In a product, one agent serves many users. Each user should only ever reach their own accounts. With BundleUp, the connection ID is the only per-user input, so this comes down to looking up the right connection IDs for the user you’re serving.
User A has connected Slack and Linear, so their agent run gets two MCP servers. User B has only connected Slack, so theirs gets one. Both runs use the same code and the same mcp.bundleup.io; only the connection IDs differ.
The pattern
- When a user connects an account, store
(user_id, provider, connection_id) in your database. Setting external_id to your user ID when you start the authorization flow lets you recover this from BundleUp too.
- When the user starts a conversation, load their connections.
- Give the agent one MCP server per connection.
- Never accept a connection ID from the client side. Always look it up server-side for the authenticated user.
Any API key in your workspace can use any connection in your workspace. BundleUp can’t know which of your users a connection belongs to. Picking the right connection for the signed-in user is your application’s job.
Several providers in one agent
Model-hosted MCP: pass one mcp tool entry per connection. The model provider keeps them apart by label.
Your own MCP client: open one client per connection. Tool names come from each provider, so two providers can both have a search tool. Prefix each tool name with its label (for example slack__search) before giving the list to the model, and route calls back by that prefix.
Two or three providers can easily add up to 60 tools, and models choose less accurately as the list grows. Give each agent only the tools it needs:
- Only attach the providers relevant to the task.
- Filter
tools/list results to an allowlist before passing them to the model.
- Narrow the integration’s scopes, so the provider exposes fewer tools in the first place.
Rate limits and cost
Each MCP message counts as one request toward the connection’s rate limit (100 per minute) and toward usage. An agent that lists tools on every turn uses more than its tool calls suggest. Cache the tool list for the length of a conversation where your client allows it.
When a user disconnects
Listen for the connection.deleted webhook and remove the connection from your database, so the next agent run doesn’t try to use it. If an MCP call fails with connection_invalid or connection_refresh_failed, prompt the user to reconnect. 
