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A single gateway endpoint can give an application one interface and one client-facing key for requests to multiple LLM providers. It does not remove the gateway’s routing policy or the upstream credentials needed to call providers. To understand what happened to any request, keep three things separate: the model requested, the provider selected, and any fallback that changed the provider or model.
What a single API key changes—and what it does not
A gateway sits between your application and model providers. The application sends a request to the gateway; the gateway applies its routing rules and calls an upstream provider. That can centralize application configuration, but it adds an operator and a policy layer to the path.
LiteLLM documents both a unified provider interface and a gateway with virtual keys and centralized controls. In its proxy setup, the client authenticates to the gateway with a virtual key or sign-in token, while the gateway uses separately configured provider credentials for upstream calls. Treat these as two credential boundaries: the key in application configuration is not necessarily the only key in the system.
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Separate the requested model, selected provider, and fallback
These are different decisions, and an audit should preserve all three.
- Requested model: the model identifier your application asks the gateway to use.
- Selected provider: the upstream service that serves that model for a particular request.
- Fallback outcome: whether a failure caused a retry through another provider for the same model or a switch to a different model.
OpenRouter’s routing documentation describes the application choosing a model while the service selects a provider unless the caller overrides provider policy. Provider-level failover can therefore preserve the requested model while changing which provider serves it. Model-level fallback is different: the service tries another model from an ordered list. OpenRouter says the response’s model attribute identifies the model ultimately used, and pricing is based on that model. A request that succeeds after fallback may consequently differ from the original request in model identity and price.
How candidate scoring can be observed
There is no single scoring standard established across gateways. The useful question is not just whether a service says it scores candidates, but whether you can inspect the candidates, the reason for selection, and the route that actually served the request.
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LLM Gateway documents request logs that show providers considered, the selected provider, the selection reason, and scores covering uptime, throughput, latency, price, priority, and cache support. Its documentation describes a hard switch away from a preferred provider when its uptime falls below 85%, and a soft switch when a competing score exceeds the preferred provider’s score by more than 0.15. These are that service’s documented routing rules—not general thresholds for gateways—and its documentation does not state a publication year for them.
The service says requests are normally scored independently. For multi-turn sessions, it also documents sticky routing that pins a session to a provider and region. That can make provider selection more consistent within a conversation; it is a different policy from independently choosing a candidate for every request.
LiteLLM: deployment routing and identification
LiteLLM documents routing strategies across configured deployments, including latency-based routing and cooldown behavior. Cooldowns apply to individual deployments, so a failing deployment can be taken out of consideration while healthy alternatives in the same model group remain eligible. LiteLLM also documents a response header that identifies the deployment serving a request.
That deployment identifier is useful operational evidence, but it is not the same thing as a detailed score breakdown. The documented LiteLLM routing information and LLM Gateway’s score dimensions are product-specific implementations; neither establishes a shared way all gateways must score candidates.
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Compare gateways by the evidence and controls they expose
| Decision area | What to establish | Documented examples |
|---|---|---|
| Operation and credentials | Who operates the gateway, where the client key is used, and where upstream provider credentials are configured. | OpenRouter documents a hosted endpoint and one key for multiple providers. LiteLLM documents a self-hosted gateway setup with a gateway-facing virtual key and separately configured provider credentials. |
| Provider controls | Whether you can allow or deny providers, set their order, or override automatic provider selection. | OpenRouter documents provider controls and caller overrides to its routing policy. |
| Fallback scope | Whether recovery tries another provider for the same model, another model, or both. | OpenRouter documents provider-level failover and ordered model-level fallback as separate layers. |
| Selection visibility | Whether logs identify candidates and selection reasons, or responses identify the deployment that served the request. | LLM Gateway documents candidate scores and selection reasons in request logs. LiteLLM documents a response header identifying the serving deployment. |
| Session behavior | Whether routing is independent for each request or pinned across a session. | LLM Gateway documents independent request scoring by default and sticky session routing as an option. |
These documented features do not establish which option will be fastest, least expensive, or most reliable for your workload. Those outcomes depend on your candidate policy and traffic; compare them with your own measurements rather than treating a gateway’s scoring labels as a universal benchmark.
Build an audit trail that explains every route
Capture enough information to reconstruct the routing decision without assuming that every gateway exposes the same fields. A practical per-request record should include:
- the requested model and the model ultimately returned, where available;
- the provider or deployment that served the request, using gateway logs or response metadata where available;
- the candidate providers considered and the recorded selection reason or score, if exposed;
- whether recovery occurred, and whether it changed providers or models;
- the outcome, latency, and spend you observe for the request.
For OpenRouter model fallback, compare the requested model with the response’s final model attribute and account for pricing based on the model actually used. For LiteLLM, retain the serving-deployment identifier from its documented response header. For LLM Gateway, use its request logs to inspect candidates, scores, and selection reasons. Do not infer a provider change solely from a successful response: the final model and the serving route answer different questions.
Test recovery without hiding changes in model behavior
OpenRouter documents rate limits, downtime, context-length validation errors, and moderation flags among the conditions that can trigger model-level fallback. A fallback can turn an availability recovery into a change in the model answering the user, so test and monitor the two effects separately.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware match- Set the intended request. Record the model and provider policy your application asks for, including any provider ordering or allowlist.
- Exercise the recovery path. In a controlled test, verify what happens when a candidate cannot serve the request. Distinguish a retry to another provider for the same model from a switch to another model.
- Inspect the evidence. Check the gateway’s logs and response metadata for selected provider or deployment, selection reason, fallback event, and final model. Record fields that are unavailable rather than guessing them.
- Compare user-facing results and cost. Check whether the final model, response behavior, and price differ from the intended route. Use workload-specific measurements for latency, success, spend, and output behavior.
Account for session affinity and the gateway operator
Independent per-request selection can distribute requests among eligible candidates, while sticky routing keeps a multi-turn session on one provider and region when the gateway supports it. LLM Gateway documents both behaviors. If session consistency or provider-side prompt caching matters to your application, evaluate the effect of pinning against the flexibility of selecting candidates independently.
A hosted gateway and a self-hosted gateway also place operational responsibility in different places. With a hosted service, the gateway operator is part of the request path; with a self-hosted proxy, your team operates that layer and configures its upstream credentials. The cited product documentation does not establish comparable data-retention, regional-processing, or compliance guarantees across these services. Check the applicable service terms and deployment settings before relying on any such guarantee.
Choose based on control and observability, not the phrase “one key”
A single application-facing key is useful when it simplifies client configuration, but it says little by itself about provider choice, fallback scope, or auditability. Before adopting a gateway, decide which controls your application needs: explicit provider restrictions, provider ordering, same-model failover, cross-model fallback, session pinning, and visibility into candidates and final routes. Then verify those controls and logs in your intended deployment and measure the behavior against your workload.
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