MCP TS SDK v2.0.0：支持2026规范，多项破坏性变更

Minor Changes

• #2501 1480241 Thanks @felixweinberger! - Export the Protocol base class and mergeCapabilities from the @modelcontextprotocol/client and @modelcontextprotocol/server package roots, restoring the v1 import for consumers that subclass Protocol (e.g. the MCP Apps SDK). The client and server packages each bundle their own compiled copy of the class, so import it from one package consistently within a process.
• The codemod now rewrites Protocol and mergeCapabilities imports from shared/protocol.js to the client or server package root, like the module's other symbols, instead of dropping them with an action-required marker.
• #2511 f60dff0 Thanks @felixweinberger! - ConnectOptions.prior accepts a cached era verdict — the new exported type PriorDiscovery. { kind: 'modern', discover } adopts a previously obtained DiscoverResult with zero round trips; { kind: 'legacy' } skips the server/discover probe and runs the plain initialize handshake directly, for servers known out-of-band to be legacy — without pinning the client to mode: 'legacy': stop supplying the verdict and connect() falls back to the configured versionNegotiation mode (under 'auto', it re-probes and rediscovers an upgraded server). Freshness is the supplying host's responsibility — a stale legacy verdict succeeds silently against an upgraded server, so hosts must date cached legacy verdicts in their own storage and stop supplying them past their policy horizon. Persisted-blob plumbing is hardened: prior: null is treated as absent, the modern arm's discover payload is schema-validated before any connection state changes, and an unrecognized shape rejects with a typed SdkError(EraNegotiationFailed) instead of a TypeError.
• #2468 5db6e38 Thanks @felixweinberger! - The response cache now stores results as JSON-serialized documents (serialize on write, parse on read) instead of live object graphs isolated with structuredClone. Same mutation isolation, but no dependency on the structuredClone global — whose absence (jest+jsdom, Node < 17) previously made every cache write throw into the store-error swallow, silently disabling caching and output-schema lookups for the session. A value without a JSON representation now fails the write loudly to the error sink, and an undecodable document in an external store is reported, dropped, and read as a miss.
• Migration for custom ResponseCacheStore implementations: CacheEntry.value (and the set() entry value) is now string — persist and return it verbatim, JSON.parse to inspect. Entries persisted by a previous SDK version fail decode once (reported, dropped) and are rewritten on the next fetch.
• #2477 8e1d2e9 Thanks @felixweinberger! - Move the schema source modules (spec schemas, OAuth schemas, protocol constants) into @modelcontextprotocol/core and resolve them from there as a regular runtime dependency instead of bundling a private copy into each package. An application importing more than one of the packages now evaluates a single shared schema graph with shared object identity. @modelcontextprotocol/core gains a ./internal subpath (SDK-internal contract; may change in any release) and the four packages now version together.
• #2513 f413763 Thanks @felixweinberger! - Align the 2026-07-28 wire with the final revision (spec PR #3002): serverInfo moves from the DiscoverResult body to the result _meta, and the per-request envelope's clientInfo demotes from required to SHOULD.
• Before this change the SDK shipped the pre-#3002 shape in both directions: the client hard-rejected a conforming server's DiscoverResult (missing body serverInfo failed parse, so the probe misclassified the server as legacy and attempted an initialize handshake against it — a hard connect failure against a modern-only server such as go-sdk v1.7.0-pre.3), and the server rejected conforming clients that omit clientInfo.
• Now:
• The 2026 wire schemas are the final revision exactly: no body serverInfo on DiscoverResult, envelope clientInfo optional (a present-but-malformed value still fails validation).
• Servers stamp _meta['io.modelcontextprotocol/serverInfo'] on every 2026-era response (spec SHOULD; a handler-authored value wins, the 2025-era wire is untouched). This includes the entry-built subscriptions/listen graceful-close results — the spec's SubscriptionsListenResultMeta extends ResultMetaObject.
• Clients keep sending clientInfo and read server identity from the discover result's _meta only. A server that stamps no identity is anonymous: getServerVersion() is undefined and the response cache partitions under a per-connection surrogate. A malformed _meta serverInfo value is treated as absent on receive (the spec marks the field self-reported, unverified, and display-only).
• Breaking type changes: DiscoverResult no longer declares serverInfo; RequestMetaEnvelope's clientInfo is optional. New public constant SERVER_INFO_META_KEY ('io.modelcontextprotocol/serverInfo').
• #2483 3f07a32 Thanks @felixweinberger! - Add preloadSchemas(), an explicit opt-in to eager wire-schema construction, and call it automatically in the Cloudflare Workers builds. The wire schemas are built lazily by default, which is the right trade on process-per-invocation runtimes — but on isolate platforms that bill request CPU while module evaluation runs during isolate warm-up, laziness moves construction into the first request each fresh isolate serves. Calling preloadSchemas() at module scope (it is synchronous and idempotent) moves that one-time cost back to module evaluation; the packages' workerd export condition now does this automatically, while the Node and browser builds stay lazy. The server package gains a dedicated browser shim for this (its browser condition previously reused the workerd shim), so browser bundles keep lazy construction.

Patch Changes

• #2402 a400259 Thanks @felixweinberger! - First beta release of SDK v2 with support for the MCP 2026-07-28 specification
• revision. See the migration guides for upgrading from v1
• (docs/migration/upgrade-to-v2.md) and adopting the 2026-07-28 revision
• (docs/migration/support-2026-07-28.md).
• #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.