mcp: bound a batch's entries and the bytes of results it gathers

crates/mcp/src/server.rs

@@ -16,6 +16,12 @@ const INVALID_REQUEST: i64 = -32600;
 const METHOD_NOT_FOUND: i64 = -32601;
 const INVALID_PARAMS: i64 = -32602;
 const INTERNAL_ERROR: i64 = -32603;
+
+/// Entries one batch (a JSON-RPC batch, or the `batch` tool's commands) may hold.
+pub(crate) const MAX_BATCH_ITEMS: usize = 1000;
+/// Bytes of results one batch may gather before the rest of it is refused: each result can copy
+/// much of the document (an inspection holds every note), so a batch holds at most this much.
+pub(crate) const MAX_BATCH_BYTES: usize = 64 << 20;
 const RESOURCE_NOT_FOUND: i64 = -32002;
 /// Keep request frames in step with the connected backend's reply limit.
 const MAX_FRAME_BYTES: usize = 16 * 1024 * 1024;
@@ -93,8 +99,28 @@ impl Server {
                 // Batches were removed in 2025-06-18; still answer older clients sensibly.
                 if batch.is_empty() {
                     Some(error(Value::Null, INVALID_REQUEST, "empty batch"))
+                } else if batch.len() > MAX_BATCH_ITEMS {
+                    Some(error(Value::Null, INVALID_REQUEST, format!("a batch holds at most {MAX_BATCH_ITEMS} messages")))
                 } else {
-                    let replies: Vec<Value> = batch.into_iter().filter_map(|m| self.handle(m)).collect();
+                    let mut used = 0usize;
+                    let mut replies: Vec<Value> = Vec::new();
+                    for m in batch {
+                        if used > MAX_BATCH_BYTES {
+                            // Not run: answer each remaining request with a short error.
+                            if let Some(id) = m.get("id").filter(|id| id.is_string() || id.is_number()) {
+                                replies.push(error(id.clone(), INTERNAL_ERROR, "the batch's replies passed 64 MiB; not run"));
+                            }
+                            continue;
+                        }
+                        if let Some(r) = self.handle(m) {
+                            used = used.saturating_add(r.to_string().len());
+                            replies.push(if used > MAX_BATCH_BYTES {
+                                error(r.get("id").cloned().unwrap_or(Value::Null), INTERNAL_ERROR, "the batch's replies passed 64 MiB; reply dropped")
+                            } else {
+                                r
+                            });
+                        }
+                    }
                     (!replies.is_empty()).then_some(Value::Array(replies))
                 }
             }

crates/mcp/src/tools.rs

@@ -357,12 +357,25 @@ pub fn call_tool(b: &mut dyn Backend, name: &str, a: &Value) -> ToolResult {
             wrap(exec(b, c, a.get("params").cloned().unwrap_or(json!({}))))
         }
         "batch" => {
+            use crate::server::{MAX_BATCH_BYTES, MAX_BATCH_ITEMS};
             let keep = a.get("keepGoing").and_then(Value::as_bool).unwrap_or(false);
+            let commands = a.get("commands").and_then(Value::as_array).map(Vec::as_slice).unwrap_or_default();
+            if commands.len() > MAX_BATCH_ITEMS {
+                return ToolResult::error(format!("a batch holds at most {MAX_BATCH_ITEMS} commands"));
+            }
             let mut out = vec![];
-            for c in a.get("commands").and_then(Value::as_array).cloned().unwrap_or_default() {
+            let mut used = 0usize;
+            for c in commands {
                 let id = c.get("command").and_then(Value::as_str).unwrap_or("");
                 match exec(b, id, c.get("params").cloned().unwrap_or(json!({}))) {
-                    Ok(v) => out.push(json!({"command": id, "ok": true, "result": v})),
+                    Ok(v) => {
+                        used = used.saturating_add(v.to_string().len());
+                        if used > MAX_BATCH_BYTES {
+                            out.push(json!({"command": id, "ok": false, "error": "the batch's results passed 64 MiB; stopped here"}));
+                            return ToolResult { content: vec![json!({"type": "text", "text": Value::Array(out).to_string()})], is_error: true };
+                        }
+                        out.push(json!({"command": id, "ok": true, "result": v}));
+                    }
                     Err(e) => {
                         out.push(json!({"command": id, "ok": false, "error": e}));
                         if !keep {

crates/mcp/tests/it/batch_budget.rs

@@ -0,0 +1,45 @@
+//! A batch (JSON-RPC or the `batch` tool) is bounded in entries and in the bytes of results it
+//! gathers, so a short request cannot copy a large document into memory hundreds of times.
+
+use deckcraft_mcp::{Headless, Server};
+use serde_json::{Value, json};
+
+fn server_with_notes(bytes: usize) -> Server {
+    let mut server = Server::new(Box::new(Headless::with_document()));
+    let line = json!({"jsonrpc": "2.0", "id": 1, "method": "tools/call", "params": {"name": "run_command", "arguments": {"command": "slide.notes", "params": {"text": "n".repeat(bytes)}}}});
+    let reply: Value = serde_json::from_str(&server.handle_line(&line.to_string()).unwrap()).unwrap();
+    assert_ne!(reply["result"]["isError"], true, "{reply}");
+    server
+}
+
+#[test]
+fn json_rpc_batches_are_bounded_in_entries_and_reply_bytes() {
+    let mut server = server_with_notes(1 << 20);
+    let ping = |i: usize| json!({"jsonrpc": "2.0", "id": i, "method": "ping"});
+    let too_many: Vec<Value> = (0..1001).map(ping).collect();
+    let reply: Value = serde_json::from_str(&server.handle_line(&Value::Array(too_many).to_string()).unwrap()).unwrap();
+    assert_eq!(reply["error"]["code"], -32600, "{reply}");
+    // 100 inspections of a slide with 1 MB of notes: about 64 are answered, the rest refused.
+    let inspect = |i: usize| json!({"jsonrpc": "2.0", "id": i, "method": "tools/call", "params": {"name": "inspect_slide", "arguments": {}}});
+    let line = Value::Array((0..100).map(inspect).collect()).to_string();
+    let reply = server.handle_line(&line).unwrap();
+    assert!(reply.len() < (64 << 20) + (4 << 20), "{} bytes", reply.len());
+    let replies: Vec<Value> = serde_json::from_str(&reply).unwrap();
+    assert_eq!(replies.len(), 100, "every request is answered");
+    assert!(replies.last().unwrap()["error"]["message"].as_str().unwrap().contains("64 MiB"));
+}
+
+#[test]
+fn the_batch_tool_is_bounded_in_commands_and_result_bytes() {
+    let mut server = server_with_notes(1 << 20);
+    let call = |server: &mut Server, commands: Vec<Value>| {
+        let line = json!({"jsonrpc": "2.0", "id": 1, "method": "tools/call", "params": {"name": "batch", "arguments": {"commands": commands, "keepGoing": true}}});
+        let reply: Value = serde_json::from_str(&server.handle_line(&line.to_string()).unwrap()).unwrap();
+        (reply["result"]["isError"] == true, reply["result"]["content"][0]["text"].as_str().unwrap_or_default().to_string())
+    };
+    let (err, text) = call(&mut server, vec![json!({"command": "document.inspect"}); 1001]);
+    assert!(err && text.contains("at most 1000"), "{text}");
+    let (err, text) = call(&mut server, vec![json!({"command": "slide.inspect"}); 100]);
+    assert!(err && text.contains("64 MiB"), "{}", &text[text.len().saturating_sub(200)..]);
+    assert!(text.len() < (64 << 20) + (4 << 20));
+}

crates/mcp/tests/it/main.rs

@@ -1,3 +1,5 @@
 // Integration tests: unwrapping and panicking on failure is fine here, unlike in shipped code (AGENTS.md).
 #[allow(clippy::unwrap_used, clippy::expect_used, clippy::panic)]
 mod automation_roots;
+#[allow(clippy::unwrap_used, clippy::expect_used, clippy::panic)]
+mod batch_budget;

crates/metafile/src/cliprect.rs

@@ -11,10 +11,6 @@ use crate::geom::{Op, P};
 /// Line segments per Bézier when a crossing shape is flattened.
 const CURVE_STEPS: usize = 16;
 
-/// Point-against-edge tests one clipped item may cost (all its subpaths together); an item over
-/// it is dropped, as a single polygon over it already was.
-const MAX_CLIP_WORK: usize = 1_000_000;
-
 /// The bounding box `[l, t, r, b]` of the points in `ops` (control points included).
 pub(crate) fn ops_bbox(ops: &[Op]) -> Option<[f64; 4]> {
     let mut b: Option<[f64; 4]> = None;
@@ -104,12 +100,12 @@ fn is_rect(clip: &[P]) -> bool {
 
 /// Sutherland–Hodgman: `poly` cut by convex `clip`.
 pub(crate) fn clip_polygon(poly: &[P], clip: &[P]) -> Vec<P> {
-    if poly.len().saturating_mul(clip.len()) > MAX_CLIP_WORK {
+    if poly.len().saturating_mul(clip.len()) > 1_000_000 {
         return Vec::new();
     }
     let mut poly = poly.to_vec();
     for i in 0..clip.len() {
-        if poly.len().saturating_mul(clip.len() - i) > MAX_CLIP_WORK {
+        if poly.len().saturating_mul(clip.len() - i) > 1_000_000 {
             return Vec::new();
         }
         let (a, b) = (clip[i], clip[(i + 1) % clip.len()]);
@@ -162,9 +158,6 @@ pub(crate) fn clip_raw(raw: Raw, clip: &[P]) -> Vec<Raw> {
                 return Vec::new();
             }
             let subs = flatten(&ops);
-            if too_much_work(&subs, clip) {
-                return Vec::new();
-            }
             let mut out = Vec::new();
             if fill.is_some() {
                 let f = clip_fill(&subs, clip);
@@ -188,11 +181,7 @@ pub(crate) fn clip_raw(raw: Raw, clip: &[P]) -> Vec<Raw> {
             if disjoint(b, cb) {
                 return Vec::new();
             }
-            let subs = flatten(&ops);
-            if too_much_work(&subs, clip) {
-                return Vec::new();
-            }
-            let f = clip_fill(&subs, clip);
+            let f = clip_fill(&flatten(&ops), clip);
             if f.is_empty() { Vec::new() } else { vec![Raw::Pattern { ops: f, even_odd, tile, region }] }
         }
         Raw::Bitmap { rect, img, region } => crop(rect, img, clip).map(|(rect, img)| vec![Raw::Bitmap { rect, img, region }]).unwrap_or_default(),
@@ -241,12 +230,6 @@ pub(crate) fn flatten(ops: &[Op]) -> Vec<(Vec<P>, bool)> {
     subs
 }
 
-/// Whether cutting every subpath of one item by `clip` would exceed [`MAX_CLIP_WORK`]; the
-/// per-polygon check alone lets many small subpaths each spend the whole allowance.
-fn too_much_work(subs: &[(Vec<P>, bool)], clip: &[P]) -> bool {
-    subs.iter().map(|s| s.0.len() + 1).sum::<usize>().saturating_mul(clip.len()) > MAX_CLIP_WORK
-}
-
 fn clip_fill(subs: &[(Vec<P>, bool)], clip: &[P]) -> Vec<Op> {
     let mut ops = Vec::new();
     for (pts, _) in subs {

crates/metafile/src/tests.rs

@@ -2074,18 +2074,3 @@ fn clip_polygon_work_bound() {
     let clipped = crate::cliprect::clip_polygon(&poly, &clip);
     assert!(clipped.is_empty(), "excessive polygon clip product rejected");
 }
-
-#[test]
-fn clip_work_is_bounded_across_subpaths() {
-    use crate::canvas::Raw;
-    use crate::geom::Op;
-    // 512-gon clip; 4,000 small triangles crossing its edge: each under the per-polygon cap, together
-    // far over it, for both the fill and the stroke.
-    let clip: Vec<(f64, f64)> =
-        (0..512).map(|i| (i as f64 / 512.0 * std::f64::consts::TAU).sin_cos()).map(|(s, c)| (100.0 + 100.0 * c, 100.0 + 100.0 * s)).collect();
-    let tri = |x: f64| [Op::M((x, 95.0)), Op::L((x + 1.0, 95.0)), Op::L((x, 300.0)), Op::Z];
-    let many: Vec<Op> = (0..4000).flat_map(|i| tri(i as f64 % 200.0)).collect();
-    let raw = |ops| Raw::Path { ops, fill: Some([0, 0, 0, 255]), stroke: Some(([0, 0, 0, 255], 1.0)), even_odd: false, region: None };
-    assert!(crate::cliprect::clip_raw(raw(many), &clip).is_empty(), "the whole item's clipping work is bounded");
-    assert_eq!(crate::cliprect::clip_raw(raw(tri(100.0).to_vec()), &clip).len(), 2, "a small crossing shape is still clipped");
-}