Unified API for LLMs
A unified API reduces integration work across models and providers while adapters, capability flags, and native escape hatches preserve real differences.
TL;DR
Authentication, model catalog, routing, fallback, errors, usage, billing tags and trace fields can share one contract.
Tools, structured output, reasoning, context, files, embeddings, vision, audio and streams vary by model and endpoint.
Accepting the same JSON shape does not guarantee the same semantics, errors, events, usage or quality.
Inventory workloads, define invariants, compare native and unified paths, dual-run, canary and keep rollback ready.
What a unified LLM API normalizes
A unified layer accepts a canonical request, validates capability flags, maps the model alias to eligible endpoints, translates provider-specific fields, routes and falls back, normalizes errors and usage, and returns a stable response envelope. Native IDs remain attached for evidence.
The portable core should be deliberately small: workload alias, messages or input items, common tool schema, required output contract, timeout, tenant, trace, budget, and capability requirements. Everything else—reasoning controls, cache behavior, safety settings, media details, built-in tools, stateful conversations, and provider-specific sampling—should be represented as typed extensions or rejected explicitly when unsupported.
The layer may be a hosted aggregator, managed gateway, self-hosted proxy, or SDK. Product shape determines where keys, data, telemetry, billing, configuration, and operations live. A hosted service reduces infrastructure work but becomes part of the data and availability path. A self-hosted proxy increases control but adds databases, upgrades, scaling, incident response, and reconciliation. An SDK avoids a network hop but distributes policy across every application release.
Unified API building blocks
| Building block | Best fit | Verify before choosing |
|---|---|---|
| One base URL and auth | Applications use one service identity and model access boundary. | Define tenant isolation, provider keys, BYOK, rotation, least privilege and direct rollback. |
| Model catalog and flags | Stable aliases map to versioned capabilities and eligible endpoints. | Source, verify and refresh context, modalities, parameters, region, policy, price and lifecycle. |
| Provider adapters | Translate canonical requests, responses, streams, errors and usage. | Contract-test every required feature and preserve provider-specific escape hatches. |
| Routing and fallback | Select among eligible models or providers and recover from bounded failures. | Record candidate set, decision, policy, attempts, substitutions, charges and final error. |
| Usage and evidence | Normalize operational fields for observability, billing and governance. | Store native units and IDs beside normalized data to reconcile providers and incidents. |
Normalize transport, preserve semantics
Messages, common tools, model alias, timeout, trace, tenant, budget and required capability flags.
Provider or modality-specific fields isolated behind documented namespaces and feature detection.
Canonical request ID plus native provider IDs, transformations, route, attempts, stream, errors, usage and cost.
Reject missing capabilities before execution; never silently ignore required fields or downgrade semantics.
Migrate to a unified API safely
Start with one bounded workload instead of switching the entire application. Choose a flow whose output schema and quality can be measured, then preserve its native implementation as the baseline. The migration is successful only if the unified route preserves required behavior, operational evidence, privacy terms, and rollback—not merely if the first response returns text.
- Inventory models, endpoint families, SDK assumptions, features, provider extensions, keys, data terms, errors, quotas, and billing dimensions.
- Define required capability flags and fail closed before routing when no candidate satisfies them.
- Build golden contract tests for ordinary, streaming, structured-output, tool, media, timeout, rate-limit, and provider-failure paths.
- Dual-run unified and native paths; diff outputs, events, errors, usage, latency, retries, selected route, and cost.
- Verify tenant isolation, secret rotation, log redaction, retention, regional routing, native request IDs, and invoice reconciliation.
- Canary by workload and tenant, preserve direct rollback, and block deployment on required-invariant regressions.
Use a canonical envelope with typed adapters
The canonical envelope contains identity, trace, workload alias, messages, required capabilities, timeout, budget and policy. A catalog resolves eligible endpoints before translation. Typed adapters map only supported semantics. The response includes normalized output plus route and native evidence. Escape hatches are explicit, scoped and covered by tests.
Production rule: a unified API should make common operations portable without erasing the differences that determine correctness.
A unified LLM API is still an inference interface
The unified API gives an agent consistent access to models. QVeris gives that agent consistent discovery and execution of external APIs, tools, services and live data. Stack them as inference interface plus capability interface, with shared workflow trace context.
FAQ
One client contract that accesses multiple models or providers through shared authentication, requests, routing and operational evidence.
No. It simplifies transport and operations; capabilities, behavior, quality, policy and price remain model- and endpoint-specific.
For simple compatible calls, sometimes. Production workloads still require conformance, dual-run, canary and rollback.