diff --git a/Sources/SocketControlSettings.swift b/Sources/SocketControlSettings.swift
index a2586136..da220b15 100644
--- a/Sources/SocketControlSettings.swift
+++ b/Sources/SocketControlSettings.swift
@@ -163,6 +163,8 @@ struct SocketControlSettings {
     static let legacyEnabledKey = "socketControlEnabled"
     static let allowSocketPathOverrideKey = "CMUX_ALLOW_SOCKET_OVERRIDE"
     static let socketPasswordEnvKey = "CMUX_SOCKET_PASSWORD"
+    static let launchTagEnvKey = "CMUX_TAG"
+    static let baseDebugBundleIdentifier = "com.cmuxterm.app.debug"
 
     private static func normalizeMode(_ raw: String) -> String {
         raw
@@ -211,6 +213,53 @@ struct SocketControlSettings {
 #endif
     }
 
+    static func launchTag(
+        environment: [String: String] = ProcessInfo.processInfo.environment
+    ) -> String? {
+        guard let raw = environment[launchTagEnvKey] else { return nil }
+        let trimmed = raw.trimmingCharacters(in: .whitespacesAndNewlines)
+        return trimmed.isEmpty ? nil : trimmed
+    }
+
+    static func shouldBlockUntaggedDebugLaunch(
+        environment: [String: String] = ProcessInfo.processInfo.environment,
+        bundleIdentifier: String? = Bundle.main.bundleIdentifier,
+        isDebugBuild: Bool = SocketControlSettings.isDebugBuild
+    ) -> Bool {
+        guard isDebugBuild else { return false }
+        if isRunningUnderXCTest(environment: environment) {
+            return false
+        }
+
+        guard let bundleIdentifier = bundleIdentifier?.trimmingCharacters(in: .whitespacesAndNewlines),
+              !bundleIdentifier.isEmpty else {
+            return false
+        }
+
+        if bundleIdentifier.hasPrefix("\(baseDebugBundleIdentifier).") {
+            return false
+        }
+
+        guard bundleIdentifier == baseDebugBundleIdentifier else {
+            return false
+        }
+
+        return launchTag(environment: environment) == nil
+    }
+
+    static func isRunningUnderXCTest(environment: [String: String]) -> Bool {
+        let indicators = [
+            "XCTestConfigurationFilePath",
+            "XCTestBundlePath",
+            "XCTestSessionIdentifier",
+            "XCInjectBundleInto",
+        ]
+        return indicators.contains { key in
+            guard let value = environment[key] else { return false }
+            return !value.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty
+        }
+    }
+
     static func socketPath(
         environment: [String: String] = ProcessInfo.processInfo.environment,
         bundleIdentifier: String? = Bundle.main.bundleIdentifier,
diff --git a/Sources/cmuxApp.swift b/Sources/cmuxApp.swift
index f857ffa0..004406e7 100644
--- a/Sources/cmuxApp.swift
+++ b/Sources/cmuxApp.swift
@@ -35,6 +35,10 @@ struct cmuxApp: App {
     @NSApplicationDelegateAdaptor(AppDelegate.self) private var appDelegate
 
     init() {
+        if SocketControlSettings.shouldBlockUntaggedDebugLaunch() {
+            Self.terminateForMissingLaunchTag()
+        }
+
         Self.configureGhosttyEnvironment()
 
         let startupAppearance = AppearanceSettings.resolvedMode()
@@ -58,6 +62,14 @@ struct cmuxApp: App {
         appDelegate.configure(tabManager: tabManager, notificationStore: notificationStore, sidebarState: sidebarState)
     }
 
+    private static func terminateForMissingLaunchTag() -> Never {
+        let message = "error: refusing to launch untagged cmux DEV; start with ./scripts/reload.sh --tag <name> (or set CMUX_TAG for test harnesses)"
+        fputs("\(message)\n", stderr)
+        fflush(stderr)
+        NSLog("%@", message)
+        Darwin.exit(64)
+    }
+
     private static func configureGhosttyEnvironment() {
         let fileManager = FileManager.default
         let ghosttyAppResources = "/Applications/Ghostty.app/Contents/Resources/ghostty"
diff --git a/cmuxTests/GhosttyConfigTests.swift b/cmuxTests/GhosttyConfigTests.swift
index 220767ba..a788043b 100644
--- a/cmuxTests/GhosttyConfigTests.swift
+++ b/cmuxTests/GhosttyConfigTests.swift
@@ -691,6 +691,56 @@ final class SocketControlSettingsTests: XCTestCase {
             "/tmp/cmux-staging.sock"
         )
     }
+
+    func testUntaggedDebugBundleBlockedWithoutLaunchTag() {
+        XCTAssertTrue(
+            SocketControlSettings.shouldBlockUntaggedDebugLaunch(
+                environment: [:],
+                bundleIdentifier: "com.cmuxterm.app.debug",
+                isDebugBuild: true
+            )
+        )
+    }
+
+    func testUntaggedDebugBundleAllowedWithLaunchTag() {
+        XCTAssertFalse(
+            SocketControlSettings.shouldBlockUntaggedDebugLaunch(
+                environment: ["CMUX_TAG": "tests-v1"],
+                bundleIdentifier: "com.cmuxterm.app.debug",
+                isDebugBuild: true
+            )
+        )
+    }
+
+    func testTaggedDebugBundleAllowedWithoutLaunchTag() {
+        XCTAssertFalse(
+            SocketControlSettings.shouldBlockUntaggedDebugLaunch(
+                environment: [:],
+                bundleIdentifier: "com.cmuxterm.app.debug.tests-v1",
+                isDebugBuild: true
+            )
+        )
+    }
+
+    func testReleaseBuildIgnoresLaunchTagGate() {
+        XCTAssertFalse(
+            SocketControlSettings.shouldBlockUntaggedDebugLaunch(
+                environment: [:],
+                bundleIdentifier: "com.cmuxterm.app.debug",
+                isDebugBuild: false
+            )
+        )
+    }
+
+    func testXCTestLaunchIgnoresLaunchTagGate() {
+        XCTAssertFalse(
+            SocketControlSettings.shouldBlockUntaggedDebugLaunch(
+                environment: ["XCTestConfigurationFilePath": "/tmp/fake.xctestconfiguration"],
+                bundleIdentifier: "com.cmuxterm.app.debug",
+                isDebugBuild: true
+            )
+        )
+    }
 }
 
 final class PostHogAnalyticsPropertiesTests: XCTestCase {
diff --git a/scripts/run-tests-v1.sh b/scripts/run-tests-v1.sh
index 317d19cf..12592e70 100755
--- a/scripts/run-tests-v1.sh
+++ b/scripts/run-tests-v1.sh
@@ -13,6 +13,7 @@ cd "$(dirname "$0")/.."
 
 DERIVED_DATA_PATH="$HOME/Library/Developer/Xcode/DerivedData/cmux-tests-v1"
 APP="$DERIVED_DATA_PATH/Build/Products/Debug/cmux DEV.app"
+RUN_TAG="tests-v1"
 
 echo "== build =="
 # Work around stale explicit-module cache artifacts (notably Sentry headers) that can
@@ -51,7 +52,7 @@ launch_and_wait() {
   defaults write com.cmuxterm.app.debug socketControlMode -string full >/dev/null 2>&1 || true
 
   # Launch directly with UI test mode enabled so startup follows deterministic test codepaths.
-  CMUX_UI_TEST_MODE=1 "$APP/Contents/MacOS/cmux DEV" >/dev/null 2>&1 &
+  CMUX_TAG="$RUN_TAG" CMUX_UI_TEST_MODE=1 "$APP/Contents/MacOS/cmux DEV" >/dev/null 2>&1 &
 
   SOCK=""
   for _ in {1..120}; do
@@ -70,7 +71,7 @@ launch_and_wait() {
   export CMUX_SOCKET="$SOCK"
 
   # Ensure LaunchServices has a visible/main window attached for rendering checks.
-  open "$APP" >/dev/null 2>&1 || true
+  CMUX_TAG="$RUN_TAG" open "$APP" >/dev/null 2>&1 || true
   sleep 0.5
 
   echo "== wait ready =="
diff --git a/scripts/run-tests-v2.sh b/scripts/run-tests-v2.sh
index 4be4e854..e17cc6c2 100755
--- a/scripts/run-tests-v2.sh
+++ b/scripts/run-tests-v2.sh
@@ -13,6 +13,7 @@ cd "$(dirname "$0")/.."
 
 DERIVED_DATA_PATH="$HOME/Library/Developer/Xcode/DerivedData/cmux-tests-v2"
 APP="$DERIVED_DATA_PATH/Build/Products/Debug/cmux DEV.app"
+RUN_TAG="tests-v2"
 
 echo "== build =="
 # Work around stale explicit-module cache artifacts (notably Sentry headers) that can
@@ -51,7 +52,7 @@ launch_and_wait() {
   defaults write com.cmuxterm.app.debug socketControlMode -string full >/dev/null 2>&1 || true
 
   # Launch directly with UI test mode enabled so startup follows deterministic test codepaths.
-  CMUX_UI_TEST_MODE=1 "$APP/Contents/MacOS/cmux DEV" >/dev/null 2>&1 &
+  CMUX_TAG="$RUN_TAG" CMUX_UI_TEST_MODE=1 "$APP/Contents/MacOS/cmux DEV" >/dev/null 2>&1 &
 
   SOCK=""
   for _ in {1..120}; do
@@ -70,7 +71,7 @@ launch_and_wait() {
   export CMUX_SOCKET="$SOCK"
 
   # Ensure LaunchServices has a visible/main window attached for rendering checks.
-  open "$APP" >/dev/null 2>&1 || true
+  CMUX_TAG="$RUN_TAG" open "$APP" >/dev/null 2>&1 || true
   sleep 0.5
 
   echo "== wait ready =="