diff --git a/api/extensions.py b/api/extensions.py
index 18cd14935..72e2327a9 100644
--- a/api/extensions.py
+++ b/api/extensions.py
@@ -42,6 +42,9 @@ _EXTENSION_SCRIPT_URLS_ENV = "HERMES_WEBUI_EXTENSION_SCRIPT_URLS"
 _EXTENSION_STYLESHEET_URLS_ENV = "HERMES_WEBUI_EXTENSION_STYLESHEET_URLS"
 _EXTENSION_MANIFEST_ENV = "HERMES_WEBUI_EXTENSION_MANIFEST"
 _ALLOWED_ASSET_PREFIXES = ("/extensions/", "/static/")
+_SIDECAR_WARNING_SOURCE = "manifest:sidecars"
+_DEFAULT_SIDECAR_HEALTH_PATH = "/health"
+_LOOPBACK_SIDECAR_HOSTS = {"127.0.0.1", "localhost", "::1"}
 
 _EXTENSION_MIME = {
     "css": "text/css",
@@ -261,6 +264,122 @@ def _manifest_asset_url(value: object) -> str:
     return EXTENSION_ROUTE_PREFIX + item
 
 
+def _manifest_entry_text(entry: Dict[str, object], key: str) -> str:
+    value = entry.get(key)
+    if not isinstance(value, str):
+        return ""
+    return value.strip()
+
+
+def _normalize_loopback_sidecar_origin(value: object) -> Optional[str]:
+    """Return a canonical loopback origin or None when unsafe.
+
+    Only browser-addressable loopback HTTP(S) origins are accepted. The returned
+    value is rebuilt from parsed components so rejected raw input is never echoed
+    into diagnostics.
+    """
+    if not isinstance(value, str):
+        return None
+    origin = value.strip()
+    if not origin or any(
+        ch in origin for ch in ("\x00", "\r", "\n", '"', "'", "<", ">", "\\")
+    ):
+        return None
+    parsed = urlsplit(origin)
+    if parsed.scheme not in ("http", "https") or not parsed.netloc:
+        return None
+    if parsed.username or parsed.password:
+        return None
+    if parsed.path or parsed.query or parsed.fragment:
+        return None
+    host = (parsed.hostname or "").lower()
+    if host not in _LOOPBACK_SIDECAR_HOSTS:
+        return None
+    try:
+        port = parsed.port
+    except ValueError:
+        return None
+    display_host = f"[{host}]" if ":" in host else host
+    return f"{parsed.scheme}://{display_host}{':' + str(port) if port is not None else ''}"
+
+
+def _normalize_sidecar_health_path(value: object) -> Optional[str]:
+    """Return a safe sidecar health path, or None when unsafe.
+
+    Health paths are same-origin paths relative to the validated sidecar origin.
+    Queries are rejected even though they are not cross-origin: health checks are
+    diagnostics, and query strings often accidentally carry tokens.
+    """
+    if not isinstance(value, str):
+        return None
+    path = value.strip()
+    if not path or not path.startswith("/") or path.startswith("//"):
+        return None
+    if any(ch in path for ch in ("\x00", "\r", "\n", '"', "'", "<", ">", "\\")):
+        return None
+    parsed = urlsplit(path)
+    if parsed.scheme or parsed.netloc or parsed.query or parsed.fragment:
+        return None
+    decoded_path = _fully_unquote_path(parsed.path)
+    if any(ch in decoded_path for ch in ("\x00", "\r", "\n", '"', "'", "<", ">", "\\")):
+        return None
+    if any(ch.isspace() for ch in decoded_path):
+        return None
+    if not decoded_path.startswith("/") or decoded_path.startswith("//"):
+        return None
+    segments = decoded_path.split("/")[1:]
+    if not segments:
+        return None
+    for segment in segments:
+        if not segment or segment in (".", ".."):
+            return None
+    return decoded_path
+
+
+def _sidecar_from_manifest_entry(
+    entry: Dict[str, object], diagnostics: Optional[Dict[str, Any]] = None
+) -> Optional[Dict[str, str]]:
+    raw = entry.get("sidecar")
+    if raw is None:
+        return None
+    if not isinstance(raw, dict):
+        _add_diagnostic_warning(diagnostics, "sidecar_invalid", _SIDECAR_WARNING_SOURCE)
+        return None
+    if raw.get("type") != "loopback":
+        _add_diagnostic_warning(
+            diagnostics, "sidecar_type_unsupported", _SIDECAR_WARNING_SOURCE
+        )
+        return None
+    origin = _normalize_loopback_sidecar_origin(raw.get("origin"))
+    if origin is None:
+        _add_diagnostic_warning(
+            diagnostics, "sidecar_origin_rejected", _SIDECAR_WARNING_SOURCE
+        )
+        return None
+    if "health_path" in raw:
+        health_path = _normalize_sidecar_health_path(raw.get("health_path"))
+        if health_path is None:
+            # Missing health_path defaults to /health; an explicitly invalid path
+            # rejects the sidecar so the browser does not probe a declaration the
+            # administrator needs to fix.
+            _add_diagnostic_warning(
+                diagnostics, "sidecar_health_path_rejected", _SIDECAR_WARNING_SOURCE
+            )
+            return None
+    else:
+        health_path = _DEFAULT_SIDECAR_HEALTH_PATH
+    sidecar_id = _manifest_entry_text(entry, "id")
+    name = _manifest_entry_text(entry, "name")
+    return {
+        "id": sidecar_id,
+        "name": name,
+        "type": "loopback",
+        "origin": origin,
+        "health_path": health_path,
+        "health_url": f"{origin}{health_path}",
+    }
+
+
 def _iter_manifest_entries(manifest: object) -> List[Tuple[str, object]]:
     entries: List[Tuple[str, object]] = []
     extension_entries: object = []
@@ -294,7 +413,7 @@ def _read_manifest_text(manifest_file: Path) -> str:
 
 def _read_manifest_urls_with_diagnostics(
     root: Path, diagnostics: Optional[Dict[str, Any]] = None
-) -> Tuple[List[str], List[str], Dict[str, Any]]:
+) -> Tuple[List[str], List[str], List[Dict[str, str]], Dict[str, Any]]:
     manifest_file, path_status = _manifest_path_with_status(root)
     manifest_status: Dict[str, Any] = {
         "configured": path_status != "not_configured",
@@ -303,28 +422,29 @@ def _read_manifest_urls_with_diagnostics(
         "entry_count": 0,
         "script_count": 0,
         "stylesheet_count": 0,
+        "sidecar_count": 0,
     }
     if manifest_file is None:
         if path_status == "invalid_path":
             _add_diagnostic_warning(diagnostics, "manifest_invalid_path", "manifest")
-        return [], [], manifest_status
+        return [], [], [], manifest_status
     try:
         if not manifest_file.exists() or not manifest_file.is_file():
             _log.warning("Configured extension manifest was not found")
             manifest_status["status"] = "missing"
             _add_diagnostic_warning(diagnostics, "manifest_missing", "manifest")
-            return [], [], manifest_status
+            return [], [], [], manifest_status
         manifest = json.loads(_read_manifest_text(manifest_file))
     except _ManifestTooLarge:
         _log.warning("Configured extension manifest exceeds %d bytes", _MAX_MANIFEST_BYTES)
         manifest_status["status"] = "oversized"
         _add_diagnostic_warning(diagnostics, "manifest_oversized", "manifest")
-        return [], [], manifest_status
+        return [], [], [], manifest_status
     except json.JSONDecodeError:
         _log.warning("Configured extension manifest is not valid JSON")
         manifest_status["status"] = "malformed"
         _add_diagnostic_warning(diagnostics, "manifest_malformed", "manifest")
-        return [], [], manifest_status
+        return [], [], [], manifest_status
     except RecursionError:
         # A <=64KB but deeply-nested manifest makes json.loads exceed the
         # interpreter recursion limit. Without this, the RecursionError escapes
@@ -332,15 +452,16 @@ def _read_manifest_urls_with_diagnostics(
         _log.warning("Configured extension manifest is too deeply nested")
         manifest_status["status"] = "too_deeply_nested"
         _add_diagnostic_warning(diagnostics, "manifest_too_deeply_nested", "manifest")
-        return [], [], manifest_status
+        return [], [], [], manifest_status
     except (OSError, UnicodeDecodeError):
         _log.warning("Configured extension manifest could not be read")
         manifest_status["status"] = "unreadable"
         _add_diagnostic_warning(diagnostics, "manifest_unreadable", "manifest")
-        return [], [], manifest_status
+        return [], [], [], manifest_status
 
     scripts: List[str] = []
     stylesheets: List[str] = []
+    sidecars: List[Dict[str, str]] = []
     entries = _iter_manifest_entries(manifest)
     manifest_status["entry_count"] = len(entries)
     scripts_full = False
@@ -348,6 +469,15 @@ def _read_manifest_urls_with_diagnostics(
     for _source, entry in entries:
         if not isinstance(entry, dict):
             continue
+        if _source.startswith("manifest.extensions["):
+            sidecar = _sidecar_from_manifest_entry(entry, diagnostics)
+            if sidecar is not None:
+                if len(sidecars) < _MAX_URL_LIST:
+                    sidecars.append(sidecar)
+                else:
+                    _add_diagnostic_warning(
+                        diagnostics, "sidecar_list_truncated", _SIDECAR_WARNING_SOURCE
+                    )
         script_source = "manifest:scripts"
         stylesheet_source = "manifest:stylesheets"
         if not scripts_full:
@@ -370,21 +500,20 @@ def _read_manifest_urls_with_diagnostics(
                 ):
                     stylesheets_full = True
                     break
-        if scripts_full and stylesheets_full:
-            break
     manifest_status.update(
         {
             "loaded": True,
             "status": "loaded",
             "script_count": len(scripts),
             "stylesheet_count": len(stylesheets),
+            "sidecar_count": len(sidecars),
         }
     )
-    return scripts, stylesheets, manifest_status
+    return scripts, stylesheets, sidecars, manifest_status
 
 
 def _read_manifest_urls(root: Path) -> Tuple[List[str], List[str]]:
-    scripts, stylesheets, _ = _read_manifest_urls_with_diagnostics(root)
+    scripts, stylesheets, _, _ = _read_manifest_urls_with_diagnostics(root)
     return scripts, stylesheets
 
 
@@ -417,6 +546,7 @@ def get_extension_status() -> Dict[str, Any]:
         "entry_count": 0,
         "script_count": 0,
         "stylesheet_count": 0,
+        "sidecar_count": 0,
     }
     if dir_configured and not dir_valid:
         _add_diagnostic_warning(diagnostics, "extension_dir_unavailable", "extension_dir")
@@ -428,12 +558,13 @@ def get_extension_status() -> Dict[str, Any]:
             "extension_dir_valid": False,
             "script_urls": [],
             "stylesheet_urls": [],
-            "counts": {"script_urls": 0, "stylesheet_urls": 0},
+            "sidecars": [],
+            "counts": {"script_urls": 0, "stylesheet_urls": 0, "sidecars": 0},
             "manifest": manifest_status,
             "warnings": diagnostics["warnings"],
         }
 
-    manifest_scripts, manifest_stylesheets, manifest_status = _read_manifest_urls_with_diagnostics(
+    manifest_scripts, manifest_stylesheets, sidecars, manifest_status = _read_manifest_urls_with_diagnostics(
         root, diagnostics
     )
     script_urls = _read_url_list(
@@ -452,9 +583,11 @@ def get_extension_status() -> Dict[str, Any]:
         "extension_dir_valid": True,
         "script_urls": script_urls,
         "stylesheet_urls": stylesheet_urls,
+        "sidecars": sidecars,
         "counts": {
             "script_urls": len(script_urls),
             "stylesheet_urls": len(stylesheet_urls),
+            "sidecars": len(sidecars),
         },
         "manifest": manifest_status,
         "warnings": diagnostics["warnings"],
diff --git a/docs/EXTENSIONS.md b/docs/EXTENSIONS.md
index 6727154db..f6997077b 100644
--- a/docs/EXTENSIONS.md
+++ b/docs/EXTENSIONS.md
@@ -95,6 +95,30 @@ may be kept in the manifest with the JSON boolean `"enabled": false`. Explicit
 `HERMES_WEBUI_EXTENSION_STYLESHEET_URLS` still work and are appended after
 manifest assets, with duplicates ignored.
 
+Extension entries may also declare a read-only loopback sidecar for diagnostics:
+
+```json
+{
+  "extensions": [
+    {
+      "id": "desktop-companion",
+      "name": "Desktop Companion",
+      "scripts": ["companion-adapter.js"],
+      "stylesheets": ["companion-adapter.css"],
+      "sidecar": {
+        "type": "loopback",
+        "origin": "http://127.0.0.1:17787",
+        "health_path": "/health"
+      }
+    }
+  ]
+}
+```
+
+Loopback sidecars do **not** change asset injection behavior. They are only
+reported by diagnostics so an operator can see that a local companion service was
+declared and optionally check its health from the browser.
+
 ## URL rules
 
 Injected asset URLs are deliberately restricted:
@@ -153,10 +177,7 @@ control, append the exact origin with `HERMES_WEBUI_CSP_CONNECT_EXTRA` before
 starting WebUI:
 
 ```bash
-HERMES_WEBUI_CSP_CONNECT_EXTRA=https://companion.example.internal \
-HERMES_WEBUI_EXTENSION_DIR=/path/to/my-extension/static \
-HERMES_WEBUI_EXTENSION_MANIFEST=extensions.json \
-./start.sh
+HERMES_WEBUI_CSP_CONNECT_EXTRA=https://companion.example.internal HERMES_WEBUI_EXTENSION_DIR=/path/to/my-extension/static HERMES_WEBUI_EXTENSION_MANIFEST=extensions.json ./start.sh
 ```
 
 `HERMES_WEBUI_CSP_CONNECT_EXTRA` accepts space-separated `http(s)://` or
@@ -164,6 +185,26 @@ HERMES_WEBUI_EXTENSION_MANIFEST=extensions.json \
 numbers. Avoid wildcard or remote origins unless you fully control the target;
 extension JavaScript runs with the logged-in WebUI session's authority.
 
+## Loopback sidecar declarations
+
+Sidecar declarations are sanitized before they appear in diagnostics:
+
+- only `"type": "loopback"` is supported
+- `origin` must be an `http` or `https` origin on `127.0.0.1`, `localhost`, or
+  `[::1]`
+- `origin` must not include a username, password, path, query string, or fragment
+- `health_path` is optional and defaults to `/health`
+- when present, `health_path` must start with `/` and must not contain a scheme,
+  host, query string, fragment, quotes, control characters, backslashes, empty
+  segments, whitespace, or path traversal
+
+Invalid sidecars are skipped with a stable warning code such as
+`sidecar_origin_rejected`, `sidecar_type_unsupported`,
+`sidecar_health_path_rejected`, or `sidecar_invalid`. Raw rejected origins and
+paths are never returned by the status endpoint. If `health_path` is omitted,
+diagnostics use `/health`; if `health_path` is present but invalid, the sidecar is
+skipped rather than probed.
+
 ## Static file serving
 
 When `HERMES_WEBUI_EXTENSION_DIR` points at an existing directory, files under
@@ -307,12 +348,22 @@ not enable, disable, install, or mutate extensions; it only fetches
 `/api/extensions/status` and offers a copy-diagnostics action.
 
 The diagnostics return the same public asset URLs that can already be injected
-into the HTML, plus coarse manifest status, asset counts, and warning codes for
-rejected or unavailable configuration. `manifest.script_count` and
-`manifest.stylesheet_count` count accepted assets from the manifest only;
-`counts.script_urls` and `counts.stylesheet_urls` count the final post-env-merge
-URLs. `manifest.entry_count` counts the loaded top-level manifest object and
-enabled extension entries that were inspected, not every extension object in the
-file. The endpoint and Settings panel do **not** return
-`HERMES_WEBUI_EXTENSION_DIR`, resolved manifest paths, raw environment values, or
-rejected URL strings.
+into the HTML, plus coarse manifest status, asset counts, sanitized declared
+loopback sidecars, and warning codes for rejected or unavailable configuration.
+`manifest.script_count` and `manifest.stylesheet_count` count accepted assets
+from the manifest only; `manifest.sidecar_count` counts accepted enabled loopback
+sidecars from the manifest. `counts.script_urls` and `counts.stylesheet_urls`
+count the final post-env-merge URLs, while `counts.sidecars` counts the sanitized
+sidecar list returned in `sidecars`. `manifest.entry_count` counts the loaded
+top-level manifest object and enabled extension entries that were inspected, not
+every extension object in the file. The endpoint and Settings panel do **not**
+return `HERMES_WEBUI_EXTENSION_DIR`, resolved manifest paths, raw environment
+values, rejected URL strings, rejected sidecar origins, or rejected health paths.
+
+When sanitized loopback sidecars are present, **Settings → Extensions** renders a
+read-only sidecar monitor card. The browser checks each declared `health_url`
+directly with `fetch(..., { credentials: 'omit', cache: 'no-store' })` and a
+short timeout. WebUI does **not** proxy sidecar requests and does not send WebUI
+cookies to sidecars. A successful HTTP response is shown as healthy, a non-OK
+HTTP response as unhealthy, and CORS/network/timeouts as unreachable or blocked;
+health response bodies are never rendered.
diff --git a/static/panels.js b/static/panels.js
index 871c18a1c..8fc6a4a0b 100644
--- a/static/panels.js
+++ b/static/panels.js
@@ -6223,6 +6223,7 @@ let _settingsIndex = null;
 let _settingsIndexPromise = null;
 let _settingsSearchSeq = 0;
 let _extensionsStatusData = null;
+let _extensionsSidecarMonitorSeq = 0;
 let _settingsSearchDismissListenerRegistered = false;
 let _settingsAppearanceAutosaveTimer = null;
 let _settingsAppearanceAutosaveRetryPayload = null;
@@ -7536,7 +7537,84 @@ function _extensionCountValue(counts,key,urls){
   return Array.isArray(urls)?urls.length:0;
 }
 
-function _renderExtensionsPanel(data){
+function _extensionSidecarHealthBadge(status,label){
+  const safeStatus=['checking','healthy','unhealthy','blocked'].includes(status)?status:'checking';
+  return `<span class="extension-sidecar-status-badge extension-sidecar-status-${safeStatus}">${esc(label||safeStatus)}</span>`;
+}
+
+function _extensionSidecarCard(sidecars){
+  const list=Array.isArray(sidecars)?sidecars:[];
+  const body=list.length?`<div class="extension-sidecar-list">${list.map((sidecar,index)=>{
+    const id=(sidecar&&sidecar.id)||'';
+    const name=(sidecar&&sidecar.name)||'';
+    const title=name||id||'Unnamed extension';
+    const meta=(name&&id)?id:(sidecar&&sidecar.type)||'loopback';
+    const origin=(sidecar&&sidecar.origin)||'';
+    const healthPath=(sidecar&&sidecar.health_path)||'';
+    const healthUrl=(sidecar&&sidecar.health_url)||'';
+    return `<div class="extension-sidecar-row" data-sidecar-index="${index}">
+      <div class="extension-sidecar-row-head">
+        <div class="extension-sidecar-title">${esc(title)}</div>
+        <span id="extensionSidecarHealth${index}" data-sidecar-health-index="${index}">${_extensionSidecarHealthBadge('checking','checking')}</span>
+      </div>
+      <div class="extension-sidecar-meta">${esc(meta)}</div>
+      <div class="extension-sidecar-fields">
+        <div><span>Origin</span><code>${esc(origin)}</code></div>
+        <div><span>Health path</span><code>${esc(healthPath)}</code></div>
+        <div><span>Health URL</span><code>${esc(healthUrl)}</code></div>
+      </div>
+    </div>`;
+  }).join('')}</div>`:'<div class="extension-url-empty">No loopback sidecars declared.</div>';
+  return `
+    <div class="provider-card extension-sidecars-card">
+      <div class="provider-card-header plugin-card-header">
+        <div class="provider-card-info">
+          <div class="provider-card-name">Loopback sidecars</div>
+          <div class="provider-card-meta">Declared local companions; health is checked directly from this browser with WebUI credentials omitted.</div>
+        </div>
+      </div>
+      <div class="provider-card-body extension-card-body">
+        ${body}
+      </div>
+    </div>`;
+}
+
+function _setExtensionSidecarHealth(index,status,label){
+  const el=document.querySelector(`[data-sidecar-health-index="${index}"]`);
+  if(el) el.innerHTML=_extensionSidecarHealthBadge(status,label);
+}
+
+async function _checkExtensionSidecarHealth(sidecar,index,seq){
+  const healthUrl=sidecar&&sidecar.health_url;
+  if(!healthUrl){
+    _setExtensionSidecarHealth(index,'blocked','unreachable / blocked');
+    return;
+  }
+  let controller=null;
+  let timeoutId=null;
+  try{
+    if(typeof AbortController!=='undefined'){
+      controller=new AbortController();
+      timeoutId=setTimeout(()=>controller.abort(),2500);
+    }
+    const res=await fetch(healthUrl,{credentials:'omit',cache:'no-store',signal:controller?controller.signal:undefined});
+    if(seq!==_extensionsSidecarMonitorSeq) return;
+    if(res.ok) _setExtensionSidecarHealth(index,'healthy','healthy');
+    else _setExtensionSidecarHealth(index,'unhealthy','unhealthy');
+  }catch(_e){
+    if(seq!==_extensionsSidecarMonitorSeq) return;
+    _setExtensionSidecarHealth(index,'blocked','unreachable / blocked');
+  }finally{
+    if(timeoutId) clearTimeout(timeoutId);
+  }
+}
+
+function _monitorExtensionSidecars(sidecars,seq){
+  if(!Array.isArray(sidecars)||sidecars.length===0) return;
+  sidecars.forEach((sidecar,index)=>_checkExtensionSidecarHealth(sidecar,index,seq));
+}
+
+function _renderExtensionsPanel(data,seq){
   const target=$('extensionsDiagnostics');
   const copyBtn=$('extensionsCopyDiagnosticsBtn');
   if(!target) return;
@@ -7546,9 +7624,11 @@ function _renderExtensionsPanel(data){
   const counts=(data&&data.counts)||{};
   const scripts=Array.isArray(data&&data.script_urls)?data.script_urls:[];
   const styles=Array.isArray(data&&data.stylesheet_urls)?data.stylesheet_urls:[];
+  const sidecars=Array.isArray(data&&data.sidecars)?data.sidecars:[];
   const statusClass=(data&&data.enabled)?'extension-card-enabled':'extension-card-disabled';
   const scriptCount=_extensionCountValue(counts,'script_urls',scripts);
   const styleCount=_extensionCountValue(counts,'stylesheet_urls',styles);
+  const sidecarCount=_extensionCountValue(counts,'sidecars',sidecars);
   target.innerHTML=`
     <div class="provider-card extension-status-card ${statusClass}">
       <div class="provider-card-header plugin-card-header">
@@ -7568,8 +7648,10 @@ function _renderExtensionsPanel(data){
           <div><span>Manifest entries inspected</span><code>${Number(manifest.entry_count)||0}</code></div>
           <div><span>Manifest script count</span><code>${Number(manifest.script_count)||0}</code></div>
           <div><span>Manifest stylesheet count</span><code>${Number(manifest.stylesheet_count)||0}</code></div>
+          <div><span>Manifest sidecar count</span><code>${Number(manifest.sidecar_count)||0}</code></div>
           <div><span>Final script count</span><code>${scriptCount}</code></div>
           <div><span>Final stylesheet count</span><code>${styleCount}</code></div>
+          <div><span>Loopback sidecar count</span><code>${sidecarCount}</code></div>
         </div>
       </div>
     </div>
@@ -7587,6 +7669,7 @@ function _renderExtensionsPanel(data){
         ${_extensionAssetList(styles)}
       </div>
     </div>
+    ${_extensionSidecarCard(sidecars)}
     <div class="provider-card extension-warnings-card">
       <div class="provider-card-header plugin-card-header">
         <div class="provider-card-info">
@@ -7599,6 +7682,7 @@ function _renderExtensionsPanel(data){
       </div>
     </div>
   `;
+  _monitorExtensionSidecars(sidecars,seq);
 }
 
 async function loadExtensionsPanel(){
@@ -7606,10 +7690,12 @@ async function loadExtensionsPanel(){
   const copyBtn=$('extensionsCopyDiagnosticsBtn');
   if(!target) return;
   if(copyBtn) copyBtn.disabled=true;
+  const seq=++_extensionsSidecarMonitorSeq;
   target.innerHTML='<div class="extensions-loading">Loading extension diagnostics…</div>';
   try{
     const data=await api('/api/extensions/status');
-    _renderExtensionsPanel(data);
+    if(seq!==_extensionsSidecarMonitorSeq) return;
+    _renderExtensionsPanel(data,seq);
   }catch(e){
     _extensionsStatusData=null;
     if(copyBtn) copyBtn.disabled=true;
diff --git a/static/style.css b/static/style.css
index 08583fab7..77c1490ca 100644
--- a/static/style.css
+++ b/static/style.css
@@ -4592,9 +4592,25 @@ main.main > #mainPlugin{display:none;}
 .extension-url-list code,.extension-warning-list code{font-family:var(--font-mono);font-size:11px;color:var(--text);overflow-wrap:anywhere;word-break:break-word;}
 .extension-warning-list span{font-size:11px;color:var(--muted);overflow-wrap:anywhere;}
 .extension-url-empty{margin-top:6px;font-size:12px;color:var(--muted);font-style:italic;}
+.extension-sidecar-list{display:flex;flex-direction:column;gap:8px;}
+.extension-sidecar-row{padding:9px 10px;border:1px solid var(--border);border-radius:8px;background:var(--surface);min-width:0;}
+.extension-sidecar-row-head{display:flex;align-items:center;justify-content:space-between;gap:10px;min-width:0;}
+.extension-sidecar-title{font-size:13px;font-weight:600;color:var(--text);overflow-wrap:anywhere;min-width:0;}
+.extension-sidecar-meta{margin-top:2px;font-size:11px;color:var(--muted);overflow-wrap:anywhere;}
+.extension-sidecar-fields{display:grid;grid-template-columns:1fr;gap:5px;margin-top:8px;}
+.extension-sidecar-fields>div{display:flex;align-items:flex-start;gap:8px;min-width:0;font-size:12px;color:var(--muted);}
+.extension-sidecar-fields span{flex:0 0 82px;}
+.extension-sidecar-fields code{font-family:var(--font-mono);font-size:11px;color:var(--text);overflow-wrap:anywhere;word-break:break-word;}
+.extension-sidecar-status-badge{display:inline-flex;align-items:center;white-space:nowrap;border-radius:999px;padding:2px 7px;font-size:10.5px;font-weight:700;font-family:var(--font-mono);line-height:1.45;background:var(--code-bg);color:var(--muted);}
+.extension-sidecar-status-healthy{background:rgba(34,197,94,.14);color:#22c55e;}
+.extension-sidecar-status-unhealthy{background:rgba(245,158,11,.14);color:#f59e0b;}
+.extension-sidecar-status-blocked{background:rgba(239,68,68,.14);color:var(--error);}
 @media (max-width: 768px){
   .extensions-copy-diagnostics-btn{width:100%;}
   .extension-summary-grid{grid-template-columns:1fr;}
+  .extension-sidecar-row-head{align-items:flex-start;flex-direction:column;}
+  .extension-sidecar-fields>div{flex-direction:column;gap:2px;}
+  .extension-sidecar-fields span{flex:0 0 auto;}
 }
 
 /* ── Provider model tags ── */
diff --git a/tests/test_extension_status_endpoint.py b/tests/test_extension_status_endpoint.py
index f174f6ec0..c62da385f 100644
--- a/tests/test_extension_status_endpoint.py
+++ b/tests/test_extension_status_endpoint.py
@@ -58,7 +58,8 @@ def test_extension_status_disabled_by_default():
         "extension_dir_valid": False,
         "script_urls": [],
         "stylesheet_urls": [],
-        "counts": {"script_urls": 0, "stylesheet_urls": 0},
+        "sidecars": [],
+        "counts": {"script_urls": 0, "stylesheet_urls": 0, "sidecars": 0},
         "manifest": {
             "configured": False,
             "loaded": False,
@@ -66,6 +67,7 @@ def test_extension_status_disabled_by_default():
             "entry_count": 0,
             "script_count": 0,
             "stylesheet_count": 0,
+            "sidecar_count": 0,
         },
         "warnings": [],
     }
@@ -124,7 +126,8 @@ def test_extension_status_reports_loaded_manifest_counts_and_urls(tmp_path, monk
         "/extensions/base.css",
         "/extensions/templates/app.css",
     ]
-    assert status["counts"] == {"script_urls": 3, "stylesheet_urls": 2}
+    assert status["sidecars"] == []
+    assert status["counts"] == {"script_urls": 3, "stylesheet_urls": 2, "sidecars": 0}
     assert status["manifest"] == {
         "configured": True,
         "loaded": True,
@@ -132,6 +135,7 @@ def test_extension_status_reports_loaded_manifest_counts_and_urls(tmp_path, monk
         "entry_count": 2,
         "script_count": 2,
         "stylesheet_count": 2,
+        "sidecar_count": 0,
     }
     assert status["warnings"] == []
 
@@ -163,6 +167,8 @@ def test_extension_status_ignores_non_dict_manifest_extensions_in_entry_count(
     assert status["script_urls"] == ["/extensions/templates/app.js"]
     assert status["manifest"]["entry_count"] == 2
     assert status["manifest"]["script_count"] == 1
+    assert status["manifest"]["sidecar_count"] == 0
+    assert status["sidecars"] == []
     assert status["warnings"] == []
 
 
@@ -332,6 +338,281 @@ def test_extension_status_reports_rejected_env_assets_without_rejected_values(tm
     assert "env.js" not in rendered
 
 
+def test_extension_status_reports_sanitized_loopback_sidecars(tmp_path, monkeypatch):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    (root / "extensions.json").write_text(
+        """
+        {
+          "extensions": [
+            {
+              "id": "desktop-companion",
+              "name": "Desktop Companion",
+              "scripts": ["companion-adapter.js"],
+              "stylesheets": ["companion-adapter.css"],
+              "sidecar": {
+                "type": "loopback",
+                "origin": "http://127.0.0.1:17787",
+                "health_path": "/health"
+              }
+            },
+            {
+              "id": "implicit-health",
+              "sidecar": {
+                "type": "loopback",
+                "origin": "http://localhost:17788"
+              }
+            },
+            {
+              "id": "ipv6-loopback",
+              "sidecar": {
+                "type": "loopback",
+                "origin": "http://[::1]:17789",
+                "health_path": "/ready"
+              }
+            }
+          ]
+        }
+        """,
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "extensions.json")
+
+    from api.extensions import get_extension_status
+
+    status = get_extension_status()
+    assert status["script_urls"] == ["/extensions/companion-adapter.js"]
+    assert status["stylesheet_urls"] == ["/extensions/companion-adapter.css"]
+    assert status["sidecars"] == [
+        {
+            "id": "desktop-companion",
+            "name": "Desktop Companion",
+            "type": "loopback",
+            "origin": "http://127.0.0.1:17787",
+            "health_path": "/health",
+            "health_url": "http://127.0.0.1:17787/health",
+        },
+        {
+            "id": "implicit-health",
+            "name": "",
+            "type": "loopback",
+            "origin": "http://localhost:17788",
+            "health_path": "/health",
+            "health_url": "http://localhost:17788/health",
+        },
+        {
+            "id": "ipv6-loopback",
+            "name": "",
+            "type": "loopback",
+            "origin": "http://[::1]:17789",
+            "health_path": "/ready",
+            "health_url": "http://[::1]:17789/ready",
+        },
+    ]
+    assert status["counts"]["sidecars"] == 3
+    assert status["manifest"]["sidecar_count"] == 3
+    assert status["warnings"] == []
+
+
+def test_extension_status_skips_disabled_sidecar_entries(tmp_path, monkeypatch):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    (root / "extensions.json").write_text(
+        """
+        {
+          "extensions": [
+            {
+              "id": "off",
+              "enabled": false,
+              "sidecar": {"type": "loopback", "origin": "http://127.0.0.1:17787"}
+            }
+          ]
+        }
+        """,
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "extensions.json")
+
+    from api.extensions import get_extension_status
+
+    status = get_extension_status()
+    assert status["sidecars"] == []
+    # The top-level manifest object is still inspected; the disabled extension
+    # entry is skipped and must not contribute a sidecar.
+    assert status["manifest"]["entry_count"] == 1
+    assert status["manifest"]["sidecar_count"] == 0
+    assert status["warnings"] == []
+
+
+def test_extension_status_rejects_non_loopback_sidecars_without_raw_value_leak(tmp_path, monkeypatch):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    (root / "extensions.json").write_text(
+        """
+        {
+          "extensions": [
+            {
+              "id": "bad-origin",
+              "sidecar": {
+                "type": "loopback",
+                "origin": "http://10.0.0.5:17787",
+                "health_path": "/health"
+              }
+            }
+          ]
+        }
+        """,
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "extensions.json")
+
+    from api.extensions import get_extension_status
+
+    status = get_extension_status()
+    assert status["sidecars"] == []
+    assert status["manifest"]["sidecar_count"] == 0
+    assert status["warnings"] == [
+        {"code": "sidecar_origin_rejected", "source": "manifest:sidecars"}
+    ]
+    rendered = repr(status)
+    assert "10.0.0.5" not in rendered
+    assert "17787" not in rendered
+
+
+def test_extension_status_rejects_invalid_sidecar_health_path_without_leak(tmp_path, monkeypatch):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    (root / "extensions.json").write_text(
+        """
+        {
+          "extensions": [
+            {
+              "id": "bad-health",
+              "sidecar": {
+                "type": "loopback",
+                "origin": "http://127.0.0.1:17787",
+                "health_path": "/../secret-health"
+              }
+            }
+          ]
+        }
+        """,
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "extensions.json")
+
+    from api.extensions import get_extension_status
+
+    status = get_extension_status()
+    assert status["sidecars"] == []
+    assert status["manifest"]["sidecar_count"] == 0
+    assert status["warnings"] == [
+        {"code": "sidecar_health_path_rejected", "source": "manifest:sidecars"}
+    ]
+    assert "secret-health" not in repr(status)
+
+
+def test_extension_status_rejects_decoded_whitespace_sidecar_health_path(tmp_path, monkeypatch):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    (root / "extensions.json").write_text(
+        """
+        {
+          "extensions": [
+            {
+              "id": "bad-health-space",
+              "sidecar": {
+                "type": "loopback",
+                "origin": "http://127.0.0.1:17787",
+                "health_path": "/health%20check"
+              }
+            }
+          ]
+        }
+        """,
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "extensions.json")
+
+    from api.extensions import get_extension_status
+
+    status = get_extension_status()
+    assert status["sidecars"] == []
+    assert status["warnings"] == [
+        {"code": "sidecar_health_path_rejected", "source": "manifest:sidecars"}
+    ]
+    rendered = repr(status)
+    assert "health check" not in rendered
+    assert "health%20check" not in rendered
+
+
+def test_extension_status_rejects_unsupported_sidecar_type_without_origin_probe(tmp_path, monkeypatch):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    (root / "extensions.json").write_text(
+        """
+        {
+          "extensions": [
+            {
+              "id": "unsupported",
+              "sidecar": {"type": "unix-socket", "origin": "http://127.0.0.1:17787"}
+            }
+          ]
+        }
+        """,
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "extensions.json")
+
+    from api.extensions import get_extension_status
+
+    status = get_extension_status()
+    assert status["sidecars"] == []
+    assert status["warnings"] == [
+        {"code": "sidecar_type_unsupported", "source": "manifest:sidecars"}
+    ]
+
+
+def test_extension_status_truncates_many_sidecars_with_sanitized_warning(tmp_path, monkeypatch):
+    import json
+
+    root = tmp_path / "extensions"
+    root.mkdir()
+    entries = [
+        {
+            "id": f"sidecar-{index}",
+            "sidecar": {
+                "type": "loopback",
+                "origin": f"http://127.0.0.1:{18000 + index}",
+            },
+        }
+        for index in range(40)
+    ]
+    (root / "extensions.json").write_text(
+        json.dumps({"extensions": entries}),
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "extensions.json")
+
+    from api.extensions import get_extension_status
+
+    status = get_extension_status()
+    assert len(status["sidecars"]) == 32
+    assert status["counts"]["sidecars"] == 32
+    assert status["manifest"]["sidecar_count"] == 32
+    assert status["warnings"] == [
+        {"code": "sidecar_list_truncated", "source": "manifest:sidecars"}
+    ]
+
+
+
 def test_extension_status_route_is_wired(monkeypatch):
     from api import routes
 
diff --git a/tests/test_extensions_settings_panel.py b/tests/test_extensions_settings_panel.py
index c0e85cdfe..5e5177aee 100644
--- a/tests/test_extensions_settings_panel.py
+++ b/tests/test_extensions_settings_panel.py
@@ -109,8 +109,11 @@ def test_extensions_panel_renders_sanitized_status_payload():
     assert "manifest.entry_count" in render_block
     assert "manifest.script_count" in render_block
     assert "manifest.stylesheet_count" in render_block
+    assert "manifest.sidecar_count" in render_block
     assert "script_urls" in render_block
     assert "stylesheet_urls" in render_block
+    assert "data&&data.sidecars" in render_block
+    assert "_extensionSidecarCard(sidecars)" in render_block
     assert "data&&data.warnings" in render_block
     assert "esc(url)" in asset_block
     assert "esc(manifest.status||'unknown')" in render_block
@@ -119,6 +122,36 @@ def test_extensions_panel_renders_sanitized_status_payload():
     assert "Rejected" not in render_block  # rejected values must never be rendered directly
 
 
+def test_extensions_panel_renders_loopback_sidecar_monitor_safely():
+    sidecar_block = _between("function _extensionSidecarCard", "function _setExtensionSidecarHealth")
+    monitor_block = _between("async function _checkExtensionSidecarHealth", "function _renderExtensionsPanel")
+    render_block = _between("function _renderExtensionsPanel", "async function loadExtensionsPanel")
+    load_block = _between("async function loadExtensionsPanel", "async function copyExtensionsDiagnostics")
+
+    assert "Loopback sidecars" in sidecar_block
+    assert "No loopback sidecars declared." in sidecar_block
+    assert "esc(title)" in sidecar_block
+    assert "esc(meta)" in sidecar_block
+    assert "esc(origin)" in sidecar_block
+    assert "esc(healthPath)" in sidecar_block
+    assert "esc(healthUrl)" in sidecar_block
+    assert "fetch(healthUrl,{credentials:'omit',cache:'no-store'" in monitor_block
+    assert "function _monitorExtensionSidecars(sidecars,seq)" in monitor_block
+    assert "const seq=_extensionsSidecarMonitorSeq" not in monitor_block
+    assert "_monitorExtensionSidecars(sidecars,seq)" in render_block
+    assert "function _renderExtensionsPanel(data,seq)" in render_block
+    assert "const seq=++_extensionsSidecarMonitorSeq" in load_block
+    assert "if(seq!==_extensionsSidecarMonitorSeq) return;" in load_block
+    assert "_renderExtensionsPanel(data,seq)" in load_block
+    assert "res.ok" in monitor_block
+    assert "res.text" not in monitor_block
+    assert "res.json" not in monitor_block
+    assert "api('/api/settings'" not in monitor_block
+    assert "api('/extensions/" not in monitor_block
+    assert "sidecar/*" not in monitor_block
+    assert not _contains_post_method(monitor_block)
+
+
 def test_copy_extensions_diagnostics_copies_current_sanitized_payload():
     copy_block = _between("function copyExtensionsDiagnostics", "// ── Plugins panel")
 
@@ -135,6 +168,8 @@ def test_extensions_styles_are_scoped_to_extensions_panel():
     assert ".extension-summary-grid" in STYLE_CSS
     assert ".extension-warning-list" in STYLE_CSS
     assert ".extension-url-list" in STYLE_CSS
+    assert ".extension-sidecar-list" in STYLE_CSS
+    assert ".extension-sidecar-status-badge" in STYLE_CSS
 
 
 def test_extensions_tab_i18n_key_exists_for_all_locales():
@@ -151,4 +186,8 @@ def test_extensions_docs_mentions_settings_panel_without_mutation_claims():
     assert "Settings → Extensions" in diagnostics_section
     assert "enable, disable, install, or mutate extensions" in diagnostics_section
     assert "`/api/extensions/status`" in diagnostics_section
-    assert "do **not** return" in diagnostics_section
+    assert "sanitized loopback sidecars" in diagnostics_section
+    assert "credentials: 'omit'" in diagnostics_section
+    assert "does **not** proxy sidecar requests" in diagnostics_section
+    assert "do **not**" in diagnostics_section
+    assert "return `HERMES_WEBUI_EXTENSION_DIR`" in diagnostics_section