MCP Server Authentication & OAuth Dynamic Client Registration

Implement flexible authentication for MCP (Model Context Protocol) server connections. For OAuth providers, auto-discover endpoints and dynamically register as a client — the user just provides the MCP server URL and clicks "Connect." For bearer/API key providers, support both admin-shared and per-agent credentials so different agents can authenticate with different accounts.

When to Use

• Building an admin UI for managing MCP server connections
• Integrating with third-party MCP providers (Linear, Sentry, Granola, Render, etc.)
• Implementing the MCP Streamable HTTP transport with authenticated tool sync
• Adding per-agent credential support so each agent can use its own account
• Adding OAuth to an existing MCP connector/server management system

Core Standards

The OAuth implementation relies on three RFCs:

1. RFC 8414 - OAuth Authorization Server Metadata Discovery via .well-known/oauth-authorization-server
2. RFC 7591 - Dynamic Client Registration at the provider's registration endpoint
3. RFC 7636 - PKCE (S256) for authorization code security

Not all MCP servers use OAuth. Some (e.g., Render) use bearer tokens with API keys and handle account/workspace selection at the MCP protocol level. The credential system must be auth-type-agnostic.

Architecture Overview

Credential Mode (Orthogonal to Auth Type)

credential_mode applies to all auth types (bearer, api_key_header, oauth), not just OAuth. This is a critical design decision — different agents may need their own credentials for the same MCP server (e.g., different Render accounts, different Linear workspaces).

credential_mode = "shared"     → Admin provides one credential, all agents use it
credential_mode = "per_agent"  → Each agent has its own credential

OAuth Flow

Admin clicks "Connect"
    |
    v
Discover OAuth metadata (RFC 8414)
    |  GET /.well-known/oauth-authorization-server
    v
Register as OAuth client (RFC 7591)
    |  POST /oauth/register
    v
Redirect to provider consent screen
    |  GET /oauth/authorize?client_id=...&code_challenge=...
    v
Provider redirects back with code
    |  GET /callback?code=...&state=...
    v
Exchange code for tokens
    |  POST /oauth/token
    v
Store tokens, sync tools

Implementation Steps

1. Database Schema

Create two tables: one for MCP server configuration (including OAuth metadata and shared tokens), and one for per-agent credentials (works for any auth type).

Refer to references/schema.md for complete migration and model setup.

Key design decisions:

• Encrypt all secrets at rest (encrypts :oauth_client_id, etc.)
• Store both shared tokens (on the server record) and per-agent tokens (join table)
• Use credential_mode ("shared" or "per_agent") — applies to ALL auth types, not just OAuth
• Store discovered_tools as a JSON array for tool name tracking
• AgentMcpConnection.access_token stores OAuth tokens, bearer tokens, or API keys

2. OAuth Discovery and Registration

Implement three model methods on the MCP server record. Refer to references/oauth_flow.md for complete implementation code.

Discovery (discover_oauth_metadata!):

• Derive .well-known/oauth-authorization-server URL from the MCP server's host
• Parse the JSON response for authorization_endpoint, token_endpoint, registration_endpoint, scopes_supported
• Skip if endpoints are already manually configured
• Not all servers support this — handle 404 gracefully

Registration (register_oauth_client!):

• POST to the registration endpoint with client_name, redirect_uris, grant_types, response_types, token_endpoint_auth_method
• Store the returned client_id and client_secret
• Skip if client_id is already present

Combined (discover_and_register_oauth!):

• Run discovery then registration in sequence, skipping either if already configured
• Accept redirect_uri parameter for the registration payload

3. Authorization Controller

Create an OAuth controller with authorize and callback actions. Refer to references/oauth_flow.md for the full controller implementation.

Critical pitfalls to avoid:

Turbo Drive cross-origin redirects: Standard redirect_to with an external URL is silently swallowed by Turbo Drive because it cannot follow cross-origin 302 redirects. The browser stays on the current page with no feedback. Render an HTML page with <meta http-equiv="refresh" content="0;url=..."> for the external OAuth redirect instead.

State parameter: Use a signed, expiring message (e.g., Rails message_verifier) containing the connector ID, PKCE code verifier, optional agent ID, and timestamp. Set 10-minute expiry.

String keys from message verifier: After verifying the state token, the payload uses string keys not symbol keys. Access with payload["connector_id"], not payload[:connector_id].

PKCE (S256): Generate a random code_verifier, compute code_challenge as URL-safe Base64 of SHA-256 digest with no padding. Send challenge in authorize request, verifier in token exchange.

Error redirects: When agent_id is present in state, redirect errors back to the agent edit page, not the connectors index. The user initiated from the agent form and should return there.

Auto-sync on first agent connection: For per-agent OAuth, the admin may not have their own account. When the callback stores the first per-agent token, auto-sync tools using that agent's token if tools haven't been discovered yet.

4. Routes

Mount the OAuth authorize as a member action on the connector resource, and the callback as a standalone route (since it doesn't carry a connector ID — that comes from state).

resources :connectors do
  member do
    get "oauth/authorize", to: "mcp_oauth#authorize", as: :mcp_oauth_authorize
  end
end
get "mcp_oauth/callback", to: "mcp_oauth#callback", as: :mcp_oauth_callback

Note on Rails route helper naming: A member route mcp_oauth_authorize on resources :connectors generates mcp_oauth_authorize_connector_path(connector) — the resource name comes last. This is a common source of NoMethodError bugs.

5. Token Management

Implement token refresh for both shared and per-agent OAuth tokens. Refer to references/oauth_flow.md for the ensure_token_fresh! pattern.

• Check expiry with a 5-minute buffer (token_expires_at < 5.minutes.from_now)
• Use with_lock for thread-safe updates on shared tokens
• Return the appropriate token based on credential mode
• Bearer/API key per-agent tokens are static (no refresh flow needed)

6. MCP Tool Sync (Streamable HTTP Protocol)

After connection, sync available tools from the MCP server. Refer to references/tool_sync.md for the complete implementation.

The MCP Streamable HTTP protocol requires a two-step handshake:

1. Send initialize JSON-RPC request to get a Mcp-Session-Id header
2. Send tools/list with the session ID header

Critical details:

• Set Accept: application/json, text/event-stream — some servers return 406 without this
• Some servers return SSE format instead of JSON — parse both formats
• The Mcp-Session-Id from the initialize response must be included on subsequent requests
• sync_tools! must accept an agent: parameter for per-agent auth token resolution
• Some servers (e.g., Render) allow unauthenticated tool listing — auth is only needed for tool execution

7. UI Considerations

Refer to references/ui_patterns.md for form and index view patterns.

Connector form:

• credential_mode radio (Shared vs Per-agent) applies to ALL auth types, not just OAuth
• When per-agent is selected for bearer/API key, hide the admin token input
• Show OAuth-specific fields (advanced config, Connect button) only for OAuth auth type
• Use Stimulus controller to toggle visibility based on both auth_type AND credential_mode

Connectors index:

• Show "Connect" button only for shared OAuth that isn't connected yet
• Show "Per-agent" label for any per-agent connector (not just OAuth)
• Show "Sync" button for everything else

Agent edit form — three states for per-agent connectors:

1. Per-agent OAuth, not connected → grayed card, "Connect" button (OAuth redirect)
2. Per-agent bearer/API key, not connected → card with inline password input for token entry
3. Connected (any type) → normal card with tool checkboxes + "Token configured" badge

Verified MCP Providers

Tested and confirmed working with:

• Linear (https://mcp.linear.app/mcp) - 45 tools, SSE response format, OAuth
• Sentry (https://mcp.sentry.dev/mcp) - 14 tools, standard JSON responses, OAuth
• Granola (https://mcp.granola.ai/mcp) - 4 tools, standard JSON responses, OAuth
• Render (https://mcp.render.com/mcp) - 24 tools, bearer token auth (no OAuth), per-agent API keys

Common Failure Modes

Symptom	Root Cause	Fix
Page stays on form after create, no redirect	Turbo Drive swallows cross-origin 302	Use HTML meta refresh instead of redirect_to
NoMethodError on route helper	Wrong helper name ordering	Member route on :connectors generates mcp_oauth_authorize_connector_path
payload[:connector_id] returns nil	Message verifier returns string keys	Use payload["connector_id"]
406 from MCP server	Missing Accept header	Add Accept: application/json, text/event-stream
400 "Mcp-Session-Id required"	Skipped initialize handshake	Send initialize first, use returned session ID
JSON parse error on tool sync	Server returns SSE format	Detect and parse both text/event-stream and JSON
Token exchange fails silently	Missing code_verifier in token request	Include PKCE verifier from signed state
OAuth discovery 404	MCP server doesn't use OAuth	Use bearer or API key auth instead; not all MCP servers support RFC 8414
Per-agent connector shows no tools	Admin can't sync without a token	Tools auto-sync on first agent connection
Error redirect goes to wrong page	agent_id not checked in rescue	Redirect to agent edit when agent_id present