feat: only load chat/notebook/mcp when their service overrides are used

[brianjenkins94]

Follows up on #827 and #828.

Problem

A workbench that doesn't use the chat, notebook or mcp service overrides still loads most of their implementation. #828 split the extension host participants, but that turned out to be only a small part of it: most of it was loaded by core code (search, terminal, SCM, tasks, getting started, the extension host...) that only needed a type, a command id or a small helper, but imported it from a module that loads the whole chat implementation or notebook editor.

Measured with the build's own module graph (all entry points except the chat, notebook, mcp, interactive, ai, speech and welcome service overrides):

	main	this PR
chat/notebook/mcp modules loaded	1183	378
their size (transformed code)	8751KB	911KB

Changes

0109-refactor-avoid-runtime-dependencies-from-core-on-cha.patch: small changes at the call sites, each one keeping the behavior:

• searchResult: type-only cast instead of instanceof NotebookEditorWidget (only NotebookEditorWidget registers itself in the notebook editor service)
• searchView: compare the editor id with NOTEBOOK_EDITOR_ID instead of instanceof NotebookEditor (nothing extends it)
• terminal decorationAddon: lazy import() of TerminalContext, only used when attaching a command to chat
• SCM and tasks: import the chat action ids from the existing chatConstants instead of chatActions
• getting started: the agent sessions welcome command id moved to agentSessionsWelcomeInput
• extension host: coerceImageBuffer and 3 builtin tool ids moved to dependency-free modules (re-exported from their previous location)

0110-refactor-split-extension-host-contribution-per-featu.patch: as suggested in #828, extensionHost.base.contribution + extensionHost.{chat,notebook,mcp}.contribution, with extensionHost.contribution still importing all of them. The feature ones are imported by their service override.

Missing main thread participants (service-override/tools/missingMainThreadCustomers, imported by the extensions service override; the Unsupported decorator moved from missing-services to tools so both use it, and check-unsupported-decorator covers both): the extension host requires every MainContext proxy to be registered, and some extHost implementations call the main thread when they are constructed (e.g. ExtHostLanguageModelTools calls $getTools()). So, like the missing services, there are typed implementations of the 16 chat/language model/notebook/mcp MainThread*Shapes: notifications and registrations do nothing, queries return nothing ([]/false), and actions (invoking a tool, opening a notebook, mcp tokens...) are @Unsupported. They are registered only for those ids and only when nothing else registered them, without depending on the import order (unnamed customers are instantiated after the named ones, and a real implementation set later replaces it).

missing-services: CustomizationHarnessService no longer calls createVSCodeHarnessDescriptor, which alone loaded ~480KB of chat.

Build check (feature-isolation-check rollup plugin): fails when more than 1000KB of chat/notebook/mcp is loaded without their service override, and always prints the 20 heaviest imports responsible, so a regression after a VSCode update points to the exact import.

A participant lost while rebasing 0110 makes the extension host fail to start (there is no missing implementation for it), and one left in extensionHost.contribution is detected by check-workbench-contributions from #831.

Behavior changes

Without the chat/lm service overrides, queries of the corresponding extension API return nothing instead of throwing: e.g. vscode.lm.selectChatModels() resolves with [] instead of rejecting with "Unsupported". Actions (e.g. vscode.lm.invokeTool, vscode.workspace.openNotebookDocument) still reject with "Unsupported".

Known remaining (~911KB)

Not addressed because they are actual dependencies rather than misplaced imports:

• editor/terminal dictation (dictationSession, chatSpeechToTextService), also used by missing-services for ChatSpeechToTextState
• go to symbol in a chat widget (ChatOutline, chatViewModel)
• extHostTypeConverters / extHostLanguageModels -> languageModels
• mainThreadEditorTabs -> chatEditorInput (tab input kinds)
• chat context keys used in menus (debug, terminal, SCM...)

Testing

• npm run build steps (compile, rollup plugins, default extensions, language packs, monaco languages), lint, check-unsupported-decorator, commitlint
• demo production build as-is (with chat/notebook/mcp): no console error, extension language model provider visible in the chat model picker (the real participants are used)
• demo production build without the chat, notebook, ai, interactive, speech and mcp service overrides, with an extension calling the chat/lm/notebook API: the extension host starts, the demo extension works, selectChatModels/tools are empty, registrations work, invokeTool and openNotebookDocument reject with "Unsupported", no console error (note: the demo dev server pre-bundles every local package, including the unused service overrides, so this has to be checked on a production build)
• in a consumer on 36.2.7 (Vite 8), simulating the same changes removed ~1.26MB of rendered chat/notebook/mcp code (-811KB minified, -191KB gzip) with the same results on a runtime smoke test

The 0109 changes could also be proposed to microsoft/vscode, which would remove most of the patch.

🤖 Generated with Claude Code

[CGNonofr]

Thanks! it looks quite promising!

[CGNonofr]

Thanks, really cool!

[github-actions]

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