Architecture overview changeddocs/2026-07-28/learn/architecture
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55Because MCP SDKs abstract away many concerns, most developers will likely find the [data layer protocol](#data-layer-protocol) section to be the most useful. It discusses how MCP servers can provide context to an AI application.
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7For specific implementation details, please refer to the documentation for your [language-specific SDK](/docs/2026-07-28/sdk).
7For specific implementation details, please refer to the documentation for your [language-specific SDK](/docs/2026-07-28/sdk.md).
88
99## Scope
1010
1111The Model Context Protocol includes the following projects:
1212
13* [MCP Specification](https://modelcontextprotocol.io/specification/latest): A specification of MCP that outlines the implementation requirements for clients and servers.
14* [MCP SDKs](/docs/2026-07-28/sdk): SDKs for different programming languages that implement MCP.
13* [MCP Specification](https://modelcontextprotocol.io/specification/latest.md): A specification of MCP that outlines the implementation requirements for clients and servers.
14* [MCP SDKs](/docs/2026-07-28/sdk.md): SDKs for different programming languages that implement MCP.
1515* **MCP Development Tools**: Tools for developing MCP servers and clients, including the [MCP Inspector](https://github.com/modelcontextprotocol/inspector)
1616* [MCP Reference Server Implementations](https://github.com/modelcontextprotocol/servers): Reference implementations of MCP servers.
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8383* **Discovery**: Lets clients query a server's supported protocol versions, capabilities, and identity through the `server/discover` request
8484* **Server features**: Enables servers to provide core functionality including tools for AI actions, resources for context data, and prompts for interaction templates from and to the client
85* **Client features**: Enables servers to elicit input from the user. Sampling is [deprecated](/specification/2026-07-28/deprecated) as of protocol version `2026-07-28`.
85* **Client features**: Enables servers to elicit input from the user. Sampling is [deprecated](/specification/2026-07-28/deprecated.md) as of protocol version `2026-07-28`.
8686* **Utility features**: Supports additional capabilities like notifications for real-time updates and progress tracking for long-running operations
8787
8888#### Transport layer
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104104
105105#### Statelessness and discovery
106106
107MCP is a stateless protocol (Every request contains all the information needed to process it, so servers infer nothing from previous requests). Every request carries the protocol version and the capabilities (Features and operations that a client or server supports, such as tools, resources, or prompts) relevant to that request in its `_meta` field, so the server can process each request on its own. Clients should also identify themselves in the same field unless configured not to. Servers advertise their supported versions and capabilities through the mandatory [`server/discover`](/specification/2026-07-28/server/discover) request, which clients may send before any other request. Detailed information can be found in the [specification](/specification/2026-07-28/basic/index#statelessness), and the [example](#example) showcases the per-request metadata and the discovery sequence.
107MCP is a stateless protocol (Every request contains all the information needed to process it, so servers infer nothing from previous requests). Every request carries the protocol version and the capabilities (Features and operations that a client or server supports, such as tools, resources, or prompts) relevant to that request in its `_meta` field, so the server can process each request on its own. Clients should also identify themselves in the same field unless configured not to. Servers advertise their supported versions and capabilities through the mandatory [`server/discover`](/specification/2026-07-28/server/discover) request, which clients may send before any other request. Detailed information can be found in the [specification](/specification/2026-07-28/basic/index.md#statelessness), and the [example](#example) showcases the per-request metadata and the discovery sequence.
108108
109109#### Primitives
110110
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121121
122122As a concrete example, consider an MCP server that provides context about a database. It can expose tools for querying the database, a resource that contains the schema of the database, and a prompt that includes few-shot examples for interacting with the tools.
123123
124For more details about server primitives see [server concepts](./server-concepts).
124For more details about server primitives see [server concepts](/docs/2026-07-28/learn/server-concepts.md).
125125
126126MCP also defines primitives that *clients* can expose. These primitives allow MCP server authors to build richer interactions.
127127
128128* **Elicitation**: Allows servers to request additional information from users. This is useful when server authors want to get more information from the user, or ask for confirmation of an action. Servers request user input with the `elicitation/create` method.
129129
130Elicitation requests are delivered through the [Multi Round-Trip Requests](/specification/2026-07-28/basic/patterns/mrtr) pattern, explained in the [elicitation overview](/docs/2026-07-28/learn/client-concepts#elicitation).
130Elicitation requests are delivered through the [Multi Round-Trip Requests](/specification/2026-07-28/basic/patterns/mrtr.md) pattern, explained in the [elicitation overview](/docs/2026-07-28/learn/client-concepts.md#elicitation).
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132132**Deprecated**: The following client primitives are deprecated as of protocol version `2026-07-28`.
133133
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134134* **Sampling**: Allows servers to request language model completions from the client's AI application. This is useful when server authors want access to a language model, but want to stay model-independent and not include a language model SDK in their MCP server. Servers request completions with the `sampling/createMessage` method, also delivered through the Multi Round-Trip Requests pattern. New implementations should integrate directly with LLM provider APIs.
135135* **Logging**: Enables servers to send log messages to clients for debugging and monitoring purposes. New implementations should log to `stderr` (stdio transport) or use OpenTelemetry.
136136
137For more details about client primitives see [client concepts](./client-concepts).
137For more details about client primitives see [client concepts](/docs/2026-07-28/learn/client-concepts.md).
138138
139Besides server and client primitives, the protocol supports optional [extensions](/extensions/overview) that build on the core protocol. For example, the [Tasks extension](/extensions/tasks/overview) lets servers return a durable handle for long-running requests, so clients can poll for status and retrieve the result later.
139Besides server and client primitives, the protocol supports optional [extensions](/extensions/overview.md) that build on the core protocol. For example, the [Tasks extension](/extensions/tasks/overview.md) lets servers return a durable handle for long-running requests, so clients can poll for status and retrieve the result later.
140140
141141#### Notifications
142142
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308308
309309#### Understanding the Tool Discovery Request
310310
311The `tools/list` request requires no parameters beyond the standard `_meta` fields that accompany every MCP request. It also accepts an optional `cursor` parameter for [pagination](/specification/2026-07-28/server/utilities/pagination), which the example above omits.
311The `tools/list` request requires no parameters beyond the standard `_meta` fields that accompany every MCP request. It also accepts an optional `cursor` parameter for [pagination](/specification/2026-07-28/server/utilities/pagination.md), which the example above omits.
312312
313313#### Understanding the Tool Discovery Response
314314
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321321* **`description`**: Detailed explanation of what the tool does and when to use it
322322* **`inputSchema`**: A JSON Schema that defines the expected input parameters, enabling type validation and providing clear documentation about required and optional parameters
323323
324The result is marked `"resultType": "complete"` and carries two caching fields. `ttlMs` is a freshness hint in milliseconds, so this tool list can be cached for five minutes. `cacheScope` indicates who may reuse the response. The specification's [caching utility](/specification/2026-07-28/server/utilities/caching) defines the full rules.
324The result is marked `"resultType": "complete"` and carries two caching fields. `ttlMs` is a freshness hint in milliseconds, so this tool list can be cached for five minutes. `cacheScope` indicates who may reuse the response. The specification's [caching utility](/specification/2026-07-28/server/utilities/caching.md) defines the full rules.
325325
326326#### How This Works in AI Applications
327327
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336336conversation.register_available_tools(available_tools)
337337```
338338
339Clients that federate many servers can use [progressive tool discovery](/docs/2026-07-28/develop/clients/client-best-practices#progressive-tool-discovery) rather than loading every tool upfront.
339Clients that federate many servers can use [progressive tool discovery](/docs/2026-07-28/develop/clients/client-best-practices.md#progressive-tool-discovery) rather than loading every tool upfront.
340340
341341**3. Tool Execution (Primitives)**
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