typescript-sdk/.changeset/codec-split-wire-break.md

@modelcontextprotocol/core-internal	@modelcontextprotocol/client	@modelcontextprotocol/server
major	major	major

Split the wire layer into per-era codecs and make protocol-revision deletions physical. Deliberate wire/schema behavior changes (see docs/migration/support-2026-07-28.md "Per-era wire codecs"):

• resultType is no longer modeled by any neutral wire schema: EmptyResultSchema (strict) now rejects {resultType} bodies; on 2025-era connections a foreign resultType is stripped before validation instead of rejected; the member exists only inside the 2026-era codec, which requires it.
• CallToolResult.content / ToolResultContent.content are required at the wire boundary (content.default([]) removed): handler results without content are rejected with -32602 instead of silently defaulted, and content-less wire results fail the client parse loudly.
• Custom (3-arg) handlers now receive _meta minus the reserved envelope keys instead of having it deleted before params validation.
• specTypeSchemas re-scoped to the neutral model: result validators no longer accept resultType; task message-type validators and RequestMetaEnvelope left the public set (SpecTypeName narrowed).
• Role aggregate types/schemas (ClientRequest, ServerResult, …) no longer carry task vocabulary; the deprecated Task* types remain importable unchanged.
• Era-mismatched spec methods fail physically: inbound era-deleted methods get -32601 even with a handler registered; outbound sends throw SdkErrorCode.MethodNotSupportedByProtocolVersion locally.
• Value guards (isCallToolResult, …) are documented as neutral-shape consumer checks, not wire validators.