diff --git a/static/index.html b/static/index.html
index d427c53..014880b 100644
--- a/static/index.html
+++ b/static/index.html
@@ -2014,6 +2014,9 @@
                     <option value="image">Image</option>
                   </select>
                 </div>
+                <div class="admin-model-form-row">
+                  <input id="adm-epLocalApiKey" type="password" placeholder="API key (optional — for protected local endpoints)" autocomplete="off" style="flex:1">
+                </div>
                 <div class="admin-model-form-row">
                   <span style="flex:1"></span>
                   <button class="admin-btn-sm" id="adm-epLocalTestBtn" style="width:55px;text-align:center;">Test</button>
diff --git a/static/js/admin.js b/static/js/admin.js
index a551df7..11d2311 100644
--- a/static/js/admin.js
+++ b/static/js/admin.js
@@ -871,11 +871,14 @@ function initEndpointForm() {
       const raw = (el('adm-epLocalUrl').value || '').trim();
       if (!raw) { msg.textContent = 'Enter a base URL to test'; msg.className = 'admin-error'; return; }
       const url = _normalizeBaseUrl(raw);
+      const keyEl = el('adm-epLocalApiKey');
+      const apiKey = keyEl ? keyEl.value.trim() : '';
       localTestBtn.disabled = true;
       localTestBtn.textContent = 'Testing...';
       try {
         const fd = new FormData();
         fd.append('base_url', url);
+        if (apiKey) fd.append('api_key', apiKey);
         const res = await fetch('/api/model-endpoints/test', { method: 'POST', body: fd, credentials: 'same-origin' });
         const d = await res.json();
         _renderEndpointTestResult(msg, res, d);
@@ -894,10 +897,13 @@ function initEndpointForm() {
       const raw = (el('adm-epLocalUrl').value || '').trim();
       if (!raw) { msg.textContent = 'Enter a base URL (e.g. http://localhost:8002/v1)'; msg.className = 'admin-error'; return; }
       const url = _normalizeBaseUrl(raw);
+      const keyEl = el('adm-epLocalApiKey');
+      const apiKey = keyEl ? keyEl.value.trim() : '';
       localAddBtn.disabled = true; localAddBtn.textContent = 'Adding...';
       try {
         const fd = new FormData();
         fd.append('base_url', url);
+        if (apiKey) fd.append('api_key', apiKey);
         const lt = el('adm-epLocalType');
         if (lt) fd.append('model_type', lt.value);
         fd.append('skip_probe', 'false');
@@ -905,6 +911,7 @@ function initEndpointForm() {
         const d = await res.json();
         if (res.ok) {
           el('adm-epLocalUrl').value = '';
+          if (keyEl) keyEl.value = '';
           if (lt) lt.value = 'llm';
           if (d.id) _recentlyAddedEpId = String(d.id);
           await loadEndpoints();
diff --git a/tests/test_local_endpoint_api_key_js.py b/tests/test_local_endpoint_api_key_js.py
new file mode 100644
index 0000000..ed04e1b
--- /dev/null
+++ b/tests/test_local_endpoint_api_key_js.py
@@ -0,0 +1,132 @@
+"""Behavioral test for issue #353 — Local LLM endpoints behind an API key.
+
+The admin "Local" add/test form previously sent only `base_url` (+ model_type),
+so a self-hosted endpoint protected by an API key could never be added — it just
+errored out. The backend `POST /api/model-endpoints` and `/model-endpoints/test`
+already accept an `api_key` form field; the fix wires the new `adm-epLocalApiKey`
+input into the local Test and Add handlers.
+
+admin.js can't be imported standalone (browser-only deps), so — same approach as
+tests/test_local_endpoint_js.py — we extract the two click-handler bodies from
+source and run them under node with mocked DOM/FormData/fetch, asserting the
+outgoing form data contains `api_key` exactly when the key field is filled.
+"""
+import json
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+_REPO = Path(__file__).resolve().parent.parent
+_ADMIN_JS = _REPO / "static" / "js" / "admin.js"
+_INDEX_HTML = _REPO / "static" / "index.html"
+_HAS_NODE = shutil.which("node") is not None
+
+
+def _extract_handler_body(src: str, marker: str) -> str:
+    """Return the body (without the outer braces) of the arrow function that
+    immediately follows `marker` in `src`, using a quote-aware brace matcher."""
+    start = src.index(marker) + len(marker)
+    brace = src.index("{", start)
+    i = brace + 1
+    depth = 1
+    quote = None
+    escaped = False
+    while i < len(src):
+        c = src[i]
+        if quote:
+            if escaped:
+                escaped = False
+            elif c == "\\":
+                escaped = True
+            elif c == quote:
+                quote = None
+        elif c in "'\"`":
+            quote = c
+        elif c == "{":
+            depth += 1
+        elif c == "}":
+            depth -= 1
+            if depth == 0:
+                return src[brace + 1:i]
+        i += 1
+    raise AssertionError(f"unbalanced braces after marker: {marker!r}")
+
+
+_HARNESS = """
+let appended = [];
+class FormData {{ append(k, v) {{ appended.push([k, String(v)]); }} }}
+const FIELDS = {fields};
+function el(id) {{
+  if (!(id in FIELDS)) return null;
+  return {{
+    get value() {{ return FIELDS[id]; }},
+    set value(x) {{ FIELDS[id] = x; }},
+    disabled: false, textContent: '',
+    classList: {{ add() {{}}, remove() {{}} }},
+  }};
+}}
+function _endpointMsg() {{ return {{ textContent: '', className: '' }}; }}
+function _normalizeBaseUrl(u) {{ return u; }}
+function _renderEndpointTestResult() {{}}
+async function loadEndpoints() {{}}
+async function _selectAddedModelInChat() {{}}
+let _recentlyAddedEpId = null;
+const localTestBtn = {{ disabled: false, textContent: '' }};
+const localAddBtn = {{ disabled: false, textContent: '' }};
+async function fetch() {{
+  return {{ ok: true, async json() {{ return {{ id: 'x', models: [], online: true, status: 'ok' }}; }} }};
+}}
+async function run() {{ {body} }}
+run().then(() => console.log(JSON.stringify(appended)))
+     .catch((e) => {{ console.error(e); process.exit(2); }});
+"""
+
+
+def _run_handler(body: str, fields: dict) -> list:
+    js = _HARNESS.format(fields=json.dumps(fields), body=body)
+    proc = subprocess.run(
+        ["node", "--input-type=module"],
+        input=js, capture_output=True, text=True, cwd=str(_REPO), timeout=30,
+    )
+    assert proc.returncode == 0, f"node failed: {proc.stderr}\n---\n{js}"
+    return json.loads(proc.stdout.strip())
+
+
+def _handler(marker: str) -> str:
+    return _extract_handler_body(_ADMIN_JS.read_text(encoding="utf-8"), marker)
+
+
+_TEST_MARKER = "localTestBtn.addEventListener('click', async () => "
+_ADD_MARKER = "localAddBtn.addEventListener('click', async () => "
+
+
+def test_local_form_has_api_key_input():
+    html = _INDEX_HTML.read_text(encoding="utf-8")
+    pos = html.find('id="adm-epLocalApiKey"')
+    assert pos != -1, "adm-epLocalApiKey input missing from index.html"
+    # Isolate the enclosing <input ...> tag and require it to be a masked field,
+    # like the cloud form's API-key input.
+    tag = html[html.rfind("<input", 0, pos):html.index(">", pos) + 1]
+    assert 'type="password"' in tag, f"local API key must be a password input: {tag}"
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+@pytest.mark.parametrize("marker", [_TEST_MARKER, _ADD_MARKER])
+def test_api_key_sent_when_filled(marker):
+    fields = {"adm-epLocalUrl": "http://localhost:8002/v1",
+              "adm-epLocalApiKey": "sk-secret", "adm-epLocalType": "llm"}
+    appended = dict(_run_handler(_handler(marker), fields))
+    assert appended.get("base_url") == "http://localhost:8002/v1"
+    assert appended.get("api_key") == "sk-secret", f"api_key not sent: {appended}"
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+@pytest.mark.parametrize("marker", [_TEST_MARKER, _ADD_MARKER])
+def test_api_key_omitted_when_blank(marker):
+    fields = {"adm-epLocalUrl": "http://localhost:8002/v1",
+              "adm-epLocalApiKey": "", "adm-epLocalType": "llm"}
+    keys = [k for k, _ in _run_handler(_handler(marker), fields)]
+    assert "base_url" in keys
+    assert "api_key" not in keys, "blank key must not be appended (avoids empty Bearer)"