diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c8101096..7f5112bc 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -523,15 +523,18 @@ jobs:
           for attempt in 1 2; do
             cleanup 2>/dev/null || true
 
-            # Launch display helper from shell (non-sandboxed)
+            # Launch display helper from shell (non-sandboxed).
+            # Use --start-delay-ms instead of --start-path because the XCTest
+            # runner is sandboxed and can't write to /tmp/ for the start signal.
+            # 10s delay gives the test time to capture baseline render stats.
             "$HELPER_PATH" \
               --modes "1920x1080,1728x1117,1600x900,1440x810" \
               --ready-path "$DISPLAY_READY" \
               --display-id-path "$DISPLAY_ID_PATH" \
-              --start-path "$DISPLAY_START" \
               --done-path "$DISPLAY_DONE" \
               --iterations 40 \
               --interval-ms 40 \
+              --start-delay-ms 10000 \
               > "$HELPER_LOG" 2>&1 &
             HELPER_PID=$!
 
@@ -619,7 +622,7 @@ jobs:
             # Write manifests so test can find the pre-launched state
             MANIFEST_PATH="/tmp/cmux-ui-test-display-harness.json"
             cat >"$MANIFEST_PATH" <<MANIFEST_EOF
-          {"readyPath":"$DISPLAY_READY","displayIDPath":"$DISPLAY_ID_PATH","startPath":"$DISPLAY_START","donePath":"$DISPLAY_DONE","logPath":"$HELPER_LOG"}
+          {"readyPath":"$DISPLAY_READY","displayIDPath":"$DISPLAY_ID_PATH","donePath":"$DISPLAY_DONE","logPath":"$HELPER_LOG"}
           MANIFEST_EOF
 
             PRELAUNCH_PATH="/tmp/cmux-ui-test-prelaunch.json"
diff --git a/cmuxUITests/DisplayResolutionRegressionUITests.swift b/cmuxUITests/DisplayResolutionRegressionUITests.swift
index 73a16c2f..2b3ef233 100644
--- a/cmuxUITests/DisplayResolutionRegressionUITests.swift
+++ b/cmuxUITests/DisplayResolutionRegressionUITests.swift
@@ -82,11 +82,15 @@ final class DisplayResolutionRegressionUITests: XCTestCase {
         var maxDiagnosticsUpdatedAt = baselineStats.diagnosticsUpdatedAt
         var lastStats = baselineStats
 
-        do {
-            try Data("start\n".utf8).write(to: URL(fileURLWithPath: displayStartPath), options: .atomic)
-        } catch {
-            XCTFail("Expected start signal file to be created at \(displayStartPath): \(error)")
-            return
+        // When pre-launched from CI, the display helper uses --start-delay-ms
+        // instead of a start signal file (sandbox prevents writing to /tmp/).
+        if prelaunch == nil {
+            do {
+                try Data("start\n".utf8).write(to: URL(fileURLWithPath: displayStartPath), options: .atomic)
+            } catch {
+                XCTFail("Expected start signal file to be created at \(displayStartPath): \(error)")
+                return
+            }
         }
 
         let deadline = Date().addingTimeInterval(30.0)
diff --git a/scripts/create-virtual-display.m b/scripts/create-virtual-display.m
index f87ab2bd..5ed92f9e 100644
--- a/scripts/create-virtual-display.m
+++ b/scripts/create-virtual-display.m
@@ -166,6 +166,8 @@ int main(int argc, const char *argv[]) {
         NSString *intervalArgument = argumentValue(arguments, @"--interval-ms");
         NSInteger intervalMs = intervalArgument.length > 0 ? intervalArgument.integerValue : 40;
         useconds_t intervalMicros = (useconds_t)(MAX(1, intervalMs) * 1000);
+        NSString *startDelayArgument = argumentValue(arguments, @"--start-delay-ms");
+        NSInteger startDelayMs = startDelayArgument.length > 0 ? startDelayArgument.integerValue : 0;
 
         unsigned int width = 0;
         unsigned int height = 0;
@@ -229,7 +231,9 @@ int main(int argc, const char *argv[]) {
 
         if (iterations > 0 && modeSpecs.count > 1) {
             dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0), ^{
-                if (startPath.length > 0) {
+                if (startDelayMs > 0) {
+                    usleep((useconds_t)(startDelayMs * 1000));
+                } else if (startPath.length > 0) {
                     while (![[NSFileManager defaultManager] fileExistsAtPath:startPath]) {
                         usleep(20 * 1000);
                     }