diff --git a/app.py b/app.py
index a89f801..c3794dd 100644
--- a/app.py
+++ b/app.py
@@ -661,6 +661,10 @@ app.include_router(setup_session_routes(session_manager, session_config, webhook
 from routes.admin_wipe_routes import setup_admin_wipe_routes
 app.include_router(setup_admin_wipe_routes(session_manager))
 
+# Addons / manifest modloader
+from routes.addon_routes import setup_addon_routes
+app.include_router(setup_addon_routes(auth_manager))
+
 # Memory
 from routes.memory.memory_routes import setup_memory_routes
 memory_router = setup_memory_routes(memory_manager, session_manager, memory_vector=memory_vector)
diff --git a/routes/addon_routes.py b/routes/addon_routes.py
new file mode 100644
index 0000000..e8b6216
--- /dev/null
+++ b/routes/addon_routes.py
@@ -0,0 +1,104 @@
+"""Admin API for Odysseus manifest addons."""
+
+from fastapi import APIRouter, HTTPException, Request
+from pydantic import BaseModel, Field
+
+from src.addons import (
+    addon_template,
+    delete_addon,
+    get_addon,
+    get_enabled_addon_contributions,
+    list_addons,
+    save_addon_manifest,
+    set_addon_enabled,
+)
+
+
+class AddonManifestRequest(BaseModel):
+    manifest: dict = Field(default_factory=dict)
+
+
+class AddonEnabledRequest(BaseModel):
+    enabled: bool
+
+
+def setup_addon_routes(auth_manager) -> APIRouter:
+    router = APIRouter(prefix="/api/addons", tags=["addons"])
+
+    def _require_admin(request: Request) -> str:
+        token = request.cookies.get("odysseus_session")
+        user = auth_manager.get_username_for_token(token)
+        if not user:
+            raise HTTPException(401, "Not authenticated")
+        if not auth_manager.is_admin(user):
+            raise HTTPException(403, "Admin only")
+        return user
+
+    @router.get("")
+    async def list_all(request: Request):
+        _require_admin(request)
+        return {
+            "addons": list_addons(),
+            "enabled_contributions": get_enabled_addon_contributions(),
+        }
+
+    @router.get("/template")
+    async def get_template(request: Request, addon_id: str = "my-addon"):
+        _require_admin(request)
+        try:
+            return {"manifest": addon_template(addon_id)}
+        except ValueError as exc:
+            raise HTTPException(400, str(exc)) from exc
+
+    @router.post("")
+    async def create_or_update(body: AddonManifestRequest, request: Request):
+        _require_admin(request)
+        try:
+            addon = save_addon_manifest(body.manifest)
+        except ValueError as exc:
+            raise HTTPException(400, str(exc)) from exc
+        return {"ok": True, "addon": addon}
+
+    @router.get("/{addon_id}")
+    async def get_one(addon_id: str, request: Request):
+        _require_admin(request)
+        try:
+            addon = get_addon(addon_id)
+        except ValueError as exc:
+            raise HTTPException(400, str(exc)) from exc
+        if not addon:
+            raise HTTPException(404, "Addon not found")
+        return {"addon": addon}
+
+    @router.patch("/{addon_id}")
+    async def update_one(addon_id: str, body: AddonManifestRequest, request: Request):
+        _require_admin(request)
+        manifest = dict(body.manifest or {})
+        manifest["id"] = addon_id
+        try:
+            addon = save_addon_manifest(manifest)
+        except ValueError as exc:
+            raise HTTPException(400, str(exc)) from exc
+        return {"ok": True, "addon": addon}
+
+    @router.post("/{addon_id}/enabled")
+    async def set_enabled(addon_id: str, body: AddonEnabledRequest, request: Request):
+        _require_admin(request)
+        try:
+            addon = set_addon_enabled(addon_id, body.enabled)
+        except KeyError as exc:
+            raise HTTPException(404, "Addon not found") from exc
+        except ValueError as exc:
+            raise HTTPException(400, str(exc)) from exc
+        return {"ok": True, "addon": addon}
+
+    @router.delete("/{addon_id}")
+    async def delete_one(addon_id: str, request: Request):
+        _require_admin(request)
+        try:
+            deleted = delete_addon(addon_id)
+        except ValueError as exc:
+            raise HTTPException(400, str(exc)) from exc
+        return {"ok": deleted}
+
+    return router
diff --git a/src/addons.py b/src/addons.py
new file mode 100644
index 0000000..a30c796
--- /dev/null
+++ b/src/addons.py
@@ -0,0 +1,280 @@
+"""Manifest-based Addon loader for Odysseus.
+
+Addons are intentionally declarative in this first layer. They can describe
+capabilities and contribute prompt/UI metadata, but Odysseus does not execute
+arbitrary addon code automatically.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import re
+import shutil
+import uuid
+from pathlib import Path
+from typing import Any, Dict, List, Optional
+
+from src.constants import ADDONS_DIR, ADDONS_FILE
+
+ADDON_SCHEMA_VERSION = 1
+_ID_RE = re.compile(r"^[a-z0-9][a-z0-9_.-]{1,63}$")
+_ALLOWED_CONTRIBUTIONS = {
+    "agent_prompt",
+    "tool_hint",
+    "settings_panel",
+    "ui_panel",
+    "api_proxy",
+    "skill_template",
+}
+
+
+def _atomic_write_json(path: str, data: Any, *, indent: Optional[int] = None) -> None:
+    os.makedirs(os.path.dirname(path) or ".", exist_ok=True)
+    tmp = f"{path}.tmp.{os.getpid()}"
+    with open(tmp, "w", encoding="utf-8") as f:
+        json.dump(data, f, indent=indent, ensure_ascii=False)
+        f.flush()
+        os.fsync(f.fileno())
+    os.replace(tmp, path)
+
+
+def _safe_addon_id(value: Any) -> str:
+    raw = str(value or "").strip().lower()
+    if "/" in raw or "\\" in raw or ".." in raw:
+        raise ValueError("Addon id must not contain path separators")
+    addon_id = raw
+    addon_id = re.sub(r"[^a-z0-9_.-]+", "-", addon_id).strip(".-_")
+    if not addon_id:
+        addon_id = f"addon-{uuid.uuid4().hex[:8]}"
+    if not _ID_RE.match(addon_id):
+        raise ValueError("Addon id must be 2-64 chars and use a-z, 0-9, dot, dash, or underscore")
+    return addon_id
+
+
+def _addon_dir(addon_id: str) -> Path:
+    safe_id = _safe_addon_id(addon_id)
+    base = Path(ADDONS_DIR).resolve()
+    path = (base / safe_id).resolve()
+    if base not in path.parents and path != base:
+        raise ValueError("Invalid addon path")
+    return path
+
+
+def _default_manifest(addon_id: str) -> Dict[str, Any]:
+    return {
+        "schema_version": ADDON_SCHEMA_VERSION,
+        "id": addon_id,
+        "name": addon_id.replace("-", " ").replace("_", " ").title(),
+        "version": "0.1.0",
+        "description": "",
+        "author": "",
+        "enabled": False,
+        "entrypoints": {},
+        "contributions": [],
+        "permissions": [],
+    }
+
+
+def _normalize_contribution(item: Any) -> Optional[Dict[str, Any]]:
+    if not isinstance(item, dict):
+        return None
+    kind = str(item.get("type") or item.get("kind") or "").strip()
+    if kind not in _ALLOWED_CONTRIBUTIONS:
+        return None
+    normalized = dict(item)
+    normalized["type"] = kind
+    if "id" in normalized:
+        normalized["id"] = str(normalized["id"])[:80]
+    return normalized
+
+
+def normalize_manifest(manifest: Dict[str, Any], *, existing_enabled: Optional[bool] = None) -> Dict[str, Any]:
+    if not isinstance(manifest, dict):
+        raise ValueError("Addon manifest must be a JSON object")
+    if not manifest.get("id") and not manifest.get("name"):
+        raise ValueError("Addon manifest requires id or name")
+    addon_id = _safe_addon_id(manifest.get("id") or manifest.get("name"))
+    normalized = _default_manifest(addon_id)
+    normalized.update({
+        "schema_version": int(manifest.get("schema_version") or ADDON_SCHEMA_VERSION),
+        "id": addon_id,
+        "name": str(manifest.get("name") or normalized["name"])[:120],
+        "version": str(manifest.get("version") or "0.1.0")[:40],
+        "description": str(manifest.get("description") or "")[:1000],
+        "author": str(manifest.get("author") or "")[:120],
+        "enabled": bool(existing_enabled if existing_enabled is not None else manifest.get("enabled", False)),
+        "entrypoints": manifest.get("entrypoints") if isinstance(manifest.get("entrypoints"), dict) else {},
+        "permissions": manifest.get("permissions") if isinstance(manifest.get("permissions"), list) else [],
+    })
+    contributions = manifest.get("contributions") if isinstance(manifest.get("contributions"), list) else []
+    normalized["contributions"] = [
+        c for c in (_normalize_contribution(item) for item in contributions) if c is not None
+    ]
+    return normalized
+
+
+def _load_registry() -> Dict[str, Dict[str, Any]]:
+    if not os.path.exists(ADDONS_FILE):
+        return {}
+    try:
+        with open(ADDONS_FILE, "r", encoding="utf-8") as f:
+            raw = json.load(f)
+    except (OSError, json.JSONDecodeError):
+        return {}
+    items = raw.get("addons", raw) if isinstance(raw, dict) else raw
+    if not isinstance(items, list):
+        return {}
+    registry: Dict[str, Dict[str, Any]] = {}
+    for item in items:
+        try:
+            addon = normalize_manifest(item)
+            registry[addon["id"]] = addon
+        except Exception:
+            continue
+    return registry
+
+
+def _save_registry(registry: Dict[str, Dict[str, Any]]) -> None:
+    _atomic_write_json(ADDONS_FILE, {"addons": list(registry.values())}, indent=2)
+
+
+def scan_addons() -> List[Dict[str, Any]]:
+    """Load addon manifests from disk and merge stored enabled state."""
+    os.makedirs(ADDONS_DIR, exist_ok=True)
+    registry = _load_registry()
+    changed = False
+    for child in sorted(Path(ADDONS_DIR).iterdir()):
+        if not child.is_dir():
+            continue
+        manifest_path = child / "addon.json"
+        if not manifest_path.exists():
+            continue
+        try:
+            with open(manifest_path, "r", encoding="utf-8") as f:
+                manifest = json.load(f)
+            addon_id = _safe_addon_id(manifest.get("id") or child.name)
+            enabled = registry.get(addon_id, {}).get("enabled")
+            addon = normalize_manifest({**manifest, "id": addon_id}, existing_enabled=enabled)
+            addon["path"] = str(manifest_path)
+            registry[addon_id] = addon
+            changed = True
+        except Exception as exc:
+            registry[child.name] = {
+                **_default_manifest(child.name),
+                "name": child.name,
+                "enabled": False,
+                "error": str(exc),
+                "path": str(manifest_path),
+            }
+            changed = True
+    if changed:
+        _save_registry(registry)
+    return list(registry.values())
+
+
+def list_addons() -> List[Dict[str, Any]]:
+    return sorted(scan_addons(), key=lambda item: item.get("name", "").lower())
+
+
+def get_addon(addon_id: str) -> Optional[Dict[str, Any]]:
+    safe_id = _safe_addon_id(addon_id)
+    for item in list_addons():
+        if item.get("id") == safe_id:
+            return item
+    return None
+
+
+def save_addon_manifest(manifest: Dict[str, Any]) -> Dict[str, Any]:
+    registry = _load_registry()
+    if not manifest.get("id") and not manifest.get("name"):
+        raise ValueError("Addon manifest requires id or name")
+    existing = registry.get(_safe_addon_id(manifest.get("id") or manifest.get("name")))
+    addon = normalize_manifest(manifest, existing_enabled=existing.get("enabled") if existing else None)
+    addon_dir = _addon_dir(addon["id"])
+    addon_dir.mkdir(parents=True, exist_ok=True)
+    manifest_path = addon_dir / "addon.json"
+    _atomic_write_json(str(manifest_path), addon, indent=2)
+    addon["path"] = str(manifest_path)
+    registry[addon["id"]] = addon
+    _save_registry(registry)
+    return addon
+
+
+def set_addon_enabled(addon_id: str, enabled: bool) -> Dict[str, Any]:
+    registry = _load_registry()
+    addon = get_addon(addon_id)
+    if not addon:
+        raise KeyError(addon_id)
+    addon["enabled"] = bool(enabled)
+    registry[addon["id"]] = addon
+    _save_registry(registry)
+    manifest_path = _addon_dir(addon["id"]) / "addon.json"
+    if manifest_path.exists():
+        persisted = dict(addon)
+        persisted.pop("path", None)
+        _atomic_write_json(str(manifest_path), persisted, indent=2)
+    return addon
+
+
+def delete_addon(addon_id: str) -> bool:
+    safe_id = _safe_addon_id(addon_id)
+    registry = _load_registry()
+    existed = safe_id in registry
+    registry.pop(safe_id, None)
+    _save_registry(registry)
+    addon_dir = _addon_dir(safe_id)
+    if addon_dir.exists():
+        shutil.rmtree(addon_dir)
+        existed = True
+    return existed
+
+
+def addon_template(addon_id: str = "my-addon") -> Dict[str, Any]:
+    safe_id = _safe_addon_id(addon_id)
+    manifest = _default_manifest(safe_id)
+    manifest.update({
+        "name": safe_id.replace("-", " ").title(),
+        "description": "Describe what this addon changes or adds.",
+        "contributions": [
+            {
+                "type": "agent_prompt",
+                "id": f"{safe_id}.guidance",
+                "text": "Short, specific guidance the agent may use when this addon is enabled.",
+            },
+            {
+                "type": "settings_panel",
+                "id": f"{safe_id}.settings",
+                "title": "Addon Settings",
+                "description": "Describe configurable options for this addon.",
+            },
+        ],
+    })
+    return manifest
+
+
+def get_enabled_addon_contributions() -> List[Dict[str, Any]]:
+    contributions: List[Dict[str, Any]] = []
+    for addon in list_addons():
+        if not addon.get("enabled") or addon.get("error"):
+            continue
+        for contribution in addon.get("contributions", []):
+            item = dict(contribution)
+            item["addon_id"] = addon["id"]
+            item["addon_name"] = addon.get("name") or addon["id"]
+            contributions.append(item)
+    return contributions
+
+
+def get_addons_prompt() -> str:
+    lines: List[str] = []
+    for item in get_enabled_addon_contributions():
+        if item.get("type") not in {"agent_prompt", "tool_hint", "skill_template"}:
+            continue
+        text = str(item.get("text") or item.get("description") or "").strip()
+        if not text:
+            continue
+        lines.append(f"- {item.get('addon_name')} ({item.get('type')}): {text[:1200]}")
+    if not lines:
+        return ""
+    return "Enabled Odysseus addons may contribute these untrusted instructions or hints:\n" + "\n".join(lines)
diff --git a/src/agent_loop.py b/src/agent_loop.py
index ebc2a99..810a25b 100644
--- a/src/agent_loop.py
+++ b/src/agent_loop.py
@@ -2005,6 +2005,18 @@ def _build_system_prompt(
         except Exception as _integ_err:
             logger.debug(f"Integration prompt injection skipped: {_integ_err}")
 
+        try:
+            from src.addons import get_addons_prompt
+            _addons_prompt = get_addons_prompt()
+            if _addons_prompt:
+                _integ_message = untrusted_context_message(
+                    "integrations and addons",
+                    ((_integ_message or {}).get("content", "") + "\n\n" + _addons_prompt).strip()
+                    if _integ_message else _addons_prompt,
+                )
+        except Exception as _addon_err:
+            logger.debug(f"Addon prompt injection skipped: {_addon_err}")
+
     # MCP tool descriptions — sourced from external servers, must not be in system role.
     if mcp_mgr:
         try:
diff --git a/src/constants.py b/src/constants.py
index eceeb6e..c38f190 100644
--- a/src/constants.py
+++ b/src/constants.py
@@ -27,6 +27,7 @@ AUTH_FILE = os.path.join(DATA_DIR, "auth.json")
 USER_PREFS_FILE = os.path.join(DATA_DIR, "user_prefs.json")
 PRESETS_FILE = os.path.join(DATA_DIR, "presets.json")
 INTEGRATIONS_FILE = os.path.join(DATA_DIR, "integrations.json")
+ADDONS_FILE = os.path.join(DATA_DIR, "addons.json")
 CONTACTS_FILE = os.path.join(DATA_DIR, "contacts.json")
 APP_KEY_FILE = os.path.join(DATA_DIR, ".app_key")
 EMBEDDING_ENDPOINT_FILE = os.path.join(DATA_DIR, "embedding_endpoint.json")
@@ -51,6 +52,7 @@ GENERATED_IMAGES_DIR = os.path.join(DATA_DIR, "generated_images")
 TTS_CACHE_DIR = os.path.join(DATA_DIR, "tts_cache")
 EMAIL_URGENCY_CACHE_DIR = os.path.join(DATA_DIR, "email_urgency_cache")
 SKILLS_DIR = os.path.join(DATA_DIR, "skills")
+ADDONS_DIR = os.path.join(DATA_DIR, "addons")
 GALLERY_DIR = os.path.join(DATA_DIR, "gallery")
 GALLERY_UPLOADS_DIR = os.path.join(DATA_DIR, "gallery_uploads")
 MEMORY_VECTORS_DIR = os.path.join(DATA_DIR, "memory_vectors")
diff --git a/static/index.html b/static/index.html
index cb30e84..898c747 100644
--- a/static/index.html
+++ b/static/index.html
@@ -1405,6 +1405,10 @@
           </button>
           <div class="settings-sidebar-divider admin-only"></div>
           <div class="settings-sidebar-label admin-only">Admin</div>
+          <button class="settings-nav-item admin-only" data-settings-tab="addons">
+            <svg width="15" height="15" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M12 2l3 6 6 .9-4.5 4.4 1.1 6.2L12 16.6 6.4 19.5l1.1-6.2L3 8.9 9 8z"/></svg>
+            <span>Addons</span>
+          </button>
           <button class="settings-nav-item admin-only" data-settings-tab="tools">
             <svg width="15" height="15" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M14.7 6.3a1 1 0 0 0 0 1.4l1.6 1.6a1 1 0 0 0 1.4 0l3.77-3.77a6 6 0 0 1-7.94 7.94l-6.91 6.91a2.12 2.12 0 0 1-3-3l6.91-6.91a6 6 0 0 1 7.94-7.94l-3.76 3.76z"/></svg>
             <span>Agent Tools</span>
@@ -2262,6 +2266,35 @@
         </div>
 
         <!-- ═══ TOOLS TAB ═══ -->
+        <!-- ADDONS TAB -->
+        <div data-settings-panel="addons" class="hidden">
+          <div class="admin-card">
+            <h2 style="display:flex;align-items:center;gap:8px;">
+              <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" style="opacity:0.6"><path d="M12 2l3 6 6 .9-4.5 4.4 1.1 6.2L12 16.6 6.4 19.5l1.1-6.2L3 8.9 9 8z"/></svg>
+              Addons
+              <span style="flex:1"></span>
+              <button class="admin-btn-sm" id="addons-refresh-btn" title="Rescan addons">Refresh</button>
+            </h2>
+            <div class="admin-toggle-sub" style="margin-bottom:12px">Manifest-based extensions loaded from <code>data/addons/&lt;id&gt;/addon.json</code>. Addons are declarative by default; Odysseus does not execute arbitrary addon code automatically.</div>
+            <div id="addons-list" class="addons-list"><div class="admin-empty">Loading addons...</div></div>
+          </div>
+          <div class="admin-card">
+            <h2 style="display:flex;align-items:center;gap:8px;">
+              <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" style="opacity:0.6"><path d="M14 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V8z"/><path d="M14 2v6h6"/><path d="M12 18v-6"/><path d="M9 15h6"/></svg>
+              Addon Builder
+            </h2>
+            <div class="admin-toggle-sub" style="margin-bottom:10px">Create or edit an addon manifest. This is the stable contract future addon code can build on.</div>
+            <div class="addon-builder-row">
+              <input id="addon-template-id" class="settings-select" type="text" value="my-addon" aria-label="Addon id" />
+              <button class="admin-btn-add" id="addon-template-btn">Load Template</button>
+              <button class="admin-btn-add" id="addon-save-btn">Save Addon</button>
+              <button class="admin-btn-sm" id="addon-clear-btn">Clear</button>
+            </div>
+            <textarea id="addon-manifest-editor" class="addon-manifest-editor" spellcheck="false" placeholder="{&#10;  &quot;id&quot;: &quot;my-addon&quot;,&#10;  &quot;name&quot;: &quot;My Addon&quot;&#10;}"></textarea>
+            <div id="addons-msg" class="addon-msg"></div>
+          </div>
+        </div>
+
         <div data-settings-panel="tools" class="hidden">
           <div class="admin-card" style="margin-bottom:12px;">
             <h2><svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" style="vertical-align:-2px;margin-right:5px;opacity:0.6"><path d="M14.7 6.3a1 1 0 0 0 0 1.4l1.6 1.6a1 1 0 0 0 1.4 0l3.77-3.77a6 6 0 0 1-7.94 7.94l-6.91 6.91a2.12 2.12 0 0 1-3-3l6.91-6.91a6 6 0 0 1 7.94-7.94l-3.76 3.76z"/></svg>Agent</h2>
diff --git a/static/js/addons.js b/static/js/addons.js
new file mode 100644
index 0000000..10f35c3
--- /dev/null
+++ b/static/js/addons.js
@@ -0,0 +1,198 @@
+// static/js/addons.js - Addon manager for manifest-based Odysseus addons
+
+import uiModule from './ui.js';
+
+let initialized = false;
+let currentManifest = null;
+
+function el(id) { return document.getElementById(id); }
+function esc(value) {
+  return uiModule && typeof uiModule.esc === 'function'
+    ? uiModule.esc(String(value ?? ''))
+    : String(value ?? '').replace(/[&<>"']/g, c => ({ '&': '&amp;', '<': '&lt;', '>': '&gt;', '"': '&quot;', "'": '&#39;' }[c]));
+}
+
+async function api(path, options = {}) {
+  const res = await fetch(path, {
+    credentials: 'same-origin',
+    headers: { 'Content-Type': 'application/json', ...(options.headers || {}) },
+    ...options,
+  });
+  const data = await res.json().catch(() => ({}));
+  if (!res.ok) throw new Error(data.detail || data.error || `HTTP ${res.status}`);
+  return data;
+}
+
+function contributionSummary(addon) {
+  const items = Array.isArray(addon.contributions) ? addon.contributions : [];
+  if (!items.length) return '<span class="addon-muted">No contributions</span>';
+  const counts = new Map();
+  items.forEach(item => counts.set(item.type || 'unknown', (counts.get(item.type || 'unknown') || 0) + 1));
+  return Array.from(counts.entries())
+    .map(([type, count]) => `<span class="addon-chip">${esc(type)}${count > 1 ? ` x${count}` : ''}</span>`)
+    .join('');
+}
+
+function renderAddons(addons) {
+  const list = el('addons-list');
+  if (!list) return;
+  if (!addons.length) {
+    list.innerHTML = '<div class="admin-empty">No addons installed yet.</div>';
+    return;
+  }
+  list.innerHTML = addons.map(addon => `
+    <div class="addon-card" data-addon-id="${esc(addon.id)}">
+      <div class="addon-card-main">
+        <div class="addon-card-title">
+          <span class="addon-status ${addon.enabled ? 'enabled' : ''}"></span>
+          <strong>${esc(addon.name || addon.id)}</strong>
+          <span class="addon-version">${esc(addon.version || '0.1.0')}</span>
+        </div>
+        <div class="addon-desc">${esc(addon.description || 'No description')}</div>
+        <div class="addon-meta">
+          <span>${esc(addon.id)}</span>
+          ${addon.author ? `<span>${esc(addon.author)}</span>` : ''}
+          ${addon.error ? `<span class="addon-error">${esc(addon.error)}</span>` : ''}
+        </div>
+        <div class="addon-contribs">${contributionSummary(addon)}</div>
+      </div>
+      <div class="addon-card-actions">
+        <button class="admin-btn-sm addon-edit-btn" data-addon-id="${esc(addon.id)}">Edit</button>
+        <button class="admin-btn-sm addon-toggle-btn" data-addon-id="${esc(addon.id)}" data-enabled="${addon.enabled ? '0' : '1'}">${addon.enabled ? 'Disable' : 'Enable'}</button>
+        <button class="admin-btn-sm danger addon-delete-btn" data-addon-id="${esc(addon.id)}">Delete</button>
+      </div>
+    </div>
+  `).join('');
+}
+
+function setMessage(message, kind = '') {
+  const node = el('addons-msg');
+  if (!node) return;
+  node.textContent = message || '';
+  node.className = kind ? `addon-msg ${kind}` : 'addon-msg';
+}
+
+function setEditorManifest(manifest) {
+  currentManifest = manifest || null;
+  const editor = el('addon-manifest-editor');
+  if (!editor) return;
+  editor.value = JSON.stringify(manifest || {}, null, 2);
+}
+
+async function loadAddons() {
+  const list = el('addons-list');
+  if (list) list.innerHTML = '<div class="admin-empty">Loading addons...</div>';
+  try {
+    const data = await api('/api/addons');
+    renderAddons(Array.isArray(data.addons) ? data.addons : []);
+    setMessage('');
+  } catch (err) {
+    if (list) list.innerHTML = `<div class="admin-empty addon-error">${esc(err.message)}</div>`;
+  }
+}
+
+async function loadTemplate() {
+  const idInput = el('addon-template-id');
+  const addonId = (idInput && idInput.value.trim()) || 'my-addon';
+  const data = await api(`/api/addons/template?addon_id=${encodeURIComponent(addonId)}`);
+  setEditorManifest(data.manifest);
+  setMessage('Template loaded. Edit the manifest and save it.', 'ok');
+}
+
+async function saveManifest() {
+  const editor = el('addon-manifest-editor');
+  if (!editor) return;
+  let manifest;
+  try {
+    manifest = JSON.parse(editor.value || '{}');
+  } catch (err) {
+    setMessage(`Invalid JSON: ${err.message}`, 'error');
+    return;
+  }
+  try {
+    const data = await api('/api/addons', {
+      method: 'POST',
+      body: JSON.stringify({ manifest }),
+    });
+    setEditorManifest(data.addon);
+    setMessage('Addon saved.', 'ok');
+    await loadAddons();
+  } catch (err) {
+    setMessage(err.message, 'error');
+  }
+}
+
+async function editAddon(addonId) {
+  try {
+    const data = await api(`/api/addons/${encodeURIComponent(addonId)}`);
+    const addon = { ...(data.addon || {}) };
+    delete addon.path;
+    setEditorManifest(addon);
+    setMessage(`Editing ${addon.id}.`, 'ok');
+  } catch (err) {
+    setMessage(err.message, 'error');
+  }
+}
+
+async function toggleAddon(addonId, enabled) {
+  try {
+    await api(`/api/addons/${encodeURIComponent(addonId)}/enabled`, {
+      method: 'POST',
+      body: JSON.stringify({ enabled }),
+    });
+    setMessage(enabled ? 'Addon enabled.' : 'Addon disabled.', 'ok');
+    await loadAddons();
+  } catch (err) {
+    setMessage(err.message, 'error');
+  }
+}
+
+async function deleteAddon(addonId) {
+  if (!confirm(`Delete addon "${addonId}"?`)) return;
+  try {
+    await api(`/api/addons/${encodeURIComponent(addonId)}`, { method: 'DELETE' });
+    if (currentManifest && currentManifest.id === addonId) setEditorManifest(null);
+    setMessage('Addon deleted.', 'ok');
+    await loadAddons();
+  } catch (err) {
+    setMessage(err.message, 'error');
+  }
+}
+
+function bindEvents() {
+  const refresh = el('addons-refresh-btn');
+  if (refresh) refresh.addEventListener('click', loadAddons);
+  const template = el('addon-template-btn');
+  if (template) template.addEventListener('click', () => loadTemplate().catch(err => setMessage(err.message, 'error')));
+  const save = el('addon-save-btn');
+  if (save) save.addEventListener('click', saveManifest);
+  const clear = el('addon-clear-btn');
+  if (clear) clear.addEventListener('click', () => {
+    setEditorManifest(null);
+    setMessage('');
+  });
+  const list = el('addons-list');
+  if (list) {
+    list.addEventListener('click', (event) => {
+      const edit = event.target.closest('.addon-edit-btn');
+      const toggle = event.target.closest('.addon-toggle-btn');
+      const del = event.target.closest('.addon-delete-btn');
+      if (edit) editAddon(edit.dataset.addonId);
+      if (toggle) toggleAddon(toggle.dataset.addonId, toggle.dataset.enabled === '1');
+      if (del) deleteAddon(del.dataset.addonId);
+    });
+  }
+}
+
+export function init() {
+  if (initialized) return;
+  initialized = true;
+  bindEvents();
+}
+
+export function open() {
+  init();
+  loadAddons();
+}
+
+export default { init, open };
diff --git a/static/js/settings.js b/static/js/settings.js
index ad2950f..e637286 100644
--- a/static/js/settings.js
+++ b/static/js/settings.js
@@ -9,6 +9,7 @@ import { sortModelIds } from './modelSort.js';
 import { providerLogo } from './providers.js';
 import { isAltGrEvent } from './platform.js';
 import { bindMenuDismiss } from './escMenuStack.js';
+import addonsModule from './addons.js';
 
 let initialized = false;
 let modalEl = null;
@@ -41,6 +42,7 @@ function initTabs() {
       document.body.classList.toggle('settings-appearance-open', tab === 'appearance');
       syncAppearanceOpacity(tab === 'appearance');
       if (tab === 'ai') refreshAiModelEndpoints();
+      if (tab === 'addons') addonsModule.open();
     });
   });
 }
@@ -2322,6 +2324,7 @@ function initAll() {
   initClose();
   initOpenPromptModalLink();
   initOpacityToggle();
+  addonsModule.init();
   initialized = true;
   initDefaultChat();
   initTeacherModel();
@@ -5731,6 +5734,7 @@ export function open(tab) {
   document.body.classList.toggle('settings-appearance-open', activeTab === 'appearance');
   syncAppearanceOpacity(activeTab === 'appearance');
   if (activeTab === 'ai') refreshAiModelEndpoints();
+  if (activeTab === 'addons') addonsModule.open();
   if (ADMIN_TABS.has(activeTab) && window.adminModule && !window.adminModule._initialized) {
     window.adminModule._initData();
   }
diff --git a/static/style.css b/static/style.css
index 640f05d..f60e682 100644
--- a/static/style.css
+++ b/static/style.css
@@ -40439,6 +40439,126 @@ body.theme-frosted .modal {
   color: var(--fg, #9cdef2);
 }
 
+/* Addon manager */
+.addons-list {
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+}
+.addon-card {
+  display: flex;
+  gap: 12px;
+  align-items: flex-start;
+  justify-content: space-between;
+  padding: 10px;
+  border: 1px solid var(--border);
+  border-radius: 6px;
+  background: color-mix(in srgb, var(--panel) 86%, var(--bg));
+}
+.addon-card-main {
+  min-width: 0;
+  display: flex;
+  flex-direction: column;
+  gap: 5px;
+}
+.addon-card-title {
+  display: flex;
+  align-items: center;
+  gap: 7px;
+  min-width: 0;
+  font-size: 13px;
+}
+.addon-card-title strong {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+.addon-status {
+  width: 8px;
+  height: 8px;
+  border-radius: 50%;
+  background: color-mix(in srgb, var(--fg) 25%, transparent);
+  flex-shrink: 0;
+}
+.addon-status.enabled {
+  background: var(--accent, #50fa7b);
+  box-shadow: 0 0 0 3px color-mix(in srgb, var(--accent, #50fa7b) 14%, transparent);
+}
+.addon-version,
+.addon-muted,
+.addon-meta {
+  color: color-mix(in srgb, var(--fg) 55%, transparent);
+  font-size: 11px;
+}
+.addon-desc {
+  font-size: 12px;
+  line-height: 1.35;
+  color: color-mix(in srgb, var(--fg) 78%, transparent);
+}
+.addon-meta {
+  display: flex;
+  gap: 8px;
+  flex-wrap: wrap;
+  font-family: var(--mono, monospace);
+}
+.addon-error,
+.addon-msg.error {
+  color: var(--red, #ff5555);
+}
+.addon-contribs {
+  display: flex;
+  gap: 5px;
+  flex-wrap: wrap;
+}
+.addon-chip {
+  padding: 2px 6px;
+  border: 1px solid var(--border);
+  border-radius: 999px;
+  font-size: 10px;
+  color: color-mix(in srgb, var(--fg) 74%, transparent);
+}
+.addon-card-actions {
+  display: flex;
+  gap: 6px;
+  flex-wrap: wrap;
+  justify-content: flex-end;
+  flex-shrink: 0;
+}
+.addon-builder-row {
+  display: flex;
+  gap: 8px;
+  flex-wrap: wrap;
+  align-items: center;
+  margin-bottom: 8px;
+}
+.addon-builder-row input {
+  min-width: 180px;
+  max-width: 260px;
+}
+.addon-manifest-editor {
+  width: 100%;
+  min-height: 300px;
+  resize: vertical;
+  box-sizing: border-box;
+  padding: 10px;
+  border: 1px solid var(--border);
+  border-radius: 6px;
+  background: var(--bg);
+  color: var(--fg);
+  font-family: var(--mono, Consolas, monospace);
+  font-size: 12px;
+  line-height: 1.45;
+}
+.addon-msg {
+  min-height: 18px;
+  margin-top: 7px;
+  font-size: 11px;
+  color: color-mix(in srgb, var(--fg) 58%, transparent);
+}
+.addon-msg.ok {
+  color: var(--accent, #50fa7b);
+}
+
 /* The model-comparison grid hard-codes 2-4 equal columns with no phone
    breakpoint that stacks them, so at 390px two models render ~178px columns and
    four render ~88px columns. Each column is a full scrolling chat (code blocks,
diff --git a/static/sw.js b/static/sw.js
index 05b3d4e..6ab0dde 100644
--- a/static/sw.js
+++ b/static/sw.js
@@ -7,7 +7,7 @@
 //   - Other static assets (images/fonts/libs): cache-first with bg refresh.
 //   - API / non-GET: never cached.
 // Bump CACHE_NAME whenever the precache list or SW logic changes.
-const CACHE_NAME = 'odysseus-v344';
+const CACHE_NAME = 'odysseus-v345';
 
 // Core shell precached on install so repeat opens are instant without any
 // network wait. Keep this list in sync with the <script type="module"> tags
@@ -44,6 +44,7 @@ const PRECACHE = [
   '/static/js/theme.js',
   '/static/js/censor.js',
   '/static/js/settings.js',
+  '/static/js/addons.js',
   '/static/js/admin.js',
   '/static/js/init.js',
   '/static/js/slashCommands.js',
diff --git a/tests/test_addons.py b/tests/test_addons.py
new file mode 100644
index 0000000..1a4aa10
--- /dev/null
+++ b/tests/test_addons.py
@@ -0,0 +1,103 @@
+import asyncio
+import json
+from types import SimpleNamespace
+
+import pytest
+
+from src import addons
+
+
+@pytest.fixture
+def addon_store(tmp_path, monkeypatch):
+    monkeypatch.setattr(addons, "ADDONS_DIR", str(tmp_path / "addons"))
+    monkeypatch.setattr(addons, "ADDONS_FILE", str(tmp_path / "addons.json"))
+    return tmp_path
+
+
+def test_addon_template_is_valid(addon_store):
+    manifest = addons.addon_template("demo-addon")
+    saved = addons.save_addon_manifest(manifest)
+
+    assert saved["id"] == "demo-addon"
+    assert saved["enabled"] is False
+    assert saved["contributions"][0]["type"] == "agent_prompt"
+    assert addons.get_addon("demo-addon")["name"] == "Demo Addon"
+
+
+def test_enable_addon_exposes_prompt_contribution(addon_store):
+    addons.save_addon_manifest({
+        "id": "prompt-addon",
+        "name": "Prompt Addon",
+        "contributions": [
+            {"type": "agent_prompt", "text": "Prefer compact summaries."},
+            {"type": "unknown", "text": "ignored"},
+        ],
+    })
+
+    assert addons.get_addons_prompt() == ""
+    addons.set_addon_enabled("prompt-addon", True)
+
+    prompt = addons.get_addons_prompt()
+    assert "Prompt Addon" in prompt
+    assert "Prefer compact summaries." in prompt
+    assert "unknown" not in prompt
+
+
+def test_addon_id_rejects_path_traversal(addon_store):
+    with pytest.raises(ValueError):
+        addons.save_addon_manifest({"id": "../bad", "name": "Bad"})
+
+
+@pytest.fixture
+def addon_routes(addon_store):
+    fastapi = pytest.importorskip("fastapi")
+    from routes.addon_routes import setup_addon_routes
+
+    class _AuthManager:
+        def get_username_for_token(self, token):
+            return "admin" if token == "session-token" else None
+
+        def is_admin(self, user):
+            return user == "admin"
+
+    router = setup_addon_routes(_AuthManager())
+
+    def endpoint(path, method):
+        for route in router.routes:
+            if getattr(route, "path", "") == path and method in getattr(route, "methods", set()):
+                return route.endpoint
+        raise AssertionError(f"{method} {path} route not registered")
+
+    return endpoint, fastapi.HTTPException
+
+
+class _Request(SimpleNamespace):
+    def __init__(self, token="session-token"):
+        super().__init__(cookies={"odysseus_session": token})
+
+
+def test_addon_routes_require_admin(addon_routes):
+    endpoint, http_exception = addon_routes
+    list_all = endpoint("/api/addons", "GET")
+
+    with pytest.raises(http_exception) as exc:
+        asyncio.run(list_all(_Request(token="bad")))
+
+    assert exc.value.status_code == 401
+
+
+def test_addon_routes_create_and_enable(addon_routes):
+    endpoint, _ = addon_routes
+    create = endpoint("/api/addons", "POST")
+    set_enabled = endpoint("/api/addons/{addon_id}/enabled", "POST")
+    list_all = endpoint("/api/addons", "GET")
+
+    body = SimpleNamespace(manifest={"id": "route-addon", "name": "Route Addon"})
+    result = asyncio.run(create(body, _Request()))
+    assert result["addon"]["id"] == "route-addon"
+
+    enabled_body = SimpleNamespace(enabled=True)
+    asyncio.run(set_enabled("route-addon", enabled_body, _Request()))
+    listed = asyncio.run(list_all(_Request()))
+    assert listed["addons"][0]["enabled"] is True
+    assert json.loads((addons._addon_dir("route-addon") / "addon.json").read_text())["enabled"] is True