fix(engine): use safe composition model.

- null represents missing/undefined

Author: ibacalu

src/workflows_mcp/engine/workflow_runner.py

@@ -1163,7 +1163,16 @@ def _merge_workflow_inputs(
 
         # Override with runtime inputs
         if runtime_inputs:
-            merged.update(runtime_inputs)
+            for key, value in runtime_inputs.items():
+                # Fix: If value is explicitly None (null), check if we have a default
+                # If we have a default, we should preserve it instead of overwriting with None
+                if (
+                    value is None
+                    and key in workflow.inputs
+                    and workflow.inputs[key].default is not None
+                ):
+                    continue
+                merged[key] = value
 
         # Validate required inputs
         missing = [

tests/snapshots/composition-child-calculator.json

@@ -0,0 +1,6 @@
+{
+  "outputs": {
+    "child_stdout": "Workspace is DEFAULT_WORKSPACE"
+  },
+  "status": "success"
+}

tests/snapshots/safe-model-parent.json

@@ -1,6 +1,6 @@
 name: composition-child-calculator
 description: Child workflow for composition testing - performs simple arithmetic
-tags: [test, core, composition, child, helper]
+
 inputs:
   number_a:
     type: num

tests/workflows/core/composition/_child-calculator.yaml

@@ -0,0 +1,21 @@
+name: safe-model-child
+description: Child workflow with default input
+version: "1.0"
+
+inputs:
+  workspace:
+    type: str
+    description: Workspace path
+    default: "DEFAULT_WORKSPACE"
+
+blocks:
+  - id: echo_workspace
+    type: Shell
+    inputs:
+      command: echo "Workspace is {{inputs.workspace}}"
+      working_dir: "{{tmp}}"
+
+outputs:
+  workspace:
+    value: "{{blocks.echo_workspace.outputs.stdout}}"
+    type: str

tests/workflows/core/composition/_comp-model-child.yaml

@@ -0,0 +1,23 @@
+name: safe-model-parent
+description: Parent workflow passing null
+version: "1.0"
+tags: [test, composition, defaults]
+
+inputs:
+  workspace:
+    type: str
+    description: Workspace path
+    required: false
+
+blocks:
+  - id: call_child
+    type: Workflow
+    inputs:
+      workflow: safe-model-child
+      inputs:
+        workspace: "{{inputs.workspace}}"
+
+outputs:
+  child_stdout:
+    value: "{{blocks.call_child.outputs.workspace}}"
+    type: str

tests/workflows/core/composition/comp-model-parent.yaml

@@ -10,6 +10,7 @@ blocks:
       inputs:
         url: "qtsone/workflows-mcp/pull/123"
         dry_run: true
+        fetch_metadata: false
 
   - id: verify_github_short
     type: Shell
@@ -43,6 +44,7 @@ blocks:
       inputs:
         url: "https://gitlab.com/owner/repo/-/merge_requests/456"
         dry_run: true
+        fetch_metadata: false
 
   - id: verify_gitlab_full
     type: Shell