From b6f5c29d190a530f362ba66a9ba8f7dd2dc7d7ff Mon Sep 17 00:00:00 2001
From: bflatastic <bombayapirat@gmail.com>
Date: Sat, 24 Oct 2026 12:58:10 +0700
Subject: [PATCH] #915: MCP get_node pages past a child too large for the limit
 and keeps the truncation marker

---
 crates/mcp/src/tools.rs                       |  7 +++--
 crates/mcp/tests/it/inspect_response_limit.rs | 31 +++++++++++++++++++
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/crates/mcp/src/tools.rs b/crates/mcp/src/tools.rs
index 19f2be35..4feebbb8 100644
--- a/crates/mcp/src/tools.rs
+++ b/crates/mcp/src/tools.rs
@@ -750,7 +750,10 @@ fn get_node(b: &mut dyn Backend, a: &Map<String, Value>) -> Result<ToolResult, S
     let total = value.get("children").and_then(Value::as_array).map_or(0, Vec::len);
     let window = children_window(&mut value, offset, budget.saturating_sub(MARKER_RESERVE));
     let shown = value.get("children").and_then(Value::as_array).map_or(0, Vec::len);
-    let next = (offset + shown < total).then_some(offset + shown);
+    // A child too large to fit on its own is passed over (a page of just the marker), so paging
+    // moves on and every answer stays within maxBytes; `get_node` on the child reads it.
+    let after = offset + shown.max(1);
+    let next = (after < total).then_some(after);
     let used_wire = wire_size(&value);
     let mut out = with_marker(
         value,
@@ -766,7 +769,7 @@ fn get_node(b: &mut dyn Backend, a: &Map<String, Value>) -> Result<ToolResult, S
             "nextOffset": next,
         })),
     );
-    if window && next.is_none() && offset == 0 && used == depth {
+    if window && shown == total && offset == 0 && used == depth && used_wire + MARKER_RESERVE <= budget {
         // Nothing was left out after all (an offset of zero that fitted whole).
         out.as_object_mut().map(|o| o.remove("truncated"));
     }
diff --git a/crates/mcp/tests/it/inspect_response_limit.rs b/crates/mcp/tests/it/inspect_response_limit.rs
index 8e52514b..764e6a50 100644
--- a/crates/mcp/tests/it/inspect_response_limit.rs
+++ b/crates/mcp/tests/it/inspect_response_limit.rs
@@ -152,3 +152,34 @@ fn get_node(c: &mut Client, args: Value) -> (Value, usize) {
     let wire = serde_json::to_string(&text).unwrap().len() + ENVELOPE;
     (serde_json::from_str(text).expect("the block is JSON"), wire)
 }
+
+/// A child too large for the limit on its own is passed over, one per page of just the marker,
+/// so paging moves on and every page stays within the limit.
+#[test]
+fn children_too_large_to_fit_page_one_at_a_time() {
+    let mut c = Client::new();
+    let frame = c.create(json!({"type": "frame", "x": 0, "y": 0, "width": 400, "height": 400}));
+    for i in 0..3 {
+        c.create(json!({"type": "rectangle", "parent": frame, "x": i * 5, "y": 5, "width": 20, "height": 20, "name": "n".repeat(3000)}));
+    }
+    let mut offset = Some(0u64);
+    let mut pages = 0;
+    while let Some(at) = offset {
+        let (page, wire) = get_node(&mut c, json!({"id": frame, "depth": 1, "maxBytes": 2500, "offset": at}));
+        assert!(wire <= 2500, "page at {at} exceeds maxBytes: {wire}");
+        assert_eq!(page["children"].as_array().map_or(0, Vec::len), 0, "{}", page["truncated"]);
+        assert!(!page["truncated"].is_null(), "the marker stays: page at {at}");
+        offset = page["truncated"]["children"]["nextOffset"].as_u64();
+        assert!(offset.is_none_or(|o| o > at), "paging moves on from {at}");
+        pages += 1;
+        assert!(pages <= 3);
+    }
+    assert_eq!(pages, 3);
+
+    // A sole child passed over still says the answer was cut.
+    let single = c.create(json!({"type": "frame", "x": 500, "y": 0, "width": 40, "height": 40}));
+    c.create(json!({"type": "rectangle", "parent": single, "x": 0, "y": 0, "width": 20, "height": 20, "name": "n".repeat(3000)}));
+    let (page, _) = get_node(&mut c, json!({"id": single, "depth": 1, "maxBytes": 2500}));
+    assert_eq!(page["children"].as_array().map_or(0, Vec::len), 0);
+    assert_eq!(page["truncated"]["children"]["total"], json!(1), "{}", page["truncated"]);
+}