diff --git a/static/boot.js b/static/boot.js
index 7bc20206..526caa24 100644
--- a/static/boot.js
+++ b/static/boot.js
@@ -2104,9 +2104,17 @@ $('btnDownload').onclick=()=>{
   const a=document.createElement('a');a.href=URL.createObjectURL(blob);
   a.download=`hermes-${S.session.session_id}.md`;a.click();URL.revokeObjectURL(a.href);
 };
+function _buildSessionExportUrl(sessionId,params){
+  const url=new URL('api/session/export',document.baseURI||location.href);
+  url.searchParams.set('session_id',String(sessionId||''));
+  Object.entries(params||{}).forEach(([key,value])=>{
+    if(value!==undefined&&value!==null)url.searchParams.set(key,String(value));
+  });
+  return url.href;
+}
 $('btnExportJSON').onclick=()=>{
   if(!S.session)return;
-  const url=`/api/session/export?session_id=${encodeURIComponent(S.session.session_id)}`;
+  const url=_buildSessionExportUrl(S.session.session_id);
   const a=document.createElement('a');a.href=url;
   a.download=`hermes-${S.session.session_id}.json`;a.click();
 };
@@ -2183,7 +2191,7 @@ function exportSessionHTML(session){
   // Drop empties so the inlined fallback keeps working for anything we couldn't read.
   const clean={};for(const k in palette){if(palette[k])clean[k]=palette[k];}
   const paletteB64=btoa(unescape(encodeURIComponent(JSON.stringify(clean))));
-  const url=`/api/session/export?session_id=${encodeURIComponent(sid)}&format=html&theme=${theme}&palette=${encodeURIComponent(paletteB64)}`;
+  const url=_buildSessionExportUrl(sid,{format:'html',theme,palette:paletteB64});
   const a=document.createElement('a');a.href=url;
   a.download=`hermes-${sid}.html`;a.click();
 }
diff --git a/tests/test_session_export_subpath_urls.py b/tests/test_session_export_subpath_urls.py
new file mode 100644
index 00000000..52e2b01a
--- /dev/null
+++ b/tests/test_session_export_subpath_urls.py
@@ -0,0 +1,165 @@
+"""Regression coverage for browser session-export URLs under subpath mounts."""
+
+from __future__ import annotations
+
+import json
+import re
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+
+ROOT = Path(__file__).parent.parent
+BOOT_JS = ROOT / "static" / "boot.js"
+INDEX_HTML = ROOT / "static" / "index.html"
+NODE = shutil.which("node")
+
+
+def _boot_source() -> str:
+    return BOOT_JS.read_text(encoding="utf-8")
+
+
+def _function_source(source: str, name: str) -> str:
+    """Extract one product function for a small, browser-free execution harness."""
+    marker = f"function {name}("
+    start = source.find(marker)
+    assert start >= 0, f"{name} must be defined in product source"
+    brace = source.find("{", start)
+    assert brace >= 0
+    depth = 0
+    for index in range(brace, len(source)):
+        if source[index] == "{":
+            depth += 1
+        elif source[index] == "}":
+            depth -= 1
+            if depth == 0:
+                return source[start : index + 1]
+    raise AssertionError(f"unterminated {name}")
+
+
+def _onclick_source(source: str, element_id: str) -> str:
+    marker = f"$('{element_id}').onclick="
+    start = source.find(marker)
+    assert start >= 0, f"{element_id} must have an export click handler"
+    arrow = source.find("()=>", start)
+    assert arrow >= 0
+    expression_start = arrow + len("()=>")
+    if source[expression_start] != "{":
+        end = source.find(";", expression_start)
+        assert end >= 0
+        return source[arrow:end]
+    brace = expression_start
+    assert brace >= 0
+    depth = 0
+    for index in range(brace, len(source)):
+        if source[index] == "{":
+            depth += 1
+        elif source[index] == "}":
+            depth -= 1
+            if depth == 0:
+                return source[arrow : index + 1]
+    raise AssertionError(f"unterminated {element_id} click handler")
+
+
+def _run_export_actions(base_uri: str) -> list[dict[str, str]]:
+    source = _boot_source()
+    helper = _function_source(source, "_buildSessionExportUrl")
+    html_export = _function_source(source, "exportSessionHTML")
+    json_handler = _onclick_source(source, "btnExportJSON")
+    html_handler = _onclick_source(source, "btnExportHTML")
+    program = json.dumps(
+        f"{helper}\n{html_export}\nreturn [{json_handler}, {html_handler}];"
+    )
+    script = f"""
+const document = {{
+  baseURI: {json.dumps(base_uri)},
+  documentElement: {{classList: {{contains: () => true}}}},
+  createElement: () => ({{click() {{ clicked.push(this); }}}}),
+}};
+const location = {{href: {json.dumps(base_uri)}}};
+const clicked = [];
+const elements = {{btnExportJSON: {{}}, btnExportHTML: {{}}}};
+const $ = (id) => elements[id];
+const S = {{session: {{session_id: 'session /?&=✓'}}}};
+const getComputedStyle = () => ({{getPropertyValue: () => ''}});
+const handlers = new Function(
+  'document', 'location', 'URL', '$', 'S', 'getComputedStyle', 'btoa', 'unescape',
+  {program},
+)(document, location, URL, $, S, getComputedStyle, btoa, unescape);
+handlers[0]();
+handlers[1]();
+const urls = clicked.map((anchor) => {{
+  const url = new URL(anchor.href);
+  return {{
+  pathname: url.pathname,
+  session_id: url.searchParams.get('session_id'),
+  format: url.searchParams.get('format') || '',
+  theme: url.searchParams.get('theme') || '',
+  palette: Boolean(url.searchParams.get('palette')),
+  }};
+}});
+console.log(JSON.stringify(urls));
+"""
+    result = subprocess.run(
+        [NODE, "-e", script],
+        cwd=ROOT,
+        check=True,
+        capture_output=True,
+        text=True,
+    )
+    return json.loads(result.stdout)
+
+
+@pytest.mark.skipif(NODE is None, reason="node is required for browser URL checks")
+def test_export_builder_keeps_root_and_subpath_mounts() -> None:
+    for base_uri, expected_path in (
+        ("https://example.test/", "/api/session/export"),
+        ("https://example.test/hermes-webui/", "/hermes-webui/api/session/export"),
+    ):
+        json_export, html_export = _run_export_actions(base_uri)
+        assert json_export == {
+            "pathname": expected_path,
+            "session_id": "session /?&=✓",
+            "format": "",
+            "theme": "",
+            "palette": False,
+        }
+        assert html_export == {
+            "pathname": expected_path,
+            "session_id": "session /?&=✓",
+            "format": "html",
+            "theme": "dark",
+            "palette": True,
+        }
+
+
+@pytest.mark.skipif(NODE is None, reason="node is required for browser URL checks")
+def test_export_actions_keep_the_app_base_from_a_session_deep_link() -> None:
+    source = INDEX_HTML.read_text(encoding="utf-8")
+    match = re.search(
+        r"<script>(\(function\(\)\{var path=location\.pathname.*?\}\)\(\))</script>",
+        source,
+    )
+    assert match, "index.html must establish its document base before app scripts load"
+    script = f"""
+let emitted = '';
+const location = {{origin: 'https://example.test', pathname: '/hermes-webui/session/deep-link'}};
+const document = {{write(value) {{ emitted += value; }}}};
+{match.group(1)};
+console.log(emitted);
+"""
+    result = subprocess.run(
+        [NODE, "-e", script],
+        cwd=ROOT,
+        check=True,
+        capture_output=True,
+        text=True,
+    )
+    base_match = re.search(r'href="([^"]+)"', result.stdout)
+    assert base_match
+
+    json_export, html_export = _run_export_actions(base_match.group(1))
+    assert json_export["pathname"] == "/hermes-webui/api/session/export"
+    assert html_export["pathname"] == "/hermes-webui/api/session/export"