diff --git a/providers/azure/services/internal/reservations/purchase.go b/providers/azure/services/internal/reservations/purchase.go
index 88c8f3ebd..fdb4da3c8 100644
--- a/providers/azure/services/internal/reservations/purchase.go
+++ b/providers/azure/services/internal/reservations/purchase.go
@@ -261,10 +261,17 @@ const (
 // DoPurchaseTwoStep executes the calculatePrice->purchase two-step flow.
 //
 // It POSTs bodyBytes to calculateURL to mint an Azure-assigned reservationOrderId,
-// then POSTs the same body to the derived purchaseURL. On a "Session timed out"
-// 400 from the purchase endpoint (Azure has retired the session) it re-runs
-// calculatePrice from scratch (up to purchaseMaxAttempts total attempts).
-// Other 4xx/5xx errors are returned immediately without retry.
+// then POSTs the same body to the derived purchaseURL. It re-runs calculatePrice
+// from scratch (up to purchaseMaxAttempts total attempts) in exactly two cases:
+//
+//  1. a "Session timed out" 400 from the purchase endpoint, meaning Azure has
+//     retired the session; and
+//  2. a 400 whose response body did not read completely, where case 1 cannot be
+//     ruled out because the fragment it matches on may be in the part that never
+//     arrived (see errRetryabilityUnknown).
+//
+// Every other response, including a cleanly read 4xx or any 5xx, is returned
+// immediately without retry.
 //
 // Returns the Azure-minted reservationOrderId on success, which the caller
 // should store as the CommitmentID.
diff --git a/providers/azure/services/internal/reservations/purchase_test.go b/providers/azure/services/internal/reservations/purchase_test.go
index 2d91086bd..d35418995 100644
--- a/providers/azure/services/internal/reservations/purchase_test.go
+++ b/providers/azure/services/internal/reservations/purchase_test.go
@@ -833,17 +833,23 @@ func (b *truncatingBody) Close() error { return nil }
 // TestDoPurchase_UnreadableBodyDoesNotSilentlyLookPermanent is the regression
 // test for the retry-classification defect.
 //
-// doPurchase's error text is the ONLY input to IsSessionTimeout, and
-// DoPurchaseTwoStep retries only when that predicate matches. When the read
-// error was discarded, a 400 whose body was truncated before the
-// "Session timed out" fragment produced a bare `status 400: <partial>` error.
-// IsSessionTimeout then returned false and the purchase was abandoned on its
-// first attempt — the precise condition purchaseMaxAttempts exists to survive —
-// with no indication that anything had gone wrong reading the response.
+// Current rule: DoPurchaseTwoStep retries when IsSessionTimeout matches the
+// error text, OR when the error carries errRetryabilityUnknown — which
+// doPurchase attaches to a 400 whose body did not read completely, and to
+// nothing else. Retryability is no longer inferred solely from text.
 //
-// The fix must not classify retryability from a body that was never fully
-// received. Pre-fix this test fails: the error carries neither the read failure
-// nor the session-timeout fragment, so both assertions below are false.
+// Why the test exists (pre-fix behavior): the error text was once the only
+// input to IsSessionTimeout, and that predicate the only retry trigger. With
+// the read error discarded, a 400 truncated before the "Session timed out"
+// fragment produced a bare `status 400: <partial>`; IsSessionTimeout returned
+// false and the purchase was abandoned on its first attempt — the precise
+// condition purchaseMaxAttempts exists to survive — with no indication that
+// anything had gone wrong reading the response.
+//
+// This test pins the surfacing and the out-of-band tag. The retry decision
+// itself is pinned by TestDoPurchaseTwoStep_TruncatedSessionTimeoutStillRetries,
+// which counts attempts, because a message assertion cannot tell a changed
+// message from changed behavior.
 func TestDoPurchase_UnreadableBodyDoesNotSilentlyLookPermanent(t *testing.T) {
 	ctx := context.Background()
 	full := `{"error":{"code":"BadRequest","message":"Session timed out - Call CalculatePrice again"}}`