modelcontextprotocol/typescript-sdk @modelcontextprotocol/client@2.0.0-beta.3

on GitHub

Patch Changes

• #2456 44797d7 Thanks @felixweinberger! - Restore the v1 parse tolerance for CallToolResult.content: an inbound legacy-era tools/call result without content defaults to [] instead of failing validation. Deployed servers — accepted by SDK v1 for years — return structuredContent-only (or otherwise content-less) results, and the strict parse turned every such call into an INVALID_RESULT error before application code could run.

  The silent-empty-success hazard the strictness guarded is preserved where it matters: the 2025 era's wire-seam schema refuses to default content for a body carrying another result family's vocabulary (task, inputRequests, requestState — the era is frozen, so the list is complete), and the 2026-era wire schemas stay strict — modern-revision servers have no legacy excuse. Task interop through an explicit result schema is untouched (including bodies that also stamp a foreign resultType), and the server-side authoring normalization refuses the same foreign-family vocabulary.

  Server-side authoring is era-independent: a handler result without content (dynamic/JS callers — the TypeScript surface requires it) is normalized to content: [] before era validation on every leg, reaching the wire spec-valid.

  Conscious call: the nested sampling ToolResultContentSchema stays spec-strict — v1 had defaulted its content too, but it is params-side (tool results a caller authors into a sampling message), deliberately not restored.

• #2431 1b90c96 Thanks @morluto! - Fix the CommonJS validators/ajv subpath so reading the exported Ajv class no longer throws ReferenceError: import_ajv is not defined. The subpath now re-exports the bundled provider's concrete Ajv value in CJS output, matching the existing ESM behavior.

• #2441 561c6d8 Thanks @felixweinberger! - POSTs whose Content-Type media type is not application/json are now
  rejected with 415 Unsupported Media Type; the header is parsed instead of
  substring-matched. Previously any value merely containing the substring
  passed the check (for example text/plain; a=application/json), case
  variants were wrongly rejected, and the 2026-07-28 entry did not inspect
  Content-Type at all — requests with a missing or non-JSON header that used
  to be served on that path now also answer 415. Values with parameters
  (application/json; charset=utf-8, including malformed parameter sections
  like application/json;) continue to work. SDK clients always send the
  correct header and are unaffected.

  The new isJsonContentType(header) helper is exported for transport and
  framework-adapter authors — custom entries composing the exported building
  blocks (classifyInboundRequest, PerRequestHTTPServerTransport) must apply
  it themselves. The hono adapter's JSON body pre-parse and the client's
  response dispatch now use the same parsed-media-type comparison.

• #2384 ce2f65d Thanks @felixweinberger! - instanceof on the SDK error classes (ProtocolError and its typed subclasses, SdkError/SdkHttpError, OAuthError, and the client's SseError, UnauthorizedError, and OAuth-client-flow error family — OAuthClientFlowError and its subclasses) now works across separately bundled copies of the SDK. The classes match by a stable brand (via Symbol.hasInstance and a registry symbol) instead of prototype identity, so a process that uses both @modelcontextprotocol/client and @modelcontextprotocol/server - a gateway, host, or in-process test - can check errors constructed by either package against the class re-exported by the other. Ordinary prototype-based instanceof is preserved as a fallback; user-defined subclasses keep plain prototype semantics. Notes: cross-bundle matching requires both copies to be at or after this release; brands assert identity, not field shape, across versions - keep reading fields defensively. As a side effect, a foreign-bundle SdkError used as an abort reason is now rethrown as-is instead of being wrapped as a RequestTimeout. Branded hierarchies additionally expose an explicit static guard, X.isInstance(value), that reads the same brand and narrows in TypeScript — an alternative for codebases that prefer predicate-style checks over instanceof. Also: UnauthorizedError now sets error.name to 'UnauthorizedError' (previously 'Error'), and per-package conformance tests enforce that every exported error class participates in branding. Version-negotiation probing now recognizes UnauthorizedError (previously a dead name-string check) and propagates it unchanged, so connect() on an auth-gated server rejects with the original UnauthorizedError (previously wrapped as the cause of an SdkError(EraNegotiationFailed)) — run finishAuth() and reconnect, and the retry probes with the token.

• #2469 9b41b56 Thanks @felixweinberger! - The Streamable HTTP client transport no longer attaches a session ID to a POST containing an initialize request — a new session starts "without a session ID attached" (2025-11-25 transports §Session Management) — and it only captures the mcp-session-id response header from a successful initialize response, since the spec assigns the session ID "at initialization time … on the HTTP response containing the InitializeResult". Previously the transport stored the header from any response, so a legacy server answering a protocol-version probe with an error that happened to carry a session ID would poison the fallback initialize, which then went out with a session ID it should not have had. A stale session ID from a previous connection is likewise no longer leaked onto the initialize handshake, and a successful initialize response that carries no session ID now clears any stale ID the transport was holding — clients include only an ID "returned by the server during initialization", so an ID the server never returned this session is outside the session model. Ignoring mcp-session-id headers mid-session is the complement of the spec's one actual rotation mechanism: a server that wants a new session terminates the old one (it "MAY terminate the session at any time") and answers 404, after which the client "MUST start a new session by sending a new InitializeRequest without a session ID attached". Rotation exists as session replacement via 404 + re-initialize, never as a header swap on a live session, so a server that rotates per the spec's own flow is handled correctly by this transport.

• #2455 cc70c5e Thanks @felixweinberger! - Version negotiation no longer discards transport handlers the caller set before connect(). The probe window now saves any pre-set onmessage/onerror/onclose, forwards error and close events to them while the probe is in flight, and restores them when the window closes — so Protocol.connect() chains them exactly as it does on a plain connect. Previously, connecting with versionNegotiation silently cleared pre-set handlers (e.g. an onerror used to detect session-expiry auth failures), leaving them permanently detached for the life of the connection.

• #2425 e8de519 Thanks @Sehlani042! - Stop advertising validator provider classes from the root client/server type declarations. The provider classes remain available from the explicit validator subpaths.