feat(#3996): reject incompatible image-output gateway requests - #4020
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Gemini image-output requests fail with opaque HTTP 400 responses when they also carry custom tools, built-in tools, or structured output. Resolve the model capability first, then reject those unsupported shapes locally before network dispatch on the gateway, direct Gemini API, and Vertex AI surfaces with a bounded error naming only fixed safe categories. Use the models.dev tool_call capability only to clarify custom-tool failures: distinguish models without tool calls from tool-capable models whose image-output request shape is incompatible, while missing catalogue data keeps the conservative message. The rejection matrix stays unchanged. Keep ordinary text requests and compatible image-output requests unchanged. Request-shape diagnostics report the resolved capability and separately note whether configuration explicitly overrode catalogue metadata. Tests cover the gate, no-dispatch behavior, safe errors, and the runtime/TUI error seam.
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What
Rejects incompatible Gemini image-output gateway requests before dispatch, with shared diagnostics and integration coverage across runtime and TUI paths.
Why
Provides a deterministic, actionable failure instead of sending unsupported response modalities to a gateway.
Validation
task test; Gemini guard and runtime/TUI integration tests.Test instructions
Run:
task testRequest image output through a Gemini gateway using an incompatible model, then using a compatible model. Expected: the incompatible request is rejected before dispatch with the categorized diagnostic, while the compatible request proceeds normally.