fix(core): preserve unicode in compaction truncation

packages/core/src/session/compaction.ts

@@ -116,8 +116,19 @@ export interface Interface extends State.Transformable<Draft> {
 
 export class Service extends Context.Service<Service, Interface>()("@opencode/SessionCompaction") {}
 
-const truncate = (value: string) =>
-  value.length <= TOOL_OUTPUT_MAX_CHARS ? value : `${value.slice(0, TOOL_OUTPUT_MAX_CHARS)}\n[truncated]`
+export const truncateToolOutput = (value: string) => {
+  if (value.length <= TOOL_OUTPUT_MAX_CHARS) return value
+  let end = 0
+  for (let count = 0; count < TOOL_OUTPUT_MAX_CHARS && end < value.length; count++) {
+    const code = value.charCodeAt(end)
+    end +=
+      code >= 0xd800 && code <= 0xdbff && value.charCodeAt(end + 1) >= 0xdc00 && value.charCodeAt(end + 1) <= 0xdfff
+        ? 2
+        : 1
+  }
+  if (end === value.length) return value
+  return `${value.slice(0, end)}\n[truncated]`
+}
 
 export const serializeToolContent = (content: SessionMessage.ToolStateCompleted["content"]) =>
   content
@@ -146,7 +157,7 @@ const serialize = (message: SessionMessage.Info) => {
         if (part.state.status === "completed")
           return [
             `[Assistant tool call]: ${part.name}(${input})`,
-            `[Tool result]: ${truncate(serializeToolContent(part.state.content))}`,
+            `[Tool result]: ${truncateToolOutput(serializeToolContent(part.state.content))}`,
           ]
         if (part.state.status === "error")
           return [`[Assistant tool call]: ${part.name}(${input})`, `[Tool error]: ${part.state.error.message}`]
@@ -157,7 +168,8 @@ const serialize = (message: SessionMessage.Info) => {
   if (message.type === "system") return `[System update]: ${message.text}`
   if (message.type === "synthetic") return `[Synthetic context]: ${message.text}`
   if (message.type === "skill") return `[Skill activated: ${message.name}]\n${message.text}`
-  if (message.type === "shell") return `[Shell]: ${message.command}\n${truncate(message.output?.output ?? "")}`
+  if (message.type === "shell")
+    return `[Shell]: ${message.command}\n${truncateToolOutput(message.output?.output ?? "")}`
   return ""
 }
 

packages/core/test/session-compaction.test.ts

@@ -110,6 +110,13 @@ test("compaction describes tool media without embedding base64", () => {
   expect(serialized).not.toContain(base64)
 })
 
+test("compaction truncation does not split surrogate pairs", () => {
+  const prefix = "a".repeat(1_999)
+
+  expect(SessionCompaction.truncateToolOutput(`${prefix}😀suffix`)).toBe(`${prefix}😀\n[truncated]`)
+  expect(SessionCompaction.truncateToolOutput("😀".repeat(2_000))).toBe("😀".repeat(2_000))
+})
+
 test("compaction prompt requires the checkpoint headings in order", () => {
   const prompt = SessionCompaction.buildPrompt({ context: ["Conversation history"] })
   expect(prompt.match(/^#{2,3} .+$/gm)).toEqual([