diff --git a/Tests/blitz_tests/ProjectAgentConfigServiceTests.swift b/Tests/blitz_tests/ProjectAgentConfigServiceTests.swift
index 5c0432b..d74b4f5 100644
--- a/Tests/blitz_tests/ProjectAgentConfigServiceTests.swift
+++ b/Tests/blitz_tests/ProjectAgentConfigServiceTests.swift
@@ -30,3 +30,39 @@ import Testing
     let blitzIphone = try #require(servers["blitz-iphone"] as? [String: Any])
     #expect(blitzIphone["command"] as? String == launcherPath)
 }
+
+@Test func detachedLauncherStopsServerWhenClientExits() throws {
+    let tempDirectory = FileManager.default.temporaryDirectory
+        .appendingPathComponent("blitz-detached-launcher-\(UUID().uuidString)", isDirectory: true)
+    try FileManager.default.createDirectory(at: tempDirectory, withIntermediateDirectories: true)
+    defer { try? FileManager.default.removeItem(at: tempDirectory) }
+
+    ProjectAgentConfigService(baseDirectory: tempDirectory).ensureMCPConfig(in: tempDirectory)
+    let launcherPath = BlitzPaths.bin.appendingPathComponent("blitz-detached-mcp").path
+    let pidFile = tempDirectory.appendingPathComponent("server.pid")
+
+    // The shell stands in for the MCP client: it starts the launcher with a stand-in server,
+    // then exits without signalling it, as a killed or crashed agent does.
+    let client = Process()
+    client.executableURL = URL(fileURLWithPath: "/bin/sh")
+    client.arguments = [
+        "-c",
+        "\"$0\" /bin/sh -c 'echo $$ > \"$0\"; exec /bin/sleep 60' \"$1\" & sleep 1",
+        launcherPath,
+        pidFile.path,
+    ]
+    client.standardOutput = FileHandle.nullDevice
+    client.standardError = FileHandle.nullDevice
+    try client.run()
+    client.waitUntilExit()
+
+    let pidText = try String(contentsOf: pidFile, encoding: .utf8)
+    let serverPID = try #require(pid_t(pidText.trimmingCharacters(in: .whitespacesAndNewlines)))
+    defer { kill(serverPID, SIGKILL) }
+
+    let deadline = Date().addingTimeInterval(10)
+    while kill(serverPID, 0) == 0 && Date() < deadline {
+        Thread.sleep(forTimeInterval: 0.2)
+    }
+    #expect(kill(serverPID, 0) != 0, "server outlived its MCP client")
+}
diff --git a/src/services/project/ProjectAgentConfigService.swift b/src/services/project/ProjectAgentConfigService.swift
index ff31e30..681c3d3 100644
--- a/src/services/project/ProjectAgentConfigService.swift
+++ b/src/services/project/ProjectAgentConfigService.swift
@@ -51,12 +51,31 @@ struct ProjectAgentConfigService {
         import os
         import signal
         import sys
+        import time
+
+        def stop_server(child_pid: int) -> None:
+            # The child is a session leader (setsid), so its pid is also its process group:
+            # this stops the server and anything it spawned.
+            try:
+                os.killpg(child_pid, signal.SIGTERM)
+            except ProcessLookupError:
+                return
+            deadline = time.monotonic() + 5
+            while time.monotonic() < deadline:
+                if os.waitpid(child_pid, os.WNOHANG)[0] == child_pid:
+                    return
+                time.sleep(0.1)
+            try:
+                os.killpg(child_pid, signal.SIGKILL)
+            except ProcessLookupError:
+                pass
 
         def main() -> int:
             if len(sys.argv) < 2:
                 print("usage: blitz-detached-mcp <command> [args...]", file=sys.stderr)
                 return 64
 
+            client_pid = os.getppid()
             child_pid = os.fork()
             if child_pid == 0:
                 os.setsid()
@@ -71,12 +90,20 @@ struct ProjectAgentConfigService {
             for sig in (signal.SIGTERM, signal.SIGINT, signal.SIGHUP):
                 signal.signal(sig, forward)
 
+            # The MCP client can exit without signalling us (killed, crashed, terminal closed).
+            # The detached server never sees that, so watch for being reparented and stop it
+            # rather than leave it running with its viewer port open.
             while True:
                 try:
-                    _, status = os.waitpid(child_pid, 0)
-                    break
+                    pid, status = os.waitpid(child_pid, os.WNOHANG)
                 except InterruptedError:
                     continue
+                if pid == child_pid:
+                    break
+                if os.getppid() != client_pid:
+                    stop_server(child_pid)
+                    return 0
+                time.sleep(1)
 
             if os.WIFEXITED(status):
                 return os.WEXITSTATUS(status)