Add General Purpose agent support

[digitarald]

Exposes a built-in "General Purpose" subagent so the model always has a named agent for parallel execution, without requiring the SubagentToolCustomAgents config flag.

Refs microsoft/vscode-internalbacklog#6750

Changes

runSubagentTool.ts

• agentName is now required in the tool schema (always present, not gated by config)
• "General Purpose" constant (GeneralPurposeAgentName) — when selected, inherits the parent's system prompt, model, and tools (same as the previous unnamed-agent behavior)
• invoke() / prepareToolInvocation() use isGeneralPurpose check to branch between built-in and custom agent paths
• BaseModelDescription always includes the agent-name instruction
• Error message references "General Purpose" as a fallback

computeAutomaticInstructions.ts

• <agents> XML block now renders whenever runSubagentTool is available (outer config gate removed)
• Hardcoded "General Purpose" entry always appears first
• Custom agents from providers still gated by SubagentToolCustomAgents config

runSubagentTool.test.ts

• Tests updated: agentName always in schema, required array includes it
• Removed config-gated test (no longer applicable)
• "General Purpose" agent test confirms inherited model/tools behavior

Testing

• 12/12 unit tests passing
• No compilation errors

[Copilot]

Pull request overview

This pull request adds support for a built-in "General Purpose" agent to the runSubagent tool. The General Purpose agent allows the tool to invoke a subagent that inherits the parent agent's model, tools, and system prompt, providing a full-capability option when custom agents don't fit the task.

Changes:

• Made agentName a required parameter in the runSubagent tool schema, with defensive handling for models that omit it
• Introduced the GeneralPurposeAgentName constant as the canonical name for the built-in general-purpose agent
• Modified automatic instructions to always include the General Purpose agent first in the agents list, followed by custom agents (when enabled)
• Updated tests to verify General Purpose agent behavior including undefined/empty string handling

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/vs/workbench/contrib/chat/common/tools/builtinTools/runSubagentTool.ts	Added GeneralPurposeAgentName constant; made agentName required in schema; updated invoke and prepareToolInvocation to handle General Purpose agent as default; simplified onDidUpdateToolData to Event.None
src/vs/workbench/contrib/chat/common/promptSyntax/computeAutomaticInstructions.ts	Modified to always include General Purpose agent first in agents list, with custom agents conditionally added when config enabled
src/vs/workbench/contrib/chat/test/common/tools/builtinTools/runSubagentTool.test.ts	Added createTool helper; added tests for undefined/empty agentName handling as General Purpose; updated existing tests to expect agentName in schema and General Purpose agent name in results
src/vs/workbench/contrib/chat/test/common/promptSyntax/computeAutomaticInstructions.test.ts	Updated test expectations to account for General Purpose agent being first; added test for disabled custom agents showing only General Purpose agent