[Feature Request] Allow subagents to override model (parity with Claude Agent SDK)

[nicbstme]

Summary

Allow custom subagents to specify their own model, independent of the orchestrator session's model. The Claude Agent SDK already supports this via a model field on AgentDefinition. Copilot's CustomAgentConfig and custom agent YAML frontmatter do not, which forces every subagent dispatch onto the orchestrator's model tier and blocks cost and latency optimization for multi agent workflows.

Motivation

Real multi agent systems benefit from heterogeneous model tiers:

• Orchestrator runs a strong planning model (Opus, GPT 5) to decompose work and delegate
• Narrow subagents (validation, extraction, format conversion, classification) run on a faster and cheaper model (Haiku, GPT 5 nano)
• Occasionally the reverse: a critical reviewer subagent wants a stronger model than the orchestrator

Today all subagents inherit the orchestrator's session model, so a session pays the tallest model's price per token across every subagent invocation. For fan out patterns (e.g., spawning N parallel validators over N slides, chunks, or rows) this is a material cost multiplier.

Current state

• CustomAgentConfig (copilot-sdk types.d.ts) exposes name, displayName, description, tools, prompt, mcpServers, infer — no model field
• Custom agent YAML frontmatter discovered via AGENTS_DIR has no way to declare a model
• Session model is picked once at createSession and inherited by every subagent dispatch in that session
• BYOK routing applies at session level only; subagents cannot target a different provider

Proposed

1. Add optional model field to CustomAgentConfig (copilot-sdk)
2. Accept model aliases and full IDs
3. Fall back to orchestrator model when omitted
4. Mirror in custom agent YAML frontmatter (copilot-cli loader)
5. For BYOK / BYOM setups, allow a provider override so a subagent can route to a different provider than the orchestrator (e.g., orchestrator on Anthropic, subagent on OpenAI)

Precedent

Claude Agent SDK's AgentDefinition.model:
https://code.claude.com/docs/en/agent-sdk/subagents

Accepts 'sonnet' | 'opus' | 'haiku' | 'inherit' or a full model ID, defaults to inherit, honored at dispatch time.

Related

• Custom Agent YAML Frontmatter Should Support Reasoning Effort #2904 — Custom Agent YAML Frontmatter Should Support Reasoning Effort (same axis, reasoning instead of model)
• [Feature request] Per-agent tool visibility for custom sub-agents copilot-sdk#1019 — Per agent tool visibility for custom sub agents (same architectural axis)
• Model name shown in UI is different from the actual model loaded when custom agent enabled #2845 — Model name shown in UI is different from actual model loaded when custom agent enabled (symptom of the gap)
• Programmatic / Automatic Model Switching in Copilot CLI #2896 — Programmatic model switching (session level, not subagent level)

[examon]

This is partially supported today. Custom subagents defined via YAML can set a per-agent model: field in their frontmatter (accepting both aliases and full model IDs), and it falls back to the session model when omitted. The SDK CustomAgentConfig type doesn't yet expose model and per-subagent BYOK provider overrides aren't wired up, so the SDK-side parity gap is real. If you'd like to track those remaining pieces separately, please comment and we can take another look.