feat(inputs): support {{tmp}} resolution in input defaults

Enables workflow input defaults to use {{tmp}} variable which resolves
to the workflow's scratch directory at runtime.

Example usage:
  inputs:
    output_dir:
      type: str
      default: "{{tmp}}/output"  # Resolves to scratch dir

Author: ibacalu

src/workflows_mcp/engine/workflow_runner.py

@@ -31,7 +31,7 @@
 from .metadata import Metadata
 from .orchestrator import BlockOrchestrator
 from .resolver import UnifiedVariableResolver
-from .schema import BlockDefinition, DependencySpec, WorkflowSchema
+from .schema import BlockDefinition, DependencySpec, InputType, WorkflowSchema
 from .secrets import EnvVarSecretProvider, SecretAuditLog, SecretRedactor
 
 logger = logging.getLogger(__name__)
@@ -955,7 +955,7 @@ def _create_initial_execution_context(
         # Create execution context (workflow-level has no metadata - only blocks have metadata)
         workflow_start_time = datetime.now(UTC).isoformat()
         exec_context = Execution(
-            inputs=self._merge_workflow_inputs(workflow, runtime_inputs),
+            inputs=self._merge_workflow_inputs(workflow, runtime_inputs, scratch_dir),
             metadata=None,  # Workflow-level Execution has no block metadata
             blocks={},
             depth=len(context.workflow_stack) if context else 0,
@@ -1076,37 +1076,41 @@ def _merge_workflow_inputs(
         self,
         workflow: WorkflowSchema,
         runtime_inputs: dict[str, Any] | None,
+        scratch_dir: Path,
     ) -> dict[str, Any]:
-        """Merge default and runtime inputs."""
+        """Merge default and runtime inputs with {{tmp}} resolution and validation."""
         merged = {}
 
-        # Apply defaults
+        # Apply defaults with {{tmp}} resolution
         for input_name, input_decl in workflow.inputs.items():
             if input_decl.default is not None:
-                merged[input_name] = input_decl.default
+                value = input_decl.default
+                if isinstance(value, str):
+                    value = value.replace("{{tmp}}", str(scratch_dir))
+                merged[input_name] = value
 
         # Override with runtime inputs
         if runtime_inputs:
             merged.update(runtime_inputs)
 
         # Validate required inputs
-        missing_inputs = []
-        empty_string_inputs = []
-        for input_name, input_decl in workflow.inputs.items():
-            is_required = getattr(input_decl, "required", False)
-            if is_required:
-                if input_name not in merged:
-                    missing_inputs.append(input_name)
-                elif input_decl.type.value == "str" and merged[input_name] == "":
-                    empty_string_inputs.append(input_name)
+        missing = [
+            name for name, decl in workflow.inputs.items() if decl.required and name not in merged
+        ]
+        empty = [
+            name
+            for name, decl in workflow.inputs.items()
+            if decl.required
+            and name in merged
+            and decl.type == InputType.STR
+            and merged[name] == ""
+        ]
 
         errors = []
-        if missing_inputs:
-            errors.append(f"Missing required inputs: {', '.join(missing_inputs)}")
-        if empty_string_inputs:
-            errors.append(
-                f"Required string inputs cannot be empty: {', '.join(empty_string_inputs)}"
-            )
+        if missing:
+            errors.append(f"Missing required inputs: {', '.join(missing)}")
+        if empty:
+            errors.append(f"Required string inputs cannot be empty: {', '.join(empty)}")
 
         if errors:
             raise ValueError("; ".join(errors))

tests/snapshots/test-tmp-in-input-defaults.json

@@ -0,0 +1,6 @@
+{
+  "outputs": {
+    "resolved": true
+  },
+  "status": "success"
+}

tests/workflows/test-tmp-in-input-defaults.yaml

@@ -0,0 +1,25 @@
+name: test-tmp-in-input-defaults
+description: Test that {{tmp}} is resolved in input defaults
+version: "1.0"
+tags: [test, tmp, input-defaults]
+
+inputs:
+  test_dir:
+    type: str
+    description: Output path with {{tmp}} default
+    default: "{{tmp}}"
+    required: false
+
+blocks:
+  - id: verify_tmp_resolved
+    type: Shell
+    inputs:
+      working_dir: "{{tmp}}"
+      command: |
+        echo "marker" > test.txt
+        [ -f "{{inputs.test_dir}}/test.txt" ]
+
+outputs:
+  resolved:
+    value: "{{blocks.verify_tmp_resolved.succeeded}}"
+    type: bool