ng-mcp-ui
GitHub
HomeAPImcpMiddleware

mcpMiddleware

Registers protocol-level middleware that runs around each MCP request and notification.

mcpMiddleware puts your code around the MCP handlers of the server. It operates on the protocol layer, not the HTTP layer. Therefore it runs on each transport.

For the concepts and the patterns, read protocol middleware. This page gives the signatures.

Signatures

mcpMiddleware(handler: McpMiddlewareFn): this
mcpMiddleware(filter: "request", handler): this
mcpMiddleware(filter: "notification", handler): this
mcpMiddleware<M extends McpMethodString>(filter: M, handler: McpTypedMiddlewareFn<M>): this
mcpMiddleware<W extends McpWildcard>(filter: W, handler): this
mcpMiddleware(filter: McpMiddlewareFilter, handler: McpMiddlewareFn): this

Each form returns the server, therefore you can chain the calls.

Handler

type McpMiddlewareFn = (
  request: { method: string; params: Record<string, unknown> },
  extra: McpExtra | undefined,
  next: () => Promise<unknown>,
) => Promise<unknown> | unknown;
Argument Contents
request The method name and the parameters. Change params before next() to change what the handler gets.
extra The request context of the SDK. It is undefined for a notification.
next Runs the next middleware, or the handler. Call it one time only.

Filters

Filter Selects
"tools/call" One method.
"tools/*" Each method with this prefix.
"request" Each request.
"notification" Each notification.
string[] Each method that one or more patterns select.
Omitted Each request and each notification.

Types

An exact method filter narrows the handler through McpTypedMiddlewareFn<M>. TypeScript then knows the type of request.params, the type of extra, and the value that next() resolves to.

Type Purpose
McpMiddlewareFn The untyped handler.
McpTypedMiddlewareFn<M> The handler for one exact method.
McpMiddlewareFilter One pattern, or an array of patterns.
McpMethodString Each MCP method that the server handles.
McpWildcard Each "prefix/*" form.
McpExtra The request context of the SDK.
McpExtraFor<M> The extra type for one method. undefined for a notification.
McpResultFor<M> The result type for one method.

Errors

Message Cause
Cannot register MCP middleware after connect() / connectStatelessTransport() has been called You registered after the connect.
next() called multiple times in middleware for "…" One handler called next() two times.
mcpMiddleware requires a handler function when a filter is provided You gave a filter and no handler.