I asked AI to tidy 20 contacts. It quietly changed 3 values.

Sources and assumptions

• Own test: GPT-5.5 via the OpenAI API, engine/ai-test.js (record.tests ids 'before' and 'after', tool chatgpt): Both outputs shown on the slides; every count (19 rows, 4 dates blanked, 3 unmentioned changes, 9 [CHECK] cells, 2 dropped '+1' prefixes) comes from comparing those outputs cell by cell with the fictional input table (checked 2026-10-11)
• Anthropic, Prompting best practices (Claude Platform Docs), 'Be clear and direct': Verbatim: 'Claude responds well to clear, explicit instructions. Being specific about your desired output can help enhance results.' and the bullet 'Be specific about the desired output format and constraints.' (Written for Claude models; used here as general prompting guidance, not as a claim about GPT-5.5.) (checked 2026-10-11)
• OpenAI, GPT-5 prompting guide (OpenAI Cookbook): Verbatim: 'GPT-5 follows prompt instructions with surgical precision' and 'poorly-constructed prompts containing contradictory or vague instructions can be more damaging to GPT-5 than to other models'. Written for GPT-5, not specifically GPT-5.5. (checked 2026-10-11)

Assumptions:

• The test ran on GPT-5.5 through the OpenAI API (label 'GPT-5.5 (OpenAI API)'), not the ChatGPT app. The idea called for Claude, but the claude CLI failed inside the research sandbox (the two failed 'claude' entries in tests.json have output null), so no Claude result exists and the slides must not claim one.
• Each prompt ran once. The same prompt can give a different result on another run or in another tool, so the post shows one real test, not a guarantee.
• The input table is fictional: invented names and companies, 555-010-xxxx phone numbers and .example email domains. The errors were planted on purpose: ambiguous dates 03/04/2026, 1/2/26 and 12/11/2025; invalid date 2026-02-30; relative date 'yesterday'; two-digit year '6 March 26'; a letter O in a phone number; a missing area code; a phone extension; the email typo 'exmaple'; a near-duplicate Maria Lopez row; a blank company cell.
• 'Content change' means any value whose meaning changed (merged, deleted, guessed, corrected or invented). Case, spacing, phone punctuation and converting an unambiguous date to YYYY-MM-DD count as formatting. Adding '+1' (before run) or dropping it (after run) is borderline and is reported as such.
• 'Quietly changed 3 values' counts changes that appear in neither the before run's notes nor its opening line: Ravi's phone O→0, Ben's email exmaple→example, and Felix's '26 read as 2026. The merge, the blanked dates, the 'yesterday' guess and the 555 area code were all disclosed.
• The before run turned 'yesterday' into 2026-10-11 because the run happened on 2026-10-12 UTC. The table gives no date for when 'yesterday' was written, so this is a guess.