fix: keep tool messages paired during trimming

src/index.mjs

@@ -156,9 +156,22 @@ export function compact(payload, opts = {}) {
       for (let j = 0; j < Math.max(0, trimmableEnd); j++) { if (!isSystem(list[j])) { idx = j; break; } }
       if (idx < 0) break; // nothing left safe to drop
       const removed = list[idx];
-      after -= blocksOf(removed.content, removed.role || "user")
-        .reduce((total, unit) => total + counter(unit.text), 0);
-      list.splice(idx, 1);
+      const pairedIndexes = new Set([idx]);
+      const removedBlocks = Array.isArray(removed.content) ? removed.content : [];
+      const removedToolUses = removedBlocks.filter((b) => b && b.type === "tool_use").map((b) => b.id).filter(Boolean);
+      if (removedToolUses.length) {
+        for (let k = 0; k < list.length; k++) {
+          if (k === idx) continue;
+          const blocks = Array.isArray(list[k].content) ? list[k].content : [];
+          if (blocks.some((b) => b && b.type === "tool_result" && removedToolUses.includes(b.tool_use_id))) pairedIndexes.add(k);
+        }
+      }
+      const removedMessages = [...pairedIndexes].sort((a, b) => b - a).map((index) => list[index]);
+      for (const message of removedMessages) {
+        after -= blocksOf(message.content, message.role || "user")
+          .reduce((total, unit) => total + counter(unit.text), 0);
+      }
+      for (const index of [...pairedIndexes].sort((a, b) => b - a)) list.splice(index, 1);
       actions.push("drop:oldest-message");
       if (++i > 1000) break;
     }

test/basic.test.mjs

@@ -172,3 +172,18 @@ test("units rejects non-array payload.messages with TypeError", () => {
     { name: "TypeError", message: /payload\.messages must be an array/ },
   );
 });
+
+test("compact does not leave tool_result without its tool_use when trimming", () => {
+  const payload = {
+    messages: [
+      { role: "assistant", content: [{ type: "tool_use", id: "call-1", input: {} }] },
+      { role: "user", content: [{ type: "tool_result", tool_use_id: "call-1", content: "result" }] },
+      ...Array.from({ length: 8 }, (_, i) => ({ role: "user", content: "old message " + i + " x".repeat(100) })),
+    ],
+  };
+  const { payload: out } = compact(payload, { maxTokens: 20, keepLastTurns: 2 });
+  const toolUses = new Set(out.messages.flatMap((m) => (Array.isArray(m.content) ? m.content : [])).filter((b) => b?.type === "tool_use").map((b) => b.id));
+  for (const b of out.messages.flatMap((m) => (Array.isArray(m.content) ? m.content : []))) {
+    if (b?.type === "tool_result") assert.ok(toolUses.has(b.tool_use_id));
+  }
+});