fix: overriding vLLM boolean flags (#1670)

deploy/sdk/src/dynamo/sdk/lib/config.py

@@ -102,12 +102,28 @@ class ServiceConfig(dict):
             # Strip prefix if needed
             arg_key = key[len(prefix) :] if prefix and key.startswith(prefix) else key
 
+            # vLLM arguments that default to True and need explicit false handling
+            # Based on https://github.com/vllm-project/vllm/blob/v0.9.1/vllm/config.py
+            vllm_true_defaults = {
+                "enable_prefix_caching": "no-enable-prefix-caching",
+                "use_tqdm_on_load": "no-use-tqdm-on-load",
+                "multi_step_stream_outputs": "no-multi-step-stream-outputs",
+            }
+
+            # Normalize key for comparison (replace hyphens with underscores)
+            normalized_key = arg_key.replace("-", "_")
+
             # Convert to CLI format
             if isinstance(value, bool):
                 if value:
-                    args.extend([f"--{arg_key}", "true"])
+                    # Always output true values as flags
+                    args.append(f"--{arg_key}")
                 else:
-                    args.extend([f"--{arg_key}", "false"])
+                    # For false values, check if this is a vLLM arg that defaults to True
+                    if normalized_key in vllm_true_defaults:
+                        # Use negative flag if available
+                        args.append(f"--{vllm_true_defaults[normalized_key]}")
+                    # For other false values, omit entirely (standard action="store_true" behavior)
             elif isinstance(value, dict):
                 args.extend([f"--{arg_key}", json.dumps(value)])
             else:

deploy/sdk/src/dynamo/sdk/tests/test_config.py

@@ -143,7 +143,7 @@ def test_service_config_override_common_configs():
 
 
 def test_explicit_boolean_arguments():
-    """Test that boolean arguments are handled with explicit true/false values"""
+    """Test that boolean arguments are handled correctly with new logic"""
     # Reset singleton instance
     ServiceConfig._instance = None
 
@@ -160,21 +160,65 @@ def test_explicit_boolean_arguments():
     }
     """
 
-    # Get arguments and verify explicit boolean handling
+    # Get arguments and verify boolean handling
     service_config = ServiceConfig.get_instance()
     vllm_worker_args = service_config.as_args("VllmWorker")
 
-    # Check that true values are passed as --arg true
+    # Check that true values are passed as flags only
     assert "--enable-prefix-caching" in vllm_worker_args
+    # Should NOT have a following "true" value
     enable_idx = vllm_worker_args.index("--enable-prefix-caching")
-    assert vllm_worker_args[enable_idx + 1] == "true"
+    assert (
+        enable_idx == len(vllm_worker_args) - 1
+        or not vllm_worker_args[enable_idx + 1] == "true"
+    )
 
-    # Check that false values are passed as --arg false
-    assert "--disable-sliding-window" in vllm_worker_args
-    disable_idx = vllm_worker_args.index("--disable-sliding-window")
-    assert vllm_worker_args[disable_idx + 1] == "false"
+    # Check that false values for standard boolean flags are omitted
+    assert "--disable-sliding-window" not in vllm_worker_args
 
-    # Check that another true value works
+    # Check that another true value works as flag
     assert "--enforce-eager" in vllm_worker_args
     enforce_idx = vllm_worker_args.index("--enforce-eager")
-    assert vllm_worker_args[enforce_idx + 1] == "true"
+    assert (
+        enforce_idx == len(vllm_worker_args) - 1
+        or not vllm_worker_args[enforce_idx + 1] == "true"
+    )
+
+
+def test_vllm_boolean_arguments_special_handling():
+    """Test that vLLM boolean arguments with special defaults are handled correctly"""
+    # Reset singleton instance
+    ServiceConfig._instance = None
+
+    # Set environment variable with vLLM boolean configs
+    os.environ[
+        "DYNAMO_SERVICE_CONFIG"
+    ] = """
+    {
+        "VllmWorker": {
+            "enable-prefix-caching": false,
+            "use-tqdm-on-load": false,
+            "multi-step-stream-outputs": false,
+            "some-other-flag": false
+        }
+    }
+    """
+
+    # Get arguments and verify vLLM special boolean handling
+    service_config = ServiceConfig.get_instance()
+    vllm_worker_args = service_config.as_args("VllmWorker")
+
+    # Check that enable-prefix-caching false uses negative flag
+    assert "--no-enable-prefix-caching" in vllm_worker_args
+    assert "--enable-prefix-caching" not in vllm_worker_args
+
+    # Check that use-tqdm-on-load false uses negative flag
+    assert "--no-use-tqdm-on-load" in vllm_worker_args
+    assert "--use-tqdm-on-load" not in vllm_worker_args
+
+    # Check that multi-step-stream-outputs false uses negative flag
+    assert "--no-multi-step-stream-outputs" in vllm_worker_args
+    assert "--multi-step-stream-outputs" not in vllm_worker_args
+
+    # Check that other false flags are omitted (standard behavior)
+    assert "--some-other-flag" not in vllm_worker_args

docs/API/sdk.md

@@ -307,9 +307,9 @@ def parse_vllm_args(service_name, prefix) -> AsyncEngineArgs:
 
     # Add custom arguments
     parser.add_argument("--router", type=str, choices=["random", "round-robin", "kv"], default="random")
-    parser.add_argument("--remote-prefill", action="store_true")
+    parser.add_argument("--remote-prefill", action="store_true", default=False)
 
-    # Add VLLM's arguments
+    # Add VLLM's arguments (ServiceConfig handles True defaults automatically)
     parser = AsyncEngineArgs.add_cli_args(parser)
 
     # Parse both custom and VLLM arguments
@@ -325,6 +325,33 @@ def parse_vllm_args(service_name, prefix) -> AsyncEngineArgs:
     return engine_args
 ```
 
+#### Boolean Argument Handling
+
+ServiceConfig uses a targeted approach for boolean arguments to maintain compatibility with different argument parsers:
+
+1. Standard Boolean Handling:
+- `true` → outputs just the flag (e.g., `--enable-feature`)
+- `false` → omitted entirely (uses parser's default)
+
+2. vLLM True-Default Arguments (targeted override support):
+- Automatically detects vLLM arguments that default to `True` and need explicit `false` handling
+- `enable-prefix-caching: false` → `--no-enable-prefix-caching` (negative flag)
+- `multi-step-stream-outputs: false` → `--no-multi-step-stream-outputs` (negative flag)
+
+```yaml
+# Example YAML configuration
+VllmWorker:
+  # Standard boolean flags (action="store_true" style)
+  enforce-eager: true          # → --enforce-eager
+  disable-logging: false       # → (omitted)
+
+  # vLLM arguments with True defaults (automatically handled)
+  enable-prefix-caching: false  # → --no-enable-prefix-caching
+
+  # Non-boolean arguments
+  max-model-len: 16384         # → --max-model-len 16384
+```
+
 #### Overriding Service Decorator with ServiceArgs
 
 The `ServiceArgs` section in YAML configuration allows you to override any parameter in the `@service` decorator: