Tools
Enable LLMs to perform actions through your server
Tools are a powerful primitive in the Model Context Protocol (MCP) that enable servers to expose executable functionality to clients. Through tools, LLMs can interact with external systems, perform computations, and take actions in the real world.
Tools are designed to be model-controlled, meaning that tools are exposed from servers to clients with the intention of the AI model being able to automatically invoke them (with a human in the loop to grant approval).
Overview
Tools in MCP allow servers to expose executable functions that can be invoked by clients and used by LLMs to perform actions. Key aspects of tools include:
- Discovery: Clients can list available tools through the
tools/list
endpoint - Invocation: Tools are called using the
tools/call
endpoint, where servers perform the requested operation and return results - Flexibility: Tools can range from simple calculations to complex API interactions
Like resources, tools are identified by unique names and can include descriptions to guide their usage. However, unlike resources, tools represent dynamic operations that can modify state or interact with external systems.
Tool definition structure
Each tool is defined with the following structure:
Implementing tools
Here’s an example of implementing a basic tool in an MCP server:
Example tool patterns
Here are some examples of types of tools that a server could provide:
System operations
Tools that interact with the local system:
API integrations
Tools that wrap external APIs:
Data processing
Tools that transform or analyze data:
Best practices
When implementing tools:
- Provide clear, descriptive names and descriptions
- Use detailed JSON Schema definitions for parameters
- Include examples in tool descriptions to demonstrate how the model should use them
- Implement proper error handling and validation
- Use progress reporting for long operations
- Keep tool operations focused and atomic
- Document expected return value structures
- Implement proper timeouts
- Consider rate limiting for resource-intensive operations
- Log tool usage for debugging and monitoring
Security considerations
When exposing tools:
Input validation
- Validate all parameters against the schema
- Sanitize file paths and system commands
- Validate URLs and external identifiers
- Check parameter sizes and ranges
- Prevent command injection
Access control
- Implement authentication where needed
- Use appropriate authorization checks
- Audit tool usage
- Rate limit requests
- Monitor for abuse
Error handling
- Don’t expose internal errors to clients
- Log security-relevant errors
- Handle timeouts appropriately
- Clean up resources after errors
- Validate return values
Tool discovery and updates
MCP supports dynamic tool discovery:
- Clients can list available tools at any time
- Servers can notify clients when tools change using
notifications/tools/list_changed
- Tools can be added or removed during runtime
- Tool definitions can be updated (though this should be done carefully)
Error handling
Tool errors should be reported within the result object, not as MCP protocol-level errors. This allows the LLM to see and potentially handle the error. When a tool encounters an error:
- Set
isError
totrue
in the result - Include error details in the
content
array
Here’s an example of proper error handling for tools:
This approach allows the LLM to see that an error occurred and potentially take corrective action or request human intervention.
Tool annotations
Tool annotations provide additional metadata about a tool’s behavior, helping clients understand how to present and manage tools. These annotations are hints that describe the nature and impact of a tool, but should not be relied upon for security decisions.
Purpose of tool annotations
Tool annotations serve several key purposes:
- Provide UX-specific information without affecting model context
- Help clients categorize and present tools appropriately
- Convey information about a tool’s potential side effects
- Assist in developing intuitive interfaces for tool approval
Available tool annotations
The MCP specification defines the following annotations for tools:
Annotation | Type | Default | Description |
---|---|---|---|
title | string | - | A human-readable title for the tool, useful for UI display |
readOnlyHint | boolean | false | If true, indicates the tool does not modify its environment |
destructiveHint | boolean | true | If true, the tool may perform destructive updates (only meaningful when readOnlyHint is false) |
idempotentHint | boolean | false | If true, calling the tool repeatedly with the same arguments has no additional effect (only meaningful when readOnlyHint is false) |
openWorldHint | boolean | true | If true, the tool may interact with an “open world” of external entities |
Example usage
Here’s how to define tools with annotations for different scenarios:
Integrating annotations in server implementation
Best practices for tool annotations
-
Be accurate about side effects: Clearly indicate whether a tool modifies its environment and whether those modifications are destructive.
-
Use descriptive titles: Provide human-friendly titles that clearly describe the tool’s purpose.
-
Indicate idempotency properly: Mark tools as idempotent only if repeated calls with the same arguments truly have no additional effect.
-
Set appropriate open/closed world hints: Indicate whether a tool interacts with a closed system (like a database) or an open system (like the web).
-
Remember annotations are hints: All properties in ToolAnnotations are hints and not guaranteed to provide a faithful description of tool behavior. Clients should never make security-critical decisions based solely on annotations.
Testing tools
A comprehensive testing strategy for MCP tools should cover:
- Functional testing: Verify tools execute correctly with valid inputs and handle invalid inputs appropriately
- Integration testing: Test tool interaction with external systems using both real and mocked dependencies
- Security testing: Validate authentication, authorization, input sanitization, and rate limiting
- Performance testing: Check behavior under load, timeout handling, and resource cleanup
- Error handling: Ensure tools properly report errors through the MCP protocol and clean up resources