The Model Context Protocol (MCP) allows you to connect external tools and services using a standardized protocol, enabling you to integrate APIs and services directly into your workflows. With MCP, you can extend Sim's capabilities by adding custom integrations that work seamlessly with your agents and workflows.
What is MCP?
MCP is an open standard that enables AI assistants to securely connect to external data sources and tools. It provides a standardized way to:
- Connect to databases, APIs, and file systems
- Access real-time data from external services
- Execute custom tools and scripts
- Maintain secure, controlled access to external resources
Adding an MCP Server as a Tool
MCP servers provide collections of tools that your agents can use.
To add one:
- Navigate to Settings → MCP Tools
- Click Add to open the configuration modal
- Enter a Server Name and Server URL
- Add any required Headers (e.g. API keys)
- Click Add MCP to save
You can also configure MCP servers directly from the toolbar in an Agent block for quick setup.
Server Configuration Options
| Field | Description |
|---|---|
| Name | Display name for the server |
| URL | The MCP server endpoint |
| Transport | Currently supports streamable-http |
| Headers | Key-value pairs for authentication or custom headers |
| Timeout | Connection timeout in milliseconds (default: 30,000) |
Environment Variables in Configuration
Server URLs and headers support environment variable substitution using {{VAR_NAME}} syntax. This keeps sensitive values like API keys out of the server configuration.
URL: https://api.example.com/mcp
Authorization: Bearer {{MCP_API_TOKEN}}When you type {{ in the URL or header fields, a dropdown appears showing available workspace environment variables.
Testing and Validation
Click Test Connection before saving to verify the server is reachable and discover available tools. The test response shows the number of tools found and the protocol version.
After saving, each server displays its available tools with parameter names, types, and required flags. If a server's tools change (e.g., after a server update), click Refresh to fetch the latest schemas. This automatically updates any agent blocks using those tools.
Tool validation badges appear on servers with issues — for example, if a tool was removed from the server but is still referenced in a workflow. Click the badge to see which workflows are affected.
Domain Allowlisting
Self-hosted deployments can restrict which MCP server domains are allowed by setting the ALLOWED_MCP_DOMAINS environment variable (comma-separated list). When set, only servers on approved domains can be added. When unset, all domains are allowed.
Refresh Tools
To auto-refresh an MCP tool already in use by an agent, go to Settings → MCP Tools, open the server's details, and click Refresh. This fetches the latest tool schemas and automatically updates any agent blocks using those tools with the new parameter definitions.
Using MCP Tools in Agents
Once MCP servers are configured, their tools become available within your agent blocks:
- Open an Agent block
- In the Tools section, click Add tool…
- Under MCP Servers, click a server to see its tools
- Select individual tools, or choose Use all N tools to add every tool from that server
- The agent can now access these tools during execution
If you haven't configured a server yet, click Add MCP Server at the top of the dropdown to open the setup modal without leaving the block.
Standalone MCP Tool Block
For more granular control, you can use the dedicated MCP Tool block to execute specific MCP tools:
The MCP Tool block allows you to:
- Execute any configured MCP tool directly
- Pass specific parameters to the tool
- Use the tool's output in subsequent workflow steps
- Chain multiple MCP tools together
When to Use MCP Tool vs Agent
| Agent with MCP tools | MCP Tool block | |
|---|---|---|
| Execution | AI decides which tools to call | Deterministic — runs the tool you pick |
| Parameters | AI chooses at runtime | You set them explicitly |
| Best for | Dynamic, conversational flows | Structured, repeatable steps |
| Reasoning | Handles complex multi-step logic | One tool, one call |
Permission Requirements
MCP functionality requires specific workspace permissions:
| Action | Required Permission |
|---|---|
| Create or update MCP servers | Write or Admin |
| Delete MCP servers | Admin |
| Use MCP tools in agents | Write or Admin |
| View available MCP tools | Read, Write, or Admin |
| Execute MCP Tool blocks | Read, Write, or Admin |
Common Use Cases
Database Integration
Connect to databases to query, insert, or update data within your workflows.
API Integrations
Access external APIs and web services that don't have built-in Sim integrations.
File System Access
Read, write, and manipulate files on local or remote file systems.
Custom Business Logic
Execute custom scripts or tools specific to your organization's needs.
Real-time Data Access
Fetch live data from external systems during workflow execution.
Security Considerations
- MCP servers run with the permissions of the user who configured them
- Always verify MCP server sources before installation
- Use environment variables for sensitive configuration data
- Review MCP server capabilities before granting access to agents
Troubleshooting
MCP Server Not Appearing
- Verify the server configuration is correct
- Check that you have the required permissions
- Ensure the MCP server is running and accessible
Tool Execution Failures
- Verify tool parameters are correctly formatted
- Check MCP server logs for error messages
- Ensure required authentication is configured
Permission Errors
- Confirm your workspace permission level
- Check if the MCP server requires additional authentication
- Verify the server is properly configured for your workspace