Function openSession

packages/hosts/mcp/src/in-memory-session-store.ts:391–438

(
    principal: Principal,
    resource: McpResource,
    request: Request,
  )

  /** Open a new session: build the server, connect a transport, drive the request. */
  const openSession = (
    principal: Principal,
    resource: McpResource,
    request: Request,
  ): Effect.Effect<McpDispatchResult> => {
    let createdSessionId: string | null = null;
    return buildServer(principal, {
      ...buildOptionsFor(request, () => createdSessionId),
      resource,
    }).pipe(
      Effect.flatMap(({ mcpServer, engine }) =>
        Effect.gen(function* () {
          const transport = new WebStandardStreamableHTTPServerTransport({
            sessionIdGenerator: () => crypto.randomUUID(),
            enableJsonResponse: true,
            onsessioninitialized: (sid) => {
              createdSessionId = sid;
              transports.set(sid, transport);
              servers.set(sid, mcpServer);
              owners.set(sid, { principal, resource });
              engines.set(sid, engine);
              lastSeen.set(sid, Date.now());
            },
            onsessionclosed: (sid) => void dispose(sid, { server: true }),
          });
          transport.onclose = () => {
            const sid = transport.sessionId;
            if (sid) void dispose(sid, { server: true });
          };
          yield* Effect.promise(() => mcpServer.connect(transport));
          // The session id is minted on the first (initialize) request, so we
          // drive `handleRequest` here; if no id results we close eagerly.
          return yield* runHandleRequest(transport, request, () => {
            // Nothing was ever registered under a session id, so `dispose` has
            // no entry to work from — release the three handles by hand, engine
            // included.
            void ignoreClose(null, "transport", () => transport.close());
            void ignoreClose(null, "server", () => mcpServer.close());
            void shutdownEngine(null, engine);
          });
        }),
      ),
      // A build failure has nowhere typed to go in the envelope; render a 500.
      Effect.catchTag("McpEngineBuildError", () =>
        Effect.succeed(jsonRpcError(500, -32603, "Internal server error")),
      ),
    );
  };

  /**
   * The session-less POST path: answer a probe for anything but `initialize`