fix: use StringDecoder for UTF-8 multi-byte stream decoding

Replace chunk.toString() with StringDecoder in stream processing to
prevent garbled characters (arrows, tree-drawing chars appearing as
mojibake) when multi-byte UTF-8 sequences are split across chunk
boundaries. StringDecoder maintains state across chunks to correctly
reassemble incomplete characters.

Fixed in bash tool, prompt shell execution, and ripgrep output.

packages/opencode/src/file/ripgrep.ts

@@ -1,5 +1,6 @@
 // Ripgrep utility functions
 import path from "path"
+import { StringDecoder } from "string_decoder" // kilocode_change - fix UTF-8 multi-byte split
 import { Global } from "../global"
 import fs from "fs/promises"
 import z from "zod"
@@ -252,11 +253,14 @@ export namespace Ripgrep {
     }
 
     let buffer = ""
+    // kilocode_change start - use StringDecoder to handle multi-byte UTF-8 characters split across chunks
+    const decoder = new StringDecoder("utf8")
     const stream = proc.stdout as AsyncIterable<Buffer | string>
     for await (const chunk of stream) {
       input.signal?.throwIfAborted()
 
-      buffer += typeof chunk === "string" ? chunk : chunk.toString()
+      buffer += typeof chunk === "string" ? chunk : decoder.write(chunk as Buffer)
+      // kilocode_change end
       // Handle both Unix (\n) and Windows (\r\n) line endings
       const lines = buffer.split(/\r?\n/)
       buffer = lines.pop() || ""

packages/opencode/src/session/prompt.ts

@@ -1,6 +1,7 @@
 import path from "path"
 import os from "os"
 import fs from "fs/promises"
+import { StringDecoder } from "string_decoder" // kilocode_change - fix UTF-8 multi-byte split
 import z from "zod"
 import { Filesystem } from "../util/filesystem"
 import { Identifier } from "../id/id"
@@ -1698,9 +1699,11 @@ NOTE: At any point in time through this workflow you should feel free to ask the
     })
 
     let output = ""
+    // kilocode_change start - use StringDecoder to handle multi-byte UTF-8 characters split across chunks
+    const decoder = new StringDecoder("utf8")
 
     proc.stdout?.on("data", (chunk) => {
-      output += chunk.toString()
+      output += decoder.write(chunk)
       if (part.state.status === "running") {
         part.state.metadata = {
           output: output,
@@ -1711,7 +1714,7 @@ NOTE: At any point in time through this workflow you should feel free to ask the
     })
 
     proc.stderr?.on("data", (chunk) => {
-      output += chunk.toString()
+      output += decoder.write(chunk)
       if (part.state.status === "running") {
         part.state.metadata = {
           output: output,
@@ -1720,6 +1723,7 @@ NOTE: At any point in time through this workflow you should feel free to ask the
         Session.updatePart(part)
       }
     })
+    // kilocode_change end
 
     let aborted = false
     let exited = false

packages/opencode/src/tool/bash.ts

@@ -1,5 +1,6 @@
 import z from "zod"
 import { spawn } from "child_process"
+import { StringDecoder } from "string_decoder" // kilocode_change - fix UTF-8 multi-byte split
 import { Tool } from "./tool"
 import path from "path"
 import DESCRIPTION from "./bash.txt"
@@ -191,8 +192,10 @@ export const BashTool = Tool.define("bash", async () => {
         },
       })
 
+      // kilocode_change start - use StringDecoder to handle multi-byte UTF-8 characters split across chunks
+      const decoder = new StringDecoder("utf8")
       const append = (chunk: Buffer) => {
-        output += chunk.toString()
+        output += decoder.write(chunk)
         ctx.metadata({
           metadata: {
             // truncate the metadata to avoid GIANT blobs of data (has nothing to do w/ what agent can access)
@@ -201,6 +204,7 @@ export const BashTool = Tool.define("bash", async () => {
           },
         })
       }
+      // kilocode_change end
 
       proc.stdout?.on("data", append)
       proc.stderr?.on("data", append)