diff --git a/cores/ardulinux/linux/LinuxSerial.cpp b/cores/ardulinux/linux/LinuxSerial.cpp
index beed04b..b7ebd74 100644
--- a/cores/ardulinux/linux/LinuxSerial.cpp
+++ b/cores/ardulinux/linux/LinuxSerial.cpp
@@ -302,8 +302,18 @@ namespace arduino {
         return -1;
     }
 
-    /** No-op: stdout is unbuffered at this level. */
-    void SimSerial::flush(void) {}
+    /**
+     * Push any buffered stdout to the underlying fd.
+     *
+     * stdout is line-buffered (see the constructor), so a completed log line
+     * leaves on its own newline.  A partial line -- Serial.print() with no
+     * trailing newline, e.g. a prompt or a progress counter -- sits in the
+     * buffer until the next newline or exit.  Arduino's flush() contract is
+     * "wait for outgoing data to be transmitted", so honour it here.
+     */
+    void SimSerial::flush(void) {
+        fflush(stdout);
+    }
 
     /**
      * Write one byte to stdout.
diff --git a/cores/ardulinux/linux/LinuxSerial.h b/cores/ardulinux/linux/LinuxSerial.h
index 1f435a8..ccd9abb 100644
--- a/cores/ardulinux/linux/LinuxSerial.h
+++ b/cores/ardulinux/linux/LinuxSerial.h
@@ -132,7 +132,7 @@ namespace arduino {
         virtual int peek(void);
         /** Always returns -1. */
         virtual int read(void);
-        /** No-op. */
+        /** Push buffered stdout out now (flushes partial, newline-less lines). */
         virtual void flush(void);
         /**
          * Write one byte to stdout.
diff --git a/platform.json b/platform.json
index 8c7fea6..6fc02e4 100644
--- a/platform.json
+++ b/platform.json
@@ -1,6 +1,6 @@
 {
   "name": "ardulinux",
-  "version": "0.2.2",
+  "version": "0.2.3",
   "title": "ArduLinux",
   "description": "ArduLinux - Arduino API for Linux",
   "keywords": ["arduino", "linux", "native", "posix", "gpio", "spi", "i2c", "portduino"],
diff --git a/tests/unit/test_sim_serial.cpp b/tests/unit/test_sim_serial.cpp
index ae59868..e1bc6cb 100644
--- a/tests/unit/test_sim_serial.cpp
+++ b/tests/unit/test_sim_serial.cpp
@@ -11,6 +11,10 @@
 
 #include <catch2/catch_test_macros.hpp>
 #include "linux/LinuxSerial.h"
+#include <string>
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
 
 // ─── SimSerial API contract ───────────────────────────────────────────────────
 
@@ -56,11 +60,6 @@ TEST_CASE("SimSerial::end is a no-op and does not crash", "[serial][sim]") {
     CHECK_NOTHROW(sim.end());
 }
 
-TEST_CASE("SimSerial::flush is a no-op and does not crash", "[serial][sim]") {
-    arduino::SimSerial sim;
-    CHECK_NOTHROW(sim.flush());
-}
-
 // ─── Global Serial instance ───────────────────────────────────────────────────
 
 TEST_CASE("arduino::Serial (global SimSerial) is always true", "[serial][sim]") {
@@ -70,3 +69,81 @@ TEST_CASE("arduino::Serial (global SimSerial) is always true", "[serial][sim]")
 TEST_CASE("arduino::Serial::available returns 0", "[serial][sim]") {
     CHECK(arduino::Serial.available() == 0);
 }
+
+// ─── stdout buffering and flush ───────────────────────────────────────────────
+//
+// SimSerial's constructor line-buffers stdout once, at static init, so Serial
+// output reaches a non-TTY sink (pipe, file, journald) on each newline instead
+// of sitting in stdio's full-buffer until exit.  flush() covers the remainder:
+// a partial, newline-less line has nothing to trigger the line flush.
+//
+// Both are asserted behaviourally, by pointing fd 1 at a pipe and reading back
+// what actually arrived.
+
+namespace {
+
+/**
+ * Run @p emit with fd 1 pointed at a pipe; return the bytes that reached the
+ * pipe by the time @p emit returned.
+ *
+ * The read is non-blocking and happens before any fflush() of our own: a
+ * courtesy flush here would push the data through even when stdout is fully
+ * buffered, and mask exactly the regression these tests exist to catch.
+ *
+ * @param emit  Callable that writes via arduino::Serial.
+ * @return      What landed in the pipe; empty if stdout held onto it.
+ */
+template <typename F>
+std::string capture_stdout_fd(F emit) {
+    // Drain anything the harness left pending so the pipe sees only emit()'s bytes.
+    fflush(stdout);
+
+    int pipefd[2];
+    REQUIRE(pipe(pipefd) == 0);
+    // Nothing may ever be written; the read has to report that rather than block.
+    REQUIRE(fcntl(pipefd[0], F_SETFL, O_NONBLOCK) == 0);
+
+    int saved = dup(STDOUT_FILENO);
+    REQUIRE(saved != -1);
+    REQUIRE(dup2(pipefd[1], STDOUT_FILENO) != -1);
+
+    emit();
+
+    char buf[128] = {0};
+    ssize_t n = read(pipefd[0], buf, sizeof(buf) - 1);
+
+    // Restore fd 1 before the caller asserts, so failures still reach the console.
+    dup2(saved, STDOUT_FILENO);
+    close(saved);
+    close(pipefd[1]);
+    close(pipefd[0]);
+
+    return (n > 0) ? std::string(buf, static_cast<size_t>(n)) : std::string();
+}
+
+}  // namespace
+
+TEST_CASE("SimSerial constructor line-buffers stdout", "[serial][sim][buffering]") {
+    // The trailing newline is what triggers the flush under _IOLBF.
+    std::string out = capture_stdout_fd([] { arduino::Serial.println("LINEBUF"); });
+    CHECK(out.find("LINEBUF") != std::string::npos);
+}
+
+TEST_CASE("SimSerial holds a partial line until flushed", "[serial][sim][buffering]") {
+    // No newline, so line buffering has nothing to act on.  This is the control
+    // for the flush test below: without it, that test would pass even if
+    // stdout were unbuffered and flush() still a no-op.
+    std::string out = capture_stdout_fd([] { arduino::Serial.print("HELD"); });
+    CHECK(out.find("HELD") == std::string::npos);
+
+    // Drop the held bytes on the real stdout rather than leaving them buffered.
+    arduino::Serial.flush();
+}
+
+TEST_CASE("SimSerial::flush pushes a partial line out", "[serial][sim][buffering]") {
+    std::string out = capture_stdout_fd([] {
+        arduino::Serial.print("PARTIAL");
+        arduino::Serial.flush();
+    });
+    CHECK(out.find("PARTIAL") != std::string::npos);
+}