diff --git a/Unity-MCP-Plugin/Packages/com.ivanmurzak.unity.mcp/Editor/Scripts/McpServerManager.cs b/Unity-MCP-Plugin/Packages/com.ivanmurzak.unity.mcp/Editor/Scripts/McpServerManager.cs
index da71b511e..b5930de78 100644
--- a/Unity-MCP-Plugin/Packages/com.ivanmurzak.unity.mcp/Editor/Scripts/McpServerManager.cs
+++ b/Unity-MCP-Plugin/Packages/com.ivanmurzak.unity.mcp/Editor/Scripts/McpServerManager.cs
@@ -52,9 +52,23 @@ public enum McpServerStatus
     [InitializeOnLoad]
     public static class McpServerManager
     {
-        const string ProcessIdKey = "McpServerManager_ProcessId";
+        // Qualified by the server port so two Editors (different projects, different ports) on
+        // the same machine never share this slot. EditorPrefs are machine-global - shared across
+        // every project of the same Editor version - so an UNqualified key let one project's
+        // CheckExistingProcess adopt the OTHER project's server PID and then tear it down on stop,
+        // making the two Editors mutually exclusive even though their ports differ. See ProcessIdKeyForPort.
+        static string ProcessIdKey => ProcessIdKeyForPort(UnityMcpPluginEditor.Port);
         const string McpServerProcessName = "gamedev-mcp-server";
 
+        /// <summary>
+        /// The EditorPrefs key under which this project's running server PID is stored, qualified by
+        /// the server <paramref name="port"/>. EditorPrefs are shared across every project of the same
+        /// Editor version on the machine, so this key MUST be port-qualified: otherwise two Editors
+        /// clobber a single global slot and adopt/terminate each other's server processes. Pure (no
+        /// Unity API access) so it can be unit-tested in EditMode.
+        /// </summary>
+        internal static string ProcessIdKeyForPort(int port) => $"McpServerManager_ProcessId_{port}";
+
         static readonly ILogger _logger = UnityLoggerFactory.LoggerFactory.CreateLogger(typeof(McpServerManager));
         static readonly ReactiveProperty<McpServerStatus> _serverStatus = new(McpServerStatus.Stopped);
         static readonly object _processMutex = new();
diff --git a/Unity-MCP-Plugin/Packages/com.ivanmurzak.unity.mcp/Tests/Editor/McpServerManagerProcessIdKeyTests.cs b/Unity-MCP-Plugin/Packages/com.ivanmurzak.unity.mcp/Tests/Editor/McpServerManagerProcessIdKeyTests.cs
new file mode 100644
index 000000000..5efa374f0
--- /dev/null
+++ b/Unity-MCP-Plugin/Packages/com.ivanmurzak.unity.mcp/Tests/Editor/McpServerManagerProcessIdKeyTests.cs
@@ -0,0 +1,53 @@
+/*
+┌──────────────────────────────────────────────────────────────────┐
+│  Author: Ivan Murzak (https://github.com/IvanMurzak)             │
+│  Repository: GitHub (https://github.com/IvanMurzak/Unity-MCP)    │
+│  Copyright (c) 2025 Ivan Murzak                                  │
+│  Licensed under the Apache License, Version 2.0.                 │
+│  See the LICENSE file in the project root for more information.  │
+└──────────────────────────────────────────────────────────────────┘
+*/
+
+#nullable enable
+using NUnit.Framework;
+
+namespace com.IvanMurzak.Unity.MCP.Editor.Tests
+{
+    /// <summary>
+    /// Regression tests: the EditorPrefs key that stores the running server PID must be qualified by
+    /// the server port. EditorPrefs are shared across every project of the same Editor version on a
+    /// machine, so an unqualified key let two Editors (different projects, different ports) clobber a
+    /// single global slot — one project's <c>CheckExistingProcess</c> would adopt the other's server
+    /// PID and terminate it on stop, making the two Editors mutually exclusive despite distinct ports.
+    ///
+    /// <see cref="McpServerManager.ProcessIdKeyForPort"/> is the pure helper behind the key.
+    /// </summary>
+    public class McpServerManagerProcessIdKeyTests
+    {
+        [Test]
+        public void ProcessIdKeyForPort_DifferentPorts_ProduceDifferentKeys()
+        {
+            Assert.AreNotEqual(
+                McpServerManager.ProcessIdKeyForPort(26645),
+                McpServerManager.ProcessIdKeyForPort(26646),
+                "Two projects on different ports must not share the same PID key, " +
+                "or they tear down each other's servers.");
+        }
+
+        [Test]
+        public void ProcessIdKeyForPort_SamePort_IsStable()
+        {
+            Assert.AreEqual(
+                McpServerManager.ProcessIdKeyForPort(26646),
+                McpServerManager.ProcessIdKeyForPort(26646),
+                "The key for a given port must be deterministic across calls so the stored PID " +
+                "can be read back and deleted.");
+        }
+
+        [Test]
+        public void ProcessIdKeyForPort_IncludesPort()
+        {
+            StringAssert.Contains("26646", McpServerManager.ProcessIdKeyForPort(26646));
+        }
+    }
+}
diff --git a/Unity-MCP-Plugin/Packages/com.ivanmurzak.unity.mcp/Tests/Editor/McpServerManagerProcessIdKeyTests.cs.meta b/Unity-MCP-Plugin/Packages/com.ivanmurzak.unity.mcp/Tests/Editor/McpServerManagerProcessIdKeyTests.cs.meta
new file mode 100644
index 000000000..7fe6e68e5
--- /dev/null
+++ b/Unity-MCP-Plugin/Packages/com.ivanmurzak.unity.mcp/Tests/Editor/McpServerManagerProcessIdKeyTests.cs.meta
@@ -0,0 +1,11 @@
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