diff --git a/claude/claude.go b/claude/claude.go
index 35da1bb..4b96f60 100644
--- a/claude/claude.go
+++ b/claude/claude.go
@@ -36,9 +36,6 @@ func trace(op string) func() {
 // top bar) from being misread as a 1 on this kind of LCD.
 const weightPrompt = `This photo shows a 3D printer filament spool sitting on a digital kitchen scale. ` +
 	`Read the weight on the scale's digital display, digit by digit, exactly as shown. ` +
-	`Note: the top edge of the display housing can come close to obscuring the TOPS of the digits, ` +
-	`so a digit that really has a top horizontal bar (like a 7) can momentarily look like a 1 — ` +
-	`look carefully for a faint top bar before deciding. ` +
 	`Report the unit shown on the display (g, kg, lb, or oz), and rate your own confidence ` +
 	`in the reading as "low", "medium", or "high". ` +
 	`Reason briefly, then on the LAST line output ONLY JSON: ` +
diff --git a/main.go b/main.go
index 6beeb9d..ac16645 100644
--- a/main.go
+++ b/main.go
@@ -217,6 +217,10 @@ func processImg(img image.Image, name string, auth claude.Auth, dryRun bool) res
 		r.Error = fmt.Sprintf("scale is set to imperial units (%s); switch it to grams", reading.Unit)
 		return r
 	}
+	if reading.Weight < 0 {
+		r.Error = fmt.Sprintf("scale read a negative weight (%g g); refusing to update", reading.Weight)
+		return r
+	}
 	r.Unit = reading.Unit
 	r.Confidence = ptr(reading.Confidence)
 	r.VoteWeights = reading.Weights
@@ -228,6 +232,10 @@ func processImg(img image.Image, name string, auth claude.Auth, dryRun bool) res
 	if info != nil {
 		r.NewWeight.Spool = ptr(info.EmptySpoolGrams)
 		r.NewWeight.Filament = ptr(reading.Weight - info.EmptySpoolGrams)
+		if reading.Weight-info.EmptySpoolGrams < 0 {
+			r.Error = fmt.Sprintf("measured total %g g is below the empty-spool weight %g g (negative filament); refusing to update", reading.Weight, info.EmptySpoolGrams)
+			return r
+		}
 	}
 
 	// Write the measured total back when it differs from the site's total — the