diff --git a/crates/op-mcp/src/batch_design_tests.rs b/crates/op-mcp/src/batch_design_tests.rs
index 21190c4ea..7d5fcc9e0 100644
--- a/crates/op-mcp/src/batch_design_tests.rs
+++ b/crates/op-mcp/src/batch_design_tests.rs
@@ -1229,8 +1229,10 @@ fn insert_node_data_hoists_node_state() {
     );
     match tool.call(&args) {
         ToolOutcome::OkWithCommand(_, EditorCommand::Batch { commands }) => {
-            assert!(matches!(&commands[0], EditorCommand::MergeAppState { plan_idx, state }
-                if *plan_idx == usize::MAX && state.contains_key("on")));
+            assert!(
+                matches!(&commands[0], EditorCommand::MergeAppState { plan_idx, state }
+                if *plan_idx == usize::MAX && state.contains_key("on"))
+            );
             assert!(matches!(&commands[1], EditorCommand::InsertSubtree { .. }));
         }
         other => panic!("expected Batch command, got {other:?}"),
@@ -1249,8 +1251,10 @@ fn design_content_hoists_node_state() {
     );
     match dispatch_design_content(&args) {
         ToolOutcome::OkWithCommand(_, EditorCommand::Batch { commands }) => {
-            assert!(matches!(&commands[0], EditorCommand::MergeAppState { plan_idx, state }
-                if *plan_idx == usize::MAX && state.contains_key("n")));
+            assert!(
+                matches!(&commands[0], EditorCommand::MergeAppState { plan_idx, state }
+                if *plan_idx == usize::MAX && state.contains_key("n"))
+            );
             assert!(matches!(&commands[1], EditorCommand::InsertSubtree { .. }));
         }
         other => panic!("expected Batch command, got {other:?}"),
diff --git a/crates/op-mcp/src/batch_program.rs b/crates/op-mcp/src/batch_program.rs
index 3af54eeca..49ddcef6e 100644
--- a/crates/op-mcp/src/batch_program.rs
+++ b/crates/op-mcp/src/batch_program.rs
@@ -261,6 +261,14 @@ fn execute_insert(binding: &str, args: &str, ctx: &mut ProgramCtx) -> Result<(),
     for (cmd, failure) in pre_commands {
         ctx.emit(cmd, failure)?;
     }
+    // Hoist node-level `state` as a SIBLING command — the program
+    // finisher batches ctx.commands itself; wrapping here would nest
+    // Batches, which apply rejects. Held until the insert below
+    // succeeds: emitting it first would leak an orphan `$app` state
+    // command into `ctx.commands` if the insert then fails (the line
+    // as a whole errors and is dropped, but a prior `ctx.emit` already
+    // recorded — state from a line that never landed must not ship).
+    let merge = super::batch_design::hoist_generation_state(&mut nodes);
     ctx.emit(
         EditorCommand::InsertAuthoredSubtree {
             nodes,
@@ -272,6 +280,20 @@ fn execute_insert(binding: &str, args: &str, ctx: &mut ProgramCtx) -> Result<(),
             parent.as_deref().unwrap_or("null")
         ),
     )?;
+    if let Some(merge) = merge {
+        // Emit AFTER the insert succeeds (a failed line must not leak
+        // orphan $app state), then swap the two recorded commands so
+        // the batch still carries MergeAppState before its insert.
+        // Sim-apply order between the two is immaterial: merge touches
+        // only doc.state, the insert only the tree. (If this merge emit
+        // itself ever failed post-insert, the line would error after
+        // its insert already landed — state dropped but never
+        // orphaned; an additive MergeAppState on the sim can't
+        // realistically fail.)
+        ctx.emit(merge, "merge generated app state")?;
+        let n = ctx.commands.len();
+        ctx.commands.swap(n - 1, n - 2);
+    }
     ctx.bind(binding, &root_id);
     Ok(())
 }
@@ -355,7 +377,13 @@ fn execute_replace(binding: &str, args: &str, ctx: &mut ProgramCtx) -> Result<()
         return Err(format!("Replace target not found: {path}"));
     };
     let old_id = old.id_str().to_string();
-    let node = parse_node_json(&args[comma + 1..], ctx.post_process)?;
+    let mut node = parse_node_json(&args[comma + 1..], ctx.post_process)?;
+    // Drain node-level `state` BEFORE the probe clone; hold the merge
+    // and emit it only after the replace below succeeds (emit applies
+    // immediately — emitting the merge first would leak an orphan
+    // `$app` state command into `ctx.commands` if the replace then
+    // fails; state from a line that never landed must not ship).
+    let merge = super::batch_design::hoist_generation_state(std::slice::from_mut(&mut node));
 
     // Predict the ids `cmd_replace_subtree` will assign: it remaps off
     // the live seed/taken BEFORE removing the old subtree — identical
@@ -375,6 +403,20 @@ fn execute_replace(binding: &str, args: &str, ctx: &mut ProgramCtx) -> Result<()
         },
         &format!("Replace failed for: {path}"),
     )?;
+    if let Some(merge) = merge {
+        // Emit AFTER the replace succeeds (a failed line must not leak
+        // orphan $app state), then swap the two recorded commands so
+        // the batch still carries MergeAppState before its replace.
+        // Sim-apply order between the two is immaterial: merge touches
+        // only doc.state, the replace only the tree. (If this merge
+        // emit itself ever failed post-replace, the line would error
+        // after its replace already landed — state dropped but never
+        // orphaned; an additive MergeAppState on the sim can't
+        // realistically fail.)
+        ctx.emit(merge, "merge generated app state")?;
+        let n = ctx.commands.len();
+        ctx.commands.swap(n - 1, n - 2);
+    }
     ctx.bind(binding, &new_id);
     Ok(())
 }
diff --git a/crates/op-mcp/src/batch_program_tests.rs b/crates/op-mcp/src/batch_program_tests.rs
index ac4e05759..e2c213c39 100644
--- a/crates/op-mcp/src/batch_program_tests.rs
+++ b/crates/op-mcp/src/batch_program_tests.rs
@@ -40,6 +40,18 @@ fn binding_id(envelope: &Value, binding: &str) -> String {
         .to_string()
 }
 
+/// Recursively search a command (unwrapping `Batch`) for a leaked
+/// `MergeAppState` — the regression guard for a failed stateful
+/// `I()`/`R()` line (see `failed_insert_line_does_not_leak_merge_app_state`
+/// / `failed_replace_line_does_not_leak_merge_app_state`).
+fn contains_merge_app_state(cmd: &EditorCommand) -> bool {
+    match cmd {
+        EditorCommand::MergeAppState { .. } => true,
+        EditorCommand::Batch { commands } => commands.iter().any(contains_merge_app_state),
+        _ => false,
+    }
+}
+
 #[test]
 fn mixed_program_executes_all_ops_with_shared_bindings_and_slash_paths() {
     let mut state = sample();
@@ -224,6 +236,122 @@ D("ghost")"##;
     assert_eq!(first.base().name.as_deref(), Some("Swapped"));
 }
 
+#[test]
+fn replace_program_hoists_node_state() {
+    let state = sample();
+    let program = r##"swap=R("n13", {"type":"frame","name":"Counter","width":120,"height":24,"state":{"n":{"type":"int","default":0}}})
+D("ghost")"##;
+    let (envelope, cmd) = call_operations(&state, program);
+    assert!(envelope.get("errors").is_none(), "{envelope}");
+    match cmd.expect("command") {
+        EditorCommand::Batch { commands } => {
+            assert!(
+                matches!(&commands[0], EditorCommand::MergeAppState { plan_idx, state }
+                if *plan_idx == usize::MAX && state.contains_key("n"))
+            );
+            let replacement = commands
+                .iter()
+                .find_map(|c| match c {
+                    EditorCommand::ReplaceSubtree { node, .. } => Some(node),
+                    _ => None,
+                })
+                .expect("ReplaceSubtree in batch");
+            let v = serde_json::to_value(replacement.as_ref()).expect("json");
+            assert!(
+                v.get("state").is_none(),
+                "replacement state must be stripped"
+            );
+        }
+        other => panic!("expected Batch, got {other:?}"),
+    }
+}
+
+#[test]
+fn failed_insert_line_does_not_leak_merge_app_state() {
+    // A stateful `I()` line whose parent doesn't resolve to a container
+    // fails at the `InsertAuthoredSubtree` emit — AFTER the node's
+    // `state` would already have been hoisted. The line's error is
+    // collected and the line dropped (TS best-effort semantics), so its
+    // hoisted `MergeAppState` must never survive into the surviving
+    // command. `U(...)` (rather than a second `I()`) is the second line
+    // so `parse_operations` can't take the single-shot Insert-only fast
+    // path and this program is guaranteed to run through the mixed-DSL
+    // executor (`run_batch_design_program`) this fix targets.
+    let mut state = sample();
+    let program = r##"a=I("nonexistent-parent", {"type":"frame","name":"Ghost","width":100,"height":50,"state":{"n":{"type":"int","default":0}}})
+U("n11", {"name":"Renamed"})"##;
+    let (envelope, cmd) = call_operations(&state, program);
+    let errors = envelope["errors"].as_array().expect("errors present");
+    assert_eq!(errors.len(), 1, "{envelope}");
+    assert!(
+        errors[0]["error"]
+            .as_str()
+            .unwrap()
+            .starts_with("Insert parent not found or not a container"),
+        "{envelope}"
+    );
+    let cmd = cmd.expect("the surviving U() line still emits a command");
+    assert!(
+        !contains_merge_app_state(&cmd),
+        "failed insert line must not leak orphan MergeAppState: {cmd:?}"
+    );
+    assert!(
+        state.apply(cmd),
+        "surviving command must still apply cleanly"
+    );
+}
+
+#[test]
+fn failed_replace_line_does_not_leak_merge_app_state() {
+    // The insert case above fails at the emit (parent-not-container is
+    // only checked inside `InsertAuthoredSubtree`'s apply). A plain
+    // `R("no-such-id", ...)` instead fails EARLIER, in `find_node_by_path`
+    // — before `hoist_generation_state` ever runs — so it can't exercise
+    // the vulnerable emit-ordering window this fix closes. To reach that
+    // window for replace, force the `ReplaceSubtree` emit ITSELF to fail
+    // post-hoist: a `pageId` that resolves to no page. The program's
+    // initial page-pin silently no-ops on the same unresolvable id
+    // (leaving the sim on its real, default active page), so
+    // `find_node_by_path` still finds "n13" and the node's `state` is
+    // hoisted — but the ReplaceSubtree command's own page stamp is then
+    // rejected at apply, reproducing exactly the failure shape codex
+    // flagged for `I()`.
+    let tool = batch_design_snapshot(&sample());
+    let mut args = BTreeMap::new();
+    args.insert("pageId".into(), "does-not-exist".into());
+    args.insert(
+        "operations".into(),
+        r##"swap=R("n13", {"type":"frame","name":"Ghost","width":100,"height":50,"state":{"n":{"type":"int","default":0}}})
+D("ghost")"##
+            .into(),
+    );
+    let (envelope, cmd): (Value, Option<EditorCommand>) = match tool.call(&args) {
+        ToolOutcome::OkJson(json) => (serde_json::from_str(&json).expect("json"), None),
+        ToolOutcome::OkJsonWithCommand(json, cmd) => {
+            (serde_json::from_str(&json).expect("json"), Some(cmd))
+        }
+        other => panic!("expected a TS result envelope, got {other:?}"),
+    };
+    let errors = envelope["errors"].as_array().expect("errors present");
+    assert_eq!(errors.len(), 1, "{envelope}");
+    assert!(
+        errors[0]["error"]
+            .as_str()
+            .unwrap()
+            .starts_with("Replace failed for:"),
+        "{envelope}"
+    );
+    // `D("ghost")` is a silent no-op (unknown id), so nothing else could
+    // legitimately contribute a command here — any surviving command
+    // must NOT be (or contain) the orphaned MergeAppState.
+    assert!(
+        cmd.as_ref()
+            .map(|c| !contains_merge_app_state(c))
+            .unwrap_or(true),
+        "failed replace line must not leak orphan MergeAppState: {cmd:?}"
+    );
+}
+
 #[test]
 fn image_op_requires_ts_quoted_syntax_and_resolves_binding_parents() {
     let mut state = sample();
diff --git a/crates/op-mcp/src/replace_node_tests.rs b/crates/op-mcp/src/replace_node_tests.rs
index c9386edfd..8fb7af2ec 100644
--- a/crates/op-mcp/src/replace_node_tests.rs
+++ b/crates/op-mcp/src/replace_node_tests.rs
@@ -131,6 +131,42 @@ fn replace_node_accepts_ts_data_tree_as_subtree() {
     }
 }
 
+#[test]
+fn replace_node_data_hoists_node_state() {
+    // A generated `data` replacement declaring node-level `state` must
+    // hoist it to a doc-root MergeAppState (unplanned priority) and
+    // strip it off the ReplaceSubtree payload — same contract as the
+    // insert paths and batch_program's R().
+    let tool = replace_node_snapshot();
+    let mut args = BTreeMap::new();
+    args.insert("nodeId".into(), "n11".into());
+    args.insert(
+        "data".into(),
+        r##"{"type":"frame","name":"Counter","width":200,"height":100,"state":{"n":{"type":"int","default":0}},"children":[{"type":"text","content":"hi"}]}"##
+            .into(),
+    );
+    match tool.call(&args) {
+        ToolOutcome::OkWithCommand(_, EditorCommand::Batch { commands }) => {
+            assert_eq!(commands.len(), 2);
+            assert!(
+                matches!(&commands[0], EditorCommand::MergeAppState { plan_idx, state }
+                if *plan_idx == usize::MAX && state.contains_key("n"))
+            );
+            match &commands[1] {
+                EditorCommand::ReplaceSubtree { node, .. } => {
+                    let v = serde_json::to_value(node.as_ref()).expect("json");
+                    assert!(
+                        v.get("state").is_none(),
+                        "replacement state must be stripped"
+                    );
+                }
+                other => panic!("expected ReplaceSubtree second, got {other:?}"),
+            }
+        }
+        other => panic!("expected Batch command, got {other:?}"),
+    }
+}
+
 #[test]
 fn replace_node_preserves_ts_leaf_data_with_extra_fields_as_subtree() {
     let mut args = BTreeMap::new();
diff --git a/crates/op-mcp/src/write_tools.rs b/crates/op-mcp/src/write_tools.rs
index 407f3a607..2dcc2e45f 100644
--- a/crates/op-mcp/src/write_tools.rs
+++ b/crates/op-mcp/src/write_tools.rs
@@ -268,11 +268,16 @@ fn ts_data_tree_command(
         .map(|s| s.trim())
         .filter(|s| !s.is_empty())
         .map(str::to_string);
-    Ok(Some(EditorCommand::InsertSubtree {
-        nodes: vec![node],
-        parent_id,
-        page_id,
-    }))
+    let mut nodes = vec![node];
+    let hoist = super::batch_design::hoist_generation_state(&mut nodes);
+    Ok(Some(super::batch_design::with_hoisted_state(
+        hoist,
+        EditorCommand::InsertSubtree {
+            nodes,
+            parent_id,
+            page_id,
+        },
+    )))
 }
 
 #[allow(clippy::result_large_err)]
@@ -564,7 +569,7 @@ impl McpTool for ReplaceNode {
             Err(e) => return e,
         };
         match ts_data_tree_node(args) {
-            Ok(Some(node)) => {
+            Ok(Some(mut node)) => {
                 let drop_children = match parse_drop_children_arg(args) {
                     Ok(v) => v,
                     Err(e) => return e,
@@ -578,14 +583,22 @@ impl McpTool for ReplaceNode {
                     .map(str::to_string);
                 let mut out = BTreeMap::new();
                 out.insert("wrote".into(), "true".into());
+                // The replacement is GENERATED node JSON — hoist any
+                // node-level `state` to the document root, same as the
+                // insert paths and batch_program's R().
+                let hoist =
+                    super::batch_design::hoist_generation_state(std::slice::from_mut(&mut node));
                 return ToolOutcome::OkWithCommand(
                     out,
-                    EditorCommand::ReplaceSubtree {
-                        node_id,
-                        node: Box::new(node),
-                        drop_children,
-                        page_id,
-                    },
+                    super::batch_design::with_hoisted_state(
+                        hoist,
+                        EditorCommand::ReplaceSubtree {
+                            node_id,
+                            node: Box::new(node),
+                            drop_children,
+                            page_id,
+                        },
+                    ),
                 );
             }
             Ok(None) => {}