Chat Customizations Editor

Split-view management pane for AI customization items across workspace, user, extension, and plugin storage. Supports harness-based filtering (Local, Copilot CLI, Claude).

Spec

src/vs/sessions/AI_CUSTOMIZATIONS.md — always read before making changes, always update after.

Key Folders

Folder	What
src/vs/workbench/contrib/chat/common/	ICustomizationHarnessService, ISectionOverride, IStorageSourceFilter — shared interfaces and filter helpers
src/vs/workbench/contrib/chat/browser/aiCustomization/	Management editor, list widgets (prompts, MCP, plugins), harness service registration
src/vs/sessions/contrib/chat/browser/	Sessions-window overrides (harness service, workspace service)
src/vs/sessions/contrib/sessions/browser/	Sessions tree view counts and toolbar

When changing harness descriptor interfaces or factory functions, verify both core and sessions registrations compile.

Key Interfaces

• IHarnessDescriptor — drives all UI behavior declaratively (hidden sections, button overrides, file filters, agent gating). See spec for full field reference.
• ISectionOverride — per-section button customization (command invocation, root file creation, type labels, file extensions).
• IStorageSourceFilter — controls which storage sources and user roots are visible per harness/type.
• IExternalCustomizationItemProvider / IExternalCustomizationItem — internal interfaces (in customizationHarnessService.ts) for extension-contributed providers that supply items directly. These mirror the proposed extension API types.

Principle: the UI widgets read everything from the descriptor — no harness-specific conditionals in widget code.

Extension API (chatSessionCustomizationProvider)

The proposed API in src/vscode-dts/vscode.proposed.chatSessionCustomizationProvider.d.ts lets extensions register customization providers. Changes to IExternalCustomizationItem or IExternalCustomizationItemProvider must be kept in sync across the full chain:

Layer	File	Type
Extension API	vscode.proposed.chatSessionCustomizationProvider.d.ts	ChatSessionCustomizationItem
IPC DTO	extHost.protocol.ts	IChatSessionCustomizationItemDto
ExtHost mapping	extHostChatAgents2.ts	$provideChatSessionCustomizations()
MainThread mapping	mainThreadChatAgents2.ts	provideChatSessionCustomizations callback
Internal interface	customizationHarnessService.ts	IExternalCustomizationItem

When adding fields to IExternalCustomizationItem, update all five layers. The proposed API .d.ts is additive-only (new optional fields are backward-compatible and do not require a version bump).

Testing

Component explorer fixtures (see component-fixtures skill): aiCustomizationListWidget.fixture.ts, aiCustomizationManagementEditor.fixture.ts under src/vs/workbench/test/browser/componentFixtures/.

Screenshotting specific tabs

The management editor fixture supports a selectedSection option to render any tab. Each tab has Dark/Light variants auto-generated by defineThemedFixtureGroup.

Available fixture IDs (use with mcp_component-exp_screenshot):

Fixture ID pattern	Tab shown
chat/aiCustomizations/aiCustomizationManagementEditor/AgentsTab/{Dark,Light}	Agents
chat/aiCustomizations/aiCustomizationManagementEditor/SkillsTab/{Dark,Light}	Skills
chat/aiCustomizations/aiCustomizationManagementEditor/InstructionsTab/{Dark,Light}	Instructions
chat/aiCustomizations/aiCustomizationManagementEditor/HooksTab/{Dark,Light}	Hooks
chat/aiCustomizations/aiCustomizationManagementEditor/PromptsTab/{Dark,Light}	Prompts
chat/aiCustomizations/aiCustomizationManagementEditor/McpServersTab/{Dark,Light}	MCP Servers
chat/aiCustomizations/aiCustomizationManagementEditor/PluginsTab/{Dark,Light}	Plugins
chat/aiCustomizations/aiCustomizationManagementEditor/LocalHarness/{Dark,Light}	Default (Agents, Local harness)
chat/aiCustomizations/aiCustomizationManagementEditor/CliHarness/{Dark,Light}	Default (Agents, CLI harness)
chat/aiCustomizations/aiCustomizationManagementEditor/ClaudeHarness/{Dark,Light}	Default (Agents, Claude harness)
chat/aiCustomizations/aiCustomizationManagementEditor/Sessions/{Dark,Light}	Sessions window variant

Adding a new tab fixture: Add a variant to the defineThemedFixtureGroup in aiCustomizationManagementEditor.fixture.ts:

MyNewTab: defineComponentFixture({
    labels: { kind: 'screenshot' },
    render: ctx => renderEditor(ctx, {
        harness: CustomizationHarness.VSCode,
        selectedSection: AICustomizationManagementSection.MySection,
    }),
}),

The selectedSection calls editor.selectSectionById() after setInput, which navigates to the specified tab and re-layouts.

Populating test data

Each customization type requires its own mock path in createMockPromptsService:

• Agents — getCustomAgents() returns agent objects
• Skills — findAgentSkills() returns IAgentSkill[]
• Prompts — getPromptSlashCommands() returns IChatPromptSlashCommand[]
• Instructions/Hooks — listPromptFiles() filtered by PromptsType
• MCP Servers — mcpWorkspaceServers/mcpUserServers arrays passed to IMcpWorkbenchService mock
• Plugins — IPluginMarketplaceService.installedPlugins and IAgentPluginService.plugins observables

All test data lives in allFiles (prompt-based items) and the mcpWorkspace/UserServers arrays. Add enough items per category (8+) to invoke scrolling.

Exercising built-in grouping

The list widget regroups items from the default chat extension under a "Built-in" header. Three things must be in place for fixtures to exercise this:

1. Include BUILTIN_STORAGE in the harness descriptor's visible sources
2. Mock IProductService.defaultChatAgent.chatExtensionId (e.g., 'GitHub.copilot-chat')
3. Give mock items extension provenance via extensionId / extensionDisplayName matching that ID

Without all three, built-in regrouping silently doesn't run and the fixture only shows flat lists.

Editor contribution service mocks

The management editor embeds a CodeEditorWidget. Electron-side editor contributions (e.g., AgentFeedbackEditorWidgetContribution) are instantiated automatically and crash if their injected services aren't registered. The fixture must mock at minimum:

• IAgentFeedbackService — needs onDidChangeFeedback, onDidChangeNavigation, onDidAddFeedback, onDidConvertFeedback, onDidAddReply, onDidSubmitFeedback as Event.None
• ICodeReviewService — needs getReviewState() / getPRReviewState() returning idle observables
• IChatEditingService — needs editingSessionsObs as empty observable
• IAgentSessionsService — needs model.sessions as empty array

These are cross-layer imports from vs/sessions/ — use // eslint-disable-next-line local/code-import-patterns on the import lines.

CI regression gates

Key fixtures have blocksCi: true in their labels. The component-fixtures.yml GitHub Action captures screenshots on every PR to main and fails the CI status check if any blocks-ci-labeled fixture's screenshot changes. This catches layout regressions automatically.

Currently gated fixtures: LocalHarness, McpServersTab, McpServersTabNarrow, AgentsTabNarrow. When adding a new section or layout-critical fixture, add blocksCi: true:

MyFixture: defineComponentFixture({
    labels: { kind: 'screenshot', blocksCi: true },
    render: ctx => renderEditor(ctx, { ... }),
}),

Don't add blocksCi to every fixture — only ones that cover critical layout paths (default view, section with list + footer, narrow viewport). Too many gated fixtures creates noisy CI.

Screenshot stability

Scrollbar fade transitions cause screenshot instability — the scrollbar shifts from visible to invisible fade class ~2 seconds after a programmatic scroll. After calling revealLastItem() or any scroll action, wait for the transition to complete before the fixture's render promise resolves:

await new Promise(resolve => setTimeout(resolve, 2400));
// Then optionally poll until .scrollbar.vertical loses the 'visible' class

Running unit tests

./scripts/test.sh --grep "applyStorageSourceFilter|customizationCounts"
npm run typecheck-client && npm run valid-layers-check

See the sessions skill for sessions-window specific guidance.

Debugging Layout in the Real Product

Component fixtures use mock data and a fixed container size. Layout bugs caused by reflow timing, real data shapes, or narrow window sizes often don't reproduce in fixtures. When a user reports a broken layout, debug in the live Code OSS product.

For launching Code OSS with CDP and connecting @playwright/cli, see the launch skill. Use --user-data-dir /tmp/code-oss-debug to avoid colliding with an already-running instance from another worktree.

Navigating to the customizations editor

After connecting, use snapshot to find the "Open Customizations" button (in the Chat panel header), then click it. To switch sections, use eval with a DOM click since sidebar items aren't interactive refs:

npx @playwright/cli eval "(() => { const items = [...document.querySelectorAll('.section-list-item')]; items.find(el => el.textContent?.includes('MCP'))?.click(); })()"

Inspecting widget layout

@playwright/cli eval wraps the expression in () => (...) — use an IIFE for multi-statement code. Use document.title as a return channel:

npx @playwright/cli eval "(() => { const w = document.querySelector('.mcp-list-widget'); \
  const lc = w?.querySelector('.mcp-list-container'); \
  const rows = lc?.querySelectorAll('.monaco-list-row'); \
  document.title = 'DBG:rows=' + (rows?.length ?? -1) \
    + ',listH=' + (lc?.offsetHeight ?? -1) \
    + ',seStH=' + (lc?.querySelector('.monaco-scrollable-element')?.style?.height ?? '') \
    + ',wH=' + (w?.offsetHeight ?? -1); })()"
npx @playwright/cli eval "document.title" 2>&1

Key diagnostics:

• rows — fewer than expected means list.layout() never received the correct viewport height.
• seStH — empty means the list was never properly laid out.
• listH vs wH — list container height should be widget height minus search bar minus footer.

Common layout issues

Symptom	Root cause	Fix pattern
List shows 0-1 rows in a tall container	layout() bailed out because offsetHeight returned 0 during display:none → visible transition	Defer layout via DOM.getWindow(this.element).requestAnimationFrame(...)
Badge or row content clips at right edge	Widget container missing overflow: hidden	Add overflow: hidden to the widget's CSS class
Items visible in fixture but not in product	Fixture uses many mock items; real product has few	Add fixture variants with fewer items or narrower dimensions (width/height options)

Fixture vs real product gaps

Fixtures render at a fixed size (default 900×600) with many mock items. They won't catch:

• Reflow timing — the real product's display:none → visible transition may not have reflowed before layout() fires
• Narrow windows — add narrow fixture variants (e.g., width: 550, height: 400)
• Real data counts — a user with 1 MCP server sees very different layout than a fixture with 12