Add optional model parameter to runSubagent tool

[hediet]

Fixes #298380
Deprecates #298161

When the parent model guesses the model name wrong, the error lists all available models:

[Copilot]

Pull request overview

Adds support for an optional model input parameter on the runSubagent chat tool so callers (notably custom agents) can request a specific language model for a subagent invocation, with multiplier-based cost fallback to the parent model.

Changes:

• Extend runSubagent tool input params and model resolution logic to accept an explicit model override.
• Add runtime error messaging that includes available model qualified names when an explicit model can’t be resolved.
• Add unit tests for schema and explicit model-resolution behaviors.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/common/tools/builtinTools/runSubagentTool.ts	Adds model?: string to tool params, updates schema, and implements explicit model resolution + fallback/error behavior.
src/vs/workbench/contrib/chat/test/common/tools/builtinTools/runSubagentTool.test.ts	Adds tests for schema exposure and explicit model resolution/fallback/error cases.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/common/tools/builtinTools/runSubagentTool.ts:507

• prepareToolInvocation resolves args.model regardless of whether SubagentToolCustomAgents is enabled. If this setting is meant to gate the feature, consider ignoring args.model (treat as undefined) unless customAgentsEnabled is true, otherwise callers can still force model selection via the parameter even when the feature is disabled.

		// Defensive: model may omit agentName despite schema requiring it
		const gpEnabled = this.configurationService.getValue<boolean>(ChatConfiguration.GeneralPurposeAgentEnabled);
		const customAgentsEnabled = this.configurationService.getValue<boolean>(ChatConfiguration.SubagentToolCustomAgents);
		const isGeneralPurpose = gpEnabled && (!args.agentName || args.agentName === GeneralPurposeAgentName);
		const subagent = (args.agentName && !isGeneralPurpose && customAgentsEnabled) ? await this.getSubAgentByName(args.agentName) : undefined;

		// Resolve the model early and cache it for invoke()
		const resolved = this.resolveSubagentModel(subagent, context.modelId, args.model);
		this._resolvedModels.set(context.toolCallId, resolved);

• Files reviewed: 2/2 changed files
• Comments generated: 3