Lookahead safety

Lookahead Safety

The single most common bug in quant research with LLMs: using data that was not yet known at the moment you claim to have known it. This skill defines the rules to keep historical analysis honest.

The core rule

The known-date is the date a piece of information was PUBLISHED, not the date the information is ABOUT.

• A 10-K covering fiscal year 2023 may be filed in March 2024. In January 2024, that 10-K did not exist.
• An S-3 effective on 2024-04-15 was unknown on 2024-04-14.
• An XBRL SharesOutstanding datapoint with period_end: 2023-12-31 is known only after the filing that contains it is published.

When asked to compute or recommend anything as of a date D, only data where filing_date <= D is admissible.

Field-by-field reference for SEC data

Field	Use as known-date?	Notes
filing_date / acceptedDate	✅ YES	The publication moment
period_end / periodOfReport	❌ NEVER	The accounting period the data covers
effectiveDate (S-3, etc.)	✅ YES	When the registration becomes usable
reportDate on XBRL facts	❌ NO	Period covered, not publication
accepted timestamp	✅ YES	Most precise — use when intra-day matters

Common traps

1. The XBRL trap. companyfacts.json returns datapoints keyed by period_end. Naively iterating these as a time series leaks future information — the datapoint with period_end: 2023-12-31 was first published months later. Always join with the originating filing's accepted date and treat THAT as availability.

2. The amendment trap. A 10-K/A (amendment) supersedes the original 10-K but is filed later. At time D between the original and the amendment, only the original was known. Don't apply restated numbers to dates before the amendment's filing_date.

3. The "as of today" trap in backtests. When using current data (e.g., current_shares_outstanding), confirm whether the source provides a point-in-time history or only the current snapshot. A snapshot is unsafe for any historical query.

4. The price-data trap. Adjusted prices are computed using SPLIT factors from splits that may not have happened yet at date D. Use raw OHLC + a split history table where each split has its own ex-date as the cutoff.

5. The earnings-revision trap. Restated earnings (a later 10-K/A correcting a previous 10-K) were unknown until restatement. Backtests that use the latest version of earnings for old dates are fictional.

Workflow when the user asks "what was X on date D"

1. Identify the data source (XBRL, filings, prices, derived metrics).
2. Determine the source's publication semantics (when did this datapoint become observable to a market participant?).
3. Filter by publication_date <= D. If publication is unknown, stop and tell the user the query is unsafe.
4. State explicitly which as_of_date was used and which records were excluded for being future-dated.

Workflow when writing backtest code

• Every read of historical state must take a query_date argument and filter on filing_date <= query_date (or equivalent).
• Never store "current" values in a structure used for historical lookup.
• When in doubt, prefer the EARLIEST plausible publication date over the latest — bias toward under-claiming what was knowable.
• Tests: include a "future-data leak" test that fails if any record with filing_date > query_date is returned by a historical query.

Phrases that should trigger this skill

• "what was the float of X on [past date]"
• "backtest this strategy"
• "shares outstanding at [past date]"
• "compute [metric] historically"
• "build a point-in-time database"
• "as of [date]"
• "lookahead-safe"
• "survivorship bias" / "look-ahead bias"

What this skill is NOT

This is not a forecasting or prediction skill. It does not tell you what will happen. It tells you what you were ALLOWED to know at a moment in time. Use it before any historical reasoning. Combine with domain skills (SEC filing types, dilution events) for full coverage.