name	chat-customizations-editor
description	Use when working on the Chat Customizations editor — the management UI for agents, skills, instructions, hooks, prompts, MCP servers, and plugins.

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:

Include BUILTIN_STORAGE in the harness descriptor's visible sources
Mock IProductService.defaultChatAgent.chatExtensionId (e.g., 'GitHub.copilot-chat')
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 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 screenshot-test.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 compile-check-ts-native && 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 agent-browser, 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 -i 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 agent-browser eval "const items = [...document.querySelectorAll('.section-list-item')]; \
  items.find(el => el.textContent?.includes('MCP'))?.click();"

Inspecting widget layout

agent-browser eval doesn't always print return values. Use document.title as a return channel:

npx agent-browser 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 agent-browser 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

name	chat-customizations-editor
description	Use when working on the Chat Customizations editor — the management UI for agents, skills, instructions, hooks, prompts, MCP servers, and plugins.