chat: replace chatSessionCustomizations with chatSessionCustomizationProvider API

[joshspicer]

Summary

Replaces the group-based chatSessionCustomizations API (merged in #304532) with a simpler, flat-item chatSessionCustomizationProvider proposed API. Extensions call chat.registerChatSessionCustomizationProvider(chatSessionType, metadata, provider) to declare what customization items (agents, skills, instructions, prompts, hooks) they support, which then appear in the customizations management editor.

API surface (chatSessionCustomizationProvider proposal)

// Registration — chatSessionType scopes the provider to a harness
chat.registerChatSessionCustomizationProvider('my-ext', { label: 'My Extension' }, provider);

// Provider
interface ChatSessionCustomizationProvider {
  provideChatSessionCustomizations(token: CancellationToken): ProviderResult<ChatSessionCustomizationItem[]>;
  onDidChangeChatSessionCustomizations?: Event<void>;
}

// Items — flat list, each item declares its type
interface ChatSessionCustomizationItem {
  readonly uri: Uri;
  readonly name: string;
  readonly type: ChatSessionCustomizationType; // .Agent, .Skill, .Instructions, .Prompt, .Hook
  readonly description?: string;
}

// Type — TaskGroup-style class, extensible
class ChatSessionCustomizationType {
  static readonly Agent, Skill, Instructions, Prompt, Hook;
  constructor(id: string); // for extension-defined types
}

Create button menu contributions

Extensions contribute create actions to the add button dropdown via contributes.menus in package.json:

"contributes": {
  "menus": {
    "chat/customizations/create": [
      {
        "command": "myext.newAgent",
        "when": "aiCustomizationManagementHarness == my-ext && aiCustomizationManagementSection == agents"
      }
    ]
  }
}

Scoping: Use aiCustomizationManagementHarness context key to target a specific session type, and aiCustomizationManagementSection to target a specific section (agents, skills, etc.).

Replacement: When a harness has menu-contributed create actions for a section, they fully replace the built-in create buttons. Switching back to the "Local" harness restores the default buttons.

Plumbing

• Ext host ↔ main thread bridge via ExtHostChatAgentsShape2 / MainThreadChatAgentsShape2
• Main thread creates IHarnessDescriptor with itemProvider and registers via ICustomizationHarnessService.registerExternalHarness()
• Provider-backed harnesses render items as a flat sorted list (no storage-based Workspace/User/Extension grouping)
• storage is optional on IAICustomizationListItem — provider items don't have it
• Feature gate: chat.customizations.providerApi.enabled (default false)
• Debug panel shows provider's raw items when an extension harness is active
• New context key aiCustomizationManagementHarness tracks the active harness ID

What was removed

The old group-based chatSessionCustomizations API from #304532:

• IChatSessionCustomizationItem/Group DTOs and converters
• registerChatSessionCustomizationsProvider from ext host sessions, main thread sessions, and chatSessionsProvider.d.ts
• getCustomizations/hasCustomizationsProvider on IChatSessionsService
• vscode.proposed.chatSessionCustomizations.d.ts (deleted)

Files changed (23)

Area	File	What
Proposal	vscode.proposed.chatSessionCustomizationProvider.d.ts	New — full type surface
	vscode.proposed.chatSessionCustomizations.d.ts	Deleted — old API
	vscode.proposed.chatSessionsProvider.d.ts	Removed old registerChatSessionCustomizationsProvider
Registry	extensionsApiProposals.ts	New chatSessionCustomizationProvider proposal
Ext host	extHostChatAgents2.ts	registerChatSessionCustomizationProvider, $provideChatSessionCustomizations
	extHostChatSessions.ts	Removed old provider code
	extHostTypes.ts	ChatSessionCustomizationType class
	extHostTypeConverters.ts	Simple .from() converter
	extHost.api.impl.ts	Wires chat.registerChatSessionCustomizationProvider
Protocol	extHost.protocol.ts	DTOs and RPC methods
Main thread	mainThreadChatAgents2.ts	Creates IHarnessDescriptor, registers via registerExternalHarness
	mainThreadChatSessions.ts	Removed old provider methods
Harness	customizationHarnessService.ts (common)	registerExternalHarness(), IExternalCustomizationItemProvider
	customizationHarnessService.ts (browser)	Simplified constructor
UI	aiCustomizationListWidget.ts	Provider flat-list path, menu-contributed create actions
	aiCustomizationManagement.ts	AICustomizationManagementCreateMenuId, harness context key
	aiCustomizationManagementEditor.ts	Harness context key binding, reactive harness tracking
	aiCustomizationDebugPanel.ts	appendExternalProviderData()
Menu extension point	menusExtensionPoint.ts	chat/customizations/create contribution point
Tests	customizationHarnessService.test.ts	New — harness registration lifecycle
	mainThreadChatSessions.test.ts	Removed old mock
Fixtures	aiCustomizationListWidget.fixture.ts	Updated mock
	aiCustomizationManagementEditor.fixture.ts	Updated mock

[Copilot]

Pull request overview

Adds a new proposed extension API to let extensions register “customization providers” that supply items directly to the AI Customizations UI, wiring the provider through ExtHost ↔ MainThread and registering it as an external harness in the customization harness service.

Changes:

• Introduces proposed API chat.registerCustomizationProvider plus related DTO/protocol plumbing.
• ExtHost/MainThread implementations register providers, forward change events, and fetch provider items over RPC.
• Updates the AI Customizations list widget to refresh on dynamic harness changes and delegate item listing to the active harness’ external provider.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/vscode-dts/vscode.proposed.chatCustomizationProvider.d.ts	New proposed API surface for registering customization providers and reporting items/metadata.
src/vs/workbench/contrib/chat/test/common/customizationHarnessService.test.ts	New unit tests for external harness registration lifecycle and workspace-subpath matching.
src/vs/workbench/contrib/chat/common/customizationHarnessService.ts	Adds external item/provider types and registerExternalHarness, and widens harness ids to strings.
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationListWidget.ts	Refreshes on dynamic harness changes and delegates listing to itemProvider when present.
src/vs/workbench/api/common/extHostTypes.ts	Adds ChatCustomizationType enum to the ExtHost API surface.
src/vs/workbench/api/common/extHostTypeConverters.ts	Converts ChatCustomizationType to internal PromptsType strings for RPC.
src/vs/workbench/api/common/extHostChatAgents2.ts	Implements provider registration, change forwarding, and $provideCustomizations.
src/vs/workbench/api/common/extHost.protocol.ts	Adds protocol DTOs and RPC methods for customization providers.
src/vs/workbench/api/common/extHost.api.impl.ts	Wires chat.registerCustomizationProvider into the extension API factory.
src/vs/workbench/api/browser/mainThreadChatAgents2.ts	Registers external harnesses backed by ExtHost providers and forwards change events to the UI.

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[Copilot]

Pull request overview

This PR replaces the previous group-based chatSessionCustomizations proposed API with a new flat-list chatSessionCustomizationProvider proposed API, and rewires the workbench “Chat Customizations” UI to consume extension-provided customization items via external harness descriptors.

Changes:

• Remove the old registerChatSessionCustomizationsProvider API + deleted vscode.proposed.chatSessionCustomizations.d.ts; add new vscode.proposed.chatSessionCustomizationProvider.d.ts + registration API.
• Add ext host ↔ main thread plumbing for the new provider and register provider-backed harnesses via ICustomizationHarnessService.registerExternalHarness.
• Update the customizations UI to handle provider-backed items (flat list) and allow extensions to contribute “Create” actions via a new menu id + context key.

Reviewed changes

Copilot reviewed 20 out of 23 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/vscode-dts/vscode.proposed.chatSessionsProvider.d.ts	Removes old customizations-provider registration from the chatSessionsProvider proposal.
src/vscode-dts/vscode.proposed.chatSessionCustomizations.d.ts	Deletes the old group-based customizations proposal types.
src/vscode-dts/vscode.proposed.chatSessionCustomizationProvider.d.ts	Adds the new flat-list customization provider proposal surface.
src/vs/workbench/test/browser/componentFixtures/aiCustomizationManagementEditor.fixture.ts	Updates fixture to new harness registration method name.
src/vs/workbench/test/browser/componentFixtures/aiCustomizationListWidget.fixture.ts	Updates fixture to new harness registration method name.
src/vs/workbench/services/actions/common/menusExtensionPoint.ts	Adds an API menu entry for create actions in the customizations editor.
src/vs/workbench/contrib/chat/test/common/customizationHarnessService.test.ts	Adds unit tests for external harness registration + workspace-subpath matching.
src/vs/workbench/contrib/chat/common/customizationHarnessService.ts	Extends harness descriptors with an optional external item provider + external harness registration.
src/vs/workbench/contrib/chat/browser/aiCustomization/customizationHarnessService.ts	Adjusts harness service wiring for core window harness list.
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationManagementEditor.ts	Adds harness context key + dynamic dropdown handling; adjusts selection telemetry/storage handling.
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationManagement.ts	Adds harness context key and a new create menu id for extension create actions.
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationListWidget.ts	Supports provider-backed items (flat list), provider change events, and menu-based create actions.
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationDebugPanel.ts	Adds debug reporting for external provider item data.
src/vs/workbench/api/test/browser/mainThreadChatSessions.test.ts	Removes old customizations-provider stubs from tests.
src/vs/workbench/api/common/extHostTypes.ts	Replaces old customizations enums with ChatSessionCustomizationType class.
src/vs/workbench/api/common/extHostTypeConverters.ts	Adds converter for ChatSessionCustomizationType; removes old group conversion.
src/vs/workbench/api/common/extHostChatSessions.ts	Removes old customizations-provider registration and RPC handler.
src/vs/workbench/api/common/extHostChatAgents2.ts	Adds new customization provider registration and $provideChatSessionCustomizations RPC.
src/vs/workbench/api/common/extHost.protocol.ts	Adds new RPC DTOs/methods on the ChatAgents2 shape; removes old groups DTOs.
src/vs/workbench/api/common/extHost.api.impl.ts	Exposes chat.registerChatSessionCustomizationProvider under the new proposal gate.
src/vs/workbench/api/browser/mainThreadChatSessions.ts	Removes old main-thread customizations provider wiring and gating.
src/vs/workbench/api/browser/mainThreadChatAgents2.ts	Registers provider-backed harnesses on the main thread via registerExternalHarness.
src/vs/platform/extensions/common/extensionsApiProposals.ts	Replaces the proposal key with chatSessionCustomizationProvider.