Plan Agent: Search subagent if explore disabled

[24anisha]

Update Plan Agent so it defaults to using search subagent if the explore subagent is disabled

[Copilot]

Pull request overview

Updates the Copilot Plan agent prompt so that when the Explore subagent is disabled, the agent falls back to using the search subagent for discovery.

Changes:

• Make Plan agent “Discovery” instructions conditional on whether Explore agent and/or Search subagent are enabled.
• Introduce a tool-name alias mapping (explore_subagent → search_subagent) in tool-name resolution.
• Refactor SearchSubagentTool.alternativeDefinition() to build an updated tool definition object before returning.

Show a summary per file

File	Description
extensions/copilot/src/extension/tools/node/searchSubagentTool.ts	Refactors how the tool definition is modified when thoroughness support is enabled.
extensions/copilot/src/extension/tools/common/toolNames.ts	Adds alias mapping intended to resolve alternative tool names back to canonical ToolNames.
extensions/copilot/src/extension/prompt/node/pseudoStartStopConversationCallback.ts	Updates imports to include getToolName (currently unused).
extensions/copilot/src/extension/agents/vscode-node/planAgentProvider.ts	Makes the Plan agent prompt vary based on Explore/Search enablement flags.

Copilot's findings

Comments suppressed due to low confidence (1)

extensions/copilot/src/extension/agents/vscode-node/planAgentProvider.ts:122

• The discovery text mixes search_subagent and explore_subagent tool names. As-is, the tool contributed in package.json is search_subagent, and explore_subagent is not a registered tool, so this prompt can lead the model to attempt calling a nonexistent tool. Make the instructions consistently reference the actual available tool name (or ensure the alias is truly registered/handled end-to-end).

			discoverySection = `## 1. Discovery

Use the *search_subagent* tool to gather context, analogous existing features to use as implementation templates, and potential blockers or ambiguities. When the task spans multiple independent areas (e.g., frontend + backend, different features, separate repos), launch **2-3 *explore_subagent* tools in parallel** — one per area — to speed up discovery.

Update the plan with your findings.`;

• Files reviewed: 2/2 changed files
• Comments generated: 1

[Copilot]

In the config-change listener, the Plan agent body now depends on ConfigKey.Advanced.SearchSubagentToolEnabled, but changes to that setting won't trigger _onDidChangeCustomAgents.fire() (only ExploreAgentEnabled was added). This can leave the cached agent definition stale until some other setting changes; include the search-subagent enablement key in this affectsConfiguration(...) set as well.

This issue also appears on line 118 of the same file.

Suggested change

	e.affectsConfiguration(ConfigKey.ExploreAgentEnabled.fullyQualifiedId)) {
	e.affectsConfiguration(ConfigKey.ExploreAgentEnabled.fullyQualifiedId) ||
	e.affectsConfiguration(ConfigKey.Advanced.SearchSubagentToolEnabled.fullyQualifiedId)) {