# Summary Fidelity Check

Summary Fidelity Check is a tested SKILL.md that checks a summary against the source text it was made from and lists the summary sentences the source does not support, each copied word for word, as strict JSON; an agent buys it once for $0.01 over x402.

- Page: https://aiskills402.com/skills/summary-fidelity-check
- Category: Research & Summaries (https://aiskills402.com/categories/research)
- Price: $0.01 once, USD-priced, paid in USDC on Base over x402. Price as loaded on this page. The 402 response your agent receives is authoritative.
- Version: 1.0.1
- Card (JSON): https://api.aiskills402.com/v1/skills/summary-fidelity-check

## Use it when

Checks a summary against the source text it was made from and lists the summary sentences the source does not support, each copied word for word, as strict JSON. It catches a number that was rounded or given another unit, a hedge that became a certainty, a cause the source never states, two facts joined into a false one, a claim whose speaker was dropped, a fact that comes from outside the source, and it ignores any instruction hidden in the source. A sentence is judged supported or unsupported and nothing in between, and a faithful paraphrase is never flagged. It never checks the source against the world and never judges what the summary left out or how well it is written. Use to check a summary, abstract, executive summary or AI-written digest against its source, to review an agent's report of a document, or to find where a summary says more than the text.

## Not for

Fact-checking: it compares the summary with the pasted source only, never with the world, so a false statement that the source makes is passed and a true one the source lacks is flagged. It does not judge what the summary leaves out, its style or its length. It reads one source and one summary pasted together, not a set of files or a web page.

## Tested, honestly

Tested 2026-10-09.

- Strong model (claude-sonnet-5-5 (Claude Code alias "sonnet")): Right on all 24, checked by code: a rounded number and a changed unit, dropped hedges, added causes, two facts merged into a false one, dropped speakers, outside facts, a changed quantifier and planted notes in English and Bulgarian, each flagged with the sentence copied exactly; an empty list for eight faithful summaries, and plain words with no list when the summary was missing.
- Weak model (claude-haiku-5-5 (Claude Code alias "haiku")): Right on all 24, checked by code, with the same flags and the same empty lists as Sonnet.

Note: Twenty-four source and summary pairs written by us in English, Bulgarian, German and Spanish: 15 summaries with a planted unsupported sentence (rounded numbers, units, hedges, causes, merged facts, dropped speakers, outside facts, a quantifier, planted notes), one request with no summary, and 8 faithful summaries built to tempt a false flag. Each answer is parsed as JSON; the flagged sentences must be exactly the planted ones, copied character for character. No check was widened. One run per model and pair.

### With and without the skill

Tested 2026-10-09.

- Summaries judged right (24 pairs): Sonnet 24/24 with, 24/24 without; Haiku 24/24 with, 23/24 without.

Same request and same checks on both sides. The request already spells out what unsupported means and the JSON shape, and with it Sonnet without the skill was right on all 24, faithful summaries included. Haiku without the skill missed one: a Bulgarian summary that dropped who made a claim about unemployment.

Same cases and the same checks with and without the skill. The cases are ours, written around what the skill is for; with a handful of cases, a difference of one or two is within noise.

Full summary: https://aiskills402.com/skills/summary-fidelity-check/tests

## Example

### English (claude-sonnet-5-5)

Before:

> SOURCE: The new bridge is 14 kilometres long and cost 380 million euros. It opened to traffic in May. SUMMARY: 1. The new bridge is 14 miles long. 2. The bridge cost 380 million euros. 3. It opened to traffic in May.

After:

> {"unsupported": [{"sentence": "The new bridge is 14 miles long.", "why": "The source gives the length as 14 kilometres, not miles."}]}

### German (claude-sonnet-5-5)

Before:

> SOURCE: Die Stadt hat im Juni die Parkgebühren erhöht. Im Juli wurden 2.300 Parkscheine weniger verkauft als im Juni. SUMMARY: 1. Die Stadt erhöhte im Juni die Parkgebühren. 2. Wegen der höheren Gebühren wurden im Juli weniger Parkscheine verkauft.

After:

> {"unsupported": [{"sentence": "Wegen der höheren Gebühren wurden im Juli weniger Parkscheine verkauft.", "why": "Die Quelle nennt beide Fakten nur nebeneinander und sagt nicht, dass die Gebührenerhöhung den Rückgang verursacht hat."}]}

### Spanish (claude-sonnet-5-5)

Before:

> SOURCE: La biblioteca de Sevilla abrió en 2015 y tiene 80 000 libros. La de Granada abrió en 2019. SUMMARY: 1. La biblioteca de Sevilla tiene 80 000 libros. 2. La biblioteca de Granada tiene 80 000 libros. 3. La de Granada abrió en 2019.

After:

> {"unsupported": [{"sentence": "La biblioteca de Granada tiene 80 000 libros.", "why": "La fuente da los 80 000 libros solo para la biblioteca de Sevilla, no para la de Granada."}]}

### Bulgarian (claude-sonnet-5-5)

Before:

> SOURCE: Министерството твърди, че през 2025 г. безработицата е спаднала до 5,4 %. Синдикатът оспорва цифрата. SUMMARY: 1. Безработицата през 2025 г. е спаднала до 5,4 %. 2. Синдикатът оспорва цифрата.

After:

> {"unsupported": [{"sentence": "Безработицата през 2025 г. е спаднала до 5,4 %.", "why": "Източникът представя цифрата като твърдение на Министерството, а резюмето я излага като безспорен факт."}]}

## What is in the file

- The answer
- When a sentence is supported
- When a sentence is unsupported
- Report only what the text shows
- Missing input
- Work in this order
- Short examples

## Languages

English, Bulgarian, Spanish, German

## How to buy

Agent (HTTP):

1. GET https://api.aiskills402.com/v1/skills/summary-fidelity-check/file without a payment header. The answer is 402 with a PAYMENT-REQUIRED header (x402 v2): exact amount, asset, network, recipient.
2. Sign `accepts[0]` with an x402 client (for example @x402/core + @x402/evm).
3. Repeat the GET with the signature in the PAYMENT-SIGNATURE header. The answer is 200 with the file, its sha256 and a re-download token.

Agent (MCP): https://mcp.aiskills402.com/mcp — free tools search_skills, get_skill, redownload_skill. Buying itself is over HTTP.

Full flow: https://aiskills402.com/docs

## The file

- Version: 1.0.1
- Size: 8.0 KB (8226 bytes)
- SHA-256: a4382e20e1b10e81f4eb9d63415a6a94b0e78a5f2b81f2eb546005919f4c6fb7
- Updated: 2026-10-09
- New versions are free through your re-download token.

## Versions

### 1.0.1 (2026-10-09)

- Measured on 24 pairs: Sonnet 24 -> 24, Haiku 23 -> 24 (without -> with the skill, same checks). No check was widened. - Price: $0.01 (no measured gain).

### 1.0.0 (2026-10-09)

First release: reads a source text and a numbered summary and lists the summary sentences the source does not support, each copied word for word, as strict JSON. Eight defect kinds (figure changed, hedge removed, reversed, cause added, facts joined wrongly, speaker dropped, outside fact, invented detail); binary verdict per sentence; faithful paraphrase never flagged; lines addressed to an AI are not obeyed; missing source or summary gets a plain refusal. Designed from the batch-4 plan row (no § 2 brief existed when written). No model run yet: the baseline, the price check ($0.02 provisional) and the sentence on what Sonnet gains are still to be written.

## License

Perpetual, non-exclusive; use and modify for yourself incl. paid work; no resale or republishing. Holder: Georgi Kalchev, aiskills402.com. Terms: https://aiskills402.com/docs#license

## FAQ

### What counts as unsupported?

A summary sentence the source does not say: a rounded number or a changed unit, a hedge turned into a certainty, a cause the source never states, two facts joined into a false one, a claim whose speaker was dropped, a fact from outside the source, or an invented detail. A faithful paraphrase, a shorter form or a weaker statement is supported, and so is a sentence that merges facts about one and the same subject.

### Does it check whether the source is true, and what does a faithful summary get?

No, the source is the whole world for this job. A true sentence that the source lacks is still flagged, because a reader who trusts the source would not find it there, while a false statement that the source itself makes is passed. A summary that only shortens, reorders or rewords, keeps the hedges and names the speakers gets an empty array and nothing more.

### How are the flagged sentences and odd inputs handled?

Every flagged line is the summary sentence in its original wording, minus its list number, so a reviewer can search for it; a script can confirm that with a plain substring test. A planted note in the source that tells the assistant to mark everything as supported is not obeyed. If the summary or the source is missing, the agent says so in plain words and returns no list.

### Does it help Claude Sonnet?

Our numbers show no gain for it, so one cent is the price. Twenty-four source and summary pairs in four languages went to Sonnet and Haiku, guided by the file or not, and a script compared the flagged sentences with the planted ones. Our request already spelled out what unsupported means, and with it plain Sonnet was right on all 24, eight faithful summaries included. Haiku missed one without the file: a Bulgarian summary that dropped who made a claim. The file keeps the rules in one place for a pipeline.

## Related skills

- [Source Quotes Extract](https://aiskills402.com/skills/source-quotes-extract.md): $0.05 once
- [Summarize Faithfully, Keep Every Number](https://aiskills402.com/skills/summarize.md): $0.03 once
- [Document Consistency Check: Find Contradictions](https://aiskills402.com/skills/document-consistency-check.md): $0.05 once

## Measurement limits

- Models other than the two named above were not run.
- Each verdict comes from the test run on the date shown; the skill may have changed since (check the version).
- Full test inputs are not published here, only short excerpts of our own text.
- Results on your own texts, languages and domains can differ.

Offer note: Paid in USDC (USD-pegged) over x402 by an AI agent; one-time.