diff --git a/agentscope-harness/src/test/java/io/agentscope/harness/agent/filesystem/local/LocalFilesystemWithShellTest.java b/agentscope-harness/src/test/java/io/agentscope/harness/agent/filesystem/local/LocalFilesystemWithShellTest.java
index 3f9ff553b9..d58e205e7d 100644
--- a/agentscope-harness/src/test/java/io/agentscope/harness/agent/filesystem/local/LocalFilesystemWithShellTest.java
+++ b/agentscope-harness/src/test/java/io/agentscope/harness/agent/filesystem/local/LocalFilesystemWithShellTest.java
@@ -17,16 +17,24 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import io.agentscope.harness.agent.filesystem.model.ExecuteResponse;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Path;
+import java.util.Locale;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.io.TempDir;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 
 class LocalFilesystemWithShellTest {
 
+    private static final int OUTPUT_LINES = 8192;
+    private static final String STDOUT_LINE = "out-" + "0123456789abcdef".repeat(8);
+    private static final String STDERR_LINE = "err-" + "fedcba9876543210".repeat(8);
+
     @Test
     void outputCharset_usesNativeEncodingOnWindows() {
         assertEquals(
@@ -46,28 +54,59 @@ void outputCharset_fallsBackToDefaultWhenWindowsNativeEncodingIsUnavailable() {
                 LocalFilesystemWithShell.outputCharset("Windows 10", null));
     }
 
+    @ParameterizedTest
+    @ValueSource(strings = {"stdout", "stderr", "both"})
+    void execute_outputLargerThanOsPipeBufferCompletesWithoutDeadlock(
+            String streams, @TempDir Path tempDir) {
+        // More than 1 MiB per stream, with room to capture both streams without truncation.
+        // Alternating writes to both pipes also catches sequential stdout/stderr readers.
+        LocalFilesystemWithShell fs =
+                new LocalFilesystemWithShell(tempDir, false, 10, 4 * 1024 * 1024, null, false);
+        ExecuteResponse resp = fs.execute(null, largeOutputCommand(streams), 10);
+
+        assertEquals(0, resp.exitCode());
+        assertFalse(resp.truncated());
+        String stdout = (STDOUT_LINE + "\n").repeat(OUTPUT_LINES);
+        String stderr = ("[stderr] " + STDERR_LINE + "\n").repeat(OUTPUT_LINES).stripTrailing();
+        String expected =
+                switch (streams) {
+                    case "stdout" -> stdout;
+                    case "stderr" -> stderr;
+                    default -> stdout + "\n" + stderr;
+                };
+        assertEquals(expected, resp.output().replace("\r\n", "\n"));
+    }
+
     @Test
-    void execute_outputLargerThanOsPipeBufferCompletesWithoutDeadlock(@TempDir Path tempDir) {
-        // ~68-72 KB of stdout: beyond the OS pipe buffer (~4 KB on Windows, 64 KB on Linux),
-        // below the default maxOutputBytes cap. Before stdout/stderr were drained concurrently
-        // with waitFor, this deadlocked and was misreported as a timeout (exit 124).
-        int lines = 4000;
-        String payload = "0123456789abcdef"; // 16 chars per line
-        boolean windows = System.getProperty("os.name").toLowerCase().contains("win");
-        String command =
-                windows
-                        ? "for /l %i in (1,1," + lines + ") do @echo " + payload
-                        : "i=0; while [ \"$i\" -lt "
-                                + lines
-                                + " ]; do echo "
-                                + payload
-                                + "; i=$((i+1)); done";
+    void execute_outputBeyondCaptureLimitStillDrainsBothPipes(@TempDir Path tempDir) {
+        LocalFilesystemWithShell fs =
+                new LocalFilesystemWithShell(tempDir, false, 10, 128, null, false);
+        ExecuteResponse resp = fs.execute(null, largeOutputCommand("both"), 10);
 
-        LocalFilesystemWithShell fs = new LocalFilesystemWithShell(tempDir);
-        ExecuteResponse resp = fs.execute(null, command, 60);
+        assertEquals(0, resp.exitCode());
+        assertTrue(resp.truncated());
+        assertEquals(
+                STDOUT_LINE.substring(0, 128) + "\n\n... Output truncated at 128 bytes.",
+                resp.output());
+    }
 
-        assertEquals(0, resp.exitCode(), "unexpected exit code, output: " + resp.output());
-        assertFalse(resp.truncated());
-        assertEquals(lines, resp.output().split(payload, -1).length - 1);
+    private static String largeOutputCommand(String streams) {
+        boolean windows = System.getProperty("os.name").toLowerCase(Locale.ROOT).contains("win");
+        String stdout = "echo " + STDOUT_LINE;
+        String stderr = windows ? "1>&2 echo " + STDERR_LINE : "echo " + STDERR_LINE + " >&2";
+        String body =
+                switch (streams) {
+                    case "stdout" -> stdout;
+                    case "stderr" -> stderr;
+                    default -> stdout + (windows ? "&" : "; ") + stderr;
+                };
+        // Shell builtins keep the reproducer independent of Python and external child processes.
+        return windows
+                ? "for /l %i in (1,1," + OUTPUT_LINES + ") do @(" + body + ")"
+                : "i=0; while [ \"$i\" -lt "
+                        + OUTPUT_LINES
+                        + " ]; do "
+                        + body
+                        + "; i=$((i+1)); done";
     }
 }