sessions: add @ and # completion providers for new chat input

[osortega]

Summary

Adds @ (agent) and # (file) completion providers to the sessions new-chat welcome widget (NewChatViewPane).

Problem

The workbench chat completions (AgentCompletions, BuiltinDynamicCompletions) register exclusively for scheme: Schemas.vscodeChatInput. The sessions new-chat input uses a different URI scheme (sessions-chat), so @ and # completions never fire in the sessions window.

Since the sessions layer cannot directly depend on the workbench chat completion infrastructure, dedicated providers are needed.

Changes

New file: chatInputCompletions.ts

• @ agent completions — queries IChatAgentService.getAgents(), shows non-default agents and their slash commands. Only allowed at the start of input (matching workbench behavior).
• # file completions — searches workspace files via ISearchService when the user types #file:pattern. On accept, adds the file as a context attachment pill via a registered command.
• Follows the established SlashCommandHandler pattern: registers on uri.scheme, uses _computeCompletionRanges() helper.
• All imports respect sessions layering rules (base, editor, platform, workbench — no reverse dependency).

newChatContextAttachments.ts

• Added public addAttachment(entry) method so the file completion command can add context pills (was previously only private _addAttachments).

newChatViewPane.ts

• Instantiates ChatInputCompletions alongside the existing SlashCommandHandler, passing the editor and context attachments.

[Copilot]

Pull request overview

This PR adds sessions-layer completion support for the new-chat input so that @ (agent) and # (file/context) completions work in the Sessions window despite using a non-workbench chat input URI scheme.

Changes:

• Added ChatInputCompletions to register @ agent and # file completion providers for the sessions new-chat editor.
• Exposed a public addAttachment(...) API on NewChatContextAttachments to allow completion accept to create attachment pills.
• Wired up the new completion registration in NewChatViewPane.

Show a summary per file

File	Description
src/vs/sessions/contrib/chat/browser/newChatViewPane.ts	Instantiates the new completions registrar alongside the existing slash command handler.
src/vs/sessions/contrib/chat/browser/newChatContextAttachments.ts	Adds a public method to append a single attachment entry and re-render.
src/vs/sessions/contrib/chat/browser/chatInputCompletions.ts	New sessions-layer completion providers for @ agents and #file search with an accept command to add attachment pills.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 4

[Copilot]

Accepting a file completion both inserts #file:… into the editor and also adds the file into attachedContext via the command. Since query is later sent as plain text alongside attachedContext, this can lead to duplicated/incorrect context (the text token may be parsed again by the workbench chat service). Consider removing the #file:… token from the input on accept (or using empty insertText and relying on the pill), similar to how the workbench reference command converts the typed token into an attachment.

Suggested change

	const text = `${typedLeader}file:${name}`;
	const uriLabel = this.labelService.getUriLabel(resource, { relative: true });
	return {
	label: { label: name, description: uriLabel },
	filterText: `${name} ${typedLeader}${name} ${uriLabel}`,
	insertText: range.varWord?.endColumn === range.replace.endColumn ? `${text} ` : text,
	const uriLabel = this.labelService.getUriLabel(resource, { relative: true });
	return {
	label: { label: name, description: uriLabel },
	filterText: `${name} ${typedLeader}${name} ${uriLabel}`,
	insertText: '',

[Copilot]

The file completion provider currently triggers for both # and @ and will run workspace file search for any @word (e.g. @w), mixing file results into agent completions and doing potentially expensive searches unnecessarily. If the intent is #file:… only, restrict triggerCharacters/regex to # and early-return unless the current word starts with #file:.

[Copilot]

Agent completions are built from chatAgentService.getAgents().filter(a => !a.isDefault) without filtering by agent locations/modes or handling restricted agent names. The workbench completion logic filters by widget location/mode and uses getFullyQualifiedId(...) when agent.name isn’t allowed; otherwise the inserted @${agent.name} can be unresolvable in chat input. Consider aligning with src/vs/workbench/contrib/chat/browser/widget/input/editor/chatInputCompletions.ts (AgentCompletions) for these filters/labels.

[Copilot]

_getOrCreateCacheKey clears the search cache only when the key expires and another completion request happens. Since this class is instantiated per new-chat widget, consider clearing the cache key in dispose() as well to avoid leaving search caches around after the widget is destroyed.

Suggested change

	private _getOrCreateCacheKey(): { key: string; time: number } {
	if (this._cacheKey && Date.now() - this._cacheKey.time > 60_000) {
	this.searchService.clearCache(this._cacheKey.key);
	this._cacheKey = undefined;
	}
	override dispose(): void {
	this._clearCacheKey();
	super.dispose();
	}
	private _clearCacheKey(): void {
	if (this._cacheKey) {
	this.searchService.clearCache(this._cacheKey.key);
	this._cacheKey = undefined;
	}
	}
	private _getOrCreateCacheKey(): { key: string; time: number } {
	if (this._cacheKey && Date.now() - this._cacheKey.time > 60_000) {
	this._clearCacheKey();
	}