diff --git a/server.py b/server.py
index c6a4cd1..ef227b7 100644
--- a/server.py
+++ b/server.py
@@ -74,6 +74,21 @@ def list_all_runs() -> list["RunStatus"]:
             runs.append(RunStatus.model_validate_json(data))
     return sorted(runs, key=lambda r: r.created_at, reverse=True)
 
+
+def find_runs_using_content(content_hash: str) -> list[tuple["RunStatus", str]]:
+    """Find all runs that use a content_hash as input or output.
+
+    Returns list of (run, role) tuples where role is 'input' or 'output'.
+    """
+    results = []
+    for run in list_all_runs():
+        if run.inputs and content_hash in run.inputs:
+            results.append((run, "input"))
+        if run.output_hash == content_hash:
+            results.append((run, "output"))
+    return results
+
+
 app = FastAPI(
     title="Art DAG L1 Server",
     description="Distributed rendering server for Art DAG",
@@ -369,15 +384,28 @@ async def discard_run(run_id: str, username: str = Depends(get_required_user)):
     if run.username not in (username, actor_id):
         raise HTTPException(403, "Access denied")
 
-    # Check if run can be discarded
-    can_discard, reason = cache_manager.can_discard_activity(run_id)
-    if not can_discard:
-        raise HTTPException(400, f"Cannot discard run: {reason}")
+    # Check if activity exists for this run
+    activity = cache_manager.get_activity(run_id)
 
-    # Discard the activity (cleans up cache entries)
-    success, msg = cache_manager.discard_activity(run_id)
-    if not success:
-        raise HTTPException(500, f"Failed to discard: {msg}")
+    if activity:
+        # Use activity manager deletion rules
+        can_discard, reason = cache_manager.can_discard_activity(run_id)
+        if not can_discard:
+            raise HTTPException(400, f"Cannot discard run: {reason}")
+
+        # Discard the activity (cleans up cache entries)
+        success, msg = cache_manager.discard_activity(run_id)
+        if not success:
+            raise HTTPException(500, f"Failed to discard: {msg}")
+    else:
+        # Legacy run without activity record - check L2 shared status manually
+        items_to_check = list(run.inputs or [])
+        if run.output_hash:
+            items_to_check.append(run.output_hash)
+
+        for content_hash in items_to_check:
+            if cache_manager.l2_checker.is_shared(content_hash):
+                raise HTTPException(400, f"Cannot discard run: item {content_hash[:16]}... is published to L2")
 
     # Remove from Redis
     redis_client.delete(f"{RUNS_KEY_PREFIX}{run_id}")
@@ -1500,7 +1528,13 @@ async def discard_cache(content_hash: str, username: str = Depends(get_required_
         pin_reason = meta.get("pin_reason", "unknown")
         raise HTTPException(400, f"Cannot discard pinned item (reason: {pin_reason})")
 
-    # Check deletion rules via cache_manager
+    # Check if used by any run (Redis runs, not just activity store)
+    runs_using = find_runs_using_content(content_hash)
+    if runs_using:
+        run, role = runs_using[0]
+        raise HTTPException(400, f"Cannot discard: item is {role} of run {run.run_id}")
+
+    # Check deletion rules via cache_manager (L2 shared status, activity store)
     can_delete, reason = cache_manager.can_delete(content_hash)
     if not can_delete:
         raise HTTPException(400, f"Cannot discard: {reason}")
@@ -1546,7 +1580,13 @@ async def ui_discard_cache(content_hash: str, request: Request):
         pin_reason = meta.get("pin_reason", "unknown")
         return f'<div class="bg-red-900/50 border border-red-700 text-red-300 px-4 py-3 rounded-lg mb-4">Cannot discard: item is pinned ({pin_reason})</div>'
 
-    # Check deletion rules via cache_manager
+    # Check if used by any run (Redis runs, not just activity store)
+    runs_using = find_runs_using_content(content_hash)
+    if runs_using:
+        run, role = runs_using[0]
+        return f'<div class="bg-red-900/50 border border-red-700 text-red-300 px-4 py-3 rounded-lg mb-4">Cannot discard: item is {role} of run {run.run_id}</div>'
+
+    # Check deletion rules via cache_manager (L2 shared status, activity store)
     can_delete, reason = cache_manager.can_delete(content_hash)
     if not can_delete:
         return f'<div class="bg-red-900/50 border border-red-700 text-red-300 px-4 py-3 rounded-lg mb-4">Cannot discard: {reason}</div>'