fix(unity): concurrent, thread-safe, window-independent HTTP server

[isekream]

Problem

The editor MCP server accepted connections but frequently returned empty bodies or timed out, and got worse under back-to-back calls. Three coupled defects:

1. Serial request handling — HandleRequests ran ProcessRequest inline on the single listener thread, which then blocks in ExecuteTool's wait loop for up to requestTimeout seconds. The next GetContext() isn't reached until the current request finishes, so concurrent bridge calls stall in the accept backlog.

2. Cross-thread EditorApplication.delayCall += — tool work was dispatched to the main thread by mutating the delayCall static delegate from the background listener thread. Delegate += is a non-atomic read-modify-write; concurrent subscription drops callbacks, so the completion flag never flips → timeout with no body. (Fragile even single-threaded — it's an editor-loop delegate being mutated off-thread.)

3. Window-coupled registry + static-ctor crash — the tool registry lived on the EditorWindow instance, so a closed window made every call return "MCP Server window is not open." Separately, LoadPreferences() called EditorPrefs.* directly in the static constructor, which Unity forbids (GetInt is not allowed to be called from a ScriptableObject constructor).

Fix

• Service each accepted request on the ThreadPool so the listener loops immediately.
• Replace the racy delayCall += dispatch with a thread-safe ConcurrentQueue<Action> drained by one persistent EditorApplication.update pump.
• Make the tool registry static and initialize it on StartServer() — the server now works headless (window closed).
• Defer LoadPreferences() out of the static constructor into EditorApplication.delayCall.
• Wrap cross-thread completion flags in a holder with volatile fields for correct visibility.

Tool bodies still run serially on the main thread (Unity API requirement); only the listener and the per-request wait move off it.

Verification

Against a live editor: scene.list_root_objects returned a full success body in ~76 ms (previously empty/timeout), and the "window not open" failure class is gone.

Single file changed: Unity/Editor/McpUnityServer.cs.

🤖 Generated with Claude Code

[isekream]

Superseded by #9, which reconciles this threading/headless rewrite with the static registry work and adds domain-reload survival, scene-building tools, Unity 6 compat, and bridge audit fixes. Safe to close.