Tool definitions sent to models that cannot call tools (Vertex Gemini image models reject every request) #41464

Bug

Tool definitions are sent to models that can't call tools, so providers that reject function calling fail every request.

Vertex Gemini image models (gemini-2.5-flash-image) return:

Unable to submit request because the model does not support function calling.

Cause

capabilities.toolcall is set from provider config (provider.ts:1236) but never read when building a request. resolveTools in session/llm/request.ts filters only on permission rules and the per-message tools map, so all ~12 tools get declared regardless of the model.

Why it's hard to work around

• The error names the model, so it looks like a provider/model problem.
• Setting tool_call: false in config appears to be the fix but does nothing, since the flag isn't consulted in this path.
• Denying tools by name in an agent isn't enough — agents start from "*": "allow", and you can't name a plugin-injected tool you don't know exists. Only permission: {"*": deny} works.

Repro

1. Configure a Vertex/Google image model, e.g. gemini-2.5-flash-image.
2. Send any message with the stock build agent.
3. Request fails with the error above. Capturing the outgoing request shows a tools array with 12 functionDeclarations.

Version

1.18.15 (dev)