The first version of any LLM app always works. Send a prompt, get a response, ship it.
Then real traffic shows up. Providers go down. Rate limits kick in. Requests hang and time out. Models return broken JSON. Users double click submit and you pay twice. Nobody can say how much you spent last month.
VernLLM exists so you don't relearn all of this the hard way. The core call stays simple, and the reliability and control features are ready to switch on when you need them. Everything is opt-in.
Here are the 14 features that make that possible.
1. Provider Fallback
Every provider has bad days. Fallback lets you define backup targets and a policy for when to switch to them. It's not "try everything until something works." It's a deliberate policy for which errors trigger a switch and in what order. Your app stays up even when one provider doesn't.
2. Circuit Breaker
Retrying a dead provider just adds load to something already struggling. A circuit breaker tracks failure patterns and, once a provider is clearly unhealthy, stops sending it traffic for a while. This works especially well alongside fallback: the circuit breaker keeps a bad provider from eating retry attempts while a healthy one picks up the slack.
3. Retries
Some failures deserve another attempt: a network blip, a temporary 500. VernLLM's retries include exponential backoff, jitter (so failures don't all retry in sync), and Retry-After awareness. The goal is to retry what's worth retrying and let permanent errors fail fast instead of burning attempts on something that was never going to work.
4. Rate Limiting
Providers cap requests, tokens, concurrency, or time based quotas. Most teams discover these limits by hitting them. VernLLM's rate limiter lets you define those limits ahead of time and coordinate traffic before the provider has to say no.
5. Usage Metering
Metering can reserve budget before a call and refund it automatically if the call fails. That refund step matters: without it, a failed request still looks like spent budget, and your internal usage numbers quietly drift from reality.
6. Caching
A lot of LLM traffic is repeats: the same prompt twice, a page refresh, an unnecessary retry. cachedCall wraps requests so identical work doesn't get redone, which means fewer provider calls, lower cost, and faster responses for anything stable enough to reuse.
7. Observability
Reliability features you can't see are hard to trust. VernLLM emits one unified event stream for retries, circuit breaker transitions, rate limit waits, and fallback events, so you can actually answer questions like why a request was slow or why the provider switched, instead of guessing.
8. Usage Tracking
Metering controls usage in the moment; tracking tells you what was actually consumed afterward. VernLLM captures token usage from provider responses, which is the raw material for dashboards, cost reports, per user or per model accounting, quotas, and billing.
9. Error Handling
Not all failures are the same, but a single generic catch block treats them that way. VernLLM's structured LLMError types let you tell a rate limit from a timeout from a broken request, so you can decide what's worth retrying and what should fail immediately.
10. Cancellation & Timeouts
Requests take time, and users don't always want to wait. VernLLM supports AbortSignal, per-attempt timeouts, and retry-aware cancellation, so a user backing out mid-request actually stops the flow instead of letting it keep retrying in the background.
11. Structured Output
A model that "usually" returns JSON isn't the same as data you can trust. VernLLM supports structured output through client-side Zod validation and provider-native JSON Schema modes, so every response gets checked against a real schema before your app touches it.
12. Tool Calling
VernLLM supports tool calling but never executes tools itself. The model can request a tool call; your application decides which tools exist, what arguments are valid, whether the call is authorized, how it runs, and what result goes back. The model proposes, your app disposes.
13. Streaming
Streaming pushes chunks to the user as they're generated instead of making everyone wait for the full response. VernLLM streams without losing the reliability layer around it, retries and caching still apply, so it's not a separate, lesser API.
14. Pluggable Logger
Every team logs differently: console output, structured JSON, an observability platform, an internal abstraction. VernLLM lets you plug in your own logger instead of forcing you to adopt its own.
The Bigger Picture
Each feature solves a narrow problem. Together, they answer a bigger one: how do you turn a raw LLM call into a production-ready capability?
Your Application
│
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┌─────────────┐
│ VernLLM │
└─────────────┘
│
┌────────────────┼────────────────┐
│ │ │
▼ ▼ ▼
Rate Limit Cache Usage Meter
│ │ │
└────────────────┼────────────────┘
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Provider Request
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┌─────────┴─────────┐
│ │
Success Failure
│ │
▼ ▼
Usage Tracking Retry /
+ Streaming Circuit Breaker
│
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Fallback
│
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Backup Provider
You don't need all 14 at once. A small project might just want structured output and streaming. A production platform might use all of it together. You choose what your app needs.
Why This Matters
Networks fail, dependencies go down, users cancel, providers have limits, and responses need validation. That was true before LLMs and it's still true now. Getting a model to generate a good answer was never the hard part. Building a system that can rely on it at scale is.
Use one feature. Use all fourteen. The architecture stays yours.
Docs: https://vernllm.vercel.app
npm install vern-llm
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