MCP TypeScript SDK (V2) / @modelcontextprotocol/server / server/serverEventBus

server/serverEventBus

Classes

InMemoryServerEventBus

Defined in: packages/server/src/server/serverEventBus.ts:56

A ServerEventBus backed by an in-process listener set.

publish() delivers synchronously to the live listener set (a listener unsubscribing itself mid-dispatch is safe; the entry's listen-router listeners never unsubscribe peers). A throwing listener does not stop delivery to the others.

Implements

ServerEventBus

Constructors

Constructor

new InMemoryServerEventBus(onerror?): InMemoryServerEventBus

Defined in: packages/server/src/server/serverEventBus.ts:63

Parameters

onerror?

(error) => void

Optional callback for errors thrown by listeners during dispatch.

Returns

InMemoryServerEventBus

Accessors

listenerCount

Get Signature

get listenerCount(): number

Defined in: packages/server/src/server/serverEventBus.ts:86

The number of currently registered listeners (test/introspection only — the routers track capacity via their own open-subscription set).

Returns

number

Methods

publish()

publish(event): void

Defined in: packages/server/src/server/serverEventBus.ts:65

Publish a change event to every registered listener.

Parameters

event

ServerEvent

Returns

void

Implementation of

ServerEventBus.publish

subscribe(listener): () => void

Defined in: packages/server/src/server/serverEventBus.ts:75

Parameters

listener

(event) => void

Returns

() => void

Implementation of

ServerEventBus.subscribe

Interfaces

ServerEventBus

Defined in: packages/server/src/server/serverEventBus.ts:37

The server-side change-event seam for subscriptions/listen.

The serving entry (createMcpHandler) owns the per-stream listen router: each open subscriptions/listen stream registers a listener via subscribe(), and consumer code (typically via handler.notify.* sugar) publishes change events via publish(). In-process servers can use the default InMemoryServerEventBus; multi-process deployments implement this interface over their own pub/sub.

The SDK owns wire semantics (ack-first, filtering, subscription-id stamping, teardown); a ServerEventBus only sources the events. It MUST NOT echo back to the listener that published an event when called from inside that listener (no surprise here — the default delivers synchronously and listeners never publish).

Methods

publish()

publish(event): void

Defined in: packages/server/src/server/serverEventBus.ts:41

Publish a change event to every registered listener.

Parameters

event

ServerEvent

Returns

void

subscribe(listener): () => void

Defined in: packages/server/src/server/serverEventBus.ts:45

Parameters

listener

(event) => void

Returns

() => void

ServerNotifier

Defined in: packages/server/src/server/serverEventBus.ts:96

Typed publish-side facade over bus.publish returned by createMcpHandler: each method publishes the corresponding ServerEvent. Prefer this over calling bus.publish directly — the names match the wire methods.

Methods

promptsChanged()

promptsChanged(): void

Defined in: packages/server/src/server/serverEventBus.ts:100

Publish notifications/prompts/list_changed to every open subscription that opted in.

Returns

void

resourcesChanged()

resourcesChanged(): void

Defined in: packages/server/src/server/serverEventBus.ts:102

Publish notifications/resources/list_changed to every open subscription that opted in.

Returns

void

resourceUpdated()

resourceUpdated(uri): void

Defined in: packages/server/src/server/serverEventBus.ts:104

Publish notifications/resources/updated for uri to every open subscription that opted in to that URI.

Parameters

uri

string

Returns

void

toolsChanged()

toolsChanged(): void

Defined in: packages/server/src/server/serverEventBus.ts:98

Publish notifications/tools/list_changed to every open subscription that opted in.

Returns

void

Type Aliases

ServerEvent

ServerEvent = { kind: "tools_list_changed"; } | { kind: "prompts_list_changed"; } | { kind: "resources_list_changed"; } | { kind: "resource_updated"; uri: string; }

Defined in: packages/server/src/server/serverEventBus.ts:15

A change event a server publishes for delivery on open subscriptions/listen streams. Each variant maps onto exactly one notification method:

tools_list_changed → notifications/tools/list_changed
prompts_list_changed → notifications/prompts/list_changed
resources_list_changed → notifications/resources/list_changed
resource_updated → notifications/resources/updated (carries the URI)

The bus carries the EVENT, not the wire shape — the entry's listen router owns subscription-id stamping and per-stream filtering.

Functions

createServerNotifier()

createServerNotifier(bus): ServerNotifier

Defined in: packages/server/src/server/serverEventBus.ts:108

Build a ServerNotifier over a bus.

Parameters

bus

ServerEventBus

Returns

ServerNotifier

honoredSubset()

honoredSubset(requested, capabilities?): object

Defined in: packages/server/src/server/serverEventBus.ts:162

The honored subset of a requested filter: keeps only the fields the client explicitly opted in to (drops false and absent fields), narrowed against the server's declared capabilities when supplied. The serving entry sends this back in notifications/subscriptions/acknowledged so the ack reflects what the server can actually deliver.

toolsListChanged is honored only when capabilities.tools.listChanged is advertised; likewise promptsListChanged / resourcesListChanged.
resourceSubscriptions is honored only when capabilities.resources.subscribe is advertised.

capabilities is optional on this pure helper for test convenience only — both wired routers REQUIRE capabilities at the call site (the HTTP router's serve() takes a required parameter; StdioListenRouter.serve() throws before setServerCapabilities() was called), so the fail-open undefined → honor everything branch is never reachable on a wired entry.

Parameters

requested

promptsListChanged?

boolean = ...

If true, receive notifications/prompts/list_changed.

resourcesListChanged?

boolean = ...

If true, receive notifications/resources/list_changed.

resourceSubscriptions?

string[] = ...

Subscribe to notifications/resources/updated for these resource URIs. Replaces the former resources/subscribe RPC on the 2026-07-28 revision.

toolsListChanged?

boolean = ...

If true, receive notifications/tools/list_changed.

capabilities?

completions?

{[key: string]: string | number | boolean | { [x: string]: string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | ... 4 more ... | null)[] | null)[] | null)[] | null)[] | null)[] | null; } | string | number | boolean | { [x: string]: string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | ... 4 more ... | null)[] | null)[] | null)[] | null)[] | null; } | (string | ... 4 more ... | null)[] | null[] | null; } = ...

Present if the server supports sending completions to the client.

experimental?

{[key: string]: object; } = ...

Experimental, non-standard capabilities that the server supports.

extensions?

{[key: string]: object; } = ...

Extensions that the server supports. Keys are extension identifiers (vendor-prefix/extension-name).

logging?

Present if the server supports sending log messages to the client.

Deprecated

Deprecated as of protocol version 2026-07-28 (SEP-2577); remains in the specification for at least twelve months. Migrate to stderr logging (STDIO servers) or OpenTelemetry.

prompts?

{ listChanged?: boolean; } = ...

Present if the server offers any prompt templates.

prompts.listChanged?

boolean = ...

Whether this server supports issuing notifications for changes to the prompt list.

resources?

{ listChanged?: boolean; subscribe?: boolean; } = ...

Present if the server offers any resources to read.

resources.listChanged?

boolean = ...

Whether this server supports issuing notifications for changes to the resource list.

boolean = ...

Whether this server supports clients subscribing to resource updates.

tasks?

{[key: string]: unknown; cancel?: {[key: string]: string | number | boolean | { [x: string]: string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | ... 4 more ... | null)[] | null)[] | null)[] | null)[] | null)[] | null; } | string | number | boolean | { [x: string]: string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | ... 4 more ... | null)[] | null)[] | null)[] | null)[] | null; } | (string | ... 4 more ... | null)[] | null[] | null; }; list?: {[key: string]: string | number | boolean | { [x: string]: string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | ... 4 more ... | null)[] | null)[] | null)[] | null)[] | null)[] | null; } | string | number | boolean | { [x: string]: string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | number | boolean | ... | (string | ... 4 more ... | null)[] | null)[] | null)[] | null)[] | null; } | (string | ... 4 more ... | null)[] | null[] | null; }; requests?: {[key: string]: unknown; tools?: {[key: string]: unknown; call?: {[key: string]: ... | ... | ... | ... | ... | ... | ...; }; }; }; } = ...

Present if the server supports task creation.

Deprecated

2025-11-25 wire vocabulary with no SDK runtime; parsed for interoperability only — servers built on this SDK never advertise it.

tasks.cancel?

Present if the server supports cancelling tasks.

tasks.list?

Present if the server supports listing tasks.

tasks.requests?

{[key: string]: unknown; tools?: {[key: string]: unknown; call?: {[key: string]: ... | ... | ... | ... | ... | ... | ...; }; }; } = ...

Capabilities for task creation on specific request types.

tasks.requests.tools?

{[key: string]: unknown; call?: {[key: string]: ... | ... | ... | ... | ... | ... | ...; }; } = ...

Task support for tool requests.

tasks.requests.tools.call?

{[key: string]: ... | ... | ... | ... | ... | ... | ...; } = ...

tools?

{ listChanged?: boolean; } = ...

Present if the server offers any tools to call.

tools.listChanged?

boolean = ...

Whether this server supports issuing notifications for changes to the tool list.

Returns

promptsListChanged?

optional promptsListChanged?: boolean

If true, receive notifications/prompts/list_changed.

resourcesListChanged?

optional resourcesListChanged?: boolean

If true, receive notifications/resources/list_changed.

resourceSubscriptions?

optional resourceSubscriptions?: string[]

Subscribe to notifications/resources/updated for these resource URIs. Replaces the former resources/subscribe RPC on the 2026-07-28 revision.

toolsListChanged?

optional toolsListChanged?: boolean

If true, receive notifications/tools/list_changed.

listenFilterAccepts()

listenFilterAccepts(filter, event): boolean

Defined in: packages/server/src/server/serverEventBus.ts:127

Whether a subscriptions/listen filter accepts a given change event.

Pure: no I/O, no mutation. The filter governs ONLY the four subscription-gated change types — non-gated notifications never reach the bus and are not modeled here.

resource_updated matches only when resourceSubscriptions is present and contains the event's URI exactly (per the spec: "for these resource URIs").

Parameters

filter

promptsListChanged?

boolean = ...

If true, receive notifications/prompts/list_changed.

resourcesListChanged?

boolean = ...

If true, receive notifications/resources/list_changed.

resourceSubscriptions?

string[] = ...

Subscribe to notifications/resources/updated for these resource URIs. Replaces the former resources/subscribe RPC on the 2026-07-28 revision.

toolsListChanged?

boolean = ...

If true, receive notifications/tools/list_changed.

event

ServerEvent

Returns

boolean

serverEventToNotification()

serverEventToNotification(event): object

Defined in: packages/server/src/server/serverEventBus.ts:179

Map a ServerEvent onto its wire notification {method, params}.

Parameters

event

ServerEvent

Returns

object

method

method: string

params?

optional params?: object

params.uri

uri: string

index

Namespaces

migrations

v1-to-v2

mappings

transforms

index

Namespaces

middleware

auth

handlers

middleware

providers

sse

server/serverEventBus ​

Classes ​

InMemoryServerEventBus ​

Implements ​

Constructors ​

Constructor ​

Parameters ​

onerror? ​

Returns ​

Accessors ​

listenerCount ​

Get Signature ​

Returns ​

Methods ​

publish() ​

Parameters ​

event ​

Returns ​

Implementation of ​

subscribe() ​

Parameters ​

listener ​

Returns ​

Implementation of ​

Interfaces ​

ServerEventBus ​

Methods ​

publish() ​

Parameters ​

event ​

Returns ​

subscribe() ​

Parameters ​

listener ​

Returns ​

ServerNotifier ​

Methods ​

promptsChanged() ​

Returns ​

resourcesChanged() ​

Returns ​

resourceUpdated() ​

Parameters ​

uri ​

Returns ​

toolsChanged() ​

Returns ​

Type Aliases ​

ServerEvent ​

Functions ​

createServerNotifier() ​

Parameters ​

bus ​

Returns ​

honoredSubset() ​

Parameters ​

requested ​

promptsListChanged? ​

resourcesListChanged? ​

resourceSubscriptions? ​

toolsListChanged? ​

capabilities? ​

completions? ​

experimental? ​

extensions? ​

logging? ​

server/serverEventBus

Classes

InMemoryServerEventBus

Implements

Constructors

Constructor

Parameters

onerror?

Returns

Accessors

listenerCount

Get Signature

Returns

Methods

publish()

Parameters

event

Returns

Implementation of

subscribe()

Parameters

listener

Returns

Implementation of

Interfaces

ServerEventBus

Methods

publish()

Parameters

event

Returns

subscribe()

Parameters

listener

Returns

ServerNotifier

Methods

promptsChanged()

Returns

resourcesChanged()

Returns

resourceUpdated()

Parameters

uri

Returns

toolsChanged()

Returns

Type Aliases

ServerEvent

Functions

createServerNotifier()

Parameters

bus

Returns

honoredSubset()

Parameters

requested

promptsListChanged?

resourcesListChanged?

resourceSubscriptions?

toolsListChanged?

capabilities?

completions?

experimental?

extensions?

logging?

prompts?

prompts.listChanged?

resources?

resources.listChanged?

resources.subscribe?

tasks?

tasks.cancel?

tasks.list?

tasks.requests?

tasks.requests.tools?

tasks.requests.tools.call?

tools?

tools.listChanged?

Returns