→ LLM integration services

LLM integration services for products that have to run in production

The model call is 10% of the work. We handle the other 90%. Caching, structured outputs, function calling, prompt versioning, eval, cost ceilings, fallback chains.

Overview

How we approach this work.

LLM integration services from Resser cover the engineering around the model call: the structured-output schemas, the eval suite, the prompt versioning, the cost telemetry, the retry logic, the fallback chains, and the observability that turn a prototype into a production feature.

We integrate Anthropic, OpenAI, Google, and open-weights LLMs into web apps, mobile apps, backend services, and existing enterprise software. Most engagements add an LLM-powered feature inside a product the customer already ships.

Most agencies sell 'GPT integration' as a single API call. The single call is the easy part. The hard parts are the prompt versioning, the eval that catches regressions, the cost ceiling that prevents a runaway loop, and the fallback when the primary provider rate-limits at 2am.

What we build

Concrete deliverables.

01

Production LLM features inside existing software

Add a summarizer, generator, extractor, or copilot into the product you already ship.

02

Structured outputs against your schema

JSON schema, Pydantic, zod. Every LLM call returns typed data your downstream code can trust.

03

Function calling and tool use

LLM-driven actions against your real APIs, with strict tool schemas and dry-run safety.

04

Streaming UIs

Token-by-token streaming, tool-call visualization, multi-turn conversation state. Vercel AI SDK end-to-end.

05

Cost and rate-limit guardrails

Per-request cost cap, per-session cap, per-cohort alerting, fallback to a secondary model on rate limit.

06

Eval and prompt versioning infrastructure

Every prompt change runs through the eval suite in CI. Rollback is a one-line change.

Stack

What we build with.

Model providers

Anthropic (Claude Haiku, Sonnet, Opus), OpenAI (GPT-4o, o-series, GPT-4.1), Google (Gemini), AWS Bedrock, Azure OpenAI Service, GCP Vertex AI.

Open-weights via vLLM

Llama 3, Mistral, Mixtral, Qwen 2.5, DeepSeek, Phi, Gemma. For private deployments where the model must stay inside your perimeter.

Frameworks and SDKs

Vercel AI SDK, Anthropic SDK, OpenAI SDK directly, LangChain / LangGraph when the orchestration is complex enough to justify it, Pydantic AI for type-safe Python.

Observability and eval

Helicone for cost and latency, LangSmith or Braintrust for eval, Promptfoo for CI-driven test suites, custom telemetry in Postgres or Clickhouse.

Outcomes

What we ship.

  • B2B SaaS feature: prompt caching cut token spend significantly with no measurable quality drop on the eval set.

  • Customer-support summarization: streaming UI with structured outputs deployed end-to-end in a few weeks.

  • Document extraction at scale: fallback chain across providers (Claude / GPT-4o / on-prem Llama) kept the feature highly available over a 90-day window.

References with names available after a scoping call.

In depth

LLM integration is not the same thing as a chatbot

A chatbot is a surface. LLM integration is plumbing. The difference matters commercially, because one of them can be bought for a few hundred euros a month and the other is a software project with a budget and a delivery date.

A chatbot sits next to your product and answers questions about it. An integration lives inside the workflow. It reads the ticket, pulls the customer record, drafts the reply in your tone, checks it against policy, and writes the result back to the system your team already uses. Nobody opens a chat window. The work simply happens faster.

If a vendor quotes you a flat monthly fee to add AI to your business, you are buying a surface. That is fine when a surface is what you need. It is the wrong purchase when the value sits in a process that already runs on your own data.

The model call is roughly a tenth of the work

Calling an LLM takes an afternoon. Everything around the call is what takes weeks, and it is also what decides whether the feature is still running in six months.

  • Structured outputs, so downstream code receives typed data instead of prose it has to parse and hope about.
  • An eval suite that runs in CI, so a prompt edit on a Tuesday cannot quietly degrade quality for every customer.
  • Prompt versioning with a one line rollback, because the fix you shipped at 2am will sometimes need undoing at 9am.
  • Cost ceilings per request, per session and per customer, so a retry loop cannot turn into a four figure invoice overnight.
  • Fallback chains across providers, because rate limits arrive without warning and your feature still has to answer.
  • Observability that shows latency, spend and failure rate per feature, not one aggregate number for the whole account.

When you should not integrate an LLM

We turn down this work more often than you might expect, and usually for one of four reasons.

  • The task has a correct answer that a rule or a query already produces. A model adds cost, latency and doubt to something deterministic.
  • Nobody can describe what a good output looks like. Without that, there is nothing to evaluate against and no way to tell improvement from noise.
  • The underlying data is a mess. A model reading bad records produces confident bad answers, which is worse than no answer at all.
  • The process is genuinely low volume. If a person handles it twelve times a month, automation will cost more than it saves for years.

Where your data goes, and why EU teams keep asking

This is the first question from most regulated buyers, so here is the direct answer. You choose the boundary, and we build inside it.

Three boundaries are common. Provider APIs under a data processing agreement with zero retention enabled, which is the fastest path and fine for most business content. EU region endpoints on AWS Bedrock, Azure OpenAI or Vertex AI, where the request never leaves European infrastructure. Or open weights models running on hardware you control, in your own VPC or on premise, where no third party ever sees the payload.

The third option costs more to build and more to run. It is the right call when the content is patient data, legal files, source code, or anything where a processor relationship is a problem rather than a formality. We are based in Croatia and work under EU law, so the contract side is not an afterthought either.

What it costs

Published ranges, because a page about a service that hides its numbers wastes everyone's time.

A single LLM feature inside software you already ship runs €10,000 to €25,000. That covers discovery, the integration itself, structured outputs, an eval suite and cost telemetry. Multi feature work with observability and fallback chains sits between €25,000 and €50,000. Running the models on your own hardware is a private AI build, which starts at €100,000.

Three things move the number more than anything else: how clean your data is, how many systems the feature has to touch, and whether the output needs to be right every time or merely right most of the time. Compliance grade accuracy is a different project from a helpful draft, and it is priced that way.

FAQ

Frequently asked.

Which LLM provider should we use?

Anthropic Claude Sonnet for reasoning-heavy multi-step work. OpenAI GPT-4o or o-series for structured extraction at scale. Open-weights for sovereignty. We benchmark on your data in discovery week before locking the choice.

Do you handle prompt versioning and rollback?

Yes. Every prompt is versioned in the codebase, runs through an eval suite in CI, and rollback is a one-line change. A hotfix on a Wednesday should not be a permanent change.

What is the typical timeline for an LLM integration?

Discovery week first. Then 2-3 weeks for a working integration in your environment with eval live. Then 2-4 weeks of hardening (cost telemetry, fallback chains, observability). Most features reach production in 5-8 weeks.

How much does an LLM integration cost?

A small LLM-integration feature into existing software starts at €10,000-€25,000. A multi-feature LLM integration with eval, observability, and structured outputs sits in the €25,000-€50,000 range.

Want to scope this for your project?

Fill the project-estimate form. We reply within one business day with a preliminary scope and a rough budget bracket.

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