diff --git a/.github/actions/ci-serialisation/action.yml b/.github/actions/ci-serialisation/action.yml
index 0b783b1..0dd41ac 100644
--- a/.github/actions/ci-serialisation/action.yml
+++ b/.github/actions/ci-serialisation/action.yml
@@ -49,9 +49,14 @@ inputs:
       serialisation on the base branch before the check refuses to diff against it and
       reports "baseline unusable" instead of naming affected types. A baseline that broken
       cannot establish what the branch changed, so any diff against it is meaningless.
-      Set by CI ownership. Observed failure rate at time of writing: 99.3% (5981 of 6024
-      on a private Revit tool repo), so anything from roughly 0.25 to 0.90 behaves identically
-      today. Raise or lower here rather than in CompareCommand.cs.
+      Set by CI ownership. HISTORICAL: the 99.3% failure rate (5981 of 6024) this value was
+      originally chosen against was the pre-fix serialiser, and the defect behind it was fixed
+      on 2026-08-03; healthy baselines since then report at or near zero failures over a
+      population of roughly six thousand. Any value from roughly 0.25 to 0.90 behaves
+      identically on everything observed so far, but only because the two states seen to date
+      sit at opposite ends of that band, not because intermediate baselines cannot occur.
+      A defect confined to one of the three converter legs lands in the low tens of percent.
+      Raise or lower here rather than in CompareCommand.cs.
     required: false
     default: '0.5'
 
diff --git a/tools/SerialiserRunner/src/SerialiserRunner/Commands/CompareCommand.cs b/tools/SerialiserRunner/src/SerialiserRunner/Commands/CompareCommand.cs
index f3ba39c..2ed559b 100644
--- a/tools/SerialiserRunner/src/SerialiserRunner/Commands/CompareCommand.cs
+++ b/tools/SerialiserRunner/src/SerialiserRunner/Commands/CompareCommand.cs
@@ -8,10 +8,21 @@ public static class CompareCommand
 {
     // POLICY VALUE, NOT A MEASUREMENT. The share of the tested population that must fail on
     // the base branch before we refuse to diff against it. Set by CI ownership, not derived
-    // from anything. Observed failure rate at time of writing: 99.3% (5981 of 6024 on
-    // a private Revit tool repo), so anything from roughly 0.25 to 0.90 behaves identically
-    // today. Overridable per-repo via the ci-serialisation action's
+    // from anything. Overridable per-repo via the ci-serialisation action's
     // implausible_baseline_ratio input; change it there, not here.
+    //
+    // HISTORICAL: the 99.3% failure rate (5981 of 6024) this value was originally chosen
+    // against was the pre-fix serialiser, and the defect behind it was fixed on 2026-08-03 by
+    // the System.Drawing.Common reference in SerialiserRunner.csproj. Healthy baselines since
+    // then report at or near zero failures over a population of roughly six thousand.
+    //
+    // Any value from roughly 0.25 to 0.90 therefore behaves identically on everything observed
+    // so far, but the reason is not that intermediate baselines are impossible. It is that the
+    // only two states seen to date sit at opposite ends of that band: near 0% with a healthy
+    // serialiser, near 100% with a globally broken one. Intermediate states are reachable:
+    // failures and population are summed across three independent converter legs, so a defect
+    // confined to one of them lands in the low tens of percent. Choosing inside the band is a
+    // choice about which partial states to refuse, not a free one.
     public const double DefaultImplausibleBaselineRatio = 0.5;
 
     public static CompareResult Compare(SerialiserResult baseline, SerialiserResult branch,