feat: add per-toolset model routing via model field on toolsets

[dgageot]

Add a 'model' field to toolset configuration that automatically routes the LLM turn processing tool results through a specified model. This enables platform-level cost optimization: cheaper/faster models handle simple tool results (file reads, knowledge-base lookups) while the agent's primary model handles complex reasoning.

Config & schema:

• Add Model field to Toolset struct in pkg/config/latest/types.go
• Add model property to Toolset in agent-schema.json

Tool plumbing:

• Add ModelOverride field to Tool struct (pkg/tools/tools.go)
• Add WithModelOverride wrapper in pkg/teamloader/model_override.go
• Wire wrapper into getToolsForAgent in pkg/teamloader/teamloader.go

Runtime:

• Extract reusable resolveModelRef from SetAgentModel in model_switcher.go with proper alloy config rejection
• Add resolveToolCallModelOverride helper (pkg/runtime/tool_model_override.go)
• In RunStream loop: after tool calls, record override; on next iteration, temporarily switch model for one turn then reset
• Clear override on agent switch to prevent cross-agent leakage

Tests:

• Unit tests for WithModelOverride wrapper
• Unit tests for resolveToolCallModelOverride
• Unit tests for resolveModelRef (alloy rejection, nil config, invalid format)
• Example config: per_tool_model_routing.yaml

Assisted-By: docker-agent

[docker-agent]

Review Summary

Assessment: 🟢 APPROVE

This PR adds per-toolset model routing to enable cost optimization by routing simple tool results through cheaper models. The implementation is clean and well-structured:

✅ Strengths:

• Proper separation of concerns: config schema → tool plumbing → runtime integration
• Correct control flow with early returns preventing fall-through bugs
• Comprehensive test coverage for new functions (resolveModelRef, WithModelOverride, resolveToolCallModelOverride)
• Good validation: resolveModelRef properly rejects alloy configs, isInlineAlloySpec validates format
• Clear override lifecycle management (set on tool calls, clear on agent switch)
• Example config demonstrating the feature

🔍 Code Quality:

• The refactoring that extracted resolveModelRef maintains the original behavior correctly
• Alloy spec handling uses mutually exclusive code paths with proper returns
• Model override wrapper pattern is clean and composable
• Error messages are descriptive

No bugs found in the changed code. The feature is ready to merge.

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Review completed by cagent