citeformer

A bulletproof way to generate verifiably cited text from language models.

What it does — one paragraph for everyone

Language models hallucinate citations. Ask GPT-4, Claude, or an open-source model to cite "source [7]" when you only gave it six sources and a solid chunk of the time it will invent [7], [8], sometimes [42]. citeformer makes that physically impossible. Before the model picks its next token, we compile a tiny grammar that only admits citation markers pointing at sources you actually supplied, and we hand that grammar to the decoder. Fabricated citations don't get generated less often — they cannot be generated at all. Bibliographies are rendered deterministically by the library in six academic styles (APA, MLA, Chicago, IEEE, Nature, Vancouver), and every emitted claim can be NLI-verified against its cited source after the fact. Try the live demo or pip install citeformer.

What makes it interesting — for the applied-AI crowd

If you've read the jsonformer source or thought about logit-layer structured output, skip to Backends.

• Logit-masked GBNF. The cite-id terminal is compiled per call to "[" ("1" | "2" | ... | "N") "]" and handed to XGrammar (default) or llguidance. Out-of-scope tokens get masked to zero probability before sampling — the sampler never sees them. This is structural, not rejection-sampled.
• Ten backends, two enforcement loci, one GenerationResult. HF + vLLM + llama.cpp enforce in-process via XGrammar / llguidance / GBNF. Fireworks drops citeformer's GBNF in unchanged via its native type: grammar mode — the same cite-id rule that masks logits in HFBackend runs inside the Fireworks runtime. OpenAI + Mistral + Gemini + OpenRouter + Together enforce inside the provider runtime via strict structured outputs (which became real token-level constrained sampling in late 2025 — see architecture.md). Anthropic is adapted from its native Citations API. All collapse to the same typed output for downstream verify / render / streaming.
• The model never touches the bibliography. Six hand-written CSL formatters (~1 kLOC, no citeproc-py dependency — see ADR-004) render references deterministically. 300 locked snapshots pin the formatter outputs.
• Verify is real, not a hit rate. result.verify() runs DeBERTa-v3-large-MNLI over every (source content, cited sentence) pair and returns a typed VerificationReport — with a coverage check for uncited-but-entailed sentences. Threshold calibration + the honest bimodal-score finding live in benchmarks/README.md#finding-4.
• 0.0 ± 0.0 fabrication across 40 runs. 4 prompt shapes × 2 models × 5 seeds in benchmarks/multiprompt_sweep.py. The stds are identically zero because there's no variance to measure — the guarantee is a contract, not a mean.

Hi, I'm Blaise — how this got built

Hi — I'm Blaise Albis-Burdige (@blaiseab). I wrote citeformer on and immediately around a trip to Ramp's NYC office. On the subway ride up I was rereading jsonformer by @1rgs — partly to sharpen my intuition for how the applied-AI folks at Ramp think about structured output, partly because jsonformer is one of those projects whose core insight ("don't prompt it; constrain the token distribution") has aged extraordinarily well. By the time I got off the train I was convinced the same move applied to RAG citations, which are — empirically, in 2026 benchmarks — wrong 14–95% of the time depending on what you measure. jsonformer has been dormant since early 2024; no successor had applied the insight to citation markers. This is that successor. The heavy lifting lives in dependencies I didn't write (XGrammar, transformers, vLLM, DeBERTa, httpx, pypdf, GROBID, readability) — citeformer's contribution is the composition plus the six §10 contracts that keep the seams honest as the surface grows. Paper-shaped write-up: PREPRINT.md.

Status: v0.1.0 on PyPI. Seven backends (HF + vLLM + llama.cpp local, OpenAI + Anthropic + Gemini + Mistral API), six hand-written CSL styles, deterministic bibliography rendering, and claim-level NLI verification. Follow CHANGELOG.md for the full change log.

Why structural, not statistical

LLM-generated citations are wrong 14–95% of the time depending on the benchmark. RAG systems still fabricate 3–13% of cited URLs. NeurIPS 2025 accepted ~50 papers with AI-generated fake references. Prompting doesn't fix it; post-hoc verification doesn't fix it. The only real fix is structural — make the invalid output token-impossible before the model reaches the decision point.

citeformer delivers that in three independent ways:

• Citation markers can't be fabricated. [N] where N > len(sources) is token-impossible to sample on local backends, and (since strict structured outputs went GA across providers in late 2025) token-impossible inside the provider's runtime on the API backends too. Proven across 40 multi-prompt runs — 0% fabrication on every prompt × model × seed triple.
• Bibliographies are rendered by the library, not the model. Six styles, deterministic output, 300 locked snapshots.
• Every citation is claim-verifiable. result.verify() runs NLI entailment per cite and returns a structured VerificationReport — not just a hit rate.

Install

# Core only — no model backend, just the types + rendering + metadata adapters.
pip install citeformer

# Local backends — masking runs in-process via XGrammar / llguidance / GBNF.
pip install 'citeformer[hf]'             # HuggingFace transformers + XGrammar
pip install 'citeformer[llamacpp]'       # llama.cpp native GBNF
pip install 'citeformer[vllm]'           # vLLM guided-decoding (Linux/CUDA only)

# API backends — masking runs inside the provider's runtime (strict structured
# outputs is real token-level constrained sampling on every modern provider).
pip install 'citeformer[openai]'         # Structured Outputs strict=true
pip install 'citeformer[anthropic]'      # Citations API adapter (with prompt-caching on)
pip install 'citeformer[openrouter]'     # Multi-provider routing (anthropic/.., openai/.., google/..)
pip install 'citeformer[fireworks]'      # Native GBNF — drops citeformer's grammar in unchanged
pip install 'citeformer[together]'       # Strict json_schema on open-weight upstreams
pip install 'citeformer[gemini]'         # response_schema constrained generation
pip install 'citeformer[mistral]'        # Strict JSON schema

# NLI verification (DeBERTa-v3-MNLI).
pip install 'citeformer[verify]'

# Cross-platform kitchen sink (HF + llama.cpp + verify; excludes vLLM).
pip install 'citeformer[all]'

Python 3.11+ (tested through 3.14). Apache-2.0.

Try it without installing. The HF Space demo runs the adversarial "100% → 0% fabrication" swing on CPU in your browser. The literature-review notebook walks end-to-end from arXiv fetch → grammar-constrained generation → NLI verification → APA-7 bibliography on a laptop-friendly 500 MB model.

Quickstart

from citeformer import Citeformer, Policy, Source
from citeformer.backends.hf import HFBackend

sources = [
    Source.from_doi("10.1038/s41586-023-06221-2"),
    Source.from_arxiv("2305.14627"),
    Source(
        metadata={
            "id": "poe-raven",
            "type": "book",
            "title": "The Raven",
            "author": [{"family": "Poe", "given": "Edgar Allan"}],
            "issued": {"date-parts": [[1845]]},
        },
        content="Once upon a midnight dreary...",
    ),
]

cf = Citeformer(
    backend=HFBackend(model="microsoft/Phi-3.5-mini-instruct"),
    style="apa-7",
    citation_policy=Policy.REQUIRED,
)
result = cf.generate(prompt="Summarize the three works.", sources=sources)

print(result.text)               # "Poe's The Raven opens... [3] BERT introduced... [2]"
for ref in result.references:
    print(ref.rendered)          # APA-7, rendered by the formatter — not the LLM

report = result.verify()         # NLI entailment per citation
print(f"{report.support_rate:.0%} of cites entailed by their source")

result.text cannot contain [4]. Not "unlikely to"; cannot, by grammar construction. Try more backends, styles, or the API tier with from citeformer.backends.openai import OpenAIBackend / anthropic import AnthropicBackend.

Backends

Ten backends, two enforcement loci ("where the masking runs"), one Backend ABC:

Backend	Extra	Enforcement	Where it lives	Notes
HFBackend	hf	In-process (XGrammar)	citeformer.backends.hf	Flagship. Grammar-level token masking.
LlamaCppBackend	llamacpp	In-process (GBNF)	citeformer.backends.llamacpp	Native GBNF via llama-cpp-python. CPU + Metal + CUDA.
VLLMBackend	vllm	In-process (XGrammar/llguidance)	citeformer.backends.vllm	vLLM guided decoding. Linux/CUDA only.
FireworksBackend	fireworks	Provider-runtime (native GBNF)	citeformer.backends.fireworks	Drops citeformer's cite-id grammar in unchanged via Fireworks's response_format={"type":"grammar"} mode. The cleanest "true logit-tier on a hosted API" backend.
OpenAIBackend	openai	Provider-runtime (strict JSON)	citeformer.backends.openai	OpenAI Structured Outputs — live verified.
AnthropicBackend	anthropic	Provider-native (Citations API)	citeformer.backends.anthropic	Live verified. Prompt-caching on by default; real messages.stream() streaming; cited_text + source_span preserved on every Citation.
OpenRouterBackend	openrouter	Provider-runtime (per-upstream)	citeformer.backends.openrouter	Multi-provider routing on the OpenAI wire format. provider.require_parameters: true keeps strict mode end-to-end. Reports per-call cost in OR credits.
TogetherBackend	together	Provider-runtime (strict json_schema)	citeformer.backends.together	Strict structured outputs on Together's open-weight upstreams (Llama / Qwen / DeepSeek).
GeminiBackend	gemini	Provider-runtime (response_schema)	citeformer.backends.gemini	Gemini's OpenAPI-subset structured output.
MistralBackend	mistral	Provider-runtime (strict JSON)	citeformer.backends.mistral	Mistral's response_format strict JSON schema.
MockBackend	(core)	Scripted	citeformer.backends.mock	For tests. Honors policies + marker styles.

All produce the same GenerationResult, so verify / render / streaming work identically across backends. OpenAI + Anthropic are live-verified against production endpoints in tests/integration/test_api_backends_live.py; Gemini + Mistral ship with fake-client coverage and the same schema contract. Full per-provider discussion: architecture.md.

API backends (quickstart)

Both API backends are live-tested against production endpoints — see tests/integration/test_api_backends_live.py.

from citeformer import Citeformer, Policy, Source
from citeformer.backends.openai import OpenAIBackend       # pip install citeformer[openai]
# from citeformer.backends.anthropic import AnthropicBackend  # pip install citeformer[anthropic]

sources = [Source(metadata={"id": "poe", "type": "book", "title": "The Raven",
                            "author": [{"family": "Poe"}],
                            "issued": {"date-parts": [[1845]]}},
                  content="Once upon a midnight dreary...")]

# OpenAI uses strict JSON-schema mode (gpt-4o-2024-08-06+ only).
# Reads OPENAI_API_KEY from env; pass `client=...` or `api_key=...` to override.
cf = Citeformer(backend=OpenAIBackend(model="gpt-4o-mini"),
                style="apa-7", citation_policy=Policy.REQUIRED)
result = cf.generate(prompt="Describe the opening in one sentence.", sources=sources)

Honest about where the masking runs: local backends mask in-process via XGrammar / llguidance / GBNF — out-of-scope citations are token-impossible to sample on your hardware. API backends (OpenAI / Mistral / OpenRouter / Gemini) hand the strict schema to the provider, which has done real token-level constrained sampling since strict structured outputs went GA in late 2025; the same guarantee, just enforced inside their runtime. Anthropic uses its own Citations API — provider-side, structurally constrained that every cite references a supplied document. All collapse to the same GenerationResult for downstream verify / render. Per-call token usage and (on OpenRouter) per-call USD cost are exposed on result.usage.

Citation policies

Policy controls where citations are grammatically required:

Policy	Shape of valid output	When to use
REQUIRED	Every sentence ends content cite-group sent-end. Cite or can't close.	Literature reviews, survey papers, anything where every claim needs provenance.
QUOTES_ONLY	Only "..." quoted spans require a trailing cite-group.	Mixed analytical prose — narrative is uncited, direct quotations are tracked.
AUTO	cite-group is allowed anywhere, never required. verify() flags uncited-but-entailed sentences post-hoc.	Open-ended generation; NLI coverage check does the policing.

Pass via Citeformer(citation_policy=Policy.REQUIRED) or per-call cf.generate(..., policy=Policy.AUTO). See Policy.

Metadata adapters

Build Source objects from real-world inputs:

Source.from_doi("10.1038/s41586-023-06221-2")      # Crossref → CSL-JSON
Source.from_arxiv("2305.14627")                     # arXiv API → CSL-JSON + abstract
Source.from_pdf("paper.pdf")                        # pypdf → title + body text
Source.from_pdf("paper.pdf", extractor="grobid")    # GROBID → author/abstract/section text
Source.from_url("https://example.com/article")      # readability-lxml + OpenGraph

# Bulk-load a library; each returns list[Source].
Source.from_bibtex("refs.bib")                      # BibTeX parser → CSL-JSON
Source.from_zotero("zotero-export.json")            # Zotero CSL JSON / Better BibTeX

All fetchers are cached on disk via diskcache (~/.cache/citeformer/metadata/, override with CITEFORMER_CACHE_DIR).

Inline marker shapes

[N] collides with Markdown link syntax. Switch it out with MarkerStyle:

from citeformer import MarkerStyle

cf = Citeformer(backend=backend, marker_style=MarkerStyle.PAREN)    # (1), (2) ...
cf = Citeformer(backend=backend, marker_style=MarkerStyle.CURLY)    # {1}, {2} ...
cf = Citeformer(backend=backend, marker_style=MarkerStyle.CARET)    # ^1, ^2 ...

The structural guarantee is identical across styles — the grammar's digit enum is bounded by range(1, len(sources) + 1) regardless of which delimiters surround it. See ADR-011.

Streaming

stream = cf.stream(prompt="...", sources=sources)
for chunk in stream:
    print(chunk, end="", flush=True)
result = stream.finalize()    # full GenerationResult with parsed citations + refs

Grammar constraints apply to every chunk. HF and llama.cpp deliver true token-by-token streaming; the API backends chunk on sentence boundaries for UI progression.

Evidence

All numbers below come from running scripts in benchmarks/ — reproducible on a commodity laptop with uv run python -m benchmarks.<script>.

Finding	Result	Script
Adversarial	100% → 0% fabrication swing when the prompt demands out-of-scope ids	adversarial.py
Sweep	0 ± 0 fabrication across 13 runs (3 models × up to 5 seeds)	sweep.py
Full-text premise	Support rate lifts with full-text NLI premise — but the number is noisy, so we report that honestly	sweep.py --premise fulltext
NLI calibration	DeBERTa-v3-large is bimodal; threshold isn't the right knob	threshold_calibration.py
Multi-prompt	0% fab across 24 runs × 4 prompt shapes — guarantee is prompt-invariant	multiprompt_sweep.py

Composition, not reinvention

citeformer's value is the composition, not the parts. The heavy lifting lives in established dependencies:

We piggyback on	For
XGrammar / llguidance	Token-level logit masking at generation time
transformers / vLLM / llama-cpp-python	Running local models
openai / anthropic SDKs	API-provider generation
lark	Authoring citation grammars before hand-off to the decoder
pydantic	Immutable output schemas with extra="forbid"
httpx + diskcache	Metadata fetchers (Crossref, arXiv) with caching
pypdf	PDF text extraction
readability-lxml	URL extraction
DeBERTa-v3-MNLI (via transformers)	NLI entailment for verify()
typer + rich	CLI + pretty output

The parts citeformer owns: citation grammar shape (§10.1), CSL-JSON source contract (§10.2), output pydantic models (§10.3), marker-to-reference coupling, the six bundled style formatters (APA 7, MLA 9, Chicago author-date, IEEE, Nature, Vancouver — ADR-004), the BibTeX parser, and the orchestration loop. Everything else is a composition.

Examples

The examples/ directory contains eight runnable scripts, each a living report:

#	File	What it shows
1	01_quickstart_mock.py	Shortest possible demo — no ML, no extras
2	02_rag_with_hf_and_verify.py	Full RAG pipeline with HF + NLI verify
3	03_standalone_rendering.py	All six styles on the same CSL-JSON item
4	04_fetch_and_render.py	DOI → Crossref → rendered reference
5	05_streaming.py	Realtime chunk streaming via cf.stream()
6	06_langchain_rag.py	LangChain Document → Source → citeformer
7	07_llamaindex_rag.py	LlamaIndex NodeWithScore → Source
8	08_literature_review.ipynb	Full academic workflow notebook (arXiv → review → verify → APA-7)
9	09_bibtex_source.py	BibTeX + Zotero ingest → APA-7 render (no network, no model)

Paper-shaped write-up

A longer design + evaluation document is in PREPRINT.md. Eight sections covering motivation, related work, design, structural-guarantee evaluation (40-run sweep), NLI calibration findings (bimodal large vs under-confident base), known limitations, and roadmap.

Is this for you?

Probably yes if:

• You're building RAG and need citations that can't hallucinate.
• You run open-weight models locally (HF / vLLM / llama.cpp) and want grammar-level guarantees.
• You call an API (OpenAI / Anthropic) and want the same GenerationResult / Citation / Reference surface across your providers.
• You need APA / MLA / Chicago / IEEE / Nature / Vancouver bibliographies rendered deterministically.
• You care about claim-level NLI verification out of the box.
• You want to ingest from BibTeX / Zotero / DOI / arXiv / PDF / URL without glue code.

Probably no if:

• You want a full agent framework — use LangChain / LlamaIndex and compose citeformer as the generation step (examples 6 & 7 show how).
• You need a TypeScript surface today — a sibling citeformer-ts may come later; not here yet.
• You need a citation style outside the six bundled — you can plug in citeproc-py yourself, or contribute a CitationFormatter subclass (see .claude/skills/add-citation-format).

Documentation

• Getting started: getting-started
• Guarantees: guarantees — what "bulletproof" actually covers.
• Architecture: reference/architecture — layers + phase plan + tiered enforcement.
• Contracts: reference/contracts — the three §10 invariants.
• ADRs: docs/decisions/ — 11 short architecture-decision records documenting major design choices.
• Benchmarks: benchmarks/README.md — the five findings with reproduction commands.

Contributing

See CONTRIBUTING.md. Short version: bug-fix PRs welcome and bump patch; feature PRs should open an issue first. The three §10 contracts (grammar shape, CSL metadata, output schemas) are deliberate ceremonies — read docs/reference/contracts.md before touching them.

License

Apache-2.0. See LICENSE.