feat: support MCP Apps

.github/instructions/general-instructions.instructions.md

@@ -36,6 +36,10 @@ applyTo: 'types/**/*.ts'
 
 - `number` types are assumed to be 64-bit integers. If a floating point values are reasonable for a field, you MUST annotate its jsdoc with `@format float`
 - For actions or commands that could be implemented by returning an array `T[]` directly, still prefer to wrap it in `{ items: T[] }` for forward compatibility. This allows adding additional fields later without breaking the shape.
+- Naming discriminants for discriminated unions:
+  - Lifecycle / state-machine unions: name the union `Foo*State` and its discriminant enum `Foo*Status`. Variant interfaces are `Foo*State` (e.g. `ToolCallState` + `ToolCallStatus` + `ToolCallStreamingState`; `McpServerState` + `McpServerStatus` + `McpServerStartingState`; `CustomizationLoadState` + `CustomizationLoadStatus`).
+  - General/typological unions (not a lifecycle): name the discriminant `Foo*Kind` (e.g. `MessageAttachment` + `MessageAttachmentKind`, `ResponsePart` + `ResponsePartKind`, `ToolCallContributor` + `ToolCallContributorKind`).
+  - Generator note: variant interface names must differ from the union wrapper names emitted by the per-language generators (e.g. Kotlin emits `value class FooStateStarting(val value: FooStartingState)`), so name variants `Foo*State` rather than `FooStatus*`.
 - After making your changes, check to make sure the documentation in `docs` is up to date. For significant new flows or features, consider adding new documentation for it. Note that Mermaid diagrams are allowed.
 - Whenever you change or add an action, you must review the reducers in `types/reducers.ts` to see if that needs to be propagated into the state. If it does, add the appropriate logic and unit tests for it.
 - Never update the protocol version unless you were instructed to do so.

CHANGELOG.md

@@ -27,10 +27,47 @@ changes accumulate. Track in-flight protocol changes via PRs touching
 
 Spec version: `0.3.0`
 
+### Added
+
+- `McpServerCustomization` now models MCP servers as first-class session
+  customizations: `enabled`, `state` (a discriminated
+  `McpServerState` union covering `starting`, `ready`, `authRequired`,
+  `error`, `stopped`), an optional `channel` URI for an `mcp://`
+  side-channel into the upstream server, and an optional `mcpApp` block
+  carrying `AhpMcpUiHostCapabilities` so clients can render
+  [MCP Apps](https://github.com/modelcontextprotocol/ext-apps).
+- `McpServerAuthRequiredState` carries `ProtectedResourceMetadata` plus
+  `reason` / `requiredScopes` / `description`, letting clients drive the
+  existing `authenticate` command for per-MCP-server auth challenges.
+- `Customization` now includes `McpServerCustomization` at the top level
+  (hosts MAY surface globally-configured MCP servers directly rather
+  than only inside a plugin or directory). MCP servers remain valid as
+  children of a container.
+- New `session/mcpServerStateChanged` action — narrow upsert of
+  `state` + `channel` on an existing `McpServerCustomization`
+  by id, intended for the high-frequency
+  `starting`/`ready`/`authRequired` transitions. Other customization
+  fields stay in `session/customizationUpdated` territory.
+- `InitializeParams.capabilities` — optional client-capability bag
+  declared during the handshake. First entry is `mcpApps?: {}`; hosts
+  SHOULD only populate `McpServerCustomization.mcpApp` / `channel` for
+  clients that declared it.
+- New guide page `docs/guide/mcp.md` (with an MCP Apps subsection) and
+  new spec page `docs/specification/mcp-channel.md`.
 - Added `status` and `error` to `ChangesetOperation` and a new
   `changeset/operationStatusChanged` action so servers can reflect an
   operation's execution lifecycle (`idle → running → error`) back into
   changeset state.
+
+### Changed
+
+- Replaced `ToolCallBase.toolClientId?: string` with a discriminated
+  `ToolCallBase.contributor?: ToolCallContributor` union
+  (`ToolCallClientContributor` / `ToolCallMcpContributor`) so MCP-served
+  tool calls can be attributed back to their originating
+  `McpServerCustomization`. `session/toolCallStart` carries the new
+  `contributor?` field in place of `toolClientId?`.
+
 - Added optional `_meta` provider metadata to `AgentCustomization`.
 - Added optional `changes` field of type `ChangesSummary` to `SessionSummary`,
   carrying optional `additions`, `deletions`, and `files` counts so servers

clients/go/CHANGELOG.md

@@ -16,6 +16,23 @@ tag whose matching `## [X.Y.Z]` heading is missing from this file.
 
 ### Added
 
+- `McpServerCustomization` now exposes the full MCP lifecycle: `Enabled`,
+  the discriminated `McpServerState` union
+  (`Starting`/`Ready`/`AuthRequired`/`Error`/`Stopped`), optional
+  `Channel` URI for the `mcp://` side-channel, and optional `McpApp`
+  block carrying `AhpMcpUiHostCapabilities` for MCP Apps.
+- `McpServerAuthRequiredState` variant carries `ProtectedResourceMetadata`
+  plus `Reason` / `RequiredScopes` / `Description` so the existing
+  `authenticate` command can drive per-server auth.
+- `Customization` top-level union now includes `McpServer` — hosts MAY
+  surface bare MCP servers directly rather than only inside a plugin or
+  directory.
+- `SessionMcpServerStateChangedAction` and matching reducer case —
+  narrow upsert of `State` + `Channel` on an existing MCP
+  server customization by id.
+- `ClientCapabilities` struct on `InitializeParams.Capabilities` with
+  first entry `McpApps`.
+### Added
 - `status` and `error` fields on `ChangesetOperation` and the
   `changeset/operationStatusChanged` action, tracking the
   `idle → running → error` lifecycle of a changeset operation.
@@ -31,6 +48,13 @@ tag whose matching `## [X.Y.Z]` heading is missing from this file.
 
 - Removed the `additions`, `deletions`, and `files` fields from `ChangesetSummary`. Aggregate counts now live on `SessionSummary.changes`; per-changeset views derive their own totals from `ChangesetState.files`.
 
+### Changed
+
+- `ToolCallBase.ToolClientId *string` replaced by
+  `ToolCallBase.Contributor *ToolCallContributor` (union with
+  `Client { ClientId }` and `Mcp { CustomizationId }` variants).
+  `SessionToolCallStartAction` carries the new `Contributor` field as
+  well. The reducer follows the rename.
 ## [0.1.0] — 2026-05-28
 
 Implements AHP `0.2.0`.

clients/go/ahp/reducers.go

@@ -70,27 +70,27 @@ func withStatusFlag(status, flag ahptypes.SessionStatus, set bool) ahptypes.Sess
 // toolCallCommon carries the fields shared by every concrete
 // [ahptypes.ToolCallState] variant.
 type toolCallCommon struct {
-	id           string
-	name         string
-	displayName  string
-	toolClientID *string
-	meta         ahptypes.JSONObject
+	id          string
+	name        string
+	displayName string
+	contributor *ahptypes.ToolCallContributor
+	meta        ahptypes.JSONObject
 }
 
 func toolCallMeta(tc ahptypes.ToolCallState) toolCallCommon {
 	switch v := tc.Value.(type) {
 	case *ahptypes.ToolCallStreamingState:
-		return toolCallCommon{v.ToolCallId, v.ToolName, v.DisplayName, v.ToolClientId, v.Meta}
+		return toolCallCommon{v.ToolCallId, v.ToolName, v.DisplayName, v.Contributor, v.Meta}
 	case *ahptypes.ToolCallPendingConfirmationState:
-		return toolCallCommon{v.ToolCallId, v.ToolName, v.DisplayName, v.ToolClientId, v.Meta}
+		return toolCallCommon{v.ToolCallId, v.ToolName, v.DisplayName, v.Contributor, v.Meta}
 	case *ahptypes.ToolCallRunningState:
-		return toolCallCommon{v.ToolCallId, v.ToolName, v.DisplayName, v.ToolClientId, v.Meta}
+		return toolCallCommon{v.ToolCallId, v.ToolName, v.DisplayName, v.Contributor, v.Meta}
 	case *ahptypes.ToolCallPendingResultConfirmationState:
-		return toolCallCommon{v.ToolCallId, v.ToolName, v.DisplayName, v.ToolClientId, v.Meta}
+		return toolCallCommon{v.ToolCallId, v.ToolName, v.DisplayName, v.Contributor, v.Meta}
 	case *ahptypes.ToolCallCompletedState:
-		return toolCallCommon{v.ToolCallId, v.ToolName, v.DisplayName, v.ToolClientId, v.Meta}
+		return toolCallCommon{v.ToolCallId, v.ToolName, v.DisplayName, v.Contributor, v.Meta}
 	case *ahptypes.ToolCallCancelledState:
-		return toolCallCommon{v.ToolCallId, v.ToolName, v.DisplayName, v.ToolClientId, v.Meta}
+		return toolCallCommon{v.ToolCallId, v.ToolName, v.DisplayName, v.Contributor, v.Meta}
 	}
 	return toolCallCommon{}
 }
@@ -189,7 +189,7 @@ func endTurn(state *ahptypes.SessionState, turnID string, turnState ahptypes.Tur
 			ToolCallId:        common.id,
 			ToolName:          common.name,
 			DisplayName:       common.displayName,
-			ToolClientId:      common.toolClientID,
+			Contributor:       common.contributor,
 			Meta:              common.meta,
 			InvocationMessage: invocation,
 			ToolInput:         toolInput,
@@ -248,6 +248,8 @@ func customizationID(c ahptypes.Customization) (string, bool) {
 		return v.Id, true
 	case *ahptypes.DirectoryCustomization:
 		return v.Id, true
+	case *ahptypes.McpServerCustomization:
+		return v.Id, true
 	}
 	return "", false
 }
@@ -286,6 +288,8 @@ func setContainerEnabled(c *ahptypes.Customization, enabled bool) {
 		v.Enabled = enabled
 	case *ahptypes.DirectoryCustomization:
 		v.Enabled = enabled
+	case *ahptypes.McpServerCustomization:
+		v.Enabled = enabled
 	}
 }
 
@@ -421,12 +425,12 @@ func ApplyActionToSession(state *ahptypes.SessionState, action ahptypes.StateAct
 		state.ActiveTurn.ResponseParts = append(state.ActiveTurn.ResponseParts, ahptypes.ResponsePart{Value: &ahptypes.ToolCallResponsePart{
 			Kind: ahptypes.ResponsePartKindToolCall,
 			ToolCall: ahptypes.ToolCallState{Value: &ahptypes.ToolCallStreamingState{
-				Status:       ahptypes.ToolCallStatusStreaming,
-				ToolCallId:   a.ToolCallId,
-				ToolName:     a.ToolName,
-				DisplayName:  a.DisplayName,
-				ToolClientId: a.ToolClientId,
-				Meta:         a.Meta,
+				Status:      ahptypes.ToolCallStatusStreaming,
+				ToolCallId:  a.ToolCallId,
+				ToolName:    a.ToolName,
+				DisplayName: a.DisplayName,
+				Contributor: a.Contributor,
+				Meta:        a.Meta,
 			}},
 		}})
 		return ReduceOutcomeApplied
@@ -586,6 +590,8 @@ func ApplyActionToSession(state *ahptypes.SessionState, action ahptypes.StateAct
 			}
 		}
 		return ReduceOutcomeNoOp
+	case *ahptypes.SessionMcpServerStateChangedAction:
+		return applyMcpServerStatusChanged(state, a)
 	case *ahptypes.SessionTruncatedAction:
 		return applyTruncated(state, a.TurnId)
 	case *ahptypes.SessionInputRequestedAction:
@@ -769,7 +775,7 @@ func applyToolCallReady(state *ahptypes.SessionState, a *ahptypes.SessionToolCal
 					ToolCallId:        common.id,
 					ToolName:          common.name,
 					DisplayName:       common.displayName,
-					ToolClientId:      common.toolClientID,
+					Contributor:       common.contributor,
 					Meta:              common.meta,
 					InvocationMessage: a.InvocationMessage,
 					ToolInput:         a.ToolInput,
@@ -781,7 +787,7 @@ func applyToolCallReady(state *ahptypes.SessionState, a *ahptypes.SessionToolCal
 				ToolCallId:        common.id,
 				ToolName:          common.name,
 				DisplayName:       common.displayName,
-				ToolClientId:      common.toolClientID,
+				Contributor:       common.contributor,
 				Meta:              common.meta,
 				InvocationMessage: a.InvocationMessage,
 				ToolInput:         a.ToolInput,
@@ -829,7 +835,7 @@ func applyToolCallConfirmed(state *ahptypes.SessionState, a *ahptypes.SessionToo
 				ToolCallId:        s.ToolCallId,
 				ToolName:          s.ToolName,
 				DisplayName:       s.DisplayName,
-				ToolClientId:      s.ToolClientId,
+				Contributor:       s.Contributor,
 				Meta:              s.Meta,
 				InvocationMessage: s.InvocationMessage,
 				ToolInput:         toolInput,
@@ -846,7 +852,7 @@ func applyToolCallConfirmed(state *ahptypes.SessionState, a *ahptypes.SessionToo
 			ToolCallId:        s.ToolCallId,
 			ToolName:          s.ToolName,
 			DisplayName:       s.DisplayName,
-			ToolClientId:      s.ToolClientId,
+			Contributor:       s.Contributor,
 			Meta:              s.Meta,
 			InvocationMessage: s.InvocationMessage,
 			ToolInput:         s.ToolInput,
@@ -886,7 +892,7 @@ func applyToolCallComplete(state *ahptypes.SessionState, a *ahptypes.SessionTool
 				ToolCallId:        common.id,
 				ToolName:          common.name,
 				DisplayName:       common.displayName,
-				ToolClientId:      common.toolClientID,
+				Contributor:       common.contributor,
 				Meta:              common.meta,
 				InvocationMessage: invocation,
 				ToolInput:         toolInput,
@@ -904,7 +910,7 @@ func applyToolCallComplete(state *ahptypes.SessionState, a *ahptypes.SessionTool
 			ToolCallId:        common.id,
 			ToolName:          common.name,
 			DisplayName:       common.displayName,
-			ToolClientId:      common.toolClientID,
+			Contributor:       common.contributor,
 			Meta:              common.meta,
 			InvocationMessage: invocation,
 			ToolInput:         toolInput,
@@ -931,7 +937,7 @@ func applyToolCallResultConfirmed(state *ahptypes.SessionState, a *ahptypes.Sess
 				ToolCallId:        s.ToolCallId,
 				ToolName:          s.ToolName,
 				DisplayName:       s.DisplayName,
-				ToolClientId:      s.ToolClientId,
+				Contributor:       s.Contributor,
 				Meta:              s.Meta,
 				InvocationMessage: s.InvocationMessage,
 				ToolInput:         s.ToolInput,
@@ -949,7 +955,7 @@ func applyToolCallResultConfirmed(state *ahptypes.SessionState, a *ahptypes.Sess
 			ToolCallId:        s.ToolCallId,
 			ToolName:          s.ToolName,
 			DisplayName:       s.DisplayName,
-			ToolClientId:      s.ToolClientId,
+			Contributor:       s.Contributor,
 			Meta:              s.Meta,
 			InvocationMessage: s.InvocationMessage,
 			ToolInput:         s.ToolInput,
@@ -959,6 +965,46 @@ func applyToolCallResultConfirmed(state *ahptypes.SessionState, a *ahptypes.Sess
 	})
 }
 
+func applyMcpServerStatusChanged(state *ahptypes.SessionState, a *ahptypes.SessionMcpServerStateChangedAction) ReduceOutcome {
+	list := state.Customizations
+	if list == nil {
+		return ReduceOutcomeNoOp
+	}
+	for i := range list {
+		got, ok := customizationID(list[i])
+		if !ok || got != a.Id {
+			continue
+		}
+		mcp, ok := list[i].Value.(*ahptypes.McpServerCustomization)
+		if !ok {
+			return ReduceOutcomeNoOp
+		}
+		mcp.State = a.State
+		mcp.Channel = a.Channel
+		return ReduceOutcomeApplied
+	}
+	for i := range list {
+		children := containerChildren(&list[i])
+		if children == nil {
+			continue
+		}
+		for j := range *children {
+			got, ok := childCustomizationID((*children)[j])
+			if !ok || got != a.Id {
+				continue
+			}
+			mcp, ok := (*children)[j].Value.(*ahptypes.McpServerCustomization)
+			if !ok {
+				return ReduceOutcomeNoOp
+			}
+			mcp.State = a.State
+			mcp.Channel = a.Channel
+			return ReduceOutcomeApplied
+		}
+	}
+	return ReduceOutcomeNoOp
+}
+
 func applyTruncated(state *ahptypes.SessionState, turnID *string) ReduceOutcome {
 	if turnID == nil {
 		state.Turns = []ahptypes.Turn{}
@@ -1110,6 +1156,7 @@ func ApplyActionToChangeset(state *ahptypes.ChangesetState, action ahptypes.Stat
 		*ahptypes.ChangesetFileSetAction,
 		*ahptypes.ChangesetFileRemovedAction,
 		*ahptypes.ChangesetOperationsChangedAction,
+		*ahptypes.ChangesetOperationStatusChangedAction,
 		*ahptypes.ChangesetClearedAction:
 		return ReduceOutcomeNoOp
 	}