diff --git a/.swiftlint.yml b/.swiftlint.yml
index aa5e824..d1d0c25 100644
--- a/.swiftlint.yml
+++ b/.swiftlint.yml
@@ -17,10 +17,10 @@ line_length:
 identifier_name:
   min_length: 1
 type_body_length:
-  warning: 600
+  warning: 700
 file_length:
-  warning: 1400
-  error: 2000
+  warning: 1500
+  error: 2200
 cyclomatic_complexity:
   warning: 15
   error: 25
diff --git a/CHANGELOG.md b/CHANGELOG.md
index c8228e4..ed72f83 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,12 @@ Format: [Keep a Changelog](https://keepachangelog.com/), versioning: [SemVer](ht
 
 ## [Unreleased]
 
+## [0.1.2] - 2026-06-04
+### Fixed
+- Ensure only one `mlx_lm.server` instance runs while switching models.
+- Wait for the previous server process and port to stop before launching the selected model.
+- Queue a follow-up restart when the model is changed again during an active transition.
+
 ## [1.3.0] - 2026-06-04
 ### Added
 - Initial public release.
@@ -24,5 +30,6 @@ Format: [Keep a Changelog](https://keepachangelog.com/), versioning: [SemVer](ht
 ### Performance
 - Polling I/O runs off the main thread (gather/apply split); idle CPU stays near zero.
 
-[Unreleased]: https://github.com/wonsss/MLXControl/compare/v1.3.0...HEAD
+[Unreleased]: https://github.com/wonsss/MLXControl/compare/v0.1.2...HEAD
+[0.1.2]: https://github.com/wonsss/MLXControl/compare/v0.1.1...v0.1.2
 [1.3.0]: https://github.com/wonsss/MLXControl/releases/tag/v1.3.0
diff --git a/Sources/MLXControl/MLXControlApp.swift b/Sources/MLXControl/MLXControlApp.swift
index c77508e..3eb2f9c 100644
--- a/Sources/MLXControl/MLXControlApp.swift
+++ b/Sources/MLXControl/MLXControlApp.swift
@@ -133,6 +133,21 @@ func allMatches(_ pattern: String, _ text: String) -> [String] {
     }
 }
 
+@MainActor
+func waitUntilFalse(
+    timeout: TimeInterval,
+    pollInterval: TimeInterval,
+    condition: @escaping () -> Bool
+) async -> Bool {
+    let deadline = Date().addingTimeInterval(timeout)
+    while condition() {
+        if Date() >= deadline { return false }
+        let sleepNs = UInt64(max(0.005, pollInterval) * 1_000_000_000)
+        try? await Task.sleep(nanoseconds: sleepNs)
+    }
+    return true
+}
+
 // AppleScript notification — all args fully escaped to block injection
 func notify(_ title: String, _ subtitle: String, _ message: String) {
     func esc(_ s: String) -> String {
@@ -252,6 +267,7 @@ final class ServerController {
     @ObservationIgnored private var lastRAMAlert = Date.distantPast
     @ObservationIgnored private var lastFreeAlert = Date.distantPast
     @ObservationIgnored private var lastActivity = Date.distantPast
+    @ObservationIgnored private var restartAfterBusy = false
 
     var isRunning: Bool { status != .stopped }
 
@@ -503,15 +519,66 @@ final class ServerController {
         }
     }
 
-    private func launchServer() {
+    private static func mlxServerPIDs() -> [Int] {
+        sh("/usr/bin/pgrep", ["-f", "mlx_lm.server"])
+            .split(separator: "\n")
+            .compactMap { Int($0.trimmingCharacters(in: .whitespacesAndNewlines)) }
+    }
+
+    private static func isMlxServerRunning() -> Bool {
+        !mlxServerPIDs().isEmpty
+    }
+
+    private func stopExistingServersAndWait() async -> Bool {
+        if let serverProcess, serverProcess.isRunning {
+            serverProcess.terminate()
+        }
+        _ = sh("/usr/bin/pkill", ["-TERM", "-f", "mlx_lm.server"])
+
+        let terminated = await waitUntilFalse(timeout: 4, pollInterval: 0.1) {
+            Self.isMlxServerRunning()
+        }
+        if terminated {
+            serverProcess = nil
+            return true
+        }
+
+        _ = sh("/usr/bin/pkill", ["-KILL", "-f", "mlx_lm.server"])
+        let killed = await waitUntilFalse(timeout: 2, pollInterval: 0.1) {
+            Self.isMlxServerRunning()
+        }
+        if killed { serverProcess = nil }
+        return killed
+    }
+
+    private func waitForPortRelease() async -> Bool {
+        await waitUntilFalse(timeout: 3, pollInterval: 0.1) {
+            Self.isPortInUse(Config.port)
+        }
+    }
+
+    private func finishServerTransition() {
+        let hasServer = Self.isMlxServerRunning()
+        busy = false
+        refresh()
+        guard restartAfterBusy, hasServer else {
+            restartAfterBusy = false
+            return
+        }
+        restartAfterBusy = false
+        restart()
+    }
+
+    @discardableResult
+    private func launchServer() -> Bool {
         guard let bin = Config.serverBin else {
             notify("⚡ MLX Control", "Start failed", "mlx_lm.server not found. pip install mlx-lm")
-            return
+            return false
         }
         // Append to log file via FileHandle kept alive in serverProcess termination handler.
         let logPath = Config.logPath
         FileManager.default.createFile(atPath: logPath, contents: nil)
-        guard let logHandle = FileHandle(forWritingAtPath: logPath) else { return }
+        guard let logHandle = FileHandle(forWritingAtPath: logPath) else { return false }
         logHandle.seekToEndOfFile()
 
         let p = Process()
@@ -522,8 +589,15 @@ final class ServerController {
         p.qualityOfService = .userInitiated
         // Close logHandle only after the process exits — prevents early interpreter shutdown.
         p.terminationHandler = { _ in try? logHandle.close() }
-        serverProcess = p
-        try? p.run()
+        do {
+            try p.run()
+            serverProcess = p
+            return true
+        } catch {
+            try? logHandle.close()
+            notify("⚡ MLX Control", "Start failed", error.localizedDescription)
+            return false
+        }
     }
 
     func start() {
@@ -532,11 +606,6 @@ final class ServerController {
             notify("⚡ MLX Control", "Start failed", "mlx_lm.server not found. pip install mlx-lm")
             return
         }
-        if Self.isPortInUse(Config.port) {
-            notify("⚡ MLX Control", "Port \(Config.port) in use",
-                   "Another app (e.g. oMLX) is already on this port. Stop it first.")
-            return
-        }
         // Memory guard: compare model disk size against available system RAM.
         let freeGB = sysTotalGB - sysUsedGB
         if let modelBytes = modelSizes[selectedModel] {
@@ -555,36 +624,68 @@ final class ServerController {
             }
         }
         busy = true; warmedUp = false; lastActivity = Date()
-        launchServer()
         Task { @MainActor in
-            try? await Task.sleep(for: .seconds(2)); self.busy = false; self.refresh()
+            guard await self.stopExistingServersAndWait() else {
+                self.finishServerTransition()
+                self.notifyStartBlockedByExistingServer()
+                return
+            }
+            guard await self.waitForPortRelease() else {
+                self.finishServerTransition()
+                notify("⚡ MLX Control", "Port \(Config.port) in use",
+                       "Stop the app using this port, then try again.")
+                return
+            }
+            self.launchServer()
+            try? await Task.sleep(for: .seconds(2))
+            self.finishServerTransition()
         }
     }
 
     func stop() {
         guard !busy else { return }
         busy = true
-        _ = sh("/usr/bin/pkill", ["-f", "mlx_lm.server"])
         Task { @MainActor in
-            try? await Task.sleep(for: .seconds(1.5)); self.busy = false; self.refresh()
+            _ = await self.stopExistingServersAndWait()
+            try? await Task.sleep(for: .seconds(1.5))
+            self.finishServerTransition()
         }
     }
 
     func restart() {
         guard !busy else { return }
         busy = true; warmedUp = false
-        _ = sh("/usr/bin/pkill", ["-f", "mlx_lm.server"])
         Task { @MainActor in
-            try? await Task.sleep(for: .seconds(2))
+            guard await self.stopExistingServersAndWait() else {
+                self.finishServerTransition()
+                self.notifyStartBlockedByExistingServer()
+                return
+            }
+            guard await self.waitForPortRelease() else {
+                self.finishServerTransition()
+                notify("⚡ MLX Control", "Port \(Config.port) in use",
+                       "Stop the app using this port, then try again.")
+                return
+            }
             self.launchServer()
             try? await Task.sleep(for: .seconds(2))
-            self.busy = false; self.refresh()
+            self.finishServerTransition()
         }
     }
 
+    private func notifyStartBlockedByExistingServer() {
+        notify("⚡ MLX Control", "Server still running",
+               "Could not stop the existing mlx_lm.server process. Stop it manually, then try again.")
+    }
+
     func switchModel(_ m: String) {
         selectedModel = m
-        if isRunning { restart() }
+        guard isRunning else { return }
+        if busy {
+            restartAfterBusy = true
+        } else {
+            restart()
+        }
     }
 
     // ── Warm-up ──
diff --git a/Tests/MLXControlTests/HelpersTests.swift b/Tests/MLXControlTests/HelpersTests.swift
index 52fe37c..532d855 100644
--- a/Tests/MLXControlTests/HelpersTests.swift
+++ b/Tests/MLXControlTests/HelpersTests.swift
@@ -33,4 +33,25 @@ final class HelpersTests: XCTestCase {
         XCTAssertFalse(valid("a/b/c"))             // two slashes
         XCTAssertFalse(valid("name with space"))
     }
+
+    @MainActor
+    func testWaitUntilFalseReturnsTrueAfterConditionClears() async {
+        var checks = 0
+        let cleared = await waitUntilFalse(timeout: 0.2, pollInterval: 0.01) {
+            checks += 1
+            return checks < 3
+        }
+
+        XCTAssertTrue(cleared)
+        XCTAssertGreaterThanOrEqual(checks, 3)
+    }
+
+    @MainActor
+    func testWaitUntilFalseReturnsFalseWhenConditionStaysTrue() async {
+        let cleared = await waitUntilFalse(timeout: 0.03, pollInterval: 0.01) {
+            true
+        }
+
+        XCTAssertFalse(cleared)
+    }
 }
diff --git a/build.sh b/build.sh
index 9bc014a..b83f656 100755
--- a/build.sh
+++ b/build.sh
@@ -3,7 +3,7 @@ set -e
 cd "$(dirname "$0")"
 
 BUNDLE_ID="io.github.wonsss.mlxcontrol"
-VERSION="1.3"
+VERSION="0.1.2"
 APP="MLXControl.app"
 
 echo "→ 아이콘 생성…"