When working on the Agent Sessions window (src/vs/sessions/ ), always follow these guidelines:

1. Read the Specification Documents First

The src/vs/sessions/ directory contains authoritative specification documents. Always read the relevant spec before making changes.

Document Path Covers

Layer spec src/vs/sessions/README.md

Layering rules, dependency constraints, folder conventions

Layout spec src/vs/sessions/LAYOUT.md

Grid structure, part positions, sizing, CSS classes, API reference

AI Customizations src/vs/sessions/AI_CUSTOMIZATIONS.md

AI customization editor and tree view design

Chat Widget src/vs/sessions/browser/widget/AGENTS_CHAT_WIDGET.md

Chat widget wrapper architecture, deferred session creation, option delivery

If you modify the implementation, you must update the corresponding spec to keep it in sync. Update the Revision History table at the bottom of LAYOUT.md with a dated entry.

2. Architecture Overview

2.1 Layering

vs/base ← Foundation utilities vs/platform ← Platform services vs/editor ← Text editor core vs/workbench ← Standard VS Code workbench vs/sessions ← Agent Sessions window (this layer)

Key constraint: vs/sessions may import from vs/workbench and all layers below it. vs/workbench must never import from vs/sessions .

2.2 Dependency Rules

• ✅ Import from vs/base , vs/platform , vs/editor , vs/workbench

• ✅ Import within vs/sessions (internal)

• ❌ Never import vs/sessions from vs/workbench

• Run npm run valid-layers-check to verify layering

2.3 How It Differs from VS Code

Aspect VS Code Workbench Agent Sessions Window

Layout Configurable part positions Fixed layout, no settings customization

Chrome Activity bar, status bar, banner Simplified — none of these

Primary UX Editor-centric Chat-first (Chat Bar is a primary part)

Editors In the grid layout Modal overlay above the workbench

Titlebar Menubar, editor actions, layout controls Session picker, run script, toggle sidebar/panel

Navigation Activity bar with viewlets Sidebar (views) + sidebar footer (account)

Entry point vs/workbench workbench class vs/sessions/browser/workbench.ts Workbench class

3. Folder Structure

src/vs/sessions/ ├── README.md # Layer specification (read first) ├── LAYOUT.md # Authoritative layout specification ├── AI_CUSTOMIZATIONS.md # AI customization design document ├── sessions.common.main.ts # Common (browser + desktop) entry point ├── sessions.desktop.main.ts # Desktop entry point (imports all contributions) ├── common/ # Shared types, context keys, and theme │ ├── categories.ts # Command categories │ ├── contextkeys.ts # ChatBar and welcome context keys │ └── theme.ts # Theme contributions ├── browser/ # Core workbench implementation │ ├── workbench.ts # Main Workbench class (implements IWorkbenchLayoutService) │ ├── menus.ts # Agent sessions menu IDs (Menus export) │ ├── layoutActions.ts # Layout toggle actions (sidebar, panel, auxiliary bar) │ ├── paneCompositePartService.ts # AgenticPaneCompositePartService │ ├── widget/ # Agent sessions chat widget │ │ └── AGENTS_CHAT_WIDGET.md # Chat widget architecture doc │ ├── parts/ # Workbench part implementations │ │ ├── parts.ts # AgenticParts enum │ │ ├── titlebarPart.ts # Titlebar (3-section toolbar layout) │ │ ├── sidebarPart.ts # Sidebar (with footer for account widget) │ │ ├── chatBarPart.ts # Chat Bar (primary chat surface) │ │ ├── auxiliaryBarPart.ts # Auxiliary Bar │ │ ├── panelPart.ts # Panel (terminal, output, etc.) │ │ ├── projectBarPart.ts # Project bar (folder entries) │ │ └── media/ # Part CSS files │ └── media/ # Layout-specific styles ├── electron-browser/ # Desktop-specific entry points │ ├── sessions.main.ts # Desktop main bootstrap │ ├── sessions.ts # Electron process entry │ ├── sessions.html # Production HTML shell │ ├── sessions-dev.html # Development HTML shell │ ├── titleService.ts # Desktop title service override │ └── parts/ │ └── titlebarPart.ts # Desktop titlebar part ├── services/ # Service overrides │ ├── configuration/browser/ # Configuration service overrides │ └── workspace/browser/ # Workspace service overrides ├── test/ # Unit tests │ └── browser/ │ └── layoutActions.test.ts └── contrib/ # Feature contributions ├── accountMenu/browser/ # Account widget for sidebar footer ├── agentFeedback/browser/ # Agent feedback attachments, overlays, hover ├── aiCustomizationTreeView/browser/ # AI customization tree view sidebar ├── applyToParentRepo/browser/ # Apply changes to parent repo ├── changesView/browser/ # File changes view ├── chat/browser/ # Chat actions (run script, branch, prompts) ├── configuration/browser/ # Configuration overrides ├── files/browser/ # File-related contributions ├── fileTreeView/browser/ # File tree view (filesystem provider) ├── gitSync/browser/ # Git sync contributions ├── logs/browser/ # Log contributions ├── sessions/browser/ # Sessions view, title bar widget, active session service ├── terminal/browser/ # Terminal contributions ├── welcome/browser/ # Welcome view contribution └── workspace/browser/ # Workspace contributions

4. Layout

Use the agent-sessions-layout skill for detailed guidance on the layout. Key points:

4.1 Visual Layout

┌─────────┬───────────────────────────────────────────────────────┐ │ │ Titlebar │ │ ├────────────────────────────────────┬──────────────────┤ │ Sidebar │ Chat Bar │ Auxiliary Bar │ │ ├────────────────────────────────────┴──────────────────┤ │ │ Panel │ └─────────┴───────────────────────────────────────────────────────┘

• Sidebar spans full window height (root grid level)

• Titlebar is inside the right section

• Chat Bar is the primary interaction surface

• Panel is hidden by default (terminal, output, etc.)

• Editor appears as a modal overlay, not in the grid

4.2 Parts

Part Default Visibility Notes

Titlebar Always visible 3-section toolbar (left/center/right)

Sidebar Visible Sessions view, AI customization tree

Chat Bar Visible Primary chat widget

Auxiliary Bar Visible Changes view, etc.

Panel Hidden Terminal, output

Editor Hidden Main part hidden; editors open via MODAL_GROUP into ModalEditorPart

Not included: Activity Bar, Status Bar, Banner.

4.3 Editor Modal

The main editor part is hidden (display:none ). All editors open via MODAL_GROUP into the standard ModalEditorPart overlay (created on-demand by EditorParts.createModalEditorPart ). The sessions configuration sets workbench.editor.useModal to 'all' , which causes findGroup() to redirect all editor opens to the modal. Click backdrop or press Escape to dismiss.

5. Chat Widget

The Agent Sessions chat experience is built around AgentSessionsChatWidget — a wrapper around the core ChatWidget that adds:

• Deferred session creation — the UI is interactive before any session resource exists; sessions are created on first message send

• Target configuration — observable state tracking which agent provider (Local, Cloud) is selected

• Welcome view — branded empty state with mascot, target buttons, option pickers, and input slot

• Initial session options — option selections travel atomically with the first request

• Configurable picker placement — pickers can appear in welcome view, input toolbar, or both

Read browser/widget/AGENTS_CHAT_WIDGET.md for the full architecture.

Key classes:

• AgentSessionsChatWidget (browser/widget/agentSessionsChatWidget.ts ) — main wrapper

• AgentSessionsChatTargetConfig (browser/widget/agentSessionsChatTargetConfig.ts ) — reactive target state

• AgentSessionsChatWelcomePart (browser/parts/agentSessionsChatWelcomePart.ts ) — welcome view

• AgentSessionsChatInputPart (browser/parts/agentSessionsChatInputPart.ts ) — standalone input adapter

6. Menus

The agent sessions window uses its own menu IDs defined in browser/menus.ts via the Menus export. Never use shared MenuId.* constants from vs/platform/actions for agent sessions UI — use the Menus.* equivalents instead.

Menu ID Purpose

Menus.ChatBarTitle

Chat bar title actions

Menus.CommandCenter

Center toolbar with session picker widget

Menus.CommandCenterCenter

Center section of command center

Menus.TitleBarContext

Titlebar context menu

Menus.TitleBarLeftLayout

Left layout toolbar

Menus.TitleBarSessionTitle

Session title in titlebar

Menus.TitleBarSessionMenu

Session menu in titlebar

Menus.TitleBarRightLayout

Right layout toolbar

Menus.PanelTitle

Panel title bar actions

Menus.SidebarTitle

Sidebar title bar actions

Menus.SidebarFooter

Sidebar footer (account widget)

Menus.SidebarCustomizations

Sidebar customizations menu

Menus.AuxiliaryBarTitle

Auxiliary bar title actions

Menus.AuxiliaryBarTitleLeft

Auxiliary bar left title actions

Menus.AgentFeedbackEditorContent

Agent feedback editor content menu

7. Context Keys

Defined in common/contextkeys.ts :

Context Key Type Purpose

activeChatBar

string

ID of the active chat bar panel

chatBarFocus

boolean

Whether chat bar has keyboard focus

chatBarVisible

boolean

Whether chat bar is visible

sessionsWelcomeVisible

boolean

Whether the sessions welcome overlay is visible

8. Contributions

Feature contributions live under contrib/<featureName>/browser/ and are registered via imports in sessions.desktop.main.ts (desktop) or sessions.common.main.ts (browser-compatible).

8.1 Key Contributions

Contribution Location Purpose

Sessions View contrib/sessions/browser/

Sessions list in sidebar, session picker, active session service

Title Bar Widget contrib/sessions/browser/sessionsTitleBarWidget.ts

Session picker in titlebar center

Account Widget contrib/accountMenu/browser/

Account button in sidebar footer

Chat Actions contrib/chat/browser/

Chat actions (run script, branch, prompts, customizations debug log)

Changes View contrib/changesView/browser/

File changes in auxiliary bar

Agent Feedback contrib/agentFeedback/browser/

Agent feedback attachments, editor overlays, hover

AI Customization Tree contrib/aiCustomizationTreeView/browser/

Sidebar tree for AI customizations

Apply to Parent Repo contrib/applyToParentRepo/browser/

Apply changes to parent repo

Files contrib/files/browser/

File-related contributions

File Tree View contrib/fileTreeView/browser/

File tree view (filesystem provider)

Git Sync contrib/gitSync/browser/

Git sync contributions

Logs contrib/logs/browser/

Log contributions

Terminal contrib/terminal/browser/

Terminal contributions

Welcome contrib/welcome/browser/

Welcome view contribution

Workspace contrib/workspace/browser/

Workspace contributions

Configuration contrib/configuration/browser/

Configuration overrides

8.2 Service Overrides

The agent sessions window registers its own implementations for:

• IPaneCompositePartService → AgenticPaneCompositePartService (creates agent-specific parts)

• IPromptsService → AgenticPromptsService (scopes prompt discovery to active session worktree)

• IActiveSessionService → ActiveSessionService (tracks active session)

Service overrides also live under services/ :

• services/configuration/browser/

• configuration service overrides

• services/workspace/browser/

• workspace service overrides

8.3 WindowVisibility.Sessions

Views and contributions that should only appear in the agent sessions window (not in regular VS Code) use WindowVisibility.Sessions in their registration.

9. Entry Points

File Purpose

sessions.common.main.ts

Common entry; imports browser-compatible services, workbench contributions

sessions.desktop.main.ts

Desktop entry; imports desktop services, electron contributions, all contrib/ modules

electron-browser/sessions.main.ts

Desktop bootstrap

electron-browser/sessions.ts

Electron process entry

electron-browser/sessions.html

Production HTML shell

electron-browser/sessions-dev.html

Development HTML shell

electron-browser/titleService.ts

Desktop title service override

electron-browser/parts/titlebarPart.ts

Desktop titlebar part

10. Development Guidelines

10.1 Adding New Features

• Core workbench code (layout, parts, services) → browser/

• Feature contributions (views, actions, editors) → contrib/<featureName>/browser/

• Register by importing in sessions.desktop.main.ts (or sessions.common.main.ts for browser-compatible)

• Use Menus.* from browser/menus.ts for menu registrations — never shared MenuId.*

• Use separate storage keys prefixed with workbench.agentsession.* or workbench.chatbar.*

• Use agent session part classes, not standard workbench parts

• Mark views with WindowVisibility.Sessions so they only appear in this window

10.2 Validating Changes

• Run npm run compile-check-ts-native to run a repo-wide TypeScript compilation check (including src/vs/sessions/ ). This is a fast way to catch TypeScript errors introduced by your changes.

• Run npm run valid-layers-check to verify layering rules are not violated.

• Use scripts/test.sh (or scripts\test.bat on Windows) for unit tests (add --grep <pattern> to filter tests)

Important do not run tsc to check for TypeScript errors always use above methods to validate TypeScript changes in src/vs/** .

10.3 Layout Changes

• Read LAYOUT.md first — it's the authoritative spec

• Use the agent-sessions-layout skill for detailed implementation guidance

• Maintain fixed positions — no settings-based customization

• Update LAYOUT.md and its Revision History after any changes

• Preserve no-op methods for unsupported features (zen mode, centered layout, etc.)

• Handle pane composite lifecycle when hiding/showing parts

10.3 Chat Widget Changes

• Read browser/widget/AGENTS_CHAT_WIDGET.md first

• Prefer composition over modifying core ChatWidget — add behavior in the wrapper

• Use IAgentChatTargetConfig observable for target state, not direct session creation

• Ensure initialSessionOptions travel atomically with the first request

• Test both first-load (extension not yet activated) and new-session flows

10.4 AI Customization Changes

• Read AI_CUSTOMIZATIONS.md first — it covers the management editor and tree view design

• Lean on existing VS Code services (IPromptsService , IMcpService , IChatService )

• Browser compatibility required — no Node.js APIs

• Active worktree comes from IActiveSessionService

10.5 Validation

• Check VS Code - Build task output for compilation errors before declaring work complete

• Run npm run valid-layers-check for layering violations

• Verify part visibility toggling (show/hide/maximize)

• Test editor modal open/close behavior

• Test sidebar footer renders with account widget