Test: Model Parameter in Run Subagent Tool

[hediet]

Refs: #298380

• anyOS @federicobrancasi
• anyOS @kycutler

Complexity: 3

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The runSubagent tool now accepts an optional model parameter that lets the LLM (or a user via system prompt) request a specific model for the subagent invocation. The model is specified as a qualified name like "Model Name (Vendor)".

Setup

• Use VS Code Insiders with Copilot
• Have multiple models available in the model picker (e.g. GPT-4o, Claude Sonnet 4.6)
• Agent mode in chat

1. Subagents use different models

• Open a chat in agent mode
• Ask the agent: "Run two subagents — one with GPT-4o (copilot) and one with Claude Sonnet 4.6 (copilot). Each subagent should report which model it is running on."
• Verify each subagent runs successfully
• Verify the two subagents report different models corresponding to what was requested

2. Explicit model overrides agent-configured model

• Create a custom agent .md file with a specific model configured (e.g. model: GPT-4o (copilot))
• Ask the LLM to invoke that agent via subagent but explicitly set model to a different model (e.g. "Claude Sonnet 4.6 (copilot)")
• Verify the explicit model parameter takes precedence over the agent's configured model

3. Multiplier constraint — model not more expensive than main model

• Select a cheaper model as your main chat model (e.g. GPT-4o Mini)
• Ask the agent to "use a subagent with model GPT-4o (copilot)" or another model with a higher multiplier
• Verify the tool returns an error message explaining the model exceeds the current model's cost tier

4. Unknown model name error

• Ask the agent to use a subagent with a model name that doesn't exist (e.g. "NonExistent Model (copilot)")
• Verify a clear error message: Requested model 'NonExistent Model (copilot)' not found.

5. No model specified — falls back to main model

• Run a subagent without specifying any model parameter
• Verify the subagent uses the same model as the parent conversation

[federicobrancasi]

Verified.

[kycutler]

Overall works well!