fix: make ACP projection truncation UTF-16 safe

steipete · steipete · commit 21f9a8103a35 · 2026-07-09T05:08:54.000+01:00
diff --git a/src/auto-reply/reply/acp-projector-truncation.test.ts b/src/auto-reply/reply/acp-projector-truncation.test.ts
diff --git a/src/auto-reply/reply/acp-projector.test.ts b/src/auto-reply/reply/acp-projector.test.ts
@@ -761,7 +761,33 @@ describe("createAcpReplyProjector", () => {
     ]);
   });
 
-  it("truncates oversized turns once and emits one truncation notice", async () => {
+  it("truncates status updates without splitting surrogate pairs", async () => {
+    const { deliveries, projector } = createProjectorHarness(
+      createLiveCfgOverrides({
+        coalesceIdleMs: 0,
+        maxChunkChars: 256,
+        maxSessionUpdateChars: 64,
+        tagVisibility: {
+          memory_summary: true,
+        },
+      }),
+    );
+
+    await projector.onEvent({
+      type: "status",
+      tag: "memory_summary",
+      text: `${"a".repeat(62)}🎉${"b".repeat(10)}`,
+    });
+
+    expect(deliveries).toEqual([
+      {
+        kind: "tool",
+        text: prefixSystemMessage(`${"a".repeat(62)}…`),
+      },
+    ]);
+  });
+
+  it("truncates oversized turns at code-point boundaries and emits one notice", async () => {
     const { deliveries, projector } = createProjectorHarness({
       acp: {
         enabled: true,
@@ -776,7 +802,7 @@ describe("createAcpReplyProjector", () => {
 
     await projector.onEvent({
       type: "text_delta",
-      text: "hello world",
+      text: "abcd😀 tail",
       tag: "agent_message_chunk",
     });
     await projector.onEvent({
@@ -787,7 +813,7 @@ describe("createAcpReplyProjector", () => {
     await projector.flush(true);
 
     expect(deliveries).toEqual([
-      { kind: "block", text: "hello" },
+      { kind: "block", text: "abcd" },
       {
         kind: "tool",
         text: prefixSystemMessage("output truncated"),
diff --git a/src/auto-reply/reply/acp-projector.ts b/src/auto-reply/reply/acp-projector.ts
@@ -463,7 +463,7 @@ export function createAcpReplyProjector(params: {
         return;
       }
       const remaining = settings.maxOutputChars - emittedOutputChars;
-      const accepted = remaining < text.length ? text.slice(0, remaining) : text;
+      const accepted = remaining < text.length ? truncateUtf16Safe(text, remaining) : text;
       if (accepted.length > 0) {
         emittedOutputChars += accepted.length;
         lastVisibleOutputTail = accepted.slice(-1);
@@ -480,6 +480,9 @@ export function createAcpReplyProjector(params: {
         }
       }
       if (accepted.length < text.length) {
+        // A split code point can leave the accepted prefix shorter than the remaining budget.
+        // Exhaust it after any drop so later deltas cannot skip past omitted text.
+        emittedOutputChars = settings.maxOutputChars;
         await emitTruncationNotice();
       }
       return;

Original file line number	Diff line number	Diff line change
`@@ -463,7 +463,7 @@ export function createAcpReplyProjector(params: {`
`463`	`463`	`return;`
`464`	`464`	`}`
`465`	`465`	`const remaining = settings.maxOutputChars - emittedOutputChars;`
`466`		`- const accepted = remaining < text.length ? text.slice(0, remaining) : text;`
	`466`	`+ const accepted = remaining < text.length ? truncateUtf16Safe(text, remaining) : text;`
`467`	`467`	`if (accepted.length > 0) {`
`468`	`468`	`emittedOutputChars += accepted.length;`
`469`	`469`	`lastVisibleOutputTail = accepted.slice(-1);`
`@@ -480,6 +480,9 @@ export function createAcpReplyProjector(params: {`
`480`	`480`	`}`
`481`	`481`	`}`
`482`	`482`	`if (accepted.length < text.length) {`
	`483`	`+ // A split code point can leave the accepted prefix shorter than the remaining budget.`
	`484`	`+ // Exhaust it after any drop so later deltas cannot skip past omitted text.`
	`485`	`+ emittedOutputChars = settings.maxOutputChars;`
`483`	`486`	`await emitTruncationNotice();`
`484`	`487`	`}`
`485`	`488`	`return;`