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The Capabilities Behind a Swappable LLM Interface

A hosted model accepted our request format but leaked reasoning markers into citation parsing. The case traces capabilities, stream errors and separate speech.

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Technical reviewer: Victor Sineglazov, AI Scientific Advisor.

Our hosted model accepted the same request format as the local adapter. Its answer still changed the meaning of our citation output. Reasoning text arrived inside ordinary content, and the citation parser counted markers from that reasoning as if they belonged to the answer.

The September 21, 2026 spike reported six cited sources even though the answer itself cited only two. Some extra markers came from passages the model had discussed as irrelevant. Moving the request to another endpoint had worked. Preserving the answer contract required more than compatible JSON.

We were building a professional learning platform for aesthetic medicine. The LLM boundary created on September 16 let the backend choose model configurations without exposing provider details throughout the product. The later spike showed which parts of that boundary needed behavioral checks.

A model registry has to change what the caller is allowed to do

The internal port offered complete and streaming responses plus model capabilities. Its registry supplied the provider model name, context and output budgets, supported options and concurrency limits. The backend could identify which configured model had answered.

These fields were useful only when code enforced them. A context budget constrained the material supplied to the model. A sampling setting determined which fields the adapter sent. A capability flag prevented an unsupported input from being quietly discarded.

Contracts that our common request shape did not settle by itself
Boundary What needed an explicit rule Failure to watch for
Input capabilities Reject images when the selected model lacks vision support. The caller receives an answer to an input that was silently reduced.
Stream content Distinguish answer text from reasoning and terminal metadata. Non-answer text changes visible output or citations.
Usage accounting Normalize input, output and cached-input counts. Different field names produce misleading internal metrics.
Failure after output begins Preserve partial output and represent the terminal error. A retry appends another generation to an already visible answer.
Speech input Return transcript segments and their time coordinates. A text-completion abstraction loses the information retrieval needs.

This table describes our design responsibilities. It is not a claim that current provider APIs expose identical features or that a capability list measures answer quality.

The parser was reading a larger text than the reader's answer

During the hosted-model spike, a reasoning block appeared in the content field. The answer's source markers and the reasoning's source markers therefore reached the same citation parser. The parser could resolve a marker correctly while applying it to the wrong part of the output.

The recorded diagnosis called for separating reasoning at the serving boundary and protecting the backend's citation parsing. We treat that as the diagnosis from the spike, rather than claiming that an endpoint change alone fixed it or that every later configuration was rechecked for this publication.

The case sharpened an existing distinction. A parser checks marker syntax and resolution; it does not establish that a claim is supported by the cited text. Our fifteen-question citation case describes that second failure. Here, even the set of text eligible for parsing needed a contract.

Retry rules depend on whether output has escaped

The adapter design distinguished a failure before the first response byte from a failure after output had begun. Before output, an eligible transient failure could be retried under the configured policy. After output, the client already held part of a generation.

The latter path preserved the delivered deltas and carried a normalized terminal error. It did not restart generation as though nothing had happened. Provider addresses and raw failure details were kept out of the public error message.

This boundary belongs to the adapter contract, but it does not settle durable conversation state. The streaming persistence case covers the separate work of saving partial answers, claiming generation and ordering a terminal event after its database write.

Extend the common port only while its semantics still fit

The later image work added image-bearing messages and an explicit vision capability. A request with images was rejected before the provider call if the selected model lacked that capability. The desired failure was visible refusal of an unsupported operation, rather than silent loss of part of the request.

The configuration also separated the model answering questions from the model describing images during indexing. Choosing an answer model did not automatically authorize it to process every uploaded source image.

Speech took another path. Transcription used its own port and registry because the operation returned segments with time coordinates and had a job lifecycle. Pretending that a transcript was merely a chat response would have hidden information the ingestion pipeline needed.

The separation followed the operation's output and lifecycle. It did not imply that every new model capability deserved another service or another public API. For image input, extending the existing message contract was workable; for transcription, the result had a different shape.

Keep the implementation decision attached to its original scale

For the September implementation, we chose a small HTTP adapter behind the internal port. The decision record tied that choice to the small provider set and left room to reconsider it when additional native integrations were needed.

That historical decision is not evidence that today's model frameworks lack a feature, or that a custom adapter is cheaper at every integration count. We have not compared current framework releases in this article.

The reusable artifact is the contract test set: inspect the outbound fields, translate usage and stop information, test unsupported input, then test a failure before and after visible output. Run a real model path as well as deterministic doubles. The hosted reasoning failure passed through the same adapter shape that already looked interchangeable.

Evidence: the September 16 LLM-port specification, September 21 hosted-model spike and later image and speech updates in our development records. Provider behavior is described for those recorded configurations. No current API comparison, quality-equivalence study or fresh model benchmark is claimed.

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Dmytro Ihnatov is the Lead Backend Developer at Pharos Production. He documents backend architecture and research experiments from the team's work on a professional learning platform for aesthetic medicine.

Dmytro Nasyrov, Founder and CTO at Pharos Production
Dmytro Nasyrov Founder & CTO Let's work together!

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