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Message exchange process between clients, servers, and LLMs

Core Idea: Tool call flow in MCP is a structured communication pattern that enables LLMs to request external data or actions through a defined sequence of messages between clients, servers, and the LLM itself.

Key Elements

Message Exchange Sequence

1. Initial User Request

   • User sends a query to the client application
   • Client forwards the query to the server
   • Server passes message to the LLM along with tool definitions

2. Tool Call Initiation

   • LLM analyzes the query and determines a tool is needed
   • LLM generates a tool call request with appropriate parameters
   • LLM returns a response indicating it needs to use a tool

3. Tool Execution

   • Server receives tool call request from LLM
   • Server either executes the tool directly (server tools) or
   • Server forwards request to an MCP server (for MCP tools)
   • Tool executes and generates result data

4. Tool Response Processing

   • Tool execution results are formatted as JSON
   • Results are added to the message history as a tool response
   • Complete message history with tool results is sent back to LLM

5. Final Response Generation

   • LLM processes the original query and tool results
   • LLM generates a comprehensive response incorporating tool data
   • Response is returned to the user via the client application

Server vs. MCP Tool Handling

• Server Tools Flow:

  • Tool definitions and execution logic reside in the main server
  • Server directly executes tools when requested by the LLM
  • No additional services or connections required

• MCP Server Tools Flow:

  • Tool definitions come from the MCP server but are passed to the LLM by the main server
  • When the LLM requests a tool call, main server forwards it to the MCP server
  • MCP server executes the tool and returns results to the main server
  • Main server adds results to message history and sends back to LLM

Flow Control Mechanisms

• Max Steps: Limits the number of consecutive tool calls in a single interaction
• Tool Selection: LLM decides which tool to call based on context and need
• Error Handling: Process for managing failed tool calls and retries
• Message History Management: How tool calls and results are recorded

Additional Connections

• Broader Context: Function Calling (foundational concept for tool calls)
• Applications: MCP Client Implementation (how clients participate in the flow)
• See Also: Model Context Protocol (the framework defining this flow)

References

1. Blue Collar Coder video on Model Context Protocol implementation
2. Model Context Protocol specification

#mcp #tool-calls #message-flow #llm-interaction #ai-tools

Connections:

Sources:

• From: Jack Herrington - Applied Model Context Protocol (MCP) In 20 Minutes