Migrate to MCP spec 2026-07-28 (stateless) #1128

Add MCP 2026-07-28 stateless support alongside the existing sessionful server. Existing clients keep their current behavior except for the separately shipped add-actor retirement.

Non-goals: Tasks on 2026-07-28 and subscriptions/listen.

Public repo sequence

• PR 0: cut off add-actor selection for new connections (breaking change) #1129 — stop selecting add-actor for new connections
• PR 2: delete add-actor implementation #1131 — delete the retired add-actor implementation
• refactor: Add protocol-neutral MCP client context #1145 — add protocol-neutral MCP client context
• refactor: Make prompt and resource services protocol-neutral #1146 — make prompt and resource services protocol-neutral
• refactor: Separate legacy MCP wiring from ActorsMcpServer #1147 — separate legacy MCP wiring from ActorsMcpServer; depends on PR 2: delete add-actor implementation #1131, refactor: Add protocol-neutral MCP client context #1145, and refactor: Make prompt and resource services protocol-neutral #1146
• feat: Add MCP 2026-07-28 stateless server #1140 — add the MCP 2026-07-28 stateless server; depends on refactor: Separate legacy MCP wiring from ActorsMcpServer #1147 and merged refactor: Extract shared tool-call engine (PR 1a — migration prerequisite) #1139
• test: Add MCP 2026-07-28 integration and conformance coverage #1132 — add the dated integration transport and official conformance coverage

#1145 and #1146 may run in parallel. #1131 remains a separate cleanup PR and must not be mixed into the refactors.

PR #1143 stays open as a spike until the replacement for #1140 exists. Do not merge it.

Hosted server sequence

Internal umbrella: apify/apify-mcp-server-internal#672.

• Internal SDK dependency work may proceed independently.
• Internal route wiring (fix: Required fields dropped from tools/list for all Actors #676) waits for feat: Add MCP 2026-07-28 stateless server #1140 to be released.
• Public test: Add MCP 2026-07-28 integration and conformance coverage #1132 and internal fix: set final statusMessage before storeTaskResult (#638) #677 must pass before the stateless route reaches production.

Use stateless, legacy, or the dated protocol revision in our names and documentation. Do not use modern except where an external SDK API requires that literal.