diff --git a/DEPLOYMENT.md b/DEPLOYMENT.md
index b10e708..192e8a3 100644
--- a/DEPLOYMENT.md
+++ b/DEPLOYMENT.md
@@ -12,7 +12,9 @@ pip install gunicorn                     # Linux / macOS
 # pip install waitress                   # Windows-friendly alternative (see below)
 
 # Multi-process (recommended): one thread per worker avoids any per-worker state surprises.
-gunicorn --factory --bind 127.0.0.1:3000 --workers 2 --threads 1 app:create_app
+# WEB_CONCURRENCY (or CURSOR_BROWSER_MULTI_WORKER=1) lets the app detect multi-worker mode so
+# POST /api/set-workspace returns 409 instead of a misleading 200 on a single worker.
+WEB_CONCURRENCY=2 gunicorn --factory --bind 127.0.0.1:3000 --workers 2 --threads 1 app:create_app
 
 # Single-process, multi-threaded: safe after the #43 lock; useful for lighter deployments.
 gunicorn --factory --bind 127.0.0.1:3000 --workers 1 --threads 4 app:create_app
@@ -66,10 +68,10 @@ gunicorn and waitress are **not** runtime dependencies; install them only when d
 
 When gunicorn runs with `--workers 2` (or more), each worker is an independent Python process:
 
-- **`POST /api/set-workspace`** only updates the worker that handled that request. Other workers keep their previous override (usually `None`). Prefer setting `WORKSPACE_PATH` or passing `--base-dir` at process start so every worker sees the same path.
+- **`POST /api/set-workspace`** cannot update every worker from a single request (each process has its own override). The API returns **HTTP 409** with code `set_workspace_multi_worker_unsupported` instead of success when multiple workers are detected (`WEB_CONCURRENCY`, `GUNICORN_WORKERS`, gunicorn `--workers` in `GUNICORN_CMD_ARGS`, or `CURSOR_BROWSER_MULTI_WORKER=1`). Set `WORKSPACE_PATH` or pass `--base-dir` at process start so every worker sees the same path.
 - **Exclusion rules** (`EXCLUSION_RULES` in app config) are loaded once at worker startup from `--exclude-rules` or the default file. Changing the rules file requires restarting workers.
 
-For a single-user localhost deployment with multiple workers, set the workspace path via environment variable rather than the Configuration page.
+For a single-user localhost deployment with multiple workers, set the workspace path via environment variable or `--base-dir` at startup; do not rely on the Configuration page `set-workspace` call.
 
 ## Path configuration
 
diff --git a/api/config_api.py b/api/config_api.py
index 9b5631f..b09a758 100644
--- a/api/config_api.py
+++ b/api/config_api.py
@@ -16,7 +16,10 @@
 from api.flask_config import api_error, json_response
 
 from utils.path_validation import WorkspacePathError, validate_workspace_path
-from utils.workspace_path import set_workspace_path_override
+from utils.workspace_path import (
+    is_multi_worker_process_deployment,
+    set_workspace_path_override,
+)
 
 bp = Blueprint("config_api", __name__)
 _logger = logging.getLogger(__name__)
@@ -172,7 +175,8 @@ def set_workspace() -> tuple[Response, int] | Response:
 
     Returns:
         ``{"success": true, "path": "..."}`` on success. 400 for invalid path or
-        body; 500 when override storage fails.
+        body; 409 when multiple WSGI worker processes are in use (override cannot
+        apply fleet-wide); 500 when override storage fails.
     """
     # Reject non-dict JSON bodies (array / string / number / null). Without
     # this, get_json returns the value directly, the truthy fallback `or {}`
@@ -194,6 +198,14 @@ def set_workspace() -> tuple[Response, int] | Response:
         return api_error("Failed to validate workspace path", "validate_workspace_path_failed", 500)
     if message is not None:
         return api_error(message, _workspace_path_error_code(message), 400)
+    if is_multi_worker_process_deployment():
+        return api_error(
+            "POST /api/set-workspace only updates this worker process. "
+            "Set WORKSPACE_PATH or pass --base-dir at startup when using "
+            "multiple gunicorn workers.",
+            "set_workspace_multi_worker_unsupported",
+            409,
+        )
     try:
         set_workspace_path_override(canonical)
     except Exception:  # noqa: BLE001 — keep the response shape structured JSON
diff --git a/templates/config.html b/templates/config.html
index 3236ad4..fb73734 100644
--- a/templates/config.html
+++ b/templates/config.html
@@ -18,8 +18,36 @@ <h1>Configuration</h1>
 </div>
 
 <script nonce="{{ csp_nonce }}">
-document.addEventListener('DOMContentLoaded', async () => {
-  document.getElementById('btn-save-config')?.addEventListener('click', validateAndSave);
+async function postSetWorkspace(path) {
+  const res = await fetch('/api/set-workspace', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ path })
+  });
+  let body = {};
+  try {
+    body = await res.json();
+  } catch (e) {
+    body = {};
+  }
+  if (res.status === 409) {
+    return {
+      ok: false,
+      message: body.error || 'Workspace path cannot be changed in this deployment.',
+      code: body.code
+    };
+  }
+  if (!res.ok) {
+    return {
+      ok: false,
+      message: body.error || 'Failed to save workspace path.',
+      code: body.code
+    };
+  }
+  return { ok: true, body };
+}
+
+document.addEventListener('DOMContentLoaded', async () => {  document.getElementById('btn-save-config')?.addEventListener('click', validateAndSave);
 
   const stored = localStorage.getItem('workspacePath');
   if (stored) {
@@ -55,13 +83,17 @@ <h1>Configuration</h1>
       });
       const valData = await valRes.json();
       if (valData.valid) {
-        localStorage.setItem('workspacePath', detected);
-        await fetch('/api/set-workspace', {
-          method: 'POST',
-          headers: { 'Content-Type': 'application/json' },
-          body: JSON.stringify({ path: detected })
-        });
-        window.location.href = '/';
+        const saved = await postSetWorkspace(detected);
+        if (saved.ok) {
+          localStorage.setItem('workspacePath', detected);
+          window.location.href = '/';
+          return;
+        }
+        const statusEl = document.getElementById('status-msg');
+        statusEl.className = 'alert alert-danger';
+        statusEl.textContent = saved.message;
+        statusEl.style.display = 'block';
+        document.getElementById('workspace-path').value = detected;
         return;
       }
     }
@@ -86,12 +118,14 @@ <h1>Configuration</h1>
     const data = await res.json();
 
     if (data.valid) {
+      const saved = await postSetWorkspace(path);
+      if (!saved.ok) {
+        statusEl.className = 'alert alert-danger';
+        statusEl.textContent = saved.message;
+        statusEl.style.display = 'block';
+        return;
+      }
       localStorage.setItem('workspacePath', path);
-      await fetch('/api/set-workspace', {
-        method: 'POST',
-        headers: { 'Content-Type': 'application/json' },
-        body: JSON.stringify({ path })
-      });
       statusEl.className = 'alert alert-success';
       statusEl.textContent = `Found ${data.workspaceCount} workspaces in the specified location`;
       statusEl.style.display = 'block';
diff --git a/tests/test_set_workspace_multiworker.py b/tests/test_set_workspace_multiworker.py
new file mode 100644
index 0000000..796d985
--- /dev/null
+++ b/tests/test_set_workspace_multiworker.py
@@ -0,0 +1,117 @@
+"""POST /api/set-workspace behavior under multi-worker WSGI deployments."""
+
+from __future__ import annotations
+
+import os
+import shutil
+import tempfile
+import unittest
+from unittest.mock import patch
+
+from tests.test_workspace_path_validation import _make_cursor_workspace_dir
+
+
+class TestSetWorkspaceMultiWorker(unittest.TestCase):
+    def setUp(self):
+        from flask import Flask
+
+        from api.config_api import bp as config_bp
+        from utils.workspace_path import set_workspace_path_override
+
+        self.tmp = tempfile.mkdtemp(prefix="cursor-multiworker-test-")
+        self.addCleanup(shutil.rmtree, self.tmp, ignore_errors=True)
+        self.addCleanup(set_workspace_path_override, None)
+
+        app = Flask(__name__)
+        app.config["TESTING"] = True
+        app.register_blueprint(config_bp)
+        self.client = app.test_client()
+        self.storage = _make_cursor_workspace_dir(self.tmp)
+
+    def test_multi_worker_returns_409_with_stable_code(self):
+        with patch(
+            "api.config_api.is_multi_worker_process_deployment",
+            return_value=True,
+        ):
+            resp = self.client.post(
+                "/api/set-workspace",
+                json={"path": self.storage},
+            )
+        self.assertEqual(resp.status_code, 409)
+        body = resp.get_json()
+        self.assertEqual(body["code"], "set_workspace_multi_worker_unsupported")
+        self.assertIn("WORKSPACE_PATH", body["error"])
+
+        from utils.workspace_path import get_workspace_path_override
+
+        self.assertIsNone(get_workspace_path_override())
+
+    def test_single_process_still_succeeds_when_not_multi_worker(self):
+        with patch(
+            "api.config_api.is_multi_worker_process_deployment",
+            return_value=False,
+        ):
+            resp = self.client.post(
+                "/api/set-workspace",
+                json={"path": self.storage},
+            )
+        self.assertEqual(resp.status_code, 200)
+        self.assertTrue(resp.get_json()["success"])
+
+
+class TestMultiWorkerDetection(unittest.TestCase):
+    def test_explicit_env_flag(self):
+        from utils.workspace_path import is_multi_worker_process_deployment
+
+        with patch.dict(os.environ, {"CURSOR_BROWSER_MULTI_WORKER": "1"}, clear=False):
+            self.assertTrue(is_multi_worker_process_deployment())
+        with patch.dict(os.environ, {"CURSOR_BROWSER_MULTI_WORKER": "0"}, clear=False):
+            self.assertFalse(is_multi_worker_process_deployment())
+
+    def test_web_concurrency_gt_one(self):
+        from utils.workspace_path import is_multi_worker_process_deployment
+
+        with patch.dict(
+            os.environ,
+            {"WEB_CONCURRENCY": "4", "CURSOR_BROWSER_MULTI_WORKER": ""},
+            clear=False,
+        ):
+            self.assertTrue(is_multi_worker_process_deployment())
+
+    def test_gunicorn_cmd_args_workers(self):
+        from utils.workspace_path import is_multi_worker_process_deployment
+
+        with patch.dict(
+            os.environ,
+            {
+                "GUNICORN_CMD_ARGS": "app:create_app --bind :5000 --workers 3",
+                "CURSOR_BROWSER_MULTI_WORKER": "",
+            },
+            clear=False,
+        ):
+            self.assertTrue(is_multi_worker_process_deployment())
+
+    def test_web_concurrency_one_does_not_block_later_multi_worker_signals(self):
+        from utils.workspace_path import is_multi_worker_process_deployment
+
+        with patch.dict(
+            os.environ,
+            {
+                "WEB_CONCURRENCY": "1",
+                "GUNICORN_WORKERS": "4",
+                "CURSOR_BROWSER_MULTI_WORKER": "",
+            },
+            clear=False,
+        ):
+            self.assertTrue(is_multi_worker_process_deployment())
+
+        with patch.dict(
+            os.environ,
+            {
+                "WEB_CONCURRENCY": "1",
+                "GUNICORN_CMD_ARGS": "app:create_app --workers 2",
+                "CURSOR_BROWSER_MULTI_WORKER": "",
+            },
+            clear=False,
+        ):
+            self.assertTrue(is_multi_worker_process_deployment())
diff --git a/utils/workspace_path.py b/utils/workspace_path.py
index 5d9b364..3adf265 100644
--- a/utils/workspace_path.py
+++ b/utils/workspace_path.py
@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import os
+import re
 import sys
 import subprocess
 import threading
@@ -40,6 +41,39 @@ def get_workspace_path_override() -> str | None:
         return _workspace_path_override
 
 
+def is_multi_worker_process_deployment() -> bool:
+    """Return True when multiple WSGI worker processes may serve requests.
+
+    Used by POST /api/set-workspace to fail loud instead of updating only one
+    process. Single-process and multi-threaded deployments return False.
+
+    Operators can force True/False with ``CURSOR_BROWSER_MULTI_WORKER``. Otherwise
+    ``WEB_CONCURRENCY``, ``GUNICORN_WORKERS``, or ``--workers`` / ``-w`` in
+    ``GUNICORN_CMD_ARGS`` are consulted when present.
+    """
+    flag = os.environ.get("CURSOR_BROWSER_MULTI_WORKER", "").strip().lower()
+    if flag in ("1", "true", "yes"):
+        return True
+    if flag in ("0", "false", "no"):
+        return False
+    for key in ("WEB_CONCURRENCY", "GUNICORN_WORKERS"):
+        raw = os.environ.get(key, "").strip()
+        if raw.isdigit():
+            count = int(raw)
+            if count > 1:
+                return True
+    cmd_args = os.environ.get("GUNICORN_CMD_ARGS", "")
+    if cmd_args:
+        for pattern in (
+            r"(?:--workers|-w)\s+(\d+)",
+            r"(?:--workers|-w)=(\d+)",
+        ):
+            for match in re.finditer(pattern, cmd_args):
+                if int(match.group(1)) > 1:
+                    return True
+    return False
+
+
 def get_default_workspace_path() -> str:
     """Detect the default Cursor workspace storage path based on OS."""
     home = os.path.expanduser("~")