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Interface versioning and deprecation policy
This explains how the MSD Host-Integration Interface (ADR-1239) evolves over
time — what is an additive change, what is breaking, how PROTOCOL_VERSION
bumps, and what deprecation window host-plugin authors can rely on. The policy
exists because a published SDK is a long-term compatibility commitment (Hyrum's
Law: once external hosts depend on it, the observable behavior is the contract).
The version knob: PROTOCOL_VERSION
PROTOCOL_VERSION is a positive integer carried by the engine and exchanged in
every negotiation (in-process and serialized). It governs the negotiated
capability set — which axes, values, and adapter shapes exist. A host
declares the version it targets; the engine negotiates down to
min(host, engine) and warns when a host declares a newer version than the
engine (capabilities beyond the engine's version are not trusted).
Additive vs. breaking
-
Additive (no version bump required): adding a new optional axis value, a new adapter option, or a new reference profile. Existing host-plugins keep working — the engine negotiates to their declared version and ignores unknowns. Additive changes are the default; the interface prefers growing the vocabulary over changing it.
-
Breaking (requires a
PROTOCOL_VERSIONbump): removing or renaming an axis, changing an adapter's call/return shape, changing the negotiation result shape, or narrowing a value set. A breaking change is never silent — it lands behind a version bump so a host that declares the old version negotiates down and gets a warning, not a crash.
Deprecation window
A value or adapter shape is deprecated, not removed, for at least one minor release cycle. During the window:
- The old form still works at its declared version.
- The reference docs mark it deprecated with the successor + the removal version.
- The negotiation emits a
warningsentry when a host uses a deprecated value.
Removal is a breaking change and follows the version-bump rule above. The
undocumented sentinel is never deprecated — it is the permanent fail-closed
fallback for a host that omits an axis.
What this means for host-plugin authors
- Pin to a
PROTOCOL_VERSIONand assert on it in your host-plugin's test. - Treat unknown axis values / adapter options as additive (ignore, don't crash).
- Watch negotiation
warnings— they surface both "host declared a newer version" and "host used a deprecated value." - The serialized handshake's consistency with the in-process negotiation is itself part of the contract: a change that breaks that consistency is breaking.
See also
- Reference: the Host-Integration Interface
- ADR-1239 (Status: Accepted)