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Schema-validated output in LLMR

Source: vignettes/about-schema.Rmd
about-schema.Rmd
knitr::opts_chunk$set(
  collapse = TRUE, comment = "#>",
  eval = identical(tolower(Sys.getenv("LLMR_RUN_VIGNETTES", "false")), "true") )

Overview

JSON mode asks the model for a JSON object; it is easy to request but gives weak guarantees about shape. Schema output supplies a JSON Schema and requests strict validation, which is more reliable when the provider enforces it. Enforcement and request shapes differ across providers, so this vignette relies on defensive parsing and local validation rather than trusting any one provider’s guarantee.

What the major providers actually support

Why prefer schema output?

  • Deterministic downstream code: predictable keys/types enable typed transforms.
  • Safer integrations: strict mode avoids extra keys, missing fields, or textual preambles.
  • Faster failure: invalid generations fail early, where retry/backoff is easy to manage.

Why JSON-only still matters

  • Broadest support across models/providers/proxies.
  • Low ceremony for exploration, labeling, and quick prototypes.

Quirks you will hit in practice

  • Models often wrap JSON in code fences or add pre/post text.
  • Arrays/objects appear where you expected scalars; ints vs doubles vary by provider/sample.
  • Safety/length caps can truncate output; detect and handle “finish_reason = length/filter.”

LLMR helpers for common parsing failures

Minimal patterns (guarded code)

All chunks use a tiny helper so your document knits even without API keys.

safe <- function(expr) tryCatch(expr, error = function(e) {message("ERROR: ", e$message); NULL})

1) JSON mode, no schema (works across OpenAI-compatible providers) 

safe({
  library(LLMR)
  cfg <- llm_config(
    provider = "deepseek",              
    model    = "deepseek-chat",
    temperature = 0
  )

  # Flip JSON mode on (OpenAI-compat shape)
  cfg_json <- enable_structured_output(cfg, schema = NULL)

  res    <- call_llm(cfg_json, 'Give me a JSON object {"ok": true, "n": 3}.')
  parsed <- llm_parse_structured(res)

  cat("Raw text:\n", as.character(res), "\n\n")
  str(parsed)
})

What could still fail? Proxies labeled “OpenAI-compatible” sometimes accept response_format but don’t strictly enforce it; LLMR’s parser recovers from fences or pre/post text.

2) Schema mode that actually works (Groq, open model)

Groq serves openai/gpt-oss-20b with OpenAI-compatible APIs. Their Structured Outputs feature enforces JSON Schema and (notably) expects all properties to be listed under required.

safe({
  library(LLMR); library(dplyr)

  # Schema: make every property required to satisfy Groq's stricter check
  schema <- list(
    type = "object",
    additionalProperties = FALSE,
    properties = list(
      title = list(type = "string"),
      year  = list(type = "integer"),
      tags  = list(type = "array", items = list(type = "string"))
    ),
    required = list("title","year","tags")
  )

  cfg <- llm_config(
    provider = "groq",
    model    = "llama-3.1-8b-instant",
    temperature = 0
  )
  cfg_strict <- enable_structured_output(cfg, schema = schema, strict = TRUE)

  df  <- tibble(x = c("BERT paper", "Vision Transformers"))
  out <- llm_fn_structured(
    df,
    prompt   = "Return JSON about '{x}' with fields title, year, tags.",
    .config  = cfg_strict,
    .schema  = schema,          # send schema to provider
    .fields  = c("title","year","tags"),
    .validate_local = TRUE
  )

  out |>
    select(structured_ok, structured_valid, title, year, tags) |>
    print(n = Inf)
})

If your key is set, you should see structured_ok = TRUE, structured_valid = TRUE, plus parsed columns.

Common gotcha: If Groq returns a 400 error complaining about required, ensure all properties are listed in the required array. Groq’s structured output implementation is stricter than OpenAI’s.

3) DeepSeek: JSON-object mode with local validation 

safe({
  library(LLMR)
  schema <- list(
    type="object",
    properties=list(answer=list(type="string"), confidence=list(type="number")),
    required=list("answer","confidence"),
    additionalProperties=FALSE
  )

  cfg <- llm_config("deepseek", "deepseek-chat")
  cfg <- enable_structured_output(cfg, schema = schema, name = "llmr_schema")

  res <- call_llm(cfg, c(
    system = "Return only the JSON object that matches the schema.",
    user   = "Answer: capital of Japan; include confidence in [0,1]."
  ))

  parsed <- llm_parse_structured(res)
  str(parsed)
})

4) Groq: another structured output example 

safe({
  library(LLMR)

  cfg <- llm_config(
    "groq", "openai/gpt-oss-20b"
  )

  schema <- list(
    type = "object",
    properties = list(name = list(type = "string"),
                      score = list(type = "number")),
    required = list("name", "score"),
    additionalProperties = FALSE
  )
  cfg_json <- enable_structured_output(cfg, schema = schema)

  res <- call_llm(cfg_json, c(
    system = "Reply as JSON only.",
    user   = "Produce fields name and score about 'MNIST'."
  ))
  str(llm_parse_structured(res))
})

Defensive patterns (no API calls) 

safe({
  library(LLMR); library(tibble)

  messy <- c(
    '```json\n{"x": 1, "y": [1,2,3]}\n```',
    'Sure! Here is JSON: {"x":"1","y":"oops"} trailing words',
    '{"x":1, "y":[2,3,4]}'
  )

  tibble(response_text = messy) |>
    llm_parse_structured_col(
      fields = c(x = "x", y = "/y/0")   # dot/bracket or JSON Pointer
    ) |>
    print(n = Inf)
})

Why this helps Works when outputs arrive fenced, with pre/post text, or when arrays sneak in. Non-scalars become list-columns (set allow_list = FALSE to force scalars only).

Parallel execution with schema validation

For production ETL workflows, combine schema validation with parallelization:

library(LLMR); library(dplyr)

cfg_with_schema = llm_config('deepseek','deepseek-chat')
  
setup_llm_parallel(workers = 10)

### Assuming there is a large data frame large_df

large_df |>
  llm_mutate_structured(
    result,
    prompt = "Extract: {text}",
    .config = cfg_with_schema,
    .schema = schema,
    .fields = c("label", "score"),
    tries = 3  # auto-retry failures
  )

reset_llm_parallel()

This processes thousands of rows efficiently with automatic retries and validation.

Choosing the mode

  • Reporting / ETL / metrics: Schema mode; fail fast and retry.
  • Exploration / ad-hoc: JSON mode + recovery parser.
  • Lightweight field extraction: Tag mode with .tags; useful when strict schema support is unavailable or unnecessarily heavy.
  • Cross-provider code: Always wrap provider toggles with enable_structured_output() and run llm_parse_structured() + local validation.

References