玩转MCP(2)-原理篇

1. 引言

这篇文章萌叔来谈谈MCP的协议的一些重要概念，以及它是如何和大模型进行交互的。

2. MCP架构

2.1 Host

Host进程充当容器和协调器，它要负责启动MCP-Client，使用Client与对应的MCP-Server进行交互。通常而言Host with Client就是第一个AI Agent，在其中必然会涉及与大模型的交互。

2.2 Client

Client由Host创建，每个Client与特定的Server具有一对一的关系

2.3 Server

通过 MCP 原语暴露resource、tool和prompt, Server可以运行在本地或者是远程服务

3. MCP协议的内容

MCP协议的核心诉求是对外说明Server具有什么能力，Client该如何使用这种能力，每种能力其实都被抽象成了某个tool，类似于函数。

为了更好的使用tool,MCP引入了resource和prompt，

3.1 `resource`

resource一般是可访问的静态或动态数据源，笔者的理解，resource是领域相关的知识库

3.2 `prompt`

用户或系统提供给模型的指令或上下文输入，用于引导模型行为。

大部分情况下，只提供tool即可，注意: 一个server通过会提供多个tool。 Client和Server采用JSON-RPC 2.0协议。

3.3 Client要获取tool信息通常会发起 `ListTools` 指令

Request

{
	"method": "tools/list",
	"params": {
		"_meta": {
			"progressToken": 3
		}
	},
	"jsonrpc": "2.0",
	"id": 3
}

Response

{
	"jsonrpc": "2.0",
	"id": 3,
	"result": {
		"tools": [{
			"name": "remember",
			"description": "Retrieve historical chat records between users and LLM.",
			"inputSchema": {
				"properties": {
					"keyword": {
						"description": "key word",
						"title": "Keyword",
						"type": "string"
					},
					"start_date": {
						"anyOf": [{
							"type": "string"
						}, {
							"type": "null"
						}],
						"default": null,
						"description": "Start date in &#039;YYYYMMDD&#039; format.When empty, automatically uses the date 3 months before today",
						"examples": ["20250620"],
						"title": "Start Date"
					},
					"end_date": {
						"anyOf": [{
							"type": "string"
						}, {
							"type": "null"
						}],
						"default": null,
						"description": "End date in &#039;YYYYMMDD&#039; format.When empty, automatically uses today&#039;s date",
						"examples": ["20250710"],
						"title": "End Date"
					},
					"max_message_count": {
						"default": 200,
						"description": "The maximum number of messages that can be returned",
						"minimum": 1,
						"title": "Max Message Count",
						"type": "integer"
					}
				},
				"required": ["keyword"],
				"type": "object"
			},
			"outputSchema": {
				"properties": {
					"result": {
						"title": "Result",
						"type": "string"
					}
				},
				"required": ["result"],
				"title": "_WrappedResult",
				"type": "object",
				"x-fastmcp-wrap-result": true
			}
		}]
	}
}

3.4 Client使用某个tool时，会使用 `callTool` 指令

Request

{
	"jsonrpc": "2.0",
	"method": "tools/call",
	"id": "tool-call-1754015372815",
	"params": {
		"name": "get_datetime",
		"arguments": {
			"format": "date_jp"
		},
		"_meta": {
			"progressToken": 0
		}
	}
}

Response

{
	"jsonrpc": "2.0",
	"id": "tool-call-1754015372815",
	"result": {
		"content": [{
			"type": "text",
			"text": "2025年08月01日"
		}],
		"isError": false
	}
}

4.与大模型的交互

我们已经通过MCP协议获知了MCP Server所能提供的能力，现在需要把这些信息告诉大模型，让大模型依据上下文以及MCP Server提供的能力，做出决策，发出指令。

下面我们以 daodao97/chatmcp 为示例，看看与大模型的交互过程

{
	"model": "deepseek-chat",
	"messages": [{
		"role": "system",
		"content": "You are an intelligent and helpful AI assistant. Please:\n1.Provide clear and concise responses\n2.If you&#039;re not sure about something, please say so\n3.When appropriate, provide examples to illustrate your points\n4.If a user messages you in a specific language, respond in that language\n5.Format responses using markdown when helpful\n6.Use mermaid to generate diagrams\n7.NOTICE: Before starting new conversation, try to get the current time(IMPORTANT)\n<system_prompt>\nYou will select appropriate tools and call them to solve user queries\n\n**CRITICAL CONSTRAINT: You MUST call only ONE tool per response. Never call multiple tools simultaneously.**\n</system_prompt>\n\nLanguage: zh\n**Tool Definitions:**\nHere are the functions available, described in JSONSchema format:\n<tool_definitions>\n[\n  {\n    \"name\": \"get_datetime\",\n    \"description\": \"Get current date and time in various formats\",\n    \"inputSchema\": {\n      \"type\": \"object\",\n      \"properties\": {\n        \"format\": {\n          \"type\": \"string\",\n          \"description\": \"\\nAvailable formats:\\n- date: 2024-12-10\\n- date_slash: 2024/12/10\\n- date_jp: 2024年12月10日\\n- datetime: 2024-12-10 00:54:01\\n- datetime_jp: 2024年12月10日 00時54分01秒\\n- datetime_t: 2024-12-10T00:54:01\\n- compact: 20241210005401\\n- compact_date: 20241210\\n- compact_time: 005401\\n- filename_md: 20241210005401.md\\n- filename_txt: 20241210005401.txt\\n- filename_log: 20241210005401.log\\n- iso: 2024-12-10T00:54:01+0900\\n- iso_basic: 20241210T005401+0900\\n- log: 2024-12-10 00:54:01.123456\\n- log_compact: 20241210_005401\\n- time: 00:54:01\\n- time_jp: 00時54分01秒\\n\"\n        }\n      },\n      \"required\": [\n        \"format\"\n      ]\n    }\n  }\n]\n</tool_definitions>\n<tool_usage_instructions>\nTOOL USE\n\nYou have access to a set of tools that are executed upon the user&#039;s approval. You can use one tool per message, and will receive the result of that tool use in the user&#039;s response. You use tools step-by-step to accomplish a given task, with each tool use informed by the result of the previous tool use.\n\n# Tool Use Formatting\n\nTool use is formatted using XML-style tags. The tool name is enclosed in opening and closing tags, and parameters must be provided in JSON format. Here&#039;s the structure:\n\n<function name=\"{tool_name}\">\n{\n  \"parameter1_name\": \"value1\",\n  \"parameter2_name\": \"value2\"\n}\n</function>\n\nFor example:\n\n<function name=\"read_file\">\n{\n  \"path\": \"src/main.js\"\n}\n</function>\n\nAlways adhere to this format for the tool use to ensure proper parsing and execution.\n</tool_usage_instructions>"
	}, {
		"role": "user",
		"content": [{
			"type": "text",
			"text": "今天几号"
		}]
	}, {
		"role": "assistant",
		"content": "<function  name=\"get_datetime\">\n{\n  \"format\": \"date_jp\"\n}\n</function>"
	}, {
		"role": "user",
		"content": "tool result: get_datetime\n[{type: text, text: 2025年08月01日}]\n"
	}, {
		"role": "assistant",
		"content": "今天是2025年8月1日。"
	}, {
		"role": "user",
		"content": [{
			"type": "text",
			"text": "明天是几号？"
		}]
	}, {
		"role": "assistant",
		"content": "<function name=\"get_datetime\">\n{\n  \"format\": \"date_jp\"\n}\n</function>"
	}, {
		"role": "user",
		"content": "tool result: get_datetime\n[{type: text, text: 2025年08月01日}]\n"
	}],
	"stream": true,
	"temperature": 1.0,
	"top_p": 1.0
}

第一条消息比较大，我们格式化一下

You are an intelligent and helpful AI assistant. Please:
1.Provide clear and concise responses
2.If you&#039;re not sure about something, please say so
3.When appropriate, provide examples to illustrate your points
4.If a user messages you in a specific language, respond in that language
5.Format responses using markdown when helpful
6.Use mermaid to generate diagrams
7.NOTICE: Before starting new conversation, try to get the current time(IMPORTANT)
<system_prompt>
You will select appropriate tools and call them to solve user queries

**CRITICAL CONSTRAINT: You MUST call only ONE tool per response. Never call multiple tools simultaneously.**
</system_prompt>

Language: zh
**Tool Definitions:**
Here are the functions available, described in JSONSchema format:
<tool_definitions>
[
  {
    "name": "get_datetime",
    "description": "Get current date and time in various formats",
    "inputSchema": {
      "type": "object",
      "properties": {
        "format": {
          "type": "string",
          "description": "\nAvailable formats:\n- date: 2024-12-10\n- date_slash: 2024/12/10\n- date_jp: 2024年12月10日\n- datetime: 2024-12-10 00:54:01\n- datetime_jp: 2024年12月10日 00時54分01秒\n- datetime_t: 2024-12-10T00:54:01\n- compact: 20241210005401\n- compact_date: 20241210\n- compact_time: 005401\n- filename_md: 20241210005401.md\n- filename_txt: 20241210005401.txt\n- filename_log: 20241210005401.log\n- iso: 2024-12-10T00:54:01+0900\n- iso_basic: 20241210T005401+0900\n- log: 2024-12-10 00:54:01.123456\n- log_compact: 20241210_005401\n- time: 00:54:01\n- time_jp: 00時54分01秒\n"
        }
      },
      "required": [
        "format"
      ]
    }
  }
]
</tool_definitions>
<tool_usage_instructions>
TOOL USE

You have access to a set of tools that are executed upon the user&#039;s approval. You can use one tool per message, and will receive the result of that tool use in the user&#039;s response. You use tools step-by-step to accomplish a given task, with each tool use informed by the result of the previous tool use.

# Tool Use Formatting

Tool use is formatted using XML-style tags. The tool name is enclosed in opening and closing tags, and parameters must be provided in JSON format. Here&#039;s the structure:

<function name="{tool_name}">
{
  "parameter1_name": "value1",
  "parameter2_name": "value2"
}
</function>

For example:

<function name="read_file">
{
  "path": "src/main.js"
}
</function>

Always adhere to this format for the tool use to ensure proper parsing and execution.
</tool_usage_instructions>

tool的定义信息是通过messages传递，而没有使用OpenAI API提供的tools参数，这种做法可能是为了在多种LLM保持兼容。
tool定义使用了特殊的XML标签<tool_definitions>，指令下发使用了XML标签<function>
最后还是要强调，LLM只能做出决策，callTool是由Host发起的，tool的执行结果稍后也通过messages传递给LLM

为什么建议LLM一次只发出一条`callTool`指令

细心的读者可能会发现

CRITICAL CONSTRAINT: You MUST call only ONE tool per response. Never call multiple tools simultaneously.

这是因为很多情况下，后一个tool的入参可能会依赖前一个tool的执行结果，针对这种情况，大模型容易处理错误。最简单的方式就是一次只执行一条指令。

示例

假定我有一个音乐MCP，提供了search_music和play_music

Q: 请为我播放周杰伦的《稻香》？

显然需要获得

1）使用search_music获取音乐文件的URL(存在无法获取到的情况)
2）如果step 1获取到了URL，使用play_music 播放音乐

5. 时序图

我整理了一张完整的时序图，对交互逻辑有不明白的地方，可以好好看看这张图。

6. 总结

得益于MCP提供的能力加持，AI agent的能力有了极大的提高，不久可能将会迎来量到质的飞跃。

参考资料

1.MCP-architecture 2.MCP-transports

作者: vearne 文章标题: 玩转MCP(2)-原理篇发表时间: 2025年8月5日文章链接: https://vearne.cc/archives/40297 版权说明: CC BY-NC-ND 4.0 DEED

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1. 引言#

2. MCP架构#

2.1 Host#

2.2 Client#

2.3 Server#

3. MCP协议的内容#

3.1 resource#

3.2 prompt#

3.3 Client要获取tool信息通常会发起 ListTools 指令#

Request#

Response#

3.4 Client使用某个tool时，会使用 callTool 指令#

Request#

Response#

4.与大模型的交互#

为什么建议LLM一次只发出一条callTool指令#

示例#

Q: 请为我播放周杰伦的《稻香》？#

5. 时序图#

6. 总结#

参考资料#

1. 引言

2. MCP架构

2.1 Host

2.2 Client

2.3 Server

3. MCP协议的内容

3.1 `resource`

3.2 `prompt`

3.3 Client要获取tool信息通常会发起 `ListTools` 指令

Request

Response

3.4 Client使用某个tool时，会使用 `callTool` 指令

Request

Response

4.与大模型的交互

为什么建议LLM一次只发出一条`callTool`指令

示例

Q: 请为我播放周杰伦的《稻香》？

5. 时序图

6. 总结

参考资料