diff --git a/server/services/process-controller.ts b/server/services/process-controller.ts
index 09430668..693243fc 100644
--- a/server/services/process-controller.ts
+++ b/server/services/process-controller.ts
@@ -38,6 +38,7 @@ export interface IProcessController {
   kill(signal?: NodeJS.Signals | number): void;
   destroySockets(): void;
   hasProcess(): boolean;
+  clearListeners(): void;
 }
 
 /**
@@ -218,4 +219,12 @@ export class ProcessController implements IProcessController {
   hasProcess(): boolean {
     return !!this.proc;
   }
+
+  clearListeners(): void {
+    this.stdoutListeners = [];
+    this.stderrListeners = [];
+    this.stderrLineListeners = [];
+    this.closeListeners = [];
+    this.errorListeners = [];
+  }
 }
diff --git a/server/services/sandbox-runner.ts b/server/services/sandbox-runner.ts
index 03bafa77..23802510 100644
--- a/server/services/sandbox-runner.ts
+++ b/server/services/sandbox-runner.ts
@@ -77,6 +77,7 @@ export class SandboxRunner {
   private totalOutputBytes = 0;
   private isSendingOutput = false;
   private flushTimer: NodeJS.Timeout | null = null;
+  private stderrFallbackBuffer = ""; // Fallback buffer for unline-buffered stderr data
   
   // Execution state
   private processStartTime: number | null = null;
@@ -724,6 +725,8 @@ export class SandboxRunner {
 
         // compile finished successfully, clear flag before running
         this.isCompiling = false;
+        // Clear listeners from previous run before spawning new process
+        this.processController.clearListeners();
         await this.processController.spawn(files.exeFile);
         this.processStartTime = Date.now();
         this.transitionTo(SimulationState.RUNNING);
@@ -827,6 +830,8 @@ export class SandboxRunner {
       command: DockerCommandBuilder.buildCompileAndRunCommand(),
     });
 
+    // Clear listeners from previous run before spawning new process
+    this.processController.clearListeners();
     await this.processController.spawn("docker", dockerArgs);
     this.logger.info("🚀 Docker: Compile + Run in single container");
     this.processStartTime = Date.now();
@@ -1029,13 +1034,13 @@ export class SandboxRunner {
     });
 
     const useFallbackParser = !this.processController.supportsStderrLineStreaming();
-    let stderrFallbackBuffer = "";
+    this.stderrFallbackBuffer = ""; // Reset buffer for this run
 
     this.processController.onStderr((data) => {
       if (useFallbackParser) {
-        stderrFallbackBuffer += data.toString();
-        const lines = stderrFallbackBuffer.split(/\r?\n/);
-        stderrFallbackBuffer = lines.pop() || "";
+        this.stderrFallbackBuffer += data.toString();
+        const lines = this.stderrFallbackBuffer.split(/\r?\n/);
+        this.stderrFallbackBuffer = lines.pop() || "";
 
         for (const line of lines) {
           if (!line) continue;
@@ -1060,6 +1065,16 @@ export class SandboxRunner {
         this.flushTimer = null;
       }
 
+      // CRITICAL: Flush any remaining data in stderr fallback buffer to prevent line loss
+      if (this.stderrFallbackBuffer) {
+        const buffered = this.stderrFallbackBuffer;
+        this.stderrFallbackBuffer = "";
+        if (buffered.trim()) {
+          const parsed = this.stderrParser.parseStderrLine(buffered, this.processStartTime);
+          this.handleParsedLine(parsed, callbacks.onPinState, callbacks.onOutput, callbacks.onError);
+        }
+      }
+
       // CRITICAL: Flush message queue before exit to prevent losing queued output
       this.flushMessageQueue();
       
diff --git a/tests/server/services/ghost-output.test.ts b/tests/server/services/ghost-output.test.ts
new file mode 100644
index 00000000..5f9be3b1
--- /dev/null
+++ b/tests/server/services/ghost-output.test.ts
@@ -0,0 +1,49 @@
+import { describe, it, expect, vi } from 'vitest';
+import { SandboxRunner } from '../../../server/services/sandbox-runner';
+
+// give the test a generous timeout because the runner needs to compile/start
+vi.setConfig({ testTimeout: 30000 });
+
+describe('Ghost Output Reproduction', () => {
+  it('should keep ProcessController listener counts stable across runs', async () => {
+    const runner = new SandboxRunner();
+
+    const ctrl: any = (runner as any).processController;
+    // initial state should have no listeners
+    expect(ctrl.stdoutListeners.length).toBe(0);
+    expect(ctrl.stderrListeners.length).toBe(0);
+    expect(ctrl.stderrLineListeners.length).toBe(0);
+
+    // first run
+    runner.runSketch({ code: 'void setup() {}', onOutput: () => {}, onIORegistry: () => {} });
+    // wait until a process exists AND listeners have been attached
+    while (!(ctrl.hasProcess() && ctrl.stderrLineListeners.length > 0)) {
+      await new Promise(r => setTimeout(r, 10));
+    }
+
+    const firstStdoutCount = ctrl.stdoutListeners.length;
+    const firstStderrCount = ctrl.stderrListeners.length;
+    const firstStderrLineCount = ctrl.stderrLineListeners.length;
+
+    console.log('[TEST] After first run:', { firstStdoutCount, firstStderrCount, firstStderrLineCount });
+
+    await runner.stop();
+
+    // second run reusing the same runner
+    runner.runSketch({ code: 'void setup() {}', onOutput: () => {}, onIORegistry: () => {} });
+    while (!(ctrl.hasProcess() && ctrl.stderrLineListeners.length > 0)) {
+      await new Promise(r => setTimeout(r, 10));
+    }
+
+    const secondStdoutCount = ctrl.stdoutListeners.length;
+    const secondStderrCount = ctrl.stderrListeners.length;
+    const secondStderrLineCount = ctrl.stderrLineListeners.length;
+
+    console.log('[TEST] After second run:', { secondStdoutCount, secondStderrCount, secondStderrLineCount });
+
+    // listener counts should not increase (no Ghost Output accumulation)
+    expect(secondStdoutCount).toBe(firstStdoutCount);
+    expect(secondStderrCount).toBe(firstStderrCount);
+    expect(secondStderrLineCount).toBe(firstStderrLineCount);
+  });
+});