Feature: inject OpenRouter provider.only for BYOK / no-fallback routing

[echarrod]

Summary

Add support for OpenRouter's provider routing field so requests can be pinned to specific upstream provider(s) — enabling BYOK-only routing and preventing silent cross-provider fallback (e.g. Anthropic → Bedrock, Google → Vertex) that bills credits.

This was previously requested in #8 and closed by the requester (they moved to a different proxy), but the feature was never implemented.

Problem

When using OpenRouter with a BYOK key, OpenRouter can fall back to a different provider serving the same model if the primary provider errors or is rate-limited. That fallback bills OpenRouter credits, even when the user only wants to use their own provider key. The "Always use for this provider" toggle only governs OpenRouter's own key for a given provider; it does not prevent cross-provider fallback.

OpenRouter's documented fix is to send a provider field in the request body:

{ "provider": { "only": ["google-ai-studio"] } }

The proxy already builds the OpenAI request body in internal/converter/converter.go and marshals it directly, so it is well placed to inject this field — but there is currently no way to configure it.

Proposed change

A new env var, OPENROUTER_PROVIDER_ONLY (comma-separated provider slugs), that the proxy injects as {"provider": {"only": [...]}} into every request body when the detected provider is OpenRouter.

Implementation (small, ~3 files):

• pkg/models/types.go — add Provider map[string]interface{} json:"provider,omitempty" to OpenAIRequest.
• internal/config/config.go — add OpenRouterProviderOnly []string, parsed from OPENROUTER_PROVIDER_ONLY.
• internal/converter/converter.go — in the OpenRouter path, set openaiReq.Provider = {"only": cfg.OpenRouterProviderOnly} when configured.

Notes:

• allow_fallback is intentionally omitted — OpenRouter rejects it as an unrecognized key inside provider, and only already implies fail-hard (no fallback).
• Provider pinning only constrains the slug. It does not prevent within-slug fallback from a BYOK endpoint to OpenRouter's own pool for the same slug; that still requires the "Always use for this provider" toggle on a Prioritized BYOK key. Worth documenting alongside the feature.

Use case

Running Claude Code through the proxy to Google Gemini via a Google AI Studio BYOK key, with zero OpenRouter credit usage and no silent rerouting.

Happy to open a PR with the implementation + tests if there's interest.

[Triumph1701]

The important edge case is preserving the distinction between “provider-only” and “same-slug fallback”: a capability that pins the upstream provider should also make the effective route observable, so a BYOK request cannot silently become an OpenRouter-key request. I’d add tests for provider.only serialization, missing/empty config, and a response/telemetry field that records the selected provider and fallback decision. Full disclosure: I’m building Your Model around this provider-abstraction problem, with one OpenAI-compatible surface across multiple providers.