diff --git a/json/internal_types.mbt b/json/internal_types.mbt
index 66fcfb8df..3d6e6ab81 100644
--- a/json/internal_types.mbt
+++ b/json/internal_types.mbt
@@ -15,13 +15,16 @@
 ///|
 priv struct ParseContext {
   mut offset : Int
-  input : String
+  input : StringView
   end_offset : Int
   mut remaining_available_depth : Int
 }
 
 ///|
-fn ParseContext::make(input : String, max_nesting_depth : Int) -> ParseContext {
+fn ParseContext::make(
+  input : StringView,
+  max_nesting_depth : Int,
+) -> ParseContext {
   {
     offset: 0,
     input,
diff --git a/json/lex_main.mbt b/json/lex_main.mbt
index 5d5e0361f..1b79d5065 100644
--- a/json/lex_main.mbt
+++ b/json/lex_main.mbt
@@ -63,12 +63,12 @@ fn ParseContext::lex_value(
         match ctx.read_char() {
           Some('0') => {
             let (n, repr) = ctx.lex_zero(start=ctx.offset - 2)
-            return Number(n, repr)
+            return Number(n, repr.map(_.to_string()))
           }
           Some(c2) => {
             if c2 is ('1'..='9') {
               let (n, repr) = ctx.lex_decimal_integer(start=ctx.offset - 2)
-              return Number(n, repr)
+              return Number(n, repr.map(_.to_string()))
             }
             ctx.invalid_char(shift=-1)
           }
@@ -76,11 +76,11 @@ fn ParseContext::lex_value(
         }
       Some('0') => {
         let (n, repr) = ctx.lex_zero(start=ctx.offset - 1)
-        return Number(n, repr)
+        return Number(n, repr.map(_.to_string()))
       }
       Some('1'..='9') => {
         let (n, repr) = ctx.lex_decimal_integer(start=ctx.offset - 1)
-        return Number(n, repr)
+        return Number(n, repr.map(_.to_string()))
       }
       Some('"') => {
         let s = ctx.lex_string()
diff --git a/json/lex_number.mbt b/json/lex_number.mbt
index 11f55af75..440f3de6b 100644
--- a/json/lex_number.mbt
+++ b/json/lex_number.mbt
@@ -16,7 +16,7 @@
 fn ParseContext::lex_decimal_integer(
   ctx : ParseContext,
   start~ : Int,
-) -> (Double, String?) raise ParseError {
+) -> (Double, StringView?) raise ParseError {
   for {
     match ctx.read_char() {
       Some('.') => return ctx.lex_decimal_point(start~)
@@ -37,7 +37,7 @@ fn ParseContext::lex_decimal_integer(
 fn ParseContext::lex_decimal_point(
   ctx : ParseContext,
   start~ : Int,
-) -> (Double, String?) raise ParseError {
+) -> (Double, StringView?) raise ParseError {
   match ctx.read_char() {
     Some(c) =>
       if c >= '0' && c <= '9' {
@@ -53,7 +53,7 @@ fn ParseContext::lex_decimal_point(
 fn ParseContext::lex_decimal_fraction(
   ctx : ParseContext,
   start~ : Int,
-) -> (Double, String?) raise ParseError {
+) -> (Double, StringView?) raise ParseError {
   for {
     match ctx.read_char() {
       Some('e' | 'E') => return ctx.lex_decimal_exponent(start~)
@@ -73,7 +73,7 @@ fn ParseContext::lex_decimal_fraction(
 fn ParseContext::lex_decimal_exponent(
   ctx : ParseContext,
   start~ : Int,
-) -> (Double, String?) raise ParseError {
+) -> (Double, StringView?) raise ParseError {
   match ctx.read_char() {
     Some('+') | Some('-') => return ctx.lex_decimal_exponent_sign(start~)
     Some(c) => {
@@ -91,7 +91,7 @@ fn ParseContext::lex_decimal_exponent(
 fn ParseContext::lex_decimal_exponent_sign(
   ctx : ParseContext,
   start~ : Int,
-) -> (Double, String?) raise ParseError {
+) -> (Double, StringView?) raise ParseError {
   match ctx.read_char() {
     Some(c) => {
       if c >= '0' && c <= '9' {
@@ -108,7 +108,7 @@ fn ParseContext::lex_decimal_exponent_sign(
 fn ParseContext::lex_decimal_exponent_integer(
   ctx : ParseContext,
   start~ : Int,
-) -> (Double, String?) {
+) -> (Double, StringView?) {
   for {
     match ctx.read_char() {
       Some(c) => {
@@ -127,7 +127,7 @@ fn ParseContext::lex_decimal_exponent_integer(
 fn ParseContext::lex_zero(
   ctx : ParseContext,
   start~ : Int,
-) -> (Double, String?) raise ParseError {
+) -> (Double, StringView?) raise ParseError {
   match ctx.read_char() {
     Some('.') => ctx.lex_decimal_point(start~)
     Some('e' | 'E') => ctx.lex_decimal_exponent(start~)
@@ -148,8 +148,8 @@ fn ParseContext::lex_number_end(
   ctx : ParseContext,
   start : Int,
   end : Int,
-) -> (Double, String?) {
-  let s = ctx.input.unsafe_substring(start~, end~)
+) -> (Double, StringView?) {
+  let s = ctx.input.view(start_offset=start, end_offset=end)
   if !s.contains(".") && !s.contains("e") && !s.contains("E") {
     // If the string does not contain a decimal point or exponent, it is likely an integer
     // We can try to parse it as an integer first
diff --git a/json/lex_string.mbt b/json/lex_string.mbt
index a8f6504f0..d27431333 100644
--- a/json/lex_string.mbt
+++ b/json/lex_string.mbt
@@ -18,7 +18,11 @@ fn ParseContext::lex_string(ctx : ParseContext) -> String raise ParseError {
   let mut start = ctx.offset
   fn flush(end : Int) {
     if start > 0 && end > start {
-      buf.write_substring(ctx.input, start, end - start)
+      buf.write_substring(
+        ctx.input.data(),
+        ctx.input.start_offset() + start,
+        end - start,
+      )
     }
   }
 
diff --git a/json/parse.mbt b/json/parse.mbt
index 0d5cb95a2..72504d2da 100644
--- a/json/parse.mbt
+++ b/json/parse.mbt
@@ -13,7 +13,7 @@
 // limitations under the License.
 
 ///|
-pub fn valid(input : String) -> Bool {
+pub fn valid(input : StringView) -> Bool {
   try {
     parse(input) |> ignore
     true
@@ -25,7 +25,7 @@ pub fn valid(input : String) -> Bool {
 ///|
 /// Parse a JSON input string into a Json value, with an optional maximum nesting depth (default is 1024)
 pub fn parse(
-  input : String,
+  input : StringView,
   max_nesting_depth? : Int = 1024,
 ) -> Json raise ParseError {
   let ctx = ParseContext::make(input, max_nesting_depth)
diff --git a/json/pkg.generated.mbti b/json/pkg.generated.mbti
index 9091f6d9e..fae4d62c4 100644
--- a/json/pkg.generated.mbti
+++ b/json/pkg.generated.mbti
@@ -7,9 +7,9 @@ fn[T : FromJson] from_json(Json, path? : JsonPath) -> T raise JsonDecodeError
 #callsite(autofill(args_loc, loc))
 fn inspect(&ToJson, content? : Json, loc~ : SourceLoc, args_loc~ : ArgsLoc) -> Unit raise InspectError
 
-fn parse(String, max_nesting_depth? : Int) -> Json raise ParseError
+fn parse(StringView, max_nesting_depth? : Int) -> Json raise ParseError
 
-fn valid(String) -> Bool
+fn valid(StringView) -> Bool
 
 // Errors
 pub(all) suberror JsonDecodeError (JsonPath, String)
diff --git a/json/utils.mbt b/json/utils.mbt
index 732bf3b9b..153d20059 100644
--- a/json/utils.mbt
+++ b/json/utils.mbt
@@ -13,7 +13,7 @@
 // limitations under the License.
 
 ///|
-fn offset_to_position(input : String, offset : Int) -> Position {
+fn offset_to_position(input : StringView, offset : Int) -> Position {
   let mut line = 1
   let mut column = 0
   for i in 0..<offset {