Enum Feature 

pub enum Feature {
    Tools,
    ToolsListChanged,
    ToolAnnotations,
    ToolStructuredOutput,
    ToolResourceLinks,
    ToolAnnotationsLastModified,
    ToolResourceLinkIcons,
    ToolAudioContent,
    ToolResultType,
    Resources,
    ResourcesListChanged,
    ResourcesSubscribe,
    Prompts,
    PromptsListChanged,
    Completions,
    Roots,
    RootsListChanged,
    Sampling,
    SamplingContext,
    SamplingTools,
    Elicitation,
    ElicitationForm,
    ElicitationUrl,
    Logging,
    JsonRpcBatching,
    StreamableHttp,
    LegacySse,
    McpProtocolVersionHeader,
    MetaExtensions,
    StatelessLifecycle,
    ServerDiscover,
    SubscriptionsListen,
    StandardRoutingHeaders,
    ResponseCaching,
    MultiRoundTripRequests,
    Ping,
    Tasks,
    Extensions,
    Experimental,
}

A protocol behaviour a session may have.

Variants

Tools

tools/list and tools/call. Every revision, every transport.

ToolsListChanged

tools.list_changed, gated on the server advertising it.

ToolAnnotations

Tool annotations — read-only, destructive, and so on. 2025-03-26.

ToolStructuredOutput

structuredContent on a tool result. 2025-06-18.

ToolResourceLinks

Resource links in a tool call result. 2025-06-18.

ToolAnnotationsLastModified

annotations.lastModified on a content block. 2025-06-18.

Separate from ToolAnnotations because the two gate different revisions: annotations as a field exist in 2024-11-05, untyped, while lastModified first appears in the typed form at 2025-06-18. A value carrying only audience or priority needs the older feature alone.

ToolResourceLinkIcons

icons on a resource link. 2025-11-25, SEP-973.

https://github.com/modelcontextprotocol/modelcontextprotocol/issues/973

ToolAudioContent

Audio content blocks. 2025-03-26.

ToolResultType

resultType on tool results. 2026-07-28, required rather than optional there.

Resources

resources/list, resources/read, resources/templates/list. Every revision.

ResourcesListChanged

resources.list_changed.

ResourcesSubscribe

Server-to-client resource change notifications. The capability survives 2026-07-28 even though the methods behind it are replaced.

Prompts

prompts/list and prompts/get. Every revision.

PromptsListChanged

prompts.list_changed.

Completions

completion/complete. 2025-03-26.

Roots

roots/list. Deprecated 2026-07-28 but still defined.

RootsListChanged

notifications/roots/list_changed. Removed in 2026-07-28.

Sampling

sampling/createMessage. Deprecated 2026-07-28 but still defined.

SamplingContext

Sampling with includeContext. 2025-11-25.

SamplingTools

Sampling with tools and toolChoice. 2025-11-25, SEP-1577.

Elicitation

elicitation/create. 2025-06-18.

ElicitationForm

Form-mode elicitation with the SEP-1330 schema shapes. 2025-11-25.

ElicitationUrl

URL-mode elicitation. 2025-11-25, SEP-1036. The completion notification behind it is removed in 2026-07-28; the capability is not.

Logging

logging/setLevel and notifications/message. Deprecated 2026-07-28, which also removes the method and moves log level into _meta.

JsonRpcBatching

JSON-RPC batching. Added 2025-03-26, removed 2025-06-18.

StreamableHttp

The Streamable HTTP transport. 2025-03-26.

LegacySse

The legacy HTTP+SSE transport. 2024-11-05 only; replaced in 2025-03-26, reclassified as Deprecated in 2026-07-28.

McpProtocolVersionHeader

The MCP-Protocol-Version header on subsequent HTTP requests. 2025-06-18.

MetaExtensions

_meta on interface types. 2025-06-18.

StatelessLifecycle

The stateless lifecycle: no initialize handshake, version and capabilities in _meta on every request. 2026-07-28, SEP-2575.

ServerDiscover

server/discover. 2026-07-28, SEP-2575.

SubscriptionsListen

subscriptions/listen, replacing the HTTP GET endpoint and resources/subscribe. 2026-07-28, SEP-2575.

StandardRoutingHeaders

Mcp-Method and Mcp-Name routing headers. 2026-07-28, SEP-2243.

ResponseCaching

ttlMs and cacheScope on cacheable results. 2026-07-28, SEP-2549.

MultiRoundTripRequests

The multi-round-trip request pattern, which is what the removal of notifications/elicitation/complete is predicated on. 2026-07-28.

Ping

ping. Removed in 2026-07-28, SEP-2575.

Tasks

The tasks capability, core-protocol only. Added 2025-11-25 (SEP-1686), removed from the core in 2026-07-28 when it moves to the io.modelcontextprotocol/tasks extension (SEP-2663).

Extensions

The extensions capability bag. 2026-07-28, SEP-2575.

Experimental

The experimental capability bag. Every revision.

Implementations

impl Feature

pub const ALL: [Self; 39]

Every feature, in declaration order.

pub const fn as_str(self) -> &'static str

The feature’s name, for logs, errors and golden files.

Trait Implementations

impl Clone for Feature

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for Feature

impl Debug for Feature

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Display for Feature

fn fmt(&self, formatter: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for Feature

impl Hash for Feature

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl Ord for Feature

fn cmp(&self, other: &Self) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

fn clamp_to<R>(self, range: R) -> Self

where Self: Sized, R: ClampBounds<Self>,

🔬This is a nightly-only experimental API. (clamp_to)

Restrict a value to a certain range.

impl PartialEq for Feature

fn eq(&self, other: &Self) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl PartialOrd for Feature

fn partial_cmp(&self, other: &Self) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl StructuralPartialEq for Feature