diff --git a/Server/src/transport/unity_instance_middleware.py b/Server/src/transport/unity_instance_middleware.py
index 82ba47fd7..238e01d54 100644
--- a/Server/src/transport/unity_instance_middleware.py
+++ b/Server/src/transport/unity_instance_middleware.py
@@ -5,6 +5,7 @@
 into the request-scoped state, allowing tools to access it via ctx.get_state("unity_instance").
 """
 from threading import RLock
+import asyncio
 import logging
 import time
 
@@ -212,6 +213,57 @@ async def _resolve_instance_value(self, value: str, ctx) -> str:
             "Read mcpforunity://instances for current sessions."
         )
 
+    async def _drop_stale_pin(self, ctx, active_instance: str) -> str:
+        """
+        Discard a pin whose editor never came back, and refuse rather than retarget.
+
+        A pin outlives the editor it names. Left in place it also suppresses
+        auto-select, so every later call fails with no_unity_session and neither
+        waiting nor relaunching the editor recovers: the editor re-registers
+        under its own name, which is not the name the pin holds.
+
+        Two things make dropping it safe. The pinned hash gets the same reconnect
+        window `_resolve_session_id` gives it, so a domain reload does not look like
+        a departure. And a departure raises instead of returning None, because
+        falling through to auto-select would land a call pinned to project A in
+        project B, report success, and re-pin B (#1023).
+
+        An empty registry means every editor is mid-reload, so the pin still stands.
+        """
+        from transport.unity_transport import _is_http_transport
+        if not (_is_http_transport() and PluginHub.is_configured()):
+            return active_instance
+
+        from transport.plugin_hub import _read_bounded_wait_env
+        max_wait_s = _read_bounded_wait_env(
+            "UNITY_MCP_SESSION_RESOLVE_MAX_WAIT_S", default_s=20.0, max_s=120.0)
+        retry_ms = float(getattr(config, "reload_retry_ms", 250))
+        sleep_seconds = max(0.05, min(0.25, retry_ms / 1000.0))
+        deadline = time.monotonic() + max_wait_s
+
+        while True:
+            instances = await self._discover_instances(ctx)
+            ids = [inst.id for inst in instances if getattr(inst, "id", None)]
+            if not ids or active_instance in ids:
+                return active_instance
+            if time.monotonic() >= deadline:
+                break
+            await asyncio.sleep(sleep_seconds)
+
+        _diag.warning(
+            "Active instance %s did not return within %.1fs; dropping the stale pin. Registered: %s",
+            active_instance,
+            max_wait_s,
+            ", ".join(ids),
+        )
+        await self.clear_active_instance(ctx)
+        raise ValueError(
+            f"Pinned Unity instance '{active_instance}' is no longer running "
+            f"(waited {max_wait_s:.0f}s). The pin has been cleared. Available: "
+            f"{', '.join(ids) or 'none'}. Pass unity_instance explicitly, or read "
+            "mcpforunity://instances for current sessions."
+        )
+
     async def _maybe_autoselect_instance(self, ctx) -> str | None:
         """
         Auto-select the sole Unity instance when no active instance is set.
@@ -351,6 +403,8 @@ async def _inject_unity_instance(self, context: MiddlewareContext) -> None:
 
         if not active_instance:
             active_instance = await self.get_active_instance(ctx)
+            if active_instance:
+                active_instance = await self._drop_stale_pin(ctx, active_instance)
         if not active_instance:
             active_instance = await self._maybe_autoselect_instance(ctx)
         if active_instance:
diff --git a/Server/tests/integration/test_instance_autoselect.py b/Server/tests/integration/test_instance_autoselect.py
index b974cb11c..036e75ea0 100644
--- a/Server/tests/integration/test_instance_autoselect.py
+++ b/Server/tests/integration/test_instance_autoselect.py
@@ -26,6 +26,7 @@ async def get_sessions(cls):
             raise AssertionError("get_sessions should be stubbed in test")
 
     plugin_hub.PluginHub = PluginHub
+    plugin_hub._read_bounded_wait_env = lambda name, default_s, max_s: 0.0
     monkeypatch.setitem(sys.modules, "transport.plugin_hub", plugin_hub)
     monkeypatch.delitem(sys.modules, "transport.unity_instance_middleware", raising=False)
 
@@ -73,6 +74,7 @@ def is_configured(cls) -> bool:
             return False
 
     plugin_hub.PluginHub = PluginHub
+    plugin_hub._read_bounded_wait_env = lambda name, default_s, max_s: 0.0
     monkeypatch.setitem(sys.modules, "transport.plugin_hub", plugin_hub)
     monkeypatch.delitem(sys.modules, "transport.unity_instance_middleware", raising=False)
 
@@ -115,6 +117,7 @@ def is_configured(cls) -> bool:
             return False
 
     plugin_hub.PluginHub = PluginHub
+    plugin_hub._read_bounded_wait_env = lambda name, default_s, max_s: 0.0
     monkeypatch.setitem(sys.modules, "transport.plugin_hub", plugin_hub)
     monkeypatch.delitem(sys.modules, "transport.unity_instance_middleware", raising=False)
 
diff --git a/Server/tests/test_stale_instance_pin.py b/Server/tests/test_stale_instance_pin.py
new file mode 100644
index 000000000..f9f1ac3d9
--- /dev/null
+++ b/Server/tests/test_stale_instance_pin.py
@@ -0,0 +1,136 @@
+"""Stale active-instance pin tests.
+
+A pin outlives the editor it names. While it stayed pinned it also suppressed
+auto-select, so every later call failed with no_unity_session and neither
+waiting nor relaunching the editor recovered: the editor re-registers under its
+own name, which is not the name the pin holds.
+
+Dropping it is only safe under two conditions, both covered here: the pinned hash
+gets the same reconnect window `_resolve_session_id` gives it, so a domain reload
+does not read as a departure; and a departure raises rather than falling through to
+auto-select, which would silently move a call from project A to project B (#1023).
+"""
+
+from types import SimpleNamespace
+
+import pytest
+
+import transport.unity_transport as unity_transport
+from transport.plugin_hub import PluginHub
+from transport.unity_instance_middleware import UnityInstanceMiddleware
+
+
+class FakeContext:
+    """Minimal ctx shim over the session-scoped state the middleware uses."""
+
+    def __init__(self, state: dict | None = None):
+        self._state = state or {}
+
+    async def get_state(self, key: str):
+        return self._state.get(key)
+
+    async def set_state(self, key: str, value) -> None:
+        self._state[key] = value
+
+
+@pytest.fixture
+def http_hub(monkeypatch):
+    monkeypatch.setattr(unity_transport, "_is_http_transport", lambda: True)
+    monkeypatch.setattr(PluginHub, "is_configured", classmethod(lambda cls: True))
+    # No reconnect window unless a test asks for one, so the suite does not sit out 20s
+    monkeypatch.setenv("UNITY_MCP_SESSION_RESOLVE_MAX_WAIT_S", "0")
+
+
+def _registered(monkeypatch, middleware, *ids):
+    async def discover(_ctx):
+        return [SimpleNamespace(id=i, hash=i.split("@")[-1], name=i.split("@")[0]) for i in ids]
+
+    monkeypatch.setattr(middleware, "_discover_instances", discover)
+
+
+def _registered_over_time(monkeypatch, middleware, *rounds):
+    """Return a different registry on each successive poll."""
+    calls = {"n": 0}
+
+    async def discover(_ctx):
+        ids = rounds[min(calls["n"], len(rounds) - 1)]
+        calls["n"] += 1
+        return [SimpleNamespace(id=i, hash=i.split("@")[-1], name=i.split("@")[0]) for i in ids]
+
+    monkeypatch.setattr(middleware, "_discover_instances", discover)
+    return calls
+
+
+@pytest.mark.asyncio
+async def test_departed_pin_is_dropped_and_refuses(http_hub, monkeypatch):
+    """The ghost pin that bricked routing must not survive a call."""
+    middleware = UnityInstanceMiddleware()
+    ctx = FakeContext({UnityInstanceMiddleware._ACTIVE_INSTANCE_STATE_KEY: "wt-display-system@bbbb"})
+    _registered(monkeypatch, middleware, "Trailblazers-1@aaaa")
+
+    with pytest.raises(ValueError) as excinfo:
+        await middleware._drop_stale_pin(ctx, "wt-display-system@bbbb")
+
+    assert "wt-display-system@bbbb" in str(excinfo.value)
+    assert await middleware.get_active_instance(ctx) is None
+
+
+@pytest.mark.asyncio
+async def test_pin_naming_a_registered_instance_is_kept(http_hub, monkeypatch):
+    """A live pin is the user's choice and must be left alone."""
+    middleware = UnityInstanceMiddleware()
+    ctx = FakeContext()
+    _registered(monkeypatch, middleware, "Trailblazers-1@aaaa", "Trailblazers-2@cccc")
+
+    resolved = await middleware._drop_stale_pin(ctx, "Trailblazers-2@cccc")
+
+    assert resolved == "Trailblazers-2@cccc"
+
+
+@pytest.mark.asyncio
+async def test_pin_is_kept_while_no_instance_is_registered(http_hub, monkeypatch):
+    """An empty registry is a domain reload in flight, not a dead instance."""
+    middleware = UnityInstanceMiddleware()
+    ctx = FakeContext()
+    _registered(monkeypatch, middleware)
+
+    resolved = await middleware._drop_stale_pin(ctx, "Trailblazers-1@aaaa")
+
+    assert resolved == "Trailblazers-1@aaaa"
+
+
+@pytest.mark.asyncio
+async def test_pin_survives_a_reload_that_ends_inside_the_wait(http_hub, monkeypatch):
+    """A reloading editor is absent from a populated registry; the wait must cover it."""
+    monkeypatch.setenv("UNITY_MCP_SESSION_RESOLVE_MAX_WAIT_S", "5")
+    middleware = UnityInstanceMiddleware()
+    ctx = FakeContext()
+    calls = _registered_over_time(
+        monkeypatch,
+        middleware,
+        ("Trailblazers-1@aaaa",),
+        ("Trailblazers-1@aaaa", "Trailblazers-2@cccc"),
+    )
+
+    resolved = await middleware._drop_stale_pin(ctx, "Trailblazers-2@cccc")
+
+    assert resolved == "Trailblazers-2@cccc"
+    assert calls["n"] > 1, "must poll again rather than drop on the first miss"
+
+
+@pytest.mark.asyncio
+async def test_dropped_pin_never_retargets_another_project(http_hub, monkeypatch):
+    """Clearing then auto-selecting the survivor is #1023 arriving through another door."""
+    middleware = UnityInstanceMiddleware()
+    ctx = FakeContext({UnityInstanceMiddleware._ACTIVE_INSTANCE_STATE_KEY: "project-a@bbbb"})
+    _registered(monkeypatch, middleware, "project-b@aaaa")
+
+    async def autoselect(_ctx):
+        pytest.fail("auto-select must not run after a pinned instance departs")
+
+    monkeypatch.setattr(middleware, "_maybe_autoselect_instance", autoselect)
+
+    with pytest.raises(ValueError):
+        await middleware._drop_stale_pin(ctx, "project-a@bbbb")
+
+    assert await middleware.get_active_instance(ctx) is None