diff --git a/src/agents/sessions/exec.test.ts b/src/agents/sessions/exec.test.ts
index 1a991e257bce..af5d3644f06a 100644
--- a/src/agents/sessions/exec.test.ts
+++ b/src/agents/sessions/exec.test.ts
@@ -93,6 +93,44 @@ describe("execCommand", () => {
     expect(result.stdoutTruncatedChars).toBe(3);
   });
 
+  it("preserves UTF-8 characters split across stdout and stderr chunks", async () => {
+    const child = createStubChild();
+    const wait = createDeferred<number | null>();
+    spawnMock.mockReturnValue(child);
+    waitForChildProcessMock.mockReturnValue(wait.promise);
+    const { execCommand } = await import("./exec.js");
+
+    const resultPromise = execCommand("cmd", [], "/tmp");
+    const stdout = Buffer.from("stdout-😀-complete", "utf8");
+    const stderr = Buffer.from("stderr-😀-complete", "utf8");
+    child.stdout.emit("data", stdout.subarray(0, 9));
+    child.stderr.emit("data", stderr.subarray(0, 9));
+    child.stdout.emit("data", stdout.subarray(9));
+    child.stderr.emit("data", stderr.subarray(9));
+    wait.resolve(0);
+
+    const result = await resultPromise;
+    expect(result.stdout).toBe("stdout-😀-complete");
+    expect(result.stderr).toBe("stderr-😀-complete");
+  });
+
+  it("flushes incomplete UTF-8 sequences when the process exits", async () => {
+    const child = createStubChild();
+    const wait = createDeferred<number | null>();
+    spawnMock.mockReturnValue(child);
+    waitForChildProcessMock.mockReturnValue(wait.promise);
+    const { execCommand } = await import("./exec.js");
+
+    const resultPromise = execCommand("cmd", [], "/tmp");
+    child.stdout.emit("data", Buffer.from([0xe2, 0x82]));
+    child.stderr.emit("data", Buffer.from([0xf0, 0x9f, 0x98]));
+    wait.resolve(0);
+
+    const result = await resultPromise;
+    expect(result.stdout).toBe("�");
+    expect(result.stderr).toBe("�");
+  });
+
   it("fails instead of silently truncating default exec output", async () => {
     const child = createStubChild();
     const wait = createDeferred<number | null>();
diff --git a/src/agents/sessions/exec.ts b/src/agents/sessions/exec.ts
index 2adc98ec50e9..e5947b374d1c 100644
--- a/src/agents/sessions/exec.ts
+++ b/src/agents/sessions/exec.ts
@@ -3,6 +3,7 @@
  */
 
 import { spawn } from "node:child_process";
+import { StringDecoder } from "node:string_decoder";
 import { waitForChildProcess } from "../utils/child-process.js";
 
 const DEFAULT_OUTPUT_LIMIT_CHARS = 16 * 1024 * 1024;
@@ -40,6 +41,10 @@ type OutputCapture = {
   truncatedChars: number;
 };
 
+function decodeCapturedOutput(decoder: StringDecoder, chunk: Buffer | string): string {
+  return Buffer.isBuffer(chunk) ? decoder.write(chunk) : `${decoder.end()}${chunk}`;
+}
+
 function clampMaxOutputChars(value: number | undefined): number {
   if (typeof value !== "number" || !Number.isFinite(value) || value <= 0) {
     return DEFAULT_OUTPUT_LIMIT_CHARS;
@@ -87,6 +92,8 @@ export async function execCommand(
 
     let stdout: OutputCapture = { text: "", truncatedChars: 0 };
     let stderr: OutputCapture = { text: "", truncatedChars: 0 };
+    const stdoutDecoder = new StringDecoder("utf8");
+    const stderrDecoder = new StringDecoder("utf8");
     let killed = false;
     let timeoutId: NodeJS.Timeout | undefined;
     let forceKillTimer: NodeJS.Timeout | undefined;
@@ -114,6 +121,16 @@ export async function execCommand(
       if (options?.signal) {
         options.signal.removeEventListener("abort", killProcess);
       }
+      const stdoutBeforeFlush = stdout.truncatedChars;
+      stdout = appendCapturedOutput(stdout, stdoutDecoder.end(), maxOutputChars, truncateOutput);
+      if (!truncateOutput && stdout.truncatedChars > stdoutBeforeFlush && !outputLimitExceeded) {
+        outputLimitExceeded = "stdout";
+      }
+      const stderrBeforeFlush = stderr.truncatedChars;
+      stderr = appendCapturedOutput(stderr, stderrDecoder.end(), maxOutputChars, truncateOutput);
+      if (!truncateOutput && stderr.truncatedChars > stderrBeforeFlush && !outputLimitExceeded) {
+        outputLimitExceeded = "stderr";
+      }
       if (outputLimitExceeded) {
         stderr = appendCapturedOutput(
           stderr,
@@ -164,7 +181,12 @@ export async function execCommand(
 
     proc.stdout?.on("data", (data) => {
       const before = stdout.truncatedChars;
-      stdout = appendCapturedOutput(stdout, data, maxOutputChars, truncateOutput);
+      stdout = appendCapturedOutput(
+        stdout,
+        decodeCapturedOutput(stdoutDecoder, data),
+        maxOutputChars,
+        truncateOutput,
+      );
       if (stdout.truncatedChars > before) {
         markOutputLimitExceeded("stdout");
       }
@@ -172,7 +194,12 @@ export async function execCommand(
 
     proc.stderr?.on("data", (data) => {
       const before = stderr.truncatedChars;
-      stderr = appendCapturedOutput(stderr, data, maxOutputChars, truncateOutput);
+      stderr = appendCapturedOutput(
+        stderr,
+        decodeCapturedOutput(stderrDecoder, data),
+        maxOutputChars,
+        truncateOutput,
+      );
       if (stderr.truncatedChars > before) {
         markOutputLimitExceeded("stderr");
       }