diff --git a/sjsonnet/src/sjsonnet/Evaluator.scala b/sjsonnet/src/sjsonnet/Evaluator.scala
index 046274f5..1e75234b 100644
--- a/sjsonnet/src/sjsonnet/Evaluator.scala
+++ b/sjsonnet/src/sjsonnet/Evaluator.scala
@@ -712,29 +712,29 @@ class Evaluator(
     (op: @switch) match {
       case Expr.BinaryOp.OP_+ =>
         val r = ld + rd
-        if (r.isNaN) Error.fail("not a number", pos)
-        if (r.isInfinite) Error.fail("overflow", pos)
+        if (r.isNaN) Error.fail("Not a number", pos)
+        if (r.isInfinite) Error.fail("Overflow", pos)
         Val.cachedNum(pos, r)
       case Expr.BinaryOp.OP_- =>
         val r = ld - rd
-        if (r.isNaN) Error.fail("not a number", pos)
-        if (r.isInfinite) Error.fail("overflow", pos)
+        if (r.isNaN) Error.fail("Not a number", pos)
+        if (r.isInfinite) Error.fail("Overflow", pos)
         Val.cachedNum(pos, r)
       case Expr.BinaryOp.OP_* =>
         val r = ld * rd
-        if (r.isNaN) Error.fail("not a number", pos)
-        if (r.isInfinite) Error.fail("overflow", pos)
+        if (r.isNaN) Error.fail("Not a number", pos)
+        if (r.isInfinite) Error.fail("Overflow", pos)
         Val.cachedNum(pos, r)
       case Expr.BinaryOp.OP_/ =>
         if (rd == 0) Error.fail("Division by zero.", pos)
         val r = ld / rd
-        if (r.isNaN) Error.fail("not a number", pos)
-        if (r.isInfinite) Error.fail("overflow", pos)
+        if (r.isNaN) Error.fail("Not a number", pos)
+        if (r.isInfinite) Error.fail("Overflow", pos)
         Val.cachedNum(pos, r)
       case Expr.BinaryOp.OP_% =>
         if (rd == 0) Error.fail("Division by zero.", pos)
         val r = ld % rd
-        if (r.isNaN) Error.fail("not a number", pos)
+        if (r.isNaN) Error.fail("Not a number", pos)
         Val.cachedNum(pos, r)
       // Use position-free static singletons for boolean results — this method is only called
       // from comprehension fast paths where position info on boolean results is unnecessary.
@@ -871,29 +871,29 @@ class Evaluator(
     (e.op: @switch) match {
       case Expr.BinaryOp.OP_* =>
         val r = visitExprAsDouble(e.lhs) * visitExprAsDouble(e.rhs)
-        if (r.isNaN) Error.fail("not a number", pos)
-        if (r.isInfinite) Error.fail("overflow", pos); r
+        if (r.isNaN) Error.fail("Not a number", pos)
+        if (r.isInfinite) Error.fail("Overflow", pos); r
       case Expr.BinaryOp.OP_/ =>
         val l = visitExprAsDouble(e.lhs)
         val r = visitExprAsDouble(e.rhs)
         if (r == 0) Error.fail("Division by zero.", pos)
         val result = l / r
-        if (result.isNaN) Error.fail("not a number", pos)
-        if (result.isInfinite) Error.fail("overflow", pos); result
+        if (result.isNaN) Error.fail("Not a number", pos)
+        if (result.isInfinite) Error.fail("Overflow", pos); result
       case Expr.BinaryOp.OP_% =>
         val l = visitExprAsDouble(e.lhs)
         val r = visitExprAsDouble(e.rhs)
         if (r == 0) Error.fail("Division by zero.", pos)
         val result = l % r
-        if (result.isNaN) Error.fail("not a number", pos); result
+        if (result.isNaN) Error.fail("Not a number", pos); result
       case Expr.BinaryOp.OP_+ =>
         val r = visitExprAsDouble(e.lhs) + visitExprAsDouble(e.rhs)
-        if (r.isNaN) Error.fail("not a number", pos)
-        if (r.isInfinite) Error.fail("overflow", pos); r
+        if (r.isNaN) Error.fail("Not a number", pos)
+        if (r.isInfinite) Error.fail("Overflow", pos); r
       case Expr.BinaryOp.OP_- =>
         val r = visitExprAsDouble(e.lhs) - visitExprAsDouble(e.rhs)
-        if (r.isNaN) Error.fail("not a number", pos)
-        if (r.isInfinite) Error.fail("overflow", pos); r
+        if (r.isNaN) Error.fail("Not a number", pos)
+        if (r.isInfinite) Error.fail("Overflow", pos); r
       case Expr.BinaryOp.OP_<< =>
         val ll = visitExprAsDouble(e.lhs).toSafeLong(pos)
         val rr = visitExprAsDouble(e.rhs).toSafeLong(pos)
@@ -1353,21 +1353,21 @@ class Evaluator(
       // Pure numeric fast path: avoid intermediate Val.Num allocation
       case Expr.BinaryOp.OP_* =>
         val r = visitExprAsDouble(e.lhs) * visitExprAsDouble(e.rhs)
-        if (r.isNaN) Error.fail("not a number", pos)
-        if (r.isInfinite) Error.fail("overflow", pos)
+        if (r.isNaN) Error.fail("Not a number", pos)
+        if (r.isInfinite) Error.fail("Overflow", pos)
         Val.cachedNum(pos, r)
       case Expr.BinaryOp.OP_- =>
         val r = visitExprAsDouble(e.lhs) - visitExprAsDouble(e.rhs)
-        if (r.isNaN) Error.fail("not a number", pos)
-        if (r.isInfinite) Error.fail("overflow", pos)
+        if (r.isNaN) Error.fail("Not a number", pos)
+        if (r.isInfinite) Error.fail("Overflow", pos)
         Val.cachedNum(pos, r)
       case Expr.BinaryOp.OP_/ =>
         val l = visitExprAsDouble(e.lhs)
         val r = visitExprAsDouble(e.rhs)
         if (r == 0) Error.fail("Division by zero.", pos)
         val result = l / r
-        if (result.isNaN) Error.fail("not a number", pos)
-        if (result.isInfinite) Error.fail("overflow", pos)
+        if (result.isNaN) Error.fail("Not a number", pos)
+        if (result.isInfinite) Error.fail("Overflow", pos)
         Val.cachedNum(pos, result)
       // Polymorphic ops: nested match avoids Tuple2 allocation; Num checked first (most common)
       case Expr.BinaryOp.OP_% =>
@@ -1379,7 +1379,7 @@ class Evaluator(
               case Val.Num(_, rd) =>
                 if (rd == 0) Error.fail("Division by zero.", pos)
                 val result = ld % rd
-                if (result.isNaN) Error.fail("not a number", pos)
+                if (result.isNaN) Error.fail("Not a number", pos)
                 Val.cachedNum(pos, result)
               case _ => failBinOp(l, e.op, r, pos)
             }
@@ -1393,8 +1393,8 @@ class Evaluator(
         (l, r) match {
           case (Val.Num(_, l), Val.Num(_, r)) =>
             val result = l + r
-            if (result.isNaN) Error.fail("not a number", pos)
-            if (result.isInfinite) Error.fail("overflow", pos)
+            if (result.isNaN) Error.fail("Not a number", pos)
+            if (result.isInfinite) Error.fail("Overflow", pos)
             Val.cachedNum(pos, result)
           case (l: Val.Str, r: Val.Str) => Val.Str.concat(pos, l, r)
           case (n: Val.Num, r: Val.Str) =>
diff --git a/sjsonnet/src/sjsonnet/Val.scala b/sjsonnet/src/sjsonnet/Val.scala
index 65fbc557..cd4e44c2 100644
--- a/sjsonnet/src/sjsonnet/Val.scala
+++ b/sjsonnet/src/sjsonnet/Val.scala
@@ -469,7 +469,7 @@ object Val {
   }
   final case class Num(var pos: Position, private val num: Double) extends Literal {
     if (num.isInfinite) {
-      Error.fail("overflow")
+      Error.fail("Overflow")
     }
 
     def prettyName = "number"
@@ -510,7 +510,7 @@ object Val {
 
     override def asDouble: Double = {
       if (num.isNaN) {
-        Error.fail("not a number")
+        Error.fail("Not a number")
       }
       num
     }
diff --git a/sjsonnet/src/sjsonnet/stdlib/ArrayModule.scala b/sjsonnet/src/sjsonnet/stdlib/ArrayModule.scala
index bc4939c8..933123af 100644
--- a/sjsonnet/src/sjsonnet/stdlib/ArrayModule.scala
+++ b/sjsonnet/src/sjsonnet/stdlib/ArrayModule.scala
@@ -1140,7 +1140,7 @@ object ArrayModule extends AbstractFunctionModule {
      */
     builtin("avg", "arr") { (_, _, arr: Val.Arr) =>
       if (arr.length == 0) {
-        Error.fail("Cannot calculate average of an empty array")
+        Error.fail("Cannot calculate average of an empty array.")
       }
       arr match {
         case r: Val.RangeArr =>
diff --git a/sjsonnet/src/sjsonnet/stdlib/MathModule.scala b/sjsonnet/src/sjsonnet/stdlib/MathModule.scala
index a021074d..815088e1 100644
--- a/sjsonnet/src/sjsonnet/stdlib/MathModule.scala
+++ b/sjsonnet/src/sjsonnet/stdlib/MathModule.scala
@@ -293,7 +293,7 @@ object MathModule extends AbstractFunctionModule {
      */
     builtin("pow", "x", "n") { (pos, ev, x: Double, n: Double) =>
       val r = math.pow(x, n)
-      if (java.lang.Double.isNaN(r)) Error.fail("not a number", pos)(ev)
+      if (java.lang.Double.isNaN(r)) Error.fail("Not a number", pos)(ev)
       r
     },
     /**
@@ -427,7 +427,7 @@ object MathModule extends AbstractFunctionModule {
      */
     builtin("asin", "x") { (pos, ev, x: Double) =>
       val r = math.asin(x)
-      if (java.lang.Double.isNaN(r)) Error.fail("not a number", pos)(ev)
+      if (java.lang.Double.isNaN(r)) Error.fail("Not a number", pos)(ev)
       r
     },
     /**
@@ -439,7 +439,7 @@ object MathModule extends AbstractFunctionModule {
      */
     builtin("acos", "x") { (pos, ev, x: Double) =>
       val r = math.acos(x)
-      if (java.lang.Double.isNaN(r)) Error.fail("not a number", pos)(ev)
+      if (java.lang.Double.isNaN(r)) Error.fail("Not a number", pos)(ev)
       r
     },
     /**
diff --git a/sjsonnet/test/resources/go_test_suite/builtin_exp3.jsonnet.golden b/sjsonnet/test/resources/go_test_suite/builtin_exp3.jsonnet.golden
index 5e22515e..c29be84f 100644
--- a/sjsonnet/test/resources/go_test_suite/builtin_exp3.jsonnet.golden
+++ b/sjsonnet/test/resources/go_test_suite/builtin_exp3.jsonnet.golden
@@ -1,3 +1,3 @@
-sjsonnet.Error: [std.exp] overflow
+sjsonnet.Error: [std.exp] Overflow
     at [<root>].(builtin_exp3.jsonnet:1:8)
 
diff --git a/sjsonnet/test/resources/go_test_suite/builtin_exp5.jsonnet.golden b/sjsonnet/test/resources/go_test_suite/builtin_exp5.jsonnet.golden
index d09fb891..41797de7 100644
--- a/sjsonnet/test/resources/go_test_suite/builtin_exp5.jsonnet.golden
+++ b/sjsonnet/test/resources/go_test_suite/builtin_exp5.jsonnet.golden
@@ -1,3 +1,3 @@
-sjsonnet.Error: [std.exp] overflow
+sjsonnet.Error: [std.exp] Overflow
     at [<root>].(builtin_exp5.jsonnet:1:8)
 
diff --git a/sjsonnet/test/resources/go_test_suite/builtin_log5.jsonnet.golden b/sjsonnet/test/resources/go_test_suite/builtin_log5.jsonnet.golden
index 2757daea..938e6f09 100644
--- a/sjsonnet/test/resources/go_test_suite/builtin_log5.jsonnet.golden
+++ b/sjsonnet/test/resources/go_test_suite/builtin_log5.jsonnet.golden
@@ -1,3 +1,3 @@
-sjsonnet.Error: [std.log] overflow
+sjsonnet.Error: [std.log] Overflow
     at [<root>].(builtin_log5.jsonnet:1:8)
 
diff --git a/sjsonnet/test/resources/go_test_suite/div4.jsonnet.golden b/sjsonnet/test/resources/go_test_suite/div4.jsonnet.golden
index 3c27737c..2d96625b 100644
--- a/sjsonnet/test/resources/go_test_suite/div4.jsonnet.golden
+++ b/sjsonnet/test/resources/go_test_suite/div4.jsonnet.golden
@@ -1,3 +1,3 @@
-sjsonnet.Error: overflow
+sjsonnet.Error: Overflow
     at [<root>].(div4.jsonnet:1:11)
 
diff --git a/sjsonnet/test/resources/go_test_suite/inf_min_number.jsonnet.golden b/sjsonnet/test/resources/go_test_suite/inf_min_number.jsonnet.golden
index 98b85281..72a38bc2 100644
--- a/sjsonnet/test/resources/go_test_suite/inf_min_number.jsonnet.golden
+++ b/sjsonnet/test/resources/go_test_suite/inf_min_number.jsonnet.golden
@@ -1,3 +1,3 @@
-sjsonnet.Error: overflow
+sjsonnet.Error: Overflow
     at [<root>].(inf_min_number.jsonnet:1:8)
 
diff --git a/sjsonnet/test/resources/go_test_suite/inf_mul_number.jsonnet.golden b/sjsonnet/test/resources/go_test_suite/inf_mul_number.jsonnet.golden
index 6d10791f..3293c15d 100644
--- a/sjsonnet/test/resources/go_test_suite/inf_mul_number.jsonnet.golden
+++ b/sjsonnet/test/resources/go_test_suite/inf_mul_number.jsonnet.golden
@@ -1,3 +1,3 @@
-sjsonnet.Error: overflow
+sjsonnet.Error: Overflow
     at [<root>].(inf_mul_number.jsonnet:1:7)
 
diff --git a/sjsonnet/test/resources/go_test_suite/inf_sum_number.jsonnet.golden b/sjsonnet/test/resources/go_test_suite/inf_sum_number.jsonnet.golden
index 4db39150..a3d08d16 100644
--- a/sjsonnet/test/resources/go_test_suite/inf_sum_number.jsonnet.golden
+++ b/sjsonnet/test/resources/go_test_suite/inf_sum_number.jsonnet.golden
@@ -1,3 +1,3 @@
-sjsonnet.Error: overflow
+sjsonnet.Error: Overflow
     at [<root>].(inf_sum_number.jsonnet:1:7)
 
diff --git a/sjsonnet/test/resources/go_test_suite/pow4.jsonnet.golden b/sjsonnet/test/resources/go_test_suite/pow4.jsonnet.golden
index b54116a4..83af3d66 100644
--- a/sjsonnet/test/resources/go_test_suite/pow4.jsonnet.golden
+++ b/sjsonnet/test/resources/go_test_suite/pow4.jsonnet.golden
@@ -1,2 +1,3 @@
-sjsonnet.Error: [std.pow] not a number
+sjsonnet.Error: [std.pow] Not a number
     at [<root>].(pow4.jsonnet:1:8)
+
diff --git a/sjsonnet/test/resources/go_test_suite/pow7.jsonnet.golden b/sjsonnet/test/resources/go_test_suite/pow7.jsonnet.golden
index 1773d54f..61a6e6d4 100644
--- a/sjsonnet/test/resources/go_test_suite/pow7.jsonnet.golden
+++ b/sjsonnet/test/resources/go_test_suite/pow7.jsonnet.golden
@@ -1,3 +1,3 @@
-sjsonnet.Error: [std.pow] overflow
+sjsonnet.Error: [std.pow] Overflow
     at [<root>].(pow7.jsonnet:2:8)
 
diff --git a/sjsonnet/test/resources/new_test_suite/error.arithmetic_overflow_addition.jsonnet.golden b/sjsonnet/test/resources/new_test_suite/error.arithmetic_overflow_addition.jsonnet.golden
index d6cfd626..998f143c 100644
--- a/sjsonnet/test/resources/new_test_suite/error.arithmetic_overflow_addition.jsonnet.golden
+++ b/sjsonnet/test/resources/new_test_suite/error.arithmetic_overflow_addition.jsonnet.golden
@@ -1,2 +1,2 @@
-sjsonnet.Error: overflow
+sjsonnet.Error: Overflow
     at [<root>].(error.arithmetic_overflow_addition.jsonnet:2:7)
diff --git a/sjsonnet/test/resources/new_test_suite/error.arithmetic_overflow_multiplication.jsonnet.golden b/sjsonnet/test/resources/new_test_suite/error.arithmetic_overflow_multiplication.jsonnet.golden
index 47400074..f1c4a392 100644
--- a/sjsonnet/test/resources/new_test_suite/error.arithmetic_overflow_multiplication.jsonnet.golden
+++ b/sjsonnet/test/resources/new_test_suite/error.arithmetic_overflow_multiplication.jsonnet.golden
@@ -1,2 +1,2 @@
-sjsonnet.Error: overflow
+sjsonnet.Error: Overflow
     at [<root>].(error.arithmetic_overflow_multiplication.jsonnet:2:7)
diff --git a/sjsonnet/test/resources/new_test_suite/error.arithmetic_overflow_subtraction.jsonnet.golden b/sjsonnet/test/resources/new_test_suite/error.arithmetic_overflow_subtraction.jsonnet.golden
index 43eeae58..52a9547a 100644
--- a/sjsonnet/test/resources/new_test_suite/error.arithmetic_overflow_subtraction.jsonnet.golden
+++ b/sjsonnet/test/resources/new_test_suite/error.arithmetic_overflow_subtraction.jsonnet.golden
@@ -1,2 +1,2 @@
-sjsonnet.Error: overflow
+sjsonnet.Error: Overflow
     at [<root>].(error.arithmetic_overflow_subtraction.jsonnet:2:8)
diff --git a/sjsonnet/test/resources/new_test_suite/error.math_acos_nan.jsonnet.golden b/sjsonnet/test/resources/new_test_suite/error.math_acos_nan.jsonnet.golden
index 08f406e0..691062a6 100644
--- a/sjsonnet/test/resources/new_test_suite/error.math_acos_nan.jsonnet.golden
+++ b/sjsonnet/test/resources/new_test_suite/error.math_acos_nan.jsonnet.golden
@@ -1 +1,3 @@
-sjsonnet.Error: [std.acos] not a number
+sjsonnet.Error: [std.acos] Not a number
+    at [<root>].(error.math_acos_nan.jsonnet:2:9)
+
diff --git a/sjsonnet/test/resources/new_test_suite/error.math_asin_nan.jsonnet.golden b/sjsonnet/test/resources/new_test_suite/error.math_asin_nan.jsonnet.golden
index 91f57ec7..b077d14e 100644
--- a/sjsonnet/test/resources/new_test_suite/error.math_asin_nan.jsonnet.golden
+++ b/sjsonnet/test/resources/new_test_suite/error.math_asin_nan.jsonnet.golden
@@ -1 +1,3 @@
-sjsonnet.Error: [std.asin] not a number
+sjsonnet.Error: [std.asin] Not a number
+    at [<root>].(error.math_asin_nan.jsonnet:2:9)
+
diff --git a/sjsonnet/test/resources/new_test_suite/error.math_pow_nan.jsonnet.golden b/sjsonnet/test/resources/new_test_suite/error.math_pow_nan.jsonnet.golden
index 6de87c33..46464ff5 100644
--- a/sjsonnet/test/resources/new_test_suite/error.math_pow_nan.jsonnet.golden
+++ b/sjsonnet/test/resources/new_test_suite/error.math_pow_nan.jsonnet.golden
@@ -1 +1,3 @@
-sjsonnet.Error: [std.pow] not a number
+sjsonnet.Error: [std.pow] Not a number
+    at [<root>].(error.math_pow_nan.jsonnet:2:8)
+
diff --git a/sjsonnet/test/resources/test_suite/error.overflow2.jsonnet.golden b/sjsonnet/test/resources/test_suite/error.overflow2.jsonnet.golden
index 98af58dc..e532b9cb 100644
--- a/sjsonnet/test/resources/test_suite/error.overflow2.jsonnet.golden
+++ b/sjsonnet/test/resources/test_suite/error.overflow2.jsonnet.golden
@@ -1,3 +1,3 @@
-sjsonnet.Error: overflow
+sjsonnet.Error: Overflow
     at [<root>].(error.overflow2.jsonnet:17:7)
 
diff --git a/sjsonnet/test/src/sjsonnet/StdMathTests.scala b/sjsonnet/test/src/sjsonnet/StdMathTests.scala
index 816a7d76..02caf315 100644
--- a/sjsonnet/test/src/sjsonnet/StdMathTests.scala
+++ b/sjsonnet/test/src/sjsonnet/StdMathTests.scala
@@ -57,9 +57,9 @@ object StdMathTests extends TestSuite {
       val errLog10 = evalErr("std.log10(-1)")
       assert(errLog10.contains("Not a number"))
       val errLog0 = evalErr("std.log(0)")
-      assert(errLog0.contains("overflow"))
+      assert(errLog0.contains("Overflow"))
       val errLog2Zero = evalErr("std.log2(0)")
-      assert(errLog2Zero.contains("overflow"))
+      assert(errLog2Zero.contains("Overflow"))
       // log(positive) must still work
       eval("std.log(1)") ==> ujson.Num(0.0)
       eval("std.log2(1)") ==> ujson.Num(0.0)
diff --git a/sjsonnet/test/src/sjsonnet/TailCallOptimizationTests.scala b/sjsonnet/test/src/sjsonnet/TailCallOptimizationTests.scala
index addb9497..3d39f2fe 100644
--- a/sjsonnet/test/src/sjsonnet/TailCallOptimizationTests.scala
+++ b/sjsonnet/test/src/sjsonnet/TailCallOptimizationTests.scala
@@ -28,7 +28,7 @@ object TailCallOptimizationTests extends TestSuite {
           |factorial(1000)
           |""".stripMargin
       )
-      assert(err.contains("overflow"))
+      assert(err.contains("Overflow"))
     }
 
     test("tailstrictDeepRecursionSum") {