diff --git a/tools/bump/flake.go b/tools/bump/flake.go
index 5297ce39..04b63fba 100644
--- a/tools/bump/flake.go
+++ b/tools/bump/flake.go
@@ -88,6 +88,15 @@ func applyFlake(ctx context.Context, r *repo) (change, error) {
 		return change{Empty: true}, nil
 	}
 
+	// The lock decides which prettier, gofumpt and nixfmt the tree is judged
+	// by, so moving it can leave files no bump touched failing the formatting
+	// check. Reformatting belongs in this commit rather than a later one: the
+	// gate below judges this tree, and the cause is this change.
+	err = runFormatters(ctx, r)
+	if err != nil {
+		return change{}, err
+	}
+
 	detail := moved
 
 	if tools, err := toolVersions(ctx, r); err == nil { //nolint:noinlineerr
diff --git a/tools/bump/generate.go b/tools/bump/generate.go
index a8bcb61f..55217a46 100644
--- a/tools/bump/generate.go
+++ b/tools/bump/generate.go
@@ -140,13 +140,7 @@ func gateGenerate(ctx context.Context, r *repo) error {
 // The first hook pass is expected to fail, because a hook that rewrites a file
 // reports failure. The second pass is the real verdict.
 func applyFormat(ctx context.Context, r *repo) (change, error) {
-	if _, err := r.nixRun(ctx, "make", "fmt"); err != nil { //nolint:noinlineerr
-		return change{}, err
-	}
-
-	_, _ = r.nixRun(ctx, "prek", "run", "--all-files")
-
-	if _, err := r.nixRun(ctx, "prek", "run", "--all-files"); err != nil { //nolint:noinlineerr
+	if err := runFormatters(ctx, r); err != nil { //nolint:noinlineerr
 		return change{}, err
 	}
 
@@ -164,3 +158,18 @@ func applyFormat(ctx context.Context, r *repo) (change, error) {
 		Detail:  touched,
 	}, nil
 }
+
+// runFormatters brings the tree up to whatever the current toolchain considers
+// formatted. The first hook pass is expected to fail, because a hook that
+// rewrites a file reports failure; the second pass is the verdict.
+func runFormatters(ctx context.Context, r *repo) error {
+	if _, err := r.nixRun(ctx, "make", "fmt"); err != nil { //nolint:noinlineerr
+		return err
+	}
+
+	_, _ = r.nixRun(ctx, "prek", "run", "--all-files")
+
+	_, err := r.nixRun(ctx, "prek", "run", "--all-files")
+
+	return err
+}