test(precision): characterize and guard the Decimal->float JSON boundary (C2)#209
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The JSON API serializes Decimals via float() (core/charts.py), so exact ledger values cross the wire as float64. The frontend renders at display precision, so typical currency values are unaffected in the UI — but exports, raw API consumers, and high-precision values see the rounding. Layer 2 of the correctness plan: pin the boundary so it can't regress and a fix is measurable. - test_typical_values_are_lossless: currency + 8dp crypto + bounded magnitudes round-trip exactly (guards the common case). - test_high_precision_is_lost: xfail(strict) documenting that values beyond ~16 significant digits are rounded on the wire; flips green when numbers are emitted exactly (needs an exact JSON encoder + frontend decimal support, which also regenerates all snapshots — that epic, not an isolated wire change). - test_api_numbers_are_json_numbers_not_strings: pins the current wire contract so any format switch is deliberate.
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Layer 2 of the correctness plan — pins the Decimal→float64 JSON boundary (C2).
Investigation showed C2 is a data-fidelity issue (exports / raw API consumers / high-precision), not a wrong-number-in-the-UI issue: the frontend renders through
Intl.NumberFormatat display precision, so typical values aren't visibly wrong. A clean exact-wire fix needs a new dep (simplejson) + regenerating all snapshots + frontend decimal support (an epic).tests/test_number_precision_boundary.py:test_typical_values_are_lossless(guards the common case — currency/8dp crypto round-trip exactly),test_high_precision_is_lost(xfail(strict)documenting the frontier; flips green when numbers are emitted exactly — the acceptance test for the epic), and a wire-contract pin. 9 passed, 3 xfailed.(Was #205, stacked on #202 which is now merged; recreated to
main.)🤖 Generated with Claude Code