diff --git a/CHANGELOG.md b/CHANGELOG.md
index b4ac9d14..4f4984dc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,12 @@
 
 ## [Unreleased]
 
+## [v0.51.545] — 2026-06-20 — Release TD (extension manifest asset bundles)
+
+### Added
+
+- **Bundle extension assets with a manifest instead of long comma-separated URL lists (#4505).** A new `HERMES_WEBUI_EXTENSION_MANIFEST` env points to a small JSON manifest (capped at 64 KB) that lists an extension's `scripts`/`stylesheets`, so self-hosters running multiple local extensions no longer have to maintain unwieldy `HERMES_WEBUI_EXTENSION_SCRIPT_URLS` strings. Per-extension `enabled: false` skips a bundle. The manifest path is traversal-guarded (must resolve inside the extension dir; absolute/`~` paths rejected), and every asset URL still goes through the exact same same-origin allowlist validator as before (`/extensions/` or `/static/` only — never third-party fetch), capped at 32 entries and deduped. Fully opt-in and default-off; with no manifest env set, behavior is unchanged. Thanks @santastabber.
+
 ## [v0.51.544] — 2026-06-20 — Release TC (opt-in: render markdown in your own messages)
 
 ### Added
diff --git a/README.md b/README.md
index e5348d96..40fcec79 100644
--- a/README.md
+++ b/README.md
@@ -344,8 +344,9 @@ Full list of environment variables:
 | `HERMES_WEBUI_CSP_CONNECT_EXTRA` | *(unset)* | Optional space-separated `http(s)://` or `ws(s)://` origins to append to the report-only CSP `connect-src` directive for reverse-proxy or tunnel deployments |
 | `HERMES_WEBUI_SSE_CHUNKED` | *(unset)* | Set truthy (`1`/`true`/`yes`/`on`) to send SSE with `Transfer-Encoding: chunked`. Needed behind buffering reverse proxies (e.g. `jupyter-server-proxy`) that otherwise buffer the whole stream; harmless but unnecessary for directly-served deployments |
 | `HERMES_WEBUI_EXTENSION_DIR` | *(unset)* | Optional local directory served at `/extensions/`; must point to an existing directory before extension injection is enabled |
-| `HERMES_WEBUI_EXTENSION_SCRIPT_URLS` | *(unset)* | Optional comma-separated same-origin script URLs to inject; see [WebUI Extensions](docs/EXTENSIONS.md) |
-| `HERMES_WEBUI_EXTENSION_STYLESHEET_URLS` | *(unset)* | Optional comma-separated same-origin stylesheet URLs to inject; see [WebUI Extensions](docs/EXTENSIONS.md) |
+| `HERMES_WEBUI_EXTENSION_MANIFEST` | *(unset)* | Optional relative JSON manifest inside `HERMES_WEBUI_EXTENSION_DIR` listing bundled scripts/styles to inject; see [WebUI Extensions](docs/EXTENSIONS.md) |
+| `HERMES_WEBUI_EXTENSION_SCRIPT_URLS` | *(unset)* | Optional comma-separated same-origin script URLs to inject; appended after manifest scripts; see [WebUI Extensions](docs/EXTENSIONS.md) |
+| `HERMES_WEBUI_EXTENSION_STYLESHEET_URLS` | *(unset)* | Optional comma-separated same-origin stylesheet URLs to inject; appended after manifest stylesheets; see [WebUI Extensions](docs/EXTENSIONS.md) |
 | `HERMES_HOME` | Windows: `%LOCALAPPDATA%\hermes`; POSIX: `~/.hermes` | Base directory for Hermes state (affects all paths) |
 | `HERMES_CONFIG_PATH` | `$HERMES_HOME/config.yaml` | Path to Hermes config file |
 | `HERMES_WEBUI_SERVER_CWD` | *(unset)* | Working directory for the server process. Defaults to the agent dir; point it at a writable workspace when the agent dir is read-only so fallback relative writes land somewhere writable |
diff --git a/api/extensions.py b/api/extensions.py
index 5a862b70..17932957 100644
--- a/api/extensions.py
+++ b/api/extensions.py
@@ -6,10 +6,11 @@ It is disabled by default and never executes or fetches third-party URLs.
 """
 
 import html
+import json
 import logging
 import os
 from pathlib import Path
-from typing import Dict, List, Optional
+from typing import Dict, List, Optional, Tuple
 from urllib.parse import unquote, urlsplit
 
 from api.helpers import _security_headers, j
@@ -22,14 +23,24 @@ _log = logging.getLogger(__name__)
 # avoids rendering tens of thousands of <script> tags into every page load.
 _MAX_URL_LIST = 32
 
+# Keep extension manifests small and auditable. The manifest is a convenience for
+# bundling static assets, not a package manager or dependency lockfile.
+_MAX_MANIFEST_BYTES = 64 * 1024
+
 # Tracks rejected URL strings we've already warned about so a misconfigured env
 # var doesn't spam the log on every request that re-reads it.
 _warned_urls: set = set()
 
+
+class _ManifestTooLarge(ValueError):
+    pass
+
+
 EXTENSION_ROUTE_PREFIX = "/extensions/"
 _EXTENSION_DIR_ENV = "HERMES_WEBUI_EXTENSION_DIR"
 _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/")
 
 _EXTENSION_MIME = {
@@ -107,47 +118,185 @@ def _is_safe_asset_url(value: str) -> bool:
     return False
 
 
-def _read_url_list(env_name: str) -> List[str]:
-    raw = os.getenv(env_name, "")
-    urls = []
-    for item in raw.split(","):
-        value = item.strip()
-        if not value:
-            continue
-        if _is_safe_asset_url(value):
-            urls.append(value)
-            if len(urls) >= _MAX_URL_LIST:
-                # Stop accumulating after the cap. Anything past this point
-                # would be silently dropped anyway; logging once makes the
-                # truncation visible to a confused operator.
-                if env_name not in _warned_urls:
-                    _warned_urls.add(env_name)
-                    _log.warning(
-                        "Extension URL list %s truncated at %d entries",
-                        env_name, _MAX_URL_LIST,
-                    )
-                break
-        elif value not in _warned_urls:
-            # First-time-seen invalid URL: log once per process so a typo
-            # in HERMES_WEBUI_EXTENSION_*_URLS doesn't disappear silently.
-            _warned_urls.add(value)
+def _warn_rejected_url(value: str, source: str) -> None:
+    if value in _warned_urls:
+        return
+    _warned_urls.add(value)
+    _log.warning(
+        "Rejected extension URL %r from %s (not a same-origin "
+        "/extensions/ or /static/ path, or contains unsafe chars)",
+        value, source,
+    )
+
+
+def _append_safe_asset_url(
+    urls: List[str], value: str, source: str, *, dedupe: bool = True
+) -> bool:
+    """Append a validated URL while preserving order and the global cap.
+
+    Returns False when the caller should stop accumulating entries for this list.
+    Manifest paths dedupe by default, while env-only lists preserve their legacy
+    behavior unless they are appending after manifest-provided assets.
+    """
+    value = value.strip() if isinstance(value, str) else ""
+    if not value:
+        return True
+    if not _is_safe_asset_url(value):
+        _warn_rejected_url(value, source)
+        return True
+    if dedupe and value in urls:
+        return True
+    if len(urls) >= _MAX_URL_LIST:
+        if source not in _warned_urls:
+            _warned_urls.add(source)
             _log.warning(
-                "Rejected extension URL %r from %s (not a same-origin "
-                "/extensions/ or /static/ path, or contains unsafe chars)",
-                value, env_name,
+                "Extension URL list %s truncated at %d entries",
+                source, _MAX_URL_LIST,
             )
+        return False
+    urls.append(value)
+    return True
+
+
+def _read_url_list(env_name: str, existing: Optional[List[str]] = None) -> List[str]:
+    raw = os.getenv(env_name, "")
+    urls = list(existing or [])
+    # Preserve legacy env-only behavior: duplicate env URLs injected twice before
+    # manifests existed. When a manifest seeds the list, dedupe appended env URLs
+    # so bundle manifests and explicit overrides do not double-load an asset.
+    dedupe = existing is not None
+    for item in raw.split(","):
+        if not _append_safe_asset_url(urls, item, env_name, dedupe=dedupe):
+            break
     return urls
 
 
+def _manifest_path(root: Path) -> Optional[Path]:
+    raw = os.getenv(_EXTENSION_MANIFEST_ENV, "").strip()
+    if not raw:
+        return None
+    if raw.startswith(("/", "~")):
+        _log.warning("Rejected extension manifest path from %s", _EXTENSION_MANIFEST_ENV)
+        return None
+    rel = _fully_unquote_path(raw)
+    if not _is_safe_relative_path(rel):
+        _log.warning("Rejected extension manifest path from %s", _EXTENSION_MANIFEST_ENV)
+        return None
+    manifest = (root / rel).resolve()
+    try:
+        manifest.relative_to(root)
+    except ValueError:
+        _log.warning("Rejected extension manifest path from %s", _EXTENSION_MANIFEST_ENV)
+        return None
+    return manifest
+
+
+def _manifest_asset_url(value: object) -> str:
+    """Normalize a manifest asset entry to the existing same-origin URL format."""
+    if not isinstance(value, str):
+        return ""
+    item = value.strip()
+    if not item:
+        return ""
+    parsed = urlsplit(item)
+    if parsed.scheme or parsed.netloc or item.startswith("//"):
+        return item
+    # Manifests are meant to make bundled local assets less noisy to list, so
+    # bare relative paths resolve under /extensions/. Absolute same-origin paths
+    # are still allowed and go through the same validator as env-configured URLs.
+    if item.startswith("/"):
+        return item
+    return EXTENSION_ROUTE_PREFIX + item
+
+
+def _iter_manifest_entries(manifest: object) -> List[Tuple[str, object]]:
+    entries: List[Tuple[str, object]] = []
+    extension_entries: object = []
+    if isinstance(manifest, dict):
+        entries.append(("manifest", manifest))
+        extension_entries = manifest.get("extensions", [])
+    elif isinstance(manifest, list):
+        extension_entries = manifest
+    if isinstance(extension_entries, list):
+        for index, extension in enumerate(extension_entries):
+            if isinstance(extension, dict) and extension.get("enabled", True) is False:
+                continue
+            entries.append((f"manifest.extensions[{index}]", extension))
+    return entries
+
+
+def _entry_asset_values(entry: Dict[str, object], key: str) -> List[object]:
+    values = entry.get(key, [])
+    return values if isinstance(values, list) else []
+
+
+def _read_manifest_text(manifest_file: Path) -> str:
+    with manifest_file.open("rb") as fh:
+        data = fh.read(_MAX_MANIFEST_BYTES + 1)
+    if len(data) > _MAX_MANIFEST_BYTES:
+        raise _ManifestTooLarge("manifest too large")
+    return data.decode("utf-8")
+
+
+def _read_manifest_urls(root: Path) -> Tuple[List[str], List[str]]:
+    manifest_file = _manifest_path(root)
+    if manifest_file is None:
+        return [], []
+    try:
+        if not manifest_file.exists() or not manifest_file.is_file():
+            _log.warning("Configured extension manifest was not found")
+            return [], []
+        manifest = json.loads(_read_manifest_text(manifest_file))
+    except _ManifestTooLarge:
+        _log.warning("Configured extension manifest exceeds %d bytes", _MAX_MANIFEST_BYTES)
+        return [], []
+    except json.JSONDecodeError:
+        _log.warning("Configured extension manifest is not valid JSON")
+        return [], []
+    except (OSError, UnicodeDecodeError):
+        _log.warning("Configured extension manifest could not be read")
+        return [], []
+
+    scripts: List[str] = []
+    stylesheets: List[str] = []
+    scripts_full = False
+    stylesheets_full = False
+    for _source, entry in _iter_manifest_entries(manifest):
+        if not isinstance(entry, dict):
+            continue
+        if not scripts_full:
+            for value in _entry_asset_values(entry, "scripts"):
+                if not _append_safe_asset_url(
+                    scripts, _manifest_asset_url(value), "manifest:scripts"
+                ):
+                    scripts_full = True
+                    break
+        if not stylesheets_full:
+            for value in _entry_asset_values(entry, "stylesheets"):
+                if not _append_safe_asset_url(
+                    stylesheets, _manifest_asset_url(value), "manifest:stylesheets"
+                ):
+                    stylesheets_full = True
+                    break
+        if scripts_full and stylesheets_full:
+            break
+    return scripts, stylesheets
+
+
 def get_extension_config() -> Dict[str, object]:
     """Return public extension config without exposing filesystem paths."""
-    enabled = _extension_root() is not None
-    if not enabled:
+    root = _extension_root()
+    if root is None:
         return {"enabled": False, "script_urls": [], "stylesheet_urls": []}
+    manifest_scripts, manifest_stylesheets = _read_manifest_urls(root)
     return {
         "enabled": True,
-        "script_urls": _read_url_list(_EXTENSION_SCRIPT_URLS_ENV),
-        "stylesheet_urls": _read_url_list(_EXTENSION_STYLESHEET_URLS_ENV),
+        "script_urls": _read_url_list(
+            _EXTENSION_SCRIPT_URLS_ENV, manifest_scripts or None
+        ),
+        "stylesheet_urls": _read_url_list(
+            _EXTENSION_STYLESHEET_URLS_ENV, manifest_stylesheets or None
+        ),
     }
 
 
diff --git a/docs/EXTENSIONS.md b/docs/EXTENSIONS.md
index 0804c79f..23b0441b 100644
--- a/docs/EXTENSIONS.md
+++ b/docs/EXTENSIONS.md
@@ -23,6 +23,7 @@ Extensions can:
 - serve files from one configured local directory at `/extensions/...`
 - inject configured same-origin stylesheets into `<head>`
 - inject configured same-origin scripts before `</body>`
+- read a local JSON manifest that lists bundled scripts/styles to inject
 - call the normal WebUI APIs available to the browser session
 
 Extensions cannot, by themselves:
@@ -53,14 +54,53 @@ export HERMES_WEBUI_EXTENSION_SCRIPT_URLS=/extensions/runtime.js,/extensions/app
 export HERMES_WEBUI_EXTENSION_STYLESHEET_URLS=/extensions/base.css,/extensions/theme.css
 ```
 
+For bundled or multi-extension installs, you may list assets in a manifest file
+inside `HERMES_WEBUI_EXTENSION_DIR` instead of maintaining long comma-separated
+environment variables:
+
+```bash
+cat > ~/.hermes/webui-extension-bundle/extensions.json <<'JSON'
+{
+  "extensions": [
+    {
+      "id": "templates",
+      "scripts": ["templates/templates.js"],
+      "stylesheets": ["templates/templates.css"]
+    },
+    {
+      "id": "sidebar-tools",
+      "scripts": ["sidebar-tools/sidebar-tools.js"],
+      "stylesheets": ["sidebar-tools/sidebar-tools.css"]
+    }
+  ]
+}
+JSON
+
+HERMES_WEBUI_EXTENSION_DIR=~/.hermes/webui-extension-bundle \
+HERMES_WEBUI_EXTENSION_MANIFEST=extensions.json \
+./start.sh
+```
+
+Manifest entries use the same URL safety rules as the environment variables.
+Bare relative entries such as `templates/templates.js` resolve to
+`/extensions/templates/templates.js`; absolute same-origin entries such as
+`/extensions/shared.js` or `/static/theme.css` are also accepted. A manifest may
+be an object with top-level `scripts` / `stylesheets`, an object with an
+`extensions` array, or a top-level array of extension objects. Disabled entries
+may be kept in the manifest with the JSON boolean `"enabled": false`. Explicit
+`HERMES_WEBUI_EXTENSION_SCRIPT_URLS` and
+`HERMES_WEBUI_EXTENSION_STYLESHEET_URLS` still work and are appended after
+manifest assets, with duplicates ignored.
+
 ## URL rules
 
 Injected asset URLs are deliberately restricted:
 
 - must be same-origin paths
-- must start with `/extensions/` or `/static/`
+- must start with `/extensions/` or `/static/` after manifest normalization
 - must not include a URL scheme, host, fragment, quote, angle bracket, newline,
   NUL byte, or backslash
+- must not contain dot-segments or dotfiles after percent-decoding
 
 Allowed examples:
 
@@ -101,6 +141,8 @@ The static handler is sandboxed:
 - dotfiles and dot-directories are not served
 - symlinks that resolve outside the extension directory are rejected
 - missing or invalid extension directories behave as disabled
+- manifest paths must be relative files inside the configured extension directory
+- malformed, missing, or oversized manifests are ignored without enabling unsafe URLs
 - failures return a generic 404 without exposing local filesystem paths
 
 ## Security notes
diff --git a/tests/test_extension_hooks.py b/tests/test_extension_hooks.py
index f598a182..d78638f2 100644
--- a/tests/test_extension_hooks.py
+++ b/tests/test_extension_hooks.py
@@ -7,8 +7,21 @@ The extension surface must stay deliberately small and safe:
 - static assets sandboxed to the configured extension directory
 """
 
+import logging
 from types import SimpleNamespace
 
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _clear_extension_env(monkeypatch):
+    for name in (
+        "HERMES_WEBUI_EXTENSION_DIR",
+        "HERMES_WEBUI_EXTENSION_MANIFEST",
+        "HERMES_WEBUI_EXTENSION_SCRIPT_URLS",
+        "HERMES_WEBUI_EXTENSION_STYLESHEET_URLS",
+    ):
+        monkeypatch.delenv(name, raising=False)
 
 class FakeHandler:
     def __init__(self):
@@ -127,6 +140,344 @@ def test_extension_route_remains_behind_webui_auth(monkeypatch):
     assert check_auth(static, SimpleNamespace(path="/static/ui.js", query="")) is True
 
 
+
+
+def test_extension_manifest_adds_bundled_assets_before_env_urls(tmp_path, monkeypatch):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    (root / "manifest.json").write_text(
+        """
+        {
+          "scripts": ["runtime.js"],
+          "stylesheets": ["base.css"],
+          "extensions": [
+            {
+              "id": "templates",
+              "scripts": ["templates/app.js", "/extensions/shared.js"],
+              "stylesheets": ["templates/app.css"]
+            },
+            {
+              "id": "disabled",
+              "enabled": false,
+              "scripts": ["disabled.js"],
+              "stylesheets": ["disabled.css"]
+            }
+          ]
+        }
+        """,
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "manifest.json")
+    monkeypatch.setenv(
+        "HERMES_WEBUI_EXTENSION_SCRIPT_URLS",
+        "/extensions/templates/app.js, /extensions/env-only.js",
+    )
+    monkeypatch.setenv(
+        "HERMES_WEBUI_EXTENSION_STYLESHEET_URLS",
+        "/extensions/env-only.css",
+    )
+
+    from api.extensions import get_extension_config
+
+    assert get_extension_config() == {
+        "enabled": True,
+        "script_urls": [
+            "/extensions/runtime.js",
+            "/extensions/templates/app.js",
+            "/extensions/shared.js",
+            "/extensions/env-only.js",
+        ],
+        "stylesheet_urls": [
+            "/extensions/base.css",
+            "/extensions/templates/app.css",
+            "/extensions/env-only.css",
+        ],
+    }
+
+
+def test_extension_manifest_reuses_url_safety_rules(tmp_path, monkeypatch):
+    monkeypatch.delenv("HERMES_WEBUI_EXTENSION_SCRIPT_URLS", raising=False)
+    monkeypatch.delenv("HERMES_WEBUI_EXTENSION_STYLESHEET_URLS", raising=False)
+    root = tmp_path / "extensions"
+    root.mkdir()
+    (root / "bundle.json").write_text(
+        """
+        {
+          "scripts": [
+            "safe.js",
+            "../escape.js",
+            "/api/session",
+            "https://example.com/evil.js",
+            "/extensions/%252e%252e/api/session"
+          ],
+          "stylesheets": ["safe.css", "nested/../escape.css"]
+        }
+        """,
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "bundle.json")
+
+    from api.extensions import get_extension_config
+
+    assert get_extension_config() == {
+        "enabled": True,
+        "script_urls": ["/extensions/safe.js"],
+        "stylesheet_urls": ["/extensions/safe.css"],
+    }
+
+
+def test_extension_manifest_path_must_stay_inside_extension_root(tmp_path, monkeypatch):
+    monkeypatch.delenv("HERMES_WEBUI_EXTENSION_STYLESHEET_URLS", raising=False)
+    root = tmp_path / "extensions"
+    root.mkdir()
+    outside = tmp_path / "outside.json"
+    outside.write_text('{"scripts":["outside.js"]}', encoding="utf-8")
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "../outside.json")
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_SCRIPT_URLS", "/extensions/env.js")
+
+    from api.extensions import get_extension_config
+
+    assert get_extension_config() == {
+        "enabled": True,
+        "script_urls": ["/extensions/env.js"],
+        "stylesheet_urls": [],
+    }
+
+
+def test_extension_manifest_url_list_shares_cap_with_env_urls(tmp_path, monkeypatch):
+    monkeypatch.delenv("HERMES_WEBUI_EXTENSION_STYLESHEET_URLS", raising=False)
+    root = tmp_path / "extensions"
+    root.mkdir()
+    scripts = ",".join(f'"script{i}.js"' for i in range(40))
+    (root / "manifest.json").write_text(f'{{"scripts":[{scripts}]}}', encoding="utf-8")
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "manifest.json")
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_SCRIPT_URLS", "/extensions/env.js")
+
+    from api.extensions import _MAX_URL_LIST, get_extension_config
+
+    config = get_extension_config()
+    assert len(config["script_urls"]) == _MAX_URL_LIST
+    assert config["script_urls"][0] == "/extensions/script0.js"
+    assert config["script_urls"][-1] == f"/extensions/script{_MAX_URL_LIST - 1}.js"
+    assert "/extensions/env.js" not in config["script_urls"]
+
+
+def test_extension_manifest_logs_invalid_json_separately_from_oversize(tmp_path, monkeypatch, caplog):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "manifest.json")
+    (root / "manifest.json").write_text('{"scripts": [', encoding="utf-8")
+
+    from api.extensions import get_extension_config
+
+    caplog.set_level(logging.WARNING, logger="api.extensions")
+
+    assert get_extension_config() == {
+        "enabled": True,
+        "script_urls": [],
+        "stylesheet_urls": [],
+    }
+    assert any("not valid JSON" in record.message for record in caplog.records)
+    assert not any("could not be read" in record.message for record in caplog.records)
+    assert not any("exceeds" in record.message for record in caplog.records)
+
+
+def test_extension_manifest_logs_invalid_utf8_as_unreadable(tmp_path, monkeypatch, caplog):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "manifest.json")
+    (root / "manifest.json").write_bytes(b"{\xff}")
+
+    from api.extensions import get_extension_config
+
+    caplog.set_level(logging.WARNING, logger="api.extensions")
+
+    assert get_extension_config() == {
+        "enabled": True,
+        "script_urls": [],
+        "stylesheet_urls": [],
+    }
+    assert any("could not be read" in record.message for record in caplog.records)
+    assert not any("exceeds" in record.message for record in caplog.records)
+
+
+def test_extension_manifest_logs_oversize_distinctly(tmp_path, monkeypatch, caplog):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "manifest.json")
+
+    from api import extensions
+
+    (root / "manifest.json").write_text(
+        '{"scripts":["pwn.js"]}' + (" " * extensions._MAX_MANIFEST_BYTES),
+        encoding="utf-8",
+    )
+    caplog.set_level(logging.WARNING, logger="api.extensions")
+
+    assert extensions.get_extension_config() == {
+        "enabled": True,
+        "script_urls": [],
+        "stylesheet_urls": [],
+    }
+    assert any("exceeds" in record.message for record in caplog.records)
+    assert not any("not valid JSON" in record.message for record in caplog.records)
+
+
+def test_extension_manifest_reads_only_bounded_size(tmp_path, monkeypatch):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "manifest.json")
+
+    from api import extensions
+
+    # If the bounded read/cap check is removed, this parses as valid JSON and
+    # would inject /extensions/pwn.js. The trailing padding makes it oversize.
+    oversize = '{"scripts":["pwn.js"]}' + (" " * extensions._MAX_MANIFEST_BYTES)
+    (root / "manifest.json").write_text(oversize, encoding="utf-8")
+
+    assert extensions.get_extension_config() == {
+        "enabled": True,
+        "script_urls": [],
+        "stylesheet_urls": [],
+    }
+
+
+
+def test_extension_manifest_multibyte_payload_is_bounded_by_bytes(tmp_path, monkeypatch):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "manifest.json")
+
+    from api import extensions
+
+    oversize_multibyte = '{"scripts":["pwn.js"]}' + ("€" * extensions._MAX_MANIFEST_BYTES)
+    (root / "manifest.json").write_text(oversize_multibyte, encoding="utf-8")
+
+    assert extensions.get_extension_config() == {
+        "enabled": True,
+        "script_urls": [],
+        "stylesheet_urls": [],
+    }
+
+
+def test_extension_manifest_cap_warning_logs_once_across_many_entries(tmp_path, monkeypatch, caplog):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    entries = [
+        {
+            "id": f"ext{i}",
+            "scripts": [f"script{i}.js"],
+            "stylesheets": [f"style{i}.css"],
+        }
+        for i in range(40)
+    ]
+    (root / "manifest.json").write_text(
+        __import__("json").dumps({"extensions": entries}), encoding="utf-8"
+    )
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "manifest.json")
+
+    from api.extensions import _MAX_URL_LIST, _warned_urls, get_extension_config
+
+    _warned_urls.clear()
+    caplog.set_level(logging.WARNING, logger="api.extensions")
+
+    config = get_extension_config()
+
+    assert len(config["script_urls"]) == _MAX_URL_LIST
+    assert len(config["stylesheet_urls"]) == _MAX_URL_LIST
+    truncation_records = [
+        record for record in caplog.records
+        if "truncated at" in record.message and "Extension URL list" in record.message
+    ]
+    assert len(truncation_records) == 2
+    assert any("manifest:scripts" in record.message for record in truncation_records)
+    assert any("manifest:stylesheets" in record.message for record in truncation_records)
+
+def test_extension_manifest_ignores_non_list_asset_fields(tmp_path, monkeypatch):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    (root / "manifest.json").write_text(
+        """
+        {
+          "scripts": "app.js",
+          "stylesheets": {"href": "app.css"},
+          "extensions": {
+            "bad": {"scripts": ["bad.js"]}
+          }
+        }
+        """,
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "manifest.json")
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_SCRIPT_URLS", "/extensions/env.js")
+    monkeypatch.delenv("HERMES_WEBUI_EXTENSION_STYLESHEET_URLS", raising=False)
+
+    from api.extensions import get_extension_config
+
+    assert get_extension_config() == {
+        "enabled": True,
+        "script_urls": ["/extensions/env.js"],
+        "stylesheet_urls": [],
+    }
+
+
+
+def test_extension_env_only_duplicate_urls_preserve_legacy_behavior(tmp_path, monkeypatch):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.delenv("HERMES_WEBUI_EXTENSION_MANIFEST", raising=False)
+    monkeypatch.setenv(
+        "HERMES_WEBUI_EXTENSION_SCRIPT_URLS",
+        "/extensions/app.js, /extensions/app.js",
+    )
+    monkeypatch.delenv("HERMES_WEBUI_EXTENSION_STYLESHEET_URLS", raising=False)
+
+    from api.extensions import get_extension_config
+
+    assert get_extension_config() == {
+        "enabled": True,
+        "script_urls": ["/extensions/app.js", "/extensions/app.js"],
+        "stylesheet_urls": [],
+    }
+
+
+def test_extension_manifest_accepts_top_level_extension_array(tmp_path, monkeypatch):
+    root = tmp_path / "extensions"
+    root.mkdir()
+    (root / "extensions.json").write_text(
+        """
+        [
+          {"id": "a", "scripts": ["a/a.js"], "stylesheets": ["a/a.css"]},
+          {"id": "b", "scripts": ["b/b.js"]}
+        ]
+        """,
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_DIR", str(root))
+    monkeypatch.setenv("HERMES_WEBUI_EXTENSION_MANIFEST", "extensions.json")
+    monkeypatch.delenv("HERMES_WEBUI_EXTENSION_SCRIPT_URLS", raising=False)
+    monkeypatch.delenv("HERMES_WEBUI_EXTENSION_STYLESHEET_URLS", raising=False)
+
+    from api.extensions import get_extension_config
+
+    assert get_extension_config() == {
+        "enabled": True,
+        "script_urls": ["/extensions/a/a.js", "/extensions/b/b.js"],
+        "stylesheet_urls": ["/extensions/a/a.css"],
+    }
+
 def test_extension_static_serving_is_sandboxed(tmp_path, monkeypatch):
     root = tmp_path / "extensions"
     root.mkdir()