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HomeDocsModel Context Protocol 29 Jul 2026
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SEP-2133: Extensions changedseps/2133-extensions

Upstream edited this page at 29 Jul 2026 22:46 UTC, give or take a minute or two: the time comes from Anthropic’s own sitemap rather than from a commit. This site recorded the change at 22 Oct 2026 20:37 UTC.

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1111> This SEP has reached Final status and is preserved as a historical record of
1212> the design as accepted. Changes made to the protocol after finalization are
1313> not reflected here. Refer to the [current
14> specification](/specification/latest) and its changelog for authoritative
14> specification](/specification/latest.md) and its changelog for authoritative
1515> requirements.
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1717| Field | Value |
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4343 
4444An MCP extension is an optional addition to the specification that defines capabilities beyond the core protocol. Extensions enable functionality that may be modular (e.g., distinct features like authentication), specialized (e.g., industry-specific logic), or experimental (e.g., features being incubated for potential core inclusion).
4545 
46Extensions are identified using a unique *extension identifier* with the format: `{vendor-prefix}/{extension-name}`, e.g. `io.modelcontextprotocol/oauth-client-credentials` or `com.example/websocket-transport`. The names follow the same rules as the [\_meta keys](https://modelcontextprotocol.io/specification/draft/basic/index#meta), except that the prefix is mandatory.
46Extensions are identified using a unique *extension identifier* with the format: `{vendor-prefix}/{extension-name}`, e.g. `io.modelcontextprotocol/oauth-client-credentials` or `com.example/websocket-transport`. The names follow the same rules as the [\_meta keys](https://modelcontextprotocol.io/specification/draft/basic/index.md#meta), except that the prefix is mandatory.
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4848To prevent identifier collisions, the vendor prefix SHOULD be a reversed domain name that the extension author owns or controls (similar to Java package naming conventions). For example, a company owning `example.com` would use `com.example/` as their prefix.
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8989 
9090Extensions MAY optionally begin as experimental extensions (see *Experimental Extensions* section) to facilitate prototyping and collaboration before formal submission. This incubation period is encouraged but not required.
9191 
92To become an official extension, extensions are created via a SEP in the [main MCP repository](https://github.com/modelcontextprotocol/modelcontextprotocol/) using the [standard SEP guidelines](https://modelcontextprotocol.io/community/sep-guidelines) but with a new type: **Extensions Track**. This type follows the same review and acceptance process as Standards Track SEPs, but clearly indicates that the proposal is for an extension rather than a core protocol addition. The SEP must identify the Working Group and Extension Maintainers that will be responsible for the extension. See [SEP-2148](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2148) for how maintainers are appointed.
92To become an official extension, extensions are created via a SEP in the [main MCP repository](https://github.com/modelcontextprotocol/modelcontextprotocol/) using the [standard SEP guidelines](https://modelcontextprotocol.io/community/sep-guidelines.md) but with a new type: **Extensions Track**. This type follows the same review and acceptance process as Standards Track SEPs, but clearly indicates that the proposal is for an extension rather than a core protocol addition. The SEP must identify the Working Group and Extension Maintainers that will be responsible for the extension. See [SEP-2148](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2148) for how maintainers are appointed.
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9494Extension SEPs:
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110110 
111111### Spec Recommendation
112112 
113Extensions will be referenced from a new page on the MCP website at [modelcontextprotocol.io/extensions](http://modelcontextprotocol.io/extensions) (to be created) with links to their specification.
113Extensions will be referenced from a new page on the MCP website at [modelcontextprotocol.io/extensions](https://modelcontextprotocol.io/extensions.md) (to be created) with links to their specification.
114114 
115Links to relevant extensions MAY also be added to the core specification as appropriate (e.g. [https://modelcontextprotocol.io/specification/draft/basic/authorization](https://modelcontextprotocol.io/specification/draft/basic/authorization) may link to ext-auth extensions), but they MUST be clearly advertised as optional extensions and SHOULD be links only (not copies of specification text).
115Links to relevant extensions MAY also be added to the core specification as appropriate (e.g. [https://modelcontextprotocol.io/specification/draft/basic/authorization](https://modelcontextprotocol.io/specification/draft/basic/authorization.md) may link to ext-auth extensions), but they MUST be clearly advertised as optional extensions and SHOULD be links only (not copies of specification text).
116116 
117117### SDK Implementation
118118 
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132132 
133133### Negotiation
134134 
135Clients and servers advertise their support for extensions in the [ClientCapabilities](https://modelcontextprotocol.io/specification/2025-06-18/schema#clientcapabilities) and [ServerCapabilities](https://modelcontextprotocol.io/specification/2025-06-18/schema#servercapabilities) fields respectively, and in the [Server Card](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1649) (currently in progress).
135Clients and servers advertise their support for extensions in the [ClientCapabilities](https://modelcontextprotocol.io/specification/2025-06-18/schema.md#clientcapabilities) and [ServerCapabilities](https://modelcontextprotocol.io/specification/2025-06-18/schema.md#servercapabilities) fields respectively, and in the [Server Card](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1649) (currently in progress).
136136 
137137A new "extensions" field will be introduced to each that is a map of *extension identifiers* to per-extension settings objects. Each extension specifies the schema of its settings object; an empty object indicates no settings.
138138 
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