SEP-2577 — Deprecation of Roots, Sampling, and Logging ¶

SEP-2577 lands in the MCP 2026-07-28 RC (locked 2026-05-21). It puts three protocol features on a deprecation path: Roots, Sampling, and Logging. mcpkit is on the 12-month annotation-only path — every existing call still works at runtime; godoc // Deprecated: blocks fire staticcheck SA1019 (and any IDE that consumes it) at call sites so consumers see the warning the moment they upgrade.

Removal target: a future major release, no earlier than the spec window below (~2027). Removal was deferred out of 0.4 (tracked in issue 850): 0.4 keeps the surfaces fully functional alongside the deprecation comments, so no behavior changes at the 0.4 cut. Removing them at 0.4 would break mcpkit’s own 12-month annotation window (closes ~2027-05-21) and drop it below 100% conformance / Tier-1 while the features remain in the targeted spec version.

TL;DR ¶

Feature	mcpkit surface	Status in 0.4.x	What to do
Roots	WithAllowedRoots, IsPathAllowed, RootsListResult, WithRootsHandler, NotifyRootsChanged	works, deprecated	move root-list semantics into application-level resource conventions; see Roots replacement
Sampling	Sample, CreateMessageRequest, SamplingMessage, ModelPreferences, WithSamplingHandler, TaskSample	works, deprecated	bring your own LLM client; see Sampling replacement
Logging	EmitLog, LogLevel, LogMessage, MCPLogHandler (slog adapter)	works, deprecated	use a standard structured-logging library; see Logging replacement

Why these three ¶

The MCP working group’s framing (paraphrased from SEP-2577 discussion): Roots, Sampling, and Logging all try to bridge between the MCP server and a host capability that has matured outside the protocol. Roots overlaps with the host’s filesystem permission model; Sampling overlaps with the host’s LLM-of-record; Logging duplicates structured-logging conventions every server runtime already has. Keeping them in the protocol surface forces every host implementer to either ship them or stub them. The deprecation removes that footgun.

Timeline ¶

Date	Event
2026-05-21	SEP-2577 trigger fired (MCP RC lock)
2026-05-31	mcpkit annotation pass lands (this doc + Deprecated: blocks)
2026-07-28	MCP 2026-07-28 GA — features remain in the spec but flagged deprecated
2027-05-21	12-month annotation window minimum closes
a future major release (≥ 2027)	mcpkit removes the deprecated symbols — no earlier than the spec window above. Deferred out of 0.4 (issue 850).

Removal was deferred out of 0.4: 0.4 keeps the surfaces working, and removing them earlier would break the annotation window above and drop mcpkit below Tier-1 while the features are still in the targeted spec version. If the spec window extends, mcpkit’s removal follows it — the deprecation doc is the source of truth, not a calendar date.

Affected symbols ¶

Roots ¶

Symbol	Replacement
server.WithAllowedRoots(roots ...string)	Application-level filesystem access control. mcpkit does not ship a drop-in.
server.WithRootsFetchTimeout(d time.Duration)	No replacement (Roots-specific).
core.IsPathAllowed(ctx, path) bool	Same — application-level check against your own allowlist.
core.BaseContext.IsPathAllowed(path) bool	Same.
core.RootsListResult	No replacement — the roots/list server-to-client request is the thing being deprecated.
core.DecodeListRootsInputResponse	MRTR helper for the deprecated roots/list flow; remove when the MRTR composition no longer needs roots input.
client.RootsHandler, client.WithRootsHandler(h)	Host wires filesystem permissions itself; mcpkit’s client no longer needs to negotiate them after removal.
(*client.Client).NotifyRootsChanged()	No replacement — roots/list_changed is part of the deprecated surface.

Migration sketch: Move from “server asks client which paths are allowed” (Roots) to “application bakes in its own filesystem capability before construction.” For tools that legitimately need user-scoped file access, model that as a resource or as an explicit tool argument, not as a protocol-level negotiation.

Sampling ¶

Symbol	Replacement
core.Sample(ctx, req) (CreateMessageResult, error)	Bring your own LLM client (anthropic-sdk-go, OpenAI SDK, etc.) and call it directly from the tool handler.
core.BaseContext.Sample(req) (CreateMessageResult, error)	Same — receive a model client via dependency injection at server construction.
core.CreateMessageRequest, core.SamplingMessage, core.ModelPreferences, core.CreateMessageResult	No replacement at the protocol surface — these wire types disappear. Application-level model abstractions replace them.
core.NewSamplingInputRequest(req), core.DecodeSamplingInputResponse	MRTR helpers for the deprecated sampling-in-MRTR flow.
(*server.TaskContext).TaskSample(req) (CreateMessageResult, error)	Same as Sample() — task continuations should hold their own LLM client.
client.SamplingHandler, client.WithSamplingHandler(h)	After removal, the host’s LLM client and the MCP client are separately wired — no protocol negotiation.

Migration sketch: Most tools that previously did ctx.Sample(...) were really asking “call the host’s LLM with this prompt.” Pass that LLM client into the server constructor; tools call it directly. mcpkit’s role narrows to tool dispatch + transport; sampling stops being a wire-level concept.

Logging ¶

Symbol	Replacement
core.EmitLog(ctx, level, logger, data)	Use slog.InfoContext(ctx, ...) (or equivalent) — write to your own log sink.
core.BaseContext.EmitLog(level, logger, data)	Same.
core.LogLevel and constants (LogDebug, LogInfo, LogNotice, LogWarning, LogError, LogCritical, LogAlert, LogEmergency)	slog.Level (or your library’s equivalent).
core.LogMessage	Wire-level type for the deprecated notifications/message; no replacement at this surface.
core.MCPLogHandler (slog → MCP bridge)	Drop the bridge; route slog to stderr / file / aggregator as you would in any other Go service.

Migration sketch: mcpkit’s logging existed to bridge tool output back to the MCP client over the wire. After deprecation, the client is no longer responsible for surfacing server logs — operators read them where every other Go service’s logs go. If you need observability inside a host UI, that’s an application-level feature now, not a protocol one.

Notes for mcpkit contributors ¶

• // Deprecated: blocks are per Go convention: blank-line-separated paragraph starting with Deprecated:. staticcheck -checks SA1019 enforces no internal call sites accidentally regress to new uses of the deprecated symbols — current call sites are grandfathered through 0.4.x but should be tracked so the eventual removal is mechanical.
• Examples (examples/mrtr, examples/apps/*, examples/stateless, examples/tasks) still demo the deprecated surfaces — they’re working illustrations of how the API behaves in 0.4.x. Each affected example carries a README banner pointing here. After the removal cut, the examples migrate or get retired with the symbols they demo.
• The deprecation surfaces are wire-level: this doc does not deprecate slog integration as a general pattern, only mcpkit’s MCP-protocol bridge for it. Same with model-client wiring — only the protocol-mediated path is leaving.