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The Initialization phase allows Clients and Agents to negotiate protocol versions, capabilities, and authentication methods.

Before a Session can be created, Clients MUST initialize the connection by calling the initialize method with:
The Agent MUST respond with the chosen protocol version, the capabilities it supports, and its implementation information:
authMethods is optional. Returning one or more valid entries advertises the Agent’s authentication surface and means the Agent MUST implement both auth/login and auth/logout. If authMethods is omitted or empty, Clients MUST NOT call either method. capabilities.auth is separate from this method surface. It advertises authentication-related extensions and does not change the availability rules for auth/login or auth/logout.

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Protocol version

The protocol versions that appear in the initialize requests and responses are a single integer that identifies a MAJOR protocol version. This version is only incremented when breaking changes are introduced. Clients and Agents MUST agree on a protocol version and act according to its specification. See Capabilities to learn how non-breaking features are introduced.

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Version Negotiation

The initialize request MUST include the latest protocol version the Client supports. If the Agent supports the requested version, it MUST respond with the same version. Otherwise, the Agent MUST respond with the latest version it supports. If the Client does not support the version specified by the Agent in the initialize response, the Client SHOULD close the connection and inform the user about it.

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Capabilities

Capabilities describe features supported by the Client and the Agent. All capabilities included in the initialize request are OPTIONAL. Clients and Agents SHOULD support all possible combinations of their peer’s capabilities. The introduction of new capabilities is not considered a breaking change. Therefore, Clients and Agents MUST treat all capabilities omitted in the initialize request as UNSUPPORTED. Capabilities may advertise top-level method surfaces or nested features that only apply within a supported surface. Capabilities may specify the availability of protocol methods, notifications, or a subset of their parameters. They may also signal behaviors of the Agent or Client implementation. Implementations can also advertise custom capabilities using the _meta field to indicate support for protocol extensions.

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Client Capabilities

Clients MAY include defined capability fields in capabilities. Omitted fields mean unsupported.

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Agent Capabilities

The Agent SHOULD specify whether it supports the following capabilities:
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session
SessionCapabilities Object
The Agent supports the session/* method surface. Omitted or null means the Agent does not support session methods. Supplying {} means the Agent supports the baseline session methods: session/new, session/list, session/resume, session/close, session/prompt, session/cancel, and session/update.
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nes
NesCapabilities Object
The Agent supports the nes/* method surface. Omitted or null means the Agent does not support NES methods. Supplying {} means the Agent supports NES sessions. NES can be advertised independently from session.
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auth
AgentAuthCapabilities Object
Authentication-related extensions supported by the Agent. This field is orthogonal to authMethods and does not advertise the availability of auth/login or auth/logout.

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Session Capabilities

Supplying session: {} means the Agent supports session/new, session/list, session/resume, session/close, session/prompt, session/cancel, and session/update. Optionally, Agents MAY support prompt extensions, MCP server transports, and additional session methods such as session/delete by specifying nested capabilities.
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prompt
PromptCapabilities Object
Object indicating the different types of content that may be included in session/prompt requests. Omitted or null means the Agent does not advertise any prompt extensions beyond the baseline text and resource-link content required by session/prompt.
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mcp
McpCapabilities Object
Object indicating the MCP server transports that may be included in session lifecycle requests. Omitted or null both mean the Agent does not advertise MCP server transport support for sessions.
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delete
SessionDeleteCapabilities Object
The session/delete method is available. Omitted or null both mean the Agent does not advertise support. Supplying an empty object means the Agent supports deleting sessions from session/list.
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additionalDirectories
SessionAdditionalDirectoriesCapabilities Object
The Agent supports additionalDirectories on supported session lifecycle requests. Omitted or null both mean the Agent does not advertise support. Supplying {} means the Agent supports additional workspace roots.

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Session Prompt Capabilities

As a baseline, Agents that advertise session MUST support ContentBlock::Text and ContentBlock::ResourceLink in session/prompt requests. Optionally, they MAY support richer types of content by specifying the following capabilities:
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image
PromptImageCapabilities Object
The prompt may include ContentBlock::Image. Omitted or null means the Agent does not advertise support. Supplying {} means the Agent supports image content in prompts.
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audio
PromptAudioCapabilities Object
The prompt may include ContentBlock::Audio. Omitted or null means the Agent does not advertise support. Supplying {} means the Agent supports audio content in prompts.
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embeddedContext
PromptEmbeddedContextCapabilities Object
The prompt may include ContentBlock::Resource. Omitted or null means the Agent does not advertise support. Supplying {} means the Agent supports embedded context in prompts.

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Session MCP Capabilities

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stdio
McpStdioCapabilities Object
The Agent supports connecting to MCP servers over stdio. Omitted or null means the Agent does not advertise support. Supplying {} means the Agent supports stdio MCP server transports.
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http
McpHttpCapabilities Object
The Agent supports connecting to MCP servers over HTTP. Omitted or null means the Agent does not advertise support. Supplying {} means the Agent supports HTTP MCP server transports.

Learn more about Authentication

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Implementation Information

Both Clients and Agents MUST provide information about their implementation in the info field. It takes the following three fields:
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name
string
Intended for programmatic or logical use, but can be used as a display name fallback if title isn’t present.
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title
string
Intended for UI and end-user contexts — optimized to be human-readable and easily understood. If not provided, the name should be used for display.
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version
string
Version of the implementation. Can be displayed to the user or used for debugging or metrics purposes.
Once the connection is initialized, you’re ready to create a session and begin the conversation with the Agent.