Fix

apps/sim/executor/utils/run-from-block.test.ts

@@ -331,9 +331,10 @@ describe('validateRunFromBlock', () => {
     expect(result.error).toContain('Upstream dependency not executed')
   })
 
-  it('rejects blocks with unexecuted transitive upstream dependencies', () => {
+  it('allows running from block when immediate predecessor was executed (ignores transitive)', () => {
     // A → X → B → C, where X is new (not executed)
-    // Running from C should fail because X in upstream chain wasn't executed
+    // Running from C is allowed because B (immediate predecessor) was executed
+    // C will use B's cached output - doesn't matter that X is new
     const dag = createDAG([
       createNode('A', [{ target: 'X' }]),
       createNode('X', [{ target: 'B' }]),
@@ -344,9 +345,8 @@ describe('validateRunFromBlock', () => {
 
     const result = validateRunFromBlock('C', dag, executedBlocks)
 
-    expect(result.valid).toBe(false)
-    expect(result.error).toContain('Upstream dependency not executed')
-    expect(result.error).toContain('X')
+    // Valid because C's immediate predecessor B was executed
+    expect(result.valid).toBe(true)
   })
 
   it('allows blocks with no dependencies even if not previously executed', () => {

apps/sim/executor/utils/run-from-block.ts

@@ -169,15 +169,18 @@ export function validateRunFromBlock(
     if (node.metadata.isSentinel) {
       return { valid: false, error: 'Cannot run from sentinel node' }
     }
-  }
 
-  // Check that ALL upstream blocks were executed (transitive check)
-  const { upstreamSet } = computeExecutionSets(dag, blockId)
-  for (const upstreamId of upstreamSet) {
-    if (!executedBlocks.has(upstreamId)) {
-      return {
-        valid: false,
-        error: `Upstream dependency not executed: ${upstreamId}`,
+    // Check immediate upstream dependencies were executed
+    for (const sourceId of node.incomingEdges) {
+      const sourceNode = dag.nodes.get(sourceId)
+      // Skip sentinel nodes - they're internal and not in executedBlocks
+      if (sourceNode?.metadata.isSentinel) continue
+
+      if (!executedBlocks.has(sourceId)) {
+        return {
+          valid: false,
+          error: `Upstream dependency not executed: ${sourceId}`,
+        }
       }
     }
   }