@ async def parse_args(args: list[str]) -> Config:
     if should_fetch_model(parsed_args, model_path):
         await fetch_model(model_path)
 
-    if is_snapshot_enabled():
+    snapshot_enabled = is_snapshot_enabled()
+    if snapshot_enabled:
         configure_snapshot_capture_env()
+        # SGLang reads these raw fields before late resolution, so snapshot mode
+        # must set them before ServerArgs creation.
+        parsed_args.enable_memory_saver = True
+        parsed_args.enable_forward_pass_metrics = False
 
     # TODO: sglang downloads the model in `from_cli_args`, which means we had to
     # fetch_model (download the model) here, in `parse_args`. `parse_args` should not
@@ -571,7 +576,11 @@ async def parse_args(args: list[str]) -> Config:
         parsed_args.enable_memory_saver = True
 
     fpm_source = _forward_pass_metrics_source(dynamo_config)
-    if fpm_source and not getattr(parsed_args, "enable_forward_pass_metrics", False):
+    if (
+        not snapshot_enabled
+        and fpm_source
+        and not getattr(parsed_args, "enable_forward_pass_metrics", False)
+    ):
         parsed_args.enable_forward_pass_metrics = True
         logging.info("Enabled forward_pass_metrics from %s", fpm_source)
 

components/src/dynamo/sglang/tests/test_sglang_unit.py

@@ -517,6 +517,77 @@ def resolve(parsed_args):
     await parse_args(sys.argv[1:])
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    ("snapshot_enabled", "expected"),
+    [
+        (False, False),
+        (True, True),
+    ],
+)
+async def test_parse_args_sets_raw_memory_saver_before_resolution(
+    monkeypatch, mock_sglang_cli, tmp_path, snapshot_enabled, expected
+):
+    monkeypatch.setattr(
+        "dynamo.sglang.args.configure_snapshot_capture_env", lambda: None
+    )
+    monkeypatch.delenv("DYN_GMS_USE_V1", raising=False)
+    if snapshot_enabled:
+        monkeypatch.setenv(SNAPSHOT_CONTROL_DIR_ENV, str(tmp_path))
+        monkeypatch.setenv("NCCL_CUMEM_ENABLE", "0")
+    else:
+        monkeypatch.delenv(SNAPSHOT_CONTROL_DIR_ENV, raising=False)
+    server_args = SimpleNamespace(
+        disaggregation_mode="null",
+        dllm_algorithm=None,
+        kv_events_config=None,
+        get_model_config=lambda: SimpleNamespace(is_multimodal=False),
+    )
+
+    def resolve(parsed_args):
+        # SGLang 0.5.19 copies this raw field unchanged; late resolution does
+        # not update it before the parent process launches the scheduler.
+        server_args.enable_memory_saver = parsed_args.enable_memory_saver
+        return server_args
+
+    monkeypatch.setattr("dynamo.sglang.args.ServerArgs.from_cli_args", resolve)
+    mock_sglang_cli(model=str(tmp_path))
+
+    config = await parse_args(sys.argv[1:])
+
+    assert config.server_args.enable_memory_saver is expected
+
+
+@pytest.mark.asyncio
+async def test_parse_args_disables_raw_fpm_for_snapshot_with_metric_port(
+    monkeypatch, mock_sglang_cli, tmp_path
+):
+    monkeypatch.setattr(
+        "dynamo.sglang.args.configure_snapshot_capture_env", lambda: None
+    )
+    monkeypatch.delenv("DYN_GMS_USE_V1", raising=False)
+    monkeypatch.setenv(SNAPSHOT_CONTROL_DIR_ENV, str(tmp_path))
+    monkeypatch.setenv("DYN_FORWARDPASS_METRIC_PORT", "23456")
+    server_args = SimpleNamespace(
+        disaggregation_mode="null",
+        dllm_algorithm=None,
+        enable_forward_pass_metrics=False,
+        kv_events_config=None,
+        get_model_config=lambda: SimpleNamespace(is_multimodal=False),
+    )
+
+    def resolve(parsed_args):
+        assert parsed_args.enable_forward_pass_metrics is False
+        return server_args
+
+    monkeypatch.setattr("dynamo.sglang.args.ServerArgs.from_cli_args", resolve)
+    mock_sglang_cli(model=str(tmp_path))
+
+    config = await parse_args(sys.argv[1:])
+
+    assert config.server_args.enable_forward_pass_metrics is False
+
+
 def test_compat_filters_async_generate_kwargs_for_older_engines():
     class OldEngine:
         async def async_generate(self, input_ids=None, sampling_params=None):