Calling the AI bluff: Adding "Do not guess" cut made-up claims from 71% to 20%

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We asked 16 AI models to extract fields from web pages where some fields were missing. Without extra instructions, they made up 405 of 573 missing fields (70.7%). Adding one sentence, Use null for any field whose value is not on the page. Do not guess., cut that to 116 of 574 (20.2%). All 16 models improved. Full table: extraction honesty benchmark.

Is that sentence special, or would any magic words do? On Hacker News, commenters compared it to make no mistakes and to a pigeon pressing a button for food. So we tested the folklore against it, and split the sentence in half to see which part does the work.

The test

Same pages as the benchmark: 42 twin pairs that differ by one row. One page shows the answer, the other does not, and both carry a decoy such as Was $493.00. We scored the 36 pages per model where the field was missing (email traps excluded, as in the benchmark).

Each phrase replaced the full sentence. The rest of the prompt stayed the same. We also split the sentence into its two halves, to see which half does the work. Five models from the top, middle and bottom of the benchmark table. The phrases, as sent:

• Make no mistakes: Make no mistakes.
• Stop bullshitting: Stop bullshitting.
• Grounded: You're not trained on this data, return exclusively grounded results.
• Quote: Also add a key "evidence": for each field you fill, quote the exact text on the page where you found it.
• Say you don't know: If you don't know, say you don't know.
• Self-check: Before you answer, check each field: did this require any guessing?
• Only "Do not guess": Do not guess.
• Only "Use null": Use null for any field whose value is not on the page.
• Full sentence: Use null for any field whose value is not on the page. Do not guess.

Results

Counts below 36 exclude errors (Gemma’s host failed often: only 24 and 26 pages scored on the two halves). “Change” is in percentage points of made-up fields, all five models pooled. Its 95% range comes from resampling the pages 10,000 times, keeping each page paired across prompts and models.

1. Make no mistakes made no difference: +2.1 points, with a range across zero. Stop bullshitting cut 4.5 points at most, at the edge of noise.
2. The boring half of the sentence does most of the work. Use null for any field whose value is not on the page alone cut 37 points. Do not guess alone cut 26. Telling the model what an honest answer looks like beat telling it what not to do.
3. The full sentence beat every other phrase. The closest, the self-check question, still left 15.8 more points of made-up fields (range 10.2 to 21.8). Both halves together beat Use null alone by 6.6 points (2.1 to 11.5).
4. Asking for a quote did not stop bluffs: some models quoted the decoy as evidence, such as Fact-checked by Omar Tamm for the author.

What this does not show

• One run per cell and five models. Repeats have not been run.
• Synthetic pages with seven page types and traps we wrote. Real sites may differ.
• One task: web extraction. A phrase that does nothing here may help elsewhere.
• All folklore runs together cost $2.29 through OpenRouter.

Have a phrase you swear by? Send it and we will run it.