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+# OpenUI OAuth "Client not registered" Fix
+
+## Problem
+
+When connecting MCP Gateway to Open-UI, you get:
+```
+{"error":"invalid_client","error_description":"Client not registered."}
+```
+
+Even though it works fine in Claude.ai.
+
+## Root Cause
+
+Your gateway uses **in-memory OAuth client registration** (`REGISTERED_CLIENTS` dict). This means:
+
+1. Each client (Claude.ai, Open-UI, etc.) registers itself via `/oauth/register`
+2. The registration is stored in RAM only
+3. When the gateway restarts → all registrations are lost
+4. Open-UI's client ID is no longer recognized during token exchange
+
+Additionally, there are **two separate OAuth flows** happening:
+- Claude.ai uses one client ID
+- Open-UI registers and gets a different client ID
+- If the gateway restarts between registration and first use, Open-UI's ID is gone
+
+## Solution Options
+
+### Option A: Persistent OAuth Client Storage (Recommended)
+Store OAuth clients in a file or database instead of RAM.
+
+**File**: `gateway-proxy/oauth_storage.py` (to be created)
+
+```python
+import json
+import os
+from typing import dict
+
+OAUTH_STORAGE_FILE = os.environ.get("OAUTH_STORAGE_FILE", "/data/oauth_clients.json")
+
+def load_oauth_clients() -> dict:
+    """Load OAuth clients from persistent storage"""
+    if os.path.exists(OAUTH_STORAGE_FILE):
+        try:
+            with open(OAUTH_STORAGE_FILE, 'r') as f:
+                return json.load(f)
+        except Exception as e:
+            logger.error(f"Failed to load OAuth clients: {e}")
+            return {}
+    return {}
+
+def save_oauth_clients(clients: dict) -> None:
+    """Save OAuth clients to persistent storage"""
+    try:
+        os.makedirs(os.path.dirname(OAUTH_STORAGE_FILE), exist_ok=True)
+        with open(OAUTH_STORAGE_FILE, 'w') as f:
+            json.dump(clients, f, indent=2)
+    except Exception as e:
+        logger.error(f"Failed to save OAuth clients: {e}")
+
+def register_client(client_info: dict) -> None:
+    """Register a new OAuth client"""
+    clients = load_oauth_clients()
+    client_id = client_info["client_id"]
+    clients[client_id] = client_info
+    save_oauth_clients(clients)
+```
+
+**Changes to gateway_proxy.py:**
+```python
+# At startup, replace:
+# REGISTERED_CLIENTS: dict[str, dict] = {}
+# with:
+from .oauth_storage import load_oauth_clients, save_oauth_clients
+REGISTERED_CLIENTS = load_oauth_clients()
+
+# In oauth_register function, after adding client:
+REGISTERED_CLIENTS[client_id] = client_info
+save_oauth_clients(REGISTERED_CLIENTS)  # Add this line
+```
+
+**docker-compose.yml update:**
+```yaml
+gateway-proxy:
+  volumes:
+    - gateway-data:/data  # Persist OAuth clients
+```
+
+### Option B: Pre-register Known Clients (Quick Fix)
+Hardcode known clients (Claude.ai, Open-UI) so they don't need dynamic registration.
+
+**In .env:**
+```bash
+# Pre-registered OAuth clients (JSON format)
+OAUTH_CLIENTS='{"claude-app":{"client_id":"claude-app","client_secret":"YOUR_SECRET","client_name":"Claude.ai"},"openui":{"client_id":"openui","client_secret":"YOUR_SECRET","client_name":"Open-UI"}}'
+```
+
+**In gateway_proxy.py:**
+```python
+import json
+
+def load_oauth_clients() -> dict:
+    """Load pre-registered clients from env"""
+    clients_json = os.environ.get("OAUTH_CLIENTS", "{}")
+    try:
+        return json.loads(clients_json)
+    except:
+        return {}
+
+REGISTERED_CLIENTS = load_oauth_clients()
+```
+
+### Option C: Disable Client Validation (Development Only)
+For testing, skip client ID validation in token endpoint:
+
+**In oauth_token function, around line 520:**
+```python
+# Change this check:
+client_info = REGISTERED_CLIENTS.get(client_id)
+if not client_info:
+    return JSONResponse({"error": "invalid_client"}, status_code=400)
+
+# To this (development only):
+client_info = REGISTERED_CLIENTS.get(client_id)
+if not client_info:
+    logger.warning(f"Client {client_id} not registered, allowing anyway (DEV MODE)")
+    # Allow unregistered clients for testing
+```
+
+## Recommended Implementation
+
+**Use Option A** (persistent storage) because:
+- ✅ Works across gateway restarts
+- ✅ Supports multiple clients (Claude.ai, Open-UI, others)
+- ✅ Secure (OAuth flow still requires proper credentials)
+- ✅ Production-ready
+- ✅ No hardcoded secrets
+
+## Implementation Steps
+
+### Step 1: Create oauth_storage.py
+Save to `gateway-proxy/oauth_storage.py` (content provided above)
+
+### Step 2: Update gateway_proxy.py
+
+Around line 27 (imports), add:
+```python
+from .oauth_storage import load_oauth_clients, save_oauth_clients
+```
+
+Around line 52, replace:
+```python
+# REGISTERED_CLIENTS: dict[str, dict] = {}
+REGISTERED_CLIENTS = load_oauth_clients()
+```
+
+In `oauth_register()` function, after line 383:
+```python
+REGISTERED_CLIENTS[client_id] = client_info
+save_oauth_clients(REGISTERED_CLIENTS)  # ADD THIS LINE
+```
+
+### Step 3: Update docker-compose.yml
+
+Add volume to gateway-proxy service:
+```yaml
+gateway-proxy:
+  volumes:
+    - gateway-data:/data
+
+volumes:
+  gateway-data:
+```
+
+### Step 4: Set environment variable (optional)
+In `.env`:
+```bash
+OAUTH_STORAGE_FILE=/data/oauth_clients.json
+```
+
+### Step 5: Restart
+```bash
+docker-compose down
+docker-compose up -d
+```
+
+## Testing
+
+1. Register with Open-UI (should succeed)
+2. Restart gateway: `docker-compose restart gateway-proxy`
+3. Try to use Open-UI again (should work - client is persisted)
+4. Check `/data/oauth_clients.json` file exists with registration
+
+## Debugging
+
+Check if clients are saved:
+```bash
+docker exec mcp-gateway cat /data/oauth_clients.json | jq '.'
+```
+
+Check gateway logs for registration:
+```bash
+docker logs mcp-gateway | grep "DCR:"
+```
+
+If you still get "Client not registered":
+1. Verify oauth_storage.py is in gateway-proxy/
+2. Check that save_oauth_clients() is being called
+3. Check file permissions on /data/ volume
+4. Check docker logs for save errors
+
+## Alternative: Use Redis or Database
+
+For production with multiple gateway instances, consider storing OAuth clients in:
+- Redis (fast, distributed)
+- PostgreSQL (persistent, queryable)
+- MongoDB (flexible schema)
+
+This ensures clients are available across all instances.