LangExtract

Extract structured data from text using LLMs, with every extraction grounded to exact byte positions in the source. An Elixir port of google/langextract.

You give it two things: a client (which LLM to call) and a template (the task definition). There is no schema DSL and no fine-tuned model — the template is a plain-language description of what to extract, plus at least one worked example: a sample text with the extractions you'd expect back from it. That example is what teaches the model your class names, your span granularity, and the output format:

client = LangExtract.new(:claude, api_key: System.get_env("ANTHROPIC_API_KEY"))

template = %LangExtract.Prompt.Template{
  description: "Extract literary works, people, and locations from the text.",
  examples: [
    %LangExtract.Prompt.ExampleData{
      text: "Dickens wrote Oliver Twist while living in London.",
      extractions: [
        %LangExtract.Extraction{class: "person", text: "Dickens"},
        %LangExtract.Extraction{class: "work", text: "Oliver Twist", attributes: %{"type" => "novel"}},
        %LangExtract.Extraction{class: "location", text: "London"}
      ]
    }
  ]
}

{:ok, {spans, _errors}} = LangExtract.run(client, "Romeo and Juliet was written by William Shakespeare.", template)

for span <- spans do
  IO.puts("#{span.class}: \"#{span.text}\" [bytes #{span.byte_start}..#{span.byte_end}] (#{span.status})")
end
# work: "Romeo and Juliet" [bytes 0..16] (exact)
# person: "William Shakespeare" [bytes 32..51] (exact)

Every extraction maps back to its exact position in the source binary via binary_part(source, span.byte_start, span.byte_end - span.byte_start).

Installation

Add lang_extract to your list of dependencies in mix.exs:

def deps do
  [
    {:lang_extract, "~> 0.5.0"}
  ]
end

LangExtract uses Req for HTTP calls. No additional adapter configuration is needed.

Quick Start

1. Create a client

client = LangExtract.new(:claude, api_key: "sk-ant-...")

Supported providers: :claude, :openai, :gemini.

Provider-specific options are passed as keyword arguments:

# OpenAI with a specific model
client = LangExtract.new(:openai, api_key: "sk-...", model: "gpt-4o")

# Gemini
client = LangExtract.new(:gemini, api_key: "gm-...")

# OpenAI-compatible endpoint (Ollama, vLLM, etc.)
client = LangExtract.new(:openai,
  api_key: "not-needed",
  base_url: "http://localhost:11434",
  json_mode: false
)

Note: the Gemini API takes the key as a URL query parameter (unlike Claude and OpenAI, which use headers), so request URLs contain the secret. Avoid logging request URLs (e.g. via custom Req steps or verbose HTTP logging) when using the Gemini provider.

2. Define a prompt template

The template is the task definition — the only place the model learns what "right" looks like. It has two parts:

• description — a plain-language instruction: what to extract.
• examples — worked examples: a sample text paired with the extractions you would expect from it.

The extractions inside each example are the answer key for its sample text. They pin down everything the description leaves open: the class vocabulary ("condition", not "diagnosis"), the span granularity ("diabetes", not "diagnosed with diabetes"), which attributes to attach, and the exact output shape — the class name becomes the YAML key in the model's reply, so the examples also teach the wire format. A description alone would leave the model to invent all of that.

Each extraction's text must appear verbatim in its example's text: the examples double as alignment ground truth, and the validator (below) checks this before you spend tokens.

template = %LangExtract.Prompt.Template{
  description: "Extract medical conditions and medications from clinical text.",
  examples: [
    %LangExtract.Prompt.ExampleData{
      text: "Patient was diagnosed with diabetes and prescribed metformin.",
      extractions: [
        %LangExtract.Extraction{
          class: "condition",
          text: "diabetes",
          attributes: %{"chronicity" => "chronic"}
        },
        %LangExtract.Extraction{
          class: "medication",
          text: "metformin",
          attributes: %{}
        }
      ]
    }
  ]
}

3. Run extraction

source = "The patient presents with hypertension and is taking lisinopril daily."

{:ok, {spans, errors}} = LangExtract.run(client, source, template)

When some chunks fail to parse, the successful spans are still returned alongside the errors. Check errors to detect failures. Infrastructure failures (task exits, timeouts) return {:error, reason} instead.

Each span contains:

Verify byte offsets round-trip:

for span <- spans, span.byte_start != nil do
  extracted = binary_part(source, span.byte_start, span.byte_end - span.byte_start)
  IO.puts("#{span.class}: #{extracted}")
end

Chunking

For documents that exceed LLM token limits, pass :max_chunk_chars to split the source into sentence-aware chunks and process them in parallel:

{:ok, {spans, errors}} = LangExtract.run(client, long_document, template,
  max_chunk_chars: 4000,
  max_concurrency: 5
)

Byte offsets in the returned spans are adjusted to reference the original source, not individual chunks.

Prompt Validation

Validate that your few-shot examples actually align with their own source text before burning LLM tokens:

# Returns :ok or {:error, [issues]}
:ok = LangExtract.Prompt.Validator.validate(template)

# Or raise on failure
:ok = LangExtract.Prompt.Validator.validate!(template)

The validator reports what it finds. You decide what to do — log, raise, or ignore. No built-in severity levels.

Alignment Without an LLM

If you already have extraction strings (e.g., from a different source), you can align them against source text directly:

spans = LangExtract.align("the quick brown fox", ["quick brown", "fox"])
# [%Span{text: "quick brown", byte_start: 4, byte_end: 15, status: :exact},
#  %Span{text: "fox", byte_start: 16, byte_end: 19, status: :exact}]

Or parse raw LLM output and align in one step:

yaml = "extractions:\n- class: animal\n  text: fox"
{:ok, spans} = LangExtract.extract("the quick brown fox", yaml)

Both canonical format (class/text/attributes keys) and dynamic-key format ("animal": "fox") are accepted. Markdown fences and <think> tags are stripped automatically.

Serialization

Convert results to plain maps for storage or interop:

map = LangExtract.Serializer.to_map(source, spans)
# %{"text" => "...", "extractions" => [%{"class" => "...", "status" => "exact", ...}]}

{:ok, {source, spans}} = LangExtract.Serializer.from_map(map)

Save and load multiple results as JSONL:

LangExtract.Serializer.save_jsonl([{source1, spans1}, {source2, spans2}], "results.jsonl")
{:ok, results} = LangExtract.Serializer.load_jsonl("results.jsonl")

Provider Options

All providers accept these common options:

Environment variable fallbacks: ANTHROPIC_API_KEY, OPENAI_API_KEY, GEMINI_API_KEY.

HTTP defaults suit LLM latency: 120s receive timeout and transient retries (429/5xx/transport errors). Override either via :req_options, e.g. req_options: [receive_timeout: 30_000, retry: false].

Provider-specific options:

How It Works

The pipeline has five stages:

1. Prompt Builder    — Renders few-shot Q&A prompt with dynamic-key examples
2. LLM Provider      — Calls Claude/OpenAI/Gemini via Req
3. Wire Format       — Strips fences/<think> tags, normalizes dynamic keys to canonical form
4. Parser            — Validates and constructs Extraction structs
5. Aligner           — Maps extraction text to byte positions (exact scan, then fuzzy fallbacks)

The aligner mirrors upstream langextract v1.6.0 (+ #485) semantics in four phases:

• Occurrence DP: Over the whole extraction list, selects one exact occurrence per extraction — order-preserving, non-overlapping, maximizing matched tokens — so repeated mentions ground to successive occurrences. Unplaced extractions fall through to the phases below.
• Exact: Linear scan for the extraction's downcased word tokens as a contiguous run in the source tokens. First occurrence wins.
• Lesser (prefix match): When the model stitches or truncates a span, the longest matching token block anchored at the extraction's first token grounds it to its opening fragment in the source (status :fuzzy; disable with accept_lesser: false).
• LCS fuzzy: Longest-common-subsequence dynamic program over normalized (stemmed, downcased) tokens. Accepts the tightest source window with coverage ≥ :fuzzy_threshold (default 0.75) and token density ≥ :min_density (default 1/3); status :fuzzy.

Architecture

lib/lang_extract/
├── alignment/              # Tokenizer, Token, Aligner, Span
├── pipeline/               # Parser, ChunkError
├── prompt/                 # Template, ExampleData, Builder, Validator
├── provider/               # Claude, OpenAI, Gemini implementations
├── client.ex               # Configured LLM client struct
├── extraction.ex           # Core extraction struct
├── wire_format.ex          # LLM wire format (encode + decode)
├── orchestrator.ex         # Pipeline wiring + chunking
├── chunker.ex              # Sentence-aware text splitting
├── pipeline.ex             # Extraction pipeline public API
└── serializer.ex           # Serialization + JSONL

Compared to the Python Original

This is an Elixir port of google/langextract. Key differences:

Not ported: visualization (HTML output), multi-pass extraction, batch Vertex AI, plugin system. See the roadmap for planned improvements.

License

MIT — see the LICENSE file for details.