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Changelog — Agent Host Protocol (Spec)

All notable changes to the protocol specification are documented here. The spec is versioned independently from the per-language client libraries; see each clients/<lang>/CHANGELOG.md for client release history.

The format is based on Keep a Changelog and the spec follows SemVer. Until 1.0.0 is reached, breaking changes may land in MINOR bumps (see docs/specification/versioning.md).

Each released entry includes a Spec version line that exactly matches the PROTOCOL_VERSION constant in types/version/registry.ts at the time of release. The publish-spec.yml workflow refuses to publish a spec/vX.Y.Z tag whose matching ## [X.Y.Z] heading is missing from this file.

[Unreleased]

The next spec release will be cut from main once breaking or notable additive changes accumulate. Track in-flight protocol changes via PRs touching types/ and the per-symbol ACTION_INTRODUCED_IN / NOTIFICATION_INTRODUCED_IN maps in types/version/registry.ts.

[0.3.0] — Unreleased

Spec version: 0.3.0

  • Renamed the UserMessage type to Message and surfaced it consistently across turn state (Turn.message, ActiveTurn.message, PendingMessage.message) and the actions that carry it (session/turnStarted, session/pendingMessageSet). The type now carries an origin field and an optional _meta object.

[0.2.0] — 2026-05-28

Spec version: 0.2.0

This is the first version released through the unified spec release pipeline (spec/v* git tags → GitHub Release with attached schema artifacts). Changes to the protocol shape that landed under this version are tracked in the commits between spec/v0.1.0 (not yet tagged) and spec/v0.2.0. Highlights:

  • Channels reorganization — every command and notification carries a top-level channel: URI; per-channel state types and action unions live under types/channels-*/ (see PR #97 and PR #152).
  • New otlp/* notifications for telemetry export (exportLogs / exportTraces / exportMetrics).
  • New session/agentChanged, session/customizationRemoved, session/changesetsChanged, and the changeset/* action family.
  • Customizations redesigned as a typed two-level tree.
  • New resourceResolve (stat + realpath; throws NotFound for the existence check) and resourceMkdir (mkdir -p semantics) requests.
  • New createResourceWatch request plus the ahp-resource-watch:/<id> channel with the resourceWatch/changed action — long-lived file-change streams over the standard subscription mechanism.
  • resourceWrite extended with mode (truncate | append | insert), position, and ifMatch for optimistic concurrency. New Conflict (-32011) error code for stale ifMatch writes.
  • The entire content-bearing resource* family is now formally bidirectional — the methods appear in ServerCommandMap and may be initiated by either peer (matches VS Code's existing implementation).
  • UserMessage._meta optional Record<string, unknown> field for provider-specific message metadata, mirroring the MCP _meta convention already used on MessageAttachmentBase, ToolDefinition, ToolCallBase, UsageInfo, and SessionState.

[0.1.0] — Pre-tagging

Spec version: 0.1.0

Initial public protocol surface. Not retroactively tagged; this entry exists for completeness so that the 0.2.0 diff is unambiguous.