diff --git a/docs/history/DEVELOPMENT_LOG.md b/docs/history/DEVELOPMENT_LOG.md
index 6aed7ed..65bf888 100644
--- a/docs/history/DEVELOPMENT_LOG.md
+++ b/docs/history/DEVELOPMENT_LOG.md
@@ -32,6 +32,69 @@ written and stay true. It is exempt from review for the same reason a receipt is
 
 ## Entries
 
+### 2026-08-18 — Batch 02 shipped: the prediction engine, and three bugs the tests found
+
+All five Batch 02 issues closed and the milestone closed with them. The app now
+ships the engine PRODUCT_PLAN.md §12 specifies rather than the prototype it was
+explicitly labelled as.
+
+**The shape is the design.** `PersonalPredictionEngine` keeps a discrete
+probability distribution over candidate start dates, not a date with a margin.
+The mode is the forecast, the window is the narrowest span holding 80% of the
+mass, and a "Not yet" is the distribution conditioned on what the user said.
+§13's requirement — that a "Not yet" updates the date, the window *and* the
+confidence rather than shifting a fixed prediction by a day — is not extra work
+in that design; it is the only thing that structure can do.
+
+**It is better, as a number.** `EngineComparisonTest` scores both engines over
+the §51 fixtures on every build: mean absolute error 0.67 against 1.00, and the
+window contained the actual start 9 times out of 9 against 7.
+
+**Coverage is the measure, not width — and measuring taught that.** The first
+version of the comparison asserted the new windows must not be wider. It failed,
+and it was the assertion that was wrong: where the personal engine is wider, it
+is right to be, and the baseline answers a history with a suspected missing
+period using a two-day window and misses. A window that misses is a broken
+promise rather than a tight forecast.
+
+**Three modelling bugs, each found by a test failing rather than by reading:**
+
+1. Median absolute deviation alone reads a user alternating 25 and 37 days as
+   perfectly consistent — half her deviations are zero. Twenty disagreeing
+   cycles came back High, which is exactly the §15 rule about volume not buying
+   confidence. Spread is now the larger of MAD and mean absolute deviation.
+2. Recency weighting assumes the recent past predicts the near future. For a
+   variable user that is false, and weighting it equally cost three days on the
+   §51 variable fixture. Recency is now trusted in proportion to how much her
+   cycles agree — which is a better statement of what recency weighting is
+   actually for.
+3. A fixed one-day floor on trend detection fired on a 42-day-cycle history
+   whose medians differed by one day. One day is a real trend at 28 and rounding
+   error at 42, so the floor is relative to the user's own spread.
+
+That is now three consecutive pieces of work — the schema guard, the boundary
+guard, and the engine — where the thing that found the defect was **running it,
+not reading it**. Worth stating as a habit rather than a coincidence.
+
+**Wired through, not just tested.** `PredictionInput` carries recent absolute
+errors and the repository feeds scored errors back in. Without that the app
+would store every error it makes and never read one back — measuring accuracy
+rather than learning from it, with §12 step 5's widening happening only in a
+unit test.
+
+- **Closed:** #10, #11, #12, #13, #14 — and the `Batch 02 — Prediction Engine`
+  milestone, which closing the last issue does not do.
+- **Next action:** Batch 03 — Core UX. The onboarding flow in §19, the designed
+  Today screen and its six dynamic states in §21–§22, period start and end
+  logging in §23–§24, the calendar in §26 and Insights in §27. The working
+  surface built in Batch 01 is deliberately ugly and says so on screen; it is
+  the thing Batch 03 replaces. Before starting, file the Batch 03 issues — the
+  milestone exists and is empty.
+- **Blockers:** Unchanged and both need a person. #8, the three branding marks,
+  which an agent must not fake. #9, the webhook, whose URL and secret are not
+  readable from this machine — until it is registered an opened `P0` raises no
+  alert at all. Neither blocks Batch 03.
+
 ### 2026-08-18 — Batch 01 foundation: seven of nine issues, and three guards that were wrong
 
 Room, DataStore, the repository layer, period CRUD end to end on a device, and