diff --git a/docs/about/changelog.md b/docs/about/changelog.md
index 1927c9499..c654bb2fe 100644
--- a/docs/about/changelog.md
+++ b/docs/about/changelog.md
@@ -10,6 +10,9 @@ priorities and future plans.
   both execution backends, and align its notifier-loop refcount expectation
   with native Perl.
 
+- Preserve buffered IPC::Open3 stdout and stderr until consumed before
+  reporting EOF, preventing IPC::Open3::Utils handler loss and pipe hangs.
+
 - Fix parsing of dense Mo::Inline expressions that use `::` as a bareword.
 
 - Preserve UTF-8 HTML octets through HTML::Parser and no-op entity decoding,
diff --git a/src/main/java/org/perlonjava/runtime/io/ProcessInputHandle.java b/src/main/java/org/perlonjava/runtime/io/ProcessInputHandle.java
index e41bded2e..c477fece1 100644
--- a/src/main/java/org/perlonjava/runtime/io/ProcessInputHandle.java
+++ b/src/main/java/org/perlonjava/runtime/io/ProcessInputHandle.java
@@ -99,25 +99,15 @@ public RuntimeScalar fileno() {
 
     @Override
     public RuntimeScalar eof() {
-        if (isClosed) return RuntimeScalarCache.scalarTrue;
-        try {
-            // Check if stream has data available or is at EOF
-            if (isEOF) return RuntimeScalarCache.scalarTrue;
-            int available = inputStream.available();
-            if (available > 0) return RuntimeScalarCache.scalarFalse;
-            
-            // Try to peek - if we get -1, it's EOF
-            inputStream.mark(1);
-            int ch = inputStream.read();
-            if (ch == -1) {
-                isEOF = true;
-                return RuntimeScalarCache.scalarTrue;
-            }
-            inputStream.reset();
-            return RuntimeScalarCache.scalarFalse;
-        } catch (IOException e) {
-            isEOF = true;
-            return RuntimeScalarCache.scalarTrue;
+        synchronized (readLock) {
+            // drainInput() is the sole reader of inputStream.  Consulting the
+            // stream here races that thread and a one-byte "peek" can block
+            // while a child waits for stdin.  More importantly, stream EOF is
+            // not Perl EOF until the bytes already drained into buffered have
+            // been returned to the caller.
+            return buffered.isEmpty() && isEOF
+                    ? RuntimeScalarCache.scalarTrue
+                    : RuntimeScalarCache.scalarFalse;
         }
     }
 
@@ -167,4 +157,3 @@ public RuntimeScalar sysread(int length) {
         }
     }
 }
-
diff --git a/src/test/resources/unit/ipc_open3_buffered_eof.t b/src/test/resources/unit/ipc_open3_buffered_eof.t
new file mode 100644
index 000000000..a0e0e90b0
--- /dev/null
+++ b/src/test/resources/unit/ipc_open3_buffered_eof.t
@@ -0,0 +1,70 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+use Test::More;
+use IPC::Open3;
+use IO::Select;
+use Symbol qw(gensym);
+
+# Regression for #1263: ProcessInputHandle drains the Java stream in a reader
+# thread.  eof must inspect that synchronized buffer, not peek the Java stream,
+# so callers such as IPC::Open3::Utils see queued stdout/stderr before EOF.
+my ($stdin, $stdout, $stderr) = (undef, undef, gensym);
+my @command = $^O eq 'MSWin32'
+    ? ('cmd.exe', '/v:on', '/c', 'set /p line=& echo out:!line!& echo err:!line! 1>&2')
+    : ('sh', '-c', 'read line; printf "out:%s\\n" "$line"; printf "err:%s\\n" "$line" >&2');
+my $pid = open3(
+    $stdin, $stdout, $stderr, @command
+);
+
+print {$stdin} "payload\n";
+ok(close($stdin), 'closing stdin lets the child complete');
+
+# Give the asynchronous readers time to transfer the child output into their
+# private buffers.  The process has exited, but buffered data is still readable.
+select undef, undef, undef, 0.05;
+ok(!eof($stdout), 'stdout with buffered child data is not EOF');
+ok(!eof($stderr), 'stderr with buffered child data is not EOF');
+
+my $selector = IO::Select->new($stdout, $stderr);
+my (%captured, $handler_calls, $short_circuit) = ((), 0, 0);
+
+READ_LOOP:
+while (my @ready = $selector->can_read(2)) {
+    for my $fh (@ready) {
+        if (eof($fh)) {
+            $selector->remove($fh);
+            close($fh);
+            next;
+        }
+
+        while (my $line = <$fh>) {
+            $handler_calls++;
+            $captured{fileno($fh)} .= $line;
+            $short_circuit = 1 if $line =~ /\Aout:payload\r?\n\z/;
+            last READ_LOOP if $short_circuit;
+        }
+    }
+}
+
+ok($handler_calls, 'buffered data invokes the read handler');
+ok($short_circuit, 'handler can short-circuit after stdout is read');
+like($captured{fileno($stdout)} // '', qr/\Aout:payload\r?\n\z/,
+    'stdout buffered data is preserved');
+
+# Finish draining stderr after the short-circuit path, mirroring the lifecycle
+# cleanup in IPC::Open3::Utils.
+my $stderr_text = '';
+while (!eof($stderr)) {
+    my $line = <$stderr>;
+    $stderr_text .= $line if defined $line;
+}
+like($stderr_text, qr/\Aerr:/,
+    'stderr buffered data is preserved');
+
+close($stdout);
+close($stderr);
+is(waitpid($pid, 0), $pid, 'child is reaped after both pipes close');
+is($? >> 8, 0, 'child exits successfully');
+
+done_testing;