feat(subagents): propose Model parameter for invoke_subagent / define…

[MattBetancourt]

Summary

Add an optional Model string parameter to invoke_subagent and define_subagent, enabling callers to specify which model a subagent should run on — rather than always inheriting the session-level model.

Full proposal: docs/proposals/subagent-model-selection.md

---

Motivation

Since v1.0.5, --model and /model let users pick a model at the session level. But subagents have no equivalent — they always inherit the parent session's model with no way to override it.

This blocks three practical architectures:

1. Cost-optimised orchestration — an orchestrator reasonably runs on a capable model (e.g. Gemini 3.1 Pro (High)), but file-scanning and research subagents only need something fast and cheap (e.g. Gemini 3.5 Flash (Low)). There is currently no way to express this split.

2. Capability-differentiated pipelines — some subtasks genuinely benefit from extended thinking (adversarial security review, complex planning). Spawning one subagent on Claude Sonnet 4.6 (Thinking) while keeping others on a standard model requires either a separate session or awkward workarounds.

3. Model benchmarking — running the same prompt against two subagents using different models for comparison is impossible today in a single session.

---

Proposed API Changes

invoke_subagent

 {
   "TypeName": "string (required)",
   "Role": "string (required)",
   "Prompt": "string (required)",
-  "Workspace": "inherit | branch | share  (optional, default: inherit)"
+  "Workspace": "inherit | branch | share  (optional, default: inherit)",
+  "Model": "string (optional) — e.g. \"Gemini 3.5 Flash (Low)\""
 }

define_subagent

 {
   "name": "string (required)",
   "description": "string (required)",
   "system_prompt": "string (required)",
   "enable_mcp_tools": "bool (optional)",
   "enable_subagent_tools": "bool (optional)",
-  "enable_write_tools": "bool (optional)"
+  "enable_write_tools": "bool (optional)",
+  "Model": "string (optional) — default model for this subagent type"
 }

Override chain (lowest → highest precedence)

session default  →  define_subagent.Model  →  invoke_subagent.Model

---

Behaviour Spec

Scenario	Expected result
Model omitted	Inherits session model — no regression
Model set in define_subagent only	All invocations of that type use the specified model
Model set in invoke_subagent only	That specific invocation uses the specified model
Both set	invoke_subagent.Model takes precedence
Invalid model identifier	Fail-fast with a clear error before the subagent starts
self-type subagent with Model	Spawns with the override, not the parent's model

---

Example

// Orchestrator (Gemini 3.1 Pro) spawns two concurrent subagents
// with different models suited to their tasks.
{
  "Subagents": [
    {
      "TypeName": "research",
      "Role": "Quick File Scanner",
      "Prompt": "List all Python files that import the requests library",
      "Workspace": "inherit",
      "Model": "Gemini 3.5 Flash (Low)"
    },
    {
      "TypeName": "self",
      "Role": "Security Reviewer",
      "Prompt": "Perform an adversarial OWASP-style review of the auth module",
      "Workspace": "branch",
      "Model": "Claude Sonnet 4.6 (Thinking)"
    }
  ]
}

Currently available models (as of v1.0.6, via agy models):

• Gemini 3.5 Flash (Low) / (Medium) / (High)
• Gemini 3.1 Pro (Low) / (High)
• Claude Sonnet 4.6 (Thinking)
• Claude Opus 4.6 (Thinking)
• GPT-OSS 120B (Medium)

---

Backwards Compatibility

Model is entirely optional. All existing invoke_subagent and define_subagent calls without it behave exactly as today — no regressions.

Out of Scope (follow-up candidates)

• Model capability validation (e.g. checking tool-calling support before assigning a model to a subagent with enable_write_tools: true)
• TUI surface for subagent model selection
• Pre-launch cost estimation hints

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[MattBetancourt]

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