Groundlens is a proofreader for what your model writes. It marks the words your sources don't back — and shows you what each one should have said. It checks RAG answers for grounding and faithfulness against their retrieved sources, the job people reach for hallucination detection, citation checking, or RAG evaluation to do — and differs in returning marks and evidence for a reviewer rather than a verdict or a score to threshold.

QUESTION    What is the invoice total?
SOURCE      ...the total amount due is 10,000 dollars, payable within 30 days...
ANSWER      The invoice total is 1,000 dollars, due in 30 days.

GROUNDLENS  1,000   nothing supports this.   Closest in invoice.pdf#p1: '10,000'

It never tells you the answer is wrong. It tells you which word to look at, and which document to open. Thirty seconds of human attention instead of five minutes.

How it works

Groundlens approaches words and numbers comparison in two different ways:

Groundlens provide the lowest score as output, not the average. Every token-similarity metric aggregates by the mean, and the mean is where single-token errors go to die.

A practical example: ten is not a hundred

A retrieved document says the total due is 10,000 dollars. The answer says 1,000 dollars. A human catches that instantly, without a finance degree.

Embedding similarity does not. Cosine between the right answer and the wrong one is about 0.99 — the error dissolves into the vector the way a drop of ink dissolves in a pool. An LLM judge does not either: it reads for plausibility, and "the total is 1,000 dollars" is a perfectly plausible sentence about an invoice. A trained span detector does not, because single-digit substitutions are rare in its training labels.

Sentence encoders organise text by vocabulary, topic and structure. Never by truth. A wrong number inside a correct sentence is, to a paraphrase-collapsing encoder, very nearly a paraphrase.

On that invoice, the mean support of the wrong answer is 0.79 — which looks fine. The weakest anchor is 0.00 — which is a mark in the margin.

Operational threshold

This library has no default threshold. A threshold is a property of a deployment, not of a method. It depends on the encoder, on your data, and on what a false positive costs you compared to a false negative. None of that is known here.

There is a measurement behind the rule. Across the operating-point grid we ran, the best false positive rate at 95 percent recall was 0.65, for every single-pass detector we tested, including this one. At the recall a regulated review actually needs, no fixed cut in that grid is usable. Shipping one would mean shipping a number we already know does not hold.

What groundlens provide is:

• A support score per word, where lower means less supported by the sources.
• Marks with receipts: the word, its span, its support, and the nearest evidence sentence, so a reviewer can check any call in seconds.
• A function calibrate(), which fits a cut on your own labelled data. It refuses to run on fewer than 200 labelled examples, because below that the cut is noise.

If you need a threshold in your pipeline, run calibrate() on your labelled data:

from groundlens import calibrate

point = calibrate(labelled, target_recall=0.95)
print(point.threshold, point.fpr, point.fpr_ci95)   # read the fpr first

calibrate() needs at least 200 labelled examples, because below that a 95%-recall threshold is estimated from a handful of points.

Install

pip install groundlens              # zero runtime dependencies. Not numpy, not torch
pip install "groundlens[encoder]"   # + the reference sentence encoder
pip install "groundlens[encoder,mcp]"   # + the MCP server, for Claude Desktop and friends

The core install pulls in no package at all, and a CI job fails the build if that ever changes. The previous version installed roughly two gigabytes of deep learning stack before you had done anything.

Quick start

from groundlens import proofread, SentenceTransformerEncoder

answer = "The invoice total is 4.75% payable within 45 days."
sources = [("policy.pdf#p3", "The rate stated in the policy is 3.90% and the term is 30 days.")]

marks = proofread(answer, sources, encoder=SentenceTransformerEncoder(), k=2)

print(marks.report())
#  4.75%   support 0.00    nearest in policy.pdf#p3: '3.90%'
#  45      support 0.00    nearest in policy.pdf#p3: '30'

Every mark carries its receipt:

for anchor in marks.weakest:
    anchor.text            # '4.75%'          the word in the answer
    anchor.span            # (21, 26)         where it sits
    anchor.kind            # 'numeral'        checked by arithmetic, not meaning
    anchor.support         # 0.0              absent from the sources
    anchor.evidence_id     # 'policy.pdf#p3'  which document to open
    anchor.evidence_text   # '3.90%'          what it should have matched

From the shell:

groundlens read --answer answer.txt --context policy.pdf#p3=policy.txt

MCP server

The same proofreader, inside your assistant. Groundlens ships an MCP server, so Claude Desktop, Claude Code, Cursor, VS Code or any other MCP client can check an answer against its sources without leaving the conversation. It runs locally over stdio. No text goes anywhere.

pip install "groundlens[encoder,mcp]"
python -m groundlens.mcp

Then point your client at it. In claude_desktop_config.json — or the equivalent mcp.json in Cursor and VS Code:

{
  "mcpServers": {
    "groundlens": {
      "command": "python",
      "args": ["-m", "groundlens.mcp"]
    }
  }
}

Use the absolute path to the Python that has Groundlens installed if it is not the one on your PATH: /path/to/venv/bin/python.

The one tool

find_unsupported_words(answer, sources, k=4, locale="und")

It returns the weakest anchors with their receipts, the floor, the encoder id and a sha256 of the finding:

{
  "weakest_anchors": [
    {
      "word": "4.75%",
      "support": 0.0,
      "checked_by": "arithmetic",
      "closest_in_sources": "3.90%",
      "source_id": "policy.pdf#p3",
      "notes": []
    }
  ],
  "floor": 0.0,
  "n_marked": 12,
  "encoder_id": "all-mpnet-base-v2@<revision-sha>",
  "sha256": "..."
}

One tool, on purpose. The previous server advertised three, and that is how one product turns into three stories before anyone has installed it.

There is no verdict and no threshold, here as everywhere else in this library. A support of 0.00 on a number means that value is absent from the sources. On a word it means no lexical anchor was found, which is ordinary in a faithful paraphrase. The server reports the marks; the reader decides.

The encoder loads on the first call, not at startup, and the model downloads once (about 420 MB) the first time it is used.

Limitations

• It cannot verify computed values — "revenue tripled" against a source saying "revenue went from 5M to 15M".
• The word channel checks whether a word is supported by the sources. It does not check that it is attached to the right thing. If an answer says "payable in 30 days" about invoice A and the 30 days belong to invoice B elsewhere in the same context, the word is supported and no mark appears.
• It cannot check reasoning. That belongs to entailment models.
• It inherits your retrieval. If the passage is wrong, so is the answer's grounding.
• Segmentation assumes space-delimited scripts, and warns rather than pretending when the text is largely CJK or Thai.

Reproducibility

• The numeral channel is exact. Decimal comparison, fixed arithmetic context, locale from an argument and never from LC_ALL. Byte-for-byte identical on any machine — CI proves it on ten OS × Python combinations under PYTHONHASHSEED=random and a Turkish locale.

• The lexical channel is a float32 cosine from a pinned encoder revision — not a model name, because a silent re-upload would change every number you ever published. It reproduces to 1e-6 across platforms and the ordering of the weakest anchors is stable. It is not bit-identical between x86 and Apple Silicon, and we make no claim that it is.

• marks.sha256 covers the structure and the numeral supports exactly, and rounds lexical supports to six decimals. Reproducing the hash reproduces the finding, not the last bits of the arithmetic.