diff --git a/apps/oxlint/src-js/generated/deserialize.js b/apps/oxlint/src-js/generated/deserialize.js
index 85d4d03f1b01a..2cdbdb74e417b 100644
--- a/apps/oxlint/src-js/generated/deserialize.js
+++ b/apps/oxlint/src-js/generated/deserialize.js
@@ -10,6 +10,7 @@ let uint8,
   sourceText,
   sourceIsAscii,
   sourceStartPos,
+  firstNonAsciiPos,
   parent = null,
   getLoc;
 
@@ -42,6 +43,14 @@ function deserializeWith(buffer, sourceTextInput, sourceByteLen, getLocInput, de
   float64 = buffer.float64;
   sourceText = sourceTextInput;
   sourceIsAscii = sourceText.length === sourceByteLen;
+  if (!sourceIsAscii) {
+    firstNonAsciiPos = sourceByteLen;
+    for (let i = sourceStartPos, e = sourceStartPos + sourceByteLen; i < e; i++)
+      if (uint8[i] >= 128) {
+        firstNonAsciiPos = i - sourceStartPos;
+        break;
+      }
+  }
   getLoc = getLocInput;
   return deserialize(uint32[536870900]);
 }
@@ -5857,11 +5866,12 @@ function deserializeStr(pos) {
     len = uint32[pos32 + 2];
   if (len === 0) return "";
   pos = uint32[pos32];
-  if (sourceIsAscii && pos >= sourceStartPos) return sourceText.substr(pos - sourceStartPos, len);
-  // Longer strings use `TextDecoder`
-  // TODO: Find best switch-over point
+  if (pos >= sourceStartPos && (sourceIsAscii || pos - sourceStartPos + len <= firstNonAsciiPos))
+    return sourceText.substr(pos - sourceStartPos, len);
+  // Use `TextDecoder` for strings longer than 9 bytes.
+  // For shorter strings, the byte-by-byte loop below avoids native call overhead.
   let end = pos + len;
-  if (len > 50) return decodeStr(uint8.subarray(pos, end));
+  if (len > 9) return decodeStr(uint8.subarray(pos, end));
   // Shorter strings decode by hand to avoid native call
   let out = "",
     c;
diff --git a/napi/parser/src-js/generated/deserialize/js.js b/napi/parser/src-js/generated/deserialize/js.js
index 04dad28fa2608..4a68737c8baef 100644
--- a/napi/parser/src-js/generated/deserialize/js.js
+++ b/napi/parser/src-js/generated/deserialize/js.js
@@ -1,7 +1,7 @@
 // Auto-generated code, DO NOT EDIT DIRECTLY!
 // To edit this generated file you have to edit `tasks/ast_tools/src/generators/raw_transfer.rs`.
 
-let uint8, uint32, float64, sourceText, sourceIsAscii, sourceEndPos;
+let uint8, uint32, float64, sourceText, sourceIsAscii, sourceEndPos, firstNonAsciiPos;
 
 const textDecoder = new TextDecoder("utf-8", { ignoreBOM: true }),
   decodeStr = textDecoder.decode.bind(textDecoder),
@@ -20,6 +20,14 @@ function deserializeWith(buffer, sourceTextInput, sourceByteLen, getLocInput, de
   float64 = buffer.float64;
   sourceText = sourceTextInput;
   sourceIsAscii = sourceText.length === sourceByteLen;
+  if (!sourceIsAscii) {
+    firstNonAsciiPos = sourceByteLen;
+    for (let i = 0; i < sourceByteLen; i++)
+      if (uint8[i] >= 128) {
+        firstNonAsciiPos = i;
+        break;
+      }
+  }
   return deserialize(uint32[536870900]);
 }
 
@@ -4513,11 +4521,12 @@ function deserializeStr(pos) {
     len = uint32[pos32 + 2];
   if (len === 0) return "";
   pos = uint32[pos32];
-  if (sourceIsAscii && pos < sourceEndPos) return sourceText.substr(pos, len);
-  // Longer strings use `TextDecoder`
-  // TODO: Find best switch-over point
+  if (pos < sourceEndPos && (sourceIsAscii || pos + len <= firstNonAsciiPos))
+    return sourceText.substr(pos, len);
+  // Use `TextDecoder` for strings longer than 9 bytes.
+  // For shorter strings, the byte-by-byte loop below avoids native call overhead.
   let end = pos + len;
-  if (len > 50) return decodeStr(uint8.subarray(pos, end));
+  if (len > 9) return decodeStr(uint8.subarray(pos, end));
   // Shorter strings decode by hand to avoid native call
   let out = "",
     c;
diff --git a/napi/parser/src-js/generated/deserialize/js_parent.js b/napi/parser/src-js/generated/deserialize/js_parent.js
index ddd97fed54b06..6ef044bdf1d5f 100644
--- a/napi/parser/src-js/generated/deserialize/js_parent.js
+++ b/napi/parser/src-js/generated/deserialize/js_parent.js
@@ -7,6 +7,7 @@ let uint8,
   sourceText,
   sourceIsAscii,
   sourceEndPos,
+  firstNonAsciiPos,
   parent = null;
 
 const textDecoder = new TextDecoder("utf-8", { ignoreBOM: true }),
@@ -26,6 +27,14 @@ function deserializeWith(buffer, sourceTextInput, sourceByteLen, getLocInput, de
   float64 = buffer.float64;
   sourceText = sourceTextInput;
   sourceIsAscii = sourceText.length === sourceByteLen;
+  if (!sourceIsAscii) {
+    firstNonAsciiPos = sourceByteLen;
+    for (let i = 0; i < sourceByteLen; i++)
+      if (uint8[i] >= 128) {
+        firstNonAsciiPos = i;
+        break;
+      }
+  }
   return deserialize(uint32[536870900]);
 }
 
@@ -5049,11 +5058,12 @@ function deserializeStr(pos) {
     len = uint32[pos32 + 2];
   if (len === 0) return "";
   pos = uint32[pos32];
-  if (sourceIsAscii && pos < sourceEndPos) return sourceText.substr(pos, len);
-  // Longer strings use `TextDecoder`
-  // TODO: Find best switch-over point
+  if (pos < sourceEndPos && (sourceIsAscii || pos + len <= firstNonAsciiPos))
+    return sourceText.substr(pos, len);
+  // Use `TextDecoder` for strings longer than 9 bytes.
+  // For shorter strings, the byte-by-byte loop below avoids native call overhead.
   let end = pos + len;
-  if (len > 50) return decodeStr(uint8.subarray(pos, end));
+  if (len > 9) return decodeStr(uint8.subarray(pos, end));
   // Shorter strings decode by hand to avoid native call
   let out = "",
     c;
diff --git a/napi/parser/src-js/generated/deserialize/js_range.js b/napi/parser/src-js/generated/deserialize/js_range.js
index 39afc4798d7fe..775d908455ae5 100644
--- a/napi/parser/src-js/generated/deserialize/js_range.js
+++ b/napi/parser/src-js/generated/deserialize/js_range.js
@@ -1,7 +1,7 @@
 // Auto-generated code, DO NOT EDIT DIRECTLY!
 // To edit this generated file you have to edit `tasks/ast_tools/src/generators/raw_transfer.rs`.
 
-let uint8, uint32, float64, sourceText, sourceIsAscii, sourceEndPos;
+let uint8, uint32, float64, sourceText, sourceIsAscii, sourceEndPos, firstNonAsciiPos;
 
 const textDecoder = new TextDecoder("utf-8", { ignoreBOM: true }),
   decodeStr = textDecoder.decode.bind(textDecoder),
@@ -20,6 +20,14 @@ function deserializeWith(buffer, sourceTextInput, sourceByteLen, getLocInput, de
   float64 = buffer.float64;
   sourceText = sourceTextInput;
   sourceIsAscii = sourceText.length === sourceByteLen;
+  if (!sourceIsAscii) {
+    firstNonAsciiPos = sourceByteLen;
+    for (let i = 0; i < sourceByteLen; i++)
+      if (uint8[i] >= 128) {
+        firstNonAsciiPos = i;
+        break;
+      }
+  }
   return deserialize(uint32[536870900]);
 }
 
@@ -5063,11 +5071,12 @@ function deserializeStr(pos) {
     len = uint32[pos32 + 2];
   if (len === 0) return "";
   pos = uint32[pos32];
-  if (sourceIsAscii && pos < sourceEndPos) return sourceText.substr(pos, len);
-  // Longer strings use `TextDecoder`
-  // TODO: Find best switch-over point
+  if (pos < sourceEndPos && (sourceIsAscii || pos + len <= firstNonAsciiPos))
+    return sourceText.substr(pos, len);
+  // Use `TextDecoder` for strings longer than 9 bytes.
+  // For shorter strings, the byte-by-byte loop below avoids native call overhead.
   let end = pos + len;
-  if (len > 50) return decodeStr(uint8.subarray(pos, end));
+  if (len > 9) return decodeStr(uint8.subarray(pos, end));
   // Shorter strings decode by hand to avoid native call
   let out = "",
     c;
diff --git a/napi/parser/src-js/generated/deserialize/js_range_parent.js b/napi/parser/src-js/generated/deserialize/js_range_parent.js
index dad53373d4ecd..40bae9adeb148 100644
--- a/napi/parser/src-js/generated/deserialize/js_range_parent.js
+++ b/napi/parser/src-js/generated/deserialize/js_range_parent.js
@@ -7,6 +7,7 @@ let uint8,
   sourceText,
   sourceIsAscii,
   sourceEndPos,
+  firstNonAsciiPos,
   parent = null;
 
 const textDecoder = new TextDecoder("utf-8", { ignoreBOM: true }),
@@ -26,6 +27,14 @@ function deserializeWith(buffer, sourceTextInput, sourceByteLen, getLocInput, de
   float64 = buffer.float64;
   sourceText = sourceTextInput;
   sourceIsAscii = sourceText.length === sourceByteLen;
+  if (!sourceIsAscii) {
+    firstNonAsciiPos = sourceByteLen;
+    for (let i = 0; i < sourceByteLen; i++)
+      if (uint8[i] >= 128) {
+        firstNonAsciiPos = i;
+        break;
+      }
+  }
   return deserialize(uint32[536870900]);
 }
 
@@ -5602,11 +5611,12 @@ function deserializeStr(pos) {
     len = uint32[pos32 + 2];
   if (len === 0) return "";
   pos = uint32[pos32];
-  if (sourceIsAscii && pos < sourceEndPos) return sourceText.substr(pos, len);
-  // Longer strings use `TextDecoder`
-  // TODO: Find best switch-over point
+  if (pos < sourceEndPos && (sourceIsAscii || pos + len <= firstNonAsciiPos))
+    return sourceText.substr(pos, len);
+  // Use `TextDecoder` for strings longer than 9 bytes.
+  // For shorter strings, the byte-by-byte loop below avoids native call overhead.
   let end = pos + len;
-  if (len > 50) return decodeStr(uint8.subarray(pos, end));
+  if (len > 9) return decodeStr(uint8.subarray(pos, end));
   // Shorter strings decode by hand to avoid native call
   let out = "",
     c;
diff --git a/napi/parser/src-js/generated/deserialize/ts.js b/napi/parser/src-js/generated/deserialize/ts.js
index 38f73fc57291b..f763b19b58931 100644
--- a/napi/parser/src-js/generated/deserialize/ts.js
+++ b/napi/parser/src-js/generated/deserialize/ts.js
@@ -1,7 +1,7 @@
 // Auto-generated code, DO NOT EDIT DIRECTLY!
 // To edit this generated file you have to edit `tasks/ast_tools/src/generators/raw_transfer.rs`.
 
-let uint8, uint32, float64, sourceText, sourceIsAscii, sourceEndPos;
+let uint8, uint32, float64, sourceText, sourceIsAscii, sourceEndPos, firstNonAsciiPos;
 
 const textDecoder = new TextDecoder("utf-8", { ignoreBOM: true }),
   decodeStr = textDecoder.decode.bind(textDecoder),
@@ -20,6 +20,14 @@ function deserializeWith(buffer, sourceTextInput, sourceByteLen, getLocInput, de
   float64 = buffer.float64;
   sourceText = sourceTextInput;
   sourceIsAscii = sourceText.length === sourceByteLen;
+  if (!sourceIsAscii) {
+    firstNonAsciiPos = sourceByteLen;
+    for (let i = 0; i < sourceByteLen; i++)
+      if (uint8[i] >= 128) {
+        firstNonAsciiPos = i;
+        break;
+      }
+  }
   return deserialize(uint32[536870900]);
 }
 
@@ -4822,11 +4830,12 @@ function deserializeStr(pos) {
     len = uint32[pos32 + 2];
   if (len === 0) return "";
   pos = uint32[pos32];
-  if (sourceIsAscii && pos < sourceEndPos) return sourceText.substr(pos, len);
-  // Longer strings use `TextDecoder`
-  // TODO: Find best switch-over point
+  if (pos < sourceEndPos && (sourceIsAscii || pos + len <= firstNonAsciiPos))
+    return sourceText.substr(pos, len);
+  // Use `TextDecoder` for strings longer than 9 bytes.
+  // For shorter strings, the byte-by-byte loop below avoids native call overhead.
   let end = pos + len;
-  if (len > 50) return decodeStr(uint8.subarray(pos, end));
+  if (len > 9) return decodeStr(uint8.subarray(pos, end));
   // Shorter strings decode by hand to avoid native call
   let out = "",
     c;
diff --git a/napi/parser/src-js/generated/deserialize/ts_parent.js b/napi/parser/src-js/generated/deserialize/ts_parent.js
index 14af741462fd8..9f1c16475f531 100644
--- a/napi/parser/src-js/generated/deserialize/ts_parent.js
+++ b/napi/parser/src-js/generated/deserialize/ts_parent.js
@@ -7,6 +7,7 @@ let uint8,
   sourceText,
   sourceIsAscii,
   sourceEndPos,
+  firstNonAsciiPos,
   parent = null;
 
 const textDecoder = new TextDecoder("utf-8", { ignoreBOM: true }),
@@ -26,6 +27,14 @@ function deserializeWith(buffer, sourceTextInput, sourceByteLen, getLocInput, de
   float64 = buffer.float64;
   sourceText = sourceTextInput;
   sourceIsAscii = sourceText.length === sourceByteLen;
+  if (!sourceIsAscii) {
+    firstNonAsciiPos = sourceByteLen;
+    for (let i = 0; i < sourceByteLen; i++)
+      if (uint8[i] >= 128) {
+        firstNonAsciiPos = i;
+        break;
+      }
+  }
   return deserialize(uint32[536870900]);
 }
 
@@ -5385,11 +5394,12 @@ function deserializeStr(pos) {
     len = uint32[pos32 + 2];
   if (len === 0) return "";
   pos = uint32[pos32];
-  if (sourceIsAscii && pos < sourceEndPos) return sourceText.substr(pos, len);
-  // Longer strings use `TextDecoder`
-  // TODO: Find best switch-over point
+  if (pos < sourceEndPos && (sourceIsAscii || pos + len <= firstNonAsciiPos))
+    return sourceText.substr(pos, len);
+  // Use `TextDecoder` for strings longer than 9 bytes.
+  // For shorter strings, the byte-by-byte loop below avoids native call overhead.
   let end = pos + len;
-  if (len > 50) return decodeStr(uint8.subarray(pos, end));
+  if (len > 9) return decodeStr(uint8.subarray(pos, end));
   // Shorter strings decode by hand to avoid native call
   let out = "",
     c;
diff --git a/napi/parser/src-js/generated/deserialize/ts_range.js b/napi/parser/src-js/generated/deserialize/ts_range.js
index 52b146779a9b5..da5c68d3dc99d 100644
--- a/napi/parser/src-js/generated/deserialize/ts_range.js
+++ b/napi/parser/src-js/generated/deserialize/ts_range.js
@@ -1,7 +1,7 @@
 // Auto-generated code, DO NOT EDIT DIRECTLY!
 // To edit this generated file you have to edit `tasks/ast_tools/src/generators/raw_transfer.rs`.
 
-let uint8, uint32, float64, sourceText, sourceIsAscii, sourceEndPos;
+let uint8, uint32, float64, sourceText, sourceIsAscii, sourceEndPos, firstNonAsciiPos;
 
 const textDecoder = new TextDecoder("utf-8", { ignoreBOM: true }),
   decodeStr = textDecoder.decode.bind(textDecoder),
@@ -20,6 +20,14 @@ function deserializeWith(buffer, sourceTextInput, sourceByteLen, getLocInput, de
   float64 = buffer.float64;
   sourceText = sourceTextInput;
   sourceIsAscii = sourceText.length === sourceByteLen;
+  if (!sourceIsAscii) {
+    firstNonAsciiPos = sourceByteLen;
+    for (let i = 0; i < sourceByteLen; i++)
+      if (uint8[i] >= 128) {
+        firstNonAsciiPos = i;
+        break;
+      }
+  }
   return deserialize(uint32[536870900]);
 }
 
@@ -5403,11 +5411,12 @@ function deserializeStr(pos) {
     len = uint32[pos32 + 2];
   if (len === 0) return "";
   pos = uint32[pos32];
-  if (sourceIsAscii && pos < sourceEndPos) return sourceText.substr(pos, len);
-  // Longer strings use `TextDecoder`
-  // TODO: Find best switch-over point
+  if (pos < sourceEndPos && (sourceIsAscii || pos + len <= firstNonAsciiPos))
+    return sourceText.substr(pos, len);
+  // Use `TextDecoder` for strings longer than 9 bytes.
+  // For shorter strings, the byte-by-byte loop below avoids native call overhead.
   let end = pos + len;
-  if (len > 50) return decodeStr(uint8.subarray(pos, end));
+  if (len > 9) return decodeStr(uint8.subarray(pos, end));
   // Shorter strings decode by hand to avoid native call
   let out = "",
     c;
diff --git a/napi/parser/src-js/generated/deserialize/ts_range_parent.js b/napi/parser/src-js/generated/deserialize/ts_range_parent.js
index 9849a612e8e0a..944adad33b26e 100644
--- a/napi/parser/src-js/generated/deserialize/ts_range_parent.js
+++ b/napi/parser/src-js/generated/deserialize/ts_range_parent.js
@@ -7,6 +7,7 @@ let uint8,
   sourceText,
   sourceIsAscii,
   sourceEndPos,
+  firstNonAsciiPos,
   parent = null;
 
 const textDecoder = new TextDecoder("utf-8", { ignoreBOM: true }),
@@ -26,6 +27,14 @@ function deserializeWith(buffer, sourceTextInput, sourceByteLen, getLocInput, de
   float64 = buffer.float64;
   sourceText = sourceTextInput;
   sourceIsAscii = sourceText.length === sourceByteLen;
+  if (!sourceIsAscii) {
+    firstNonAsciiPos = sourceByteLen;
+    for (let i = 0; i < sourceByteLen; i++)
+      if (uint8[i] >= 128) {
+        firstNonAsciiPos = i;
+        break;
+      }
+  }
   return deserialize(uint32[536870900]);
 }
 
@@ -5966,11 +5975,12 @@ function deserializeStr(pos) {
     len = uint32[pos32 + 2];
   if (len === 0) return "";
   pos = uint32[pos32];
-  if (sourceIsAscii && pos < sourceEndPos) return sourceText.substr(pos, len);
-  // Longer strings use `TextDecoder`
-  // TODO: Find best switch-over point
+  if (pos < sourceEndPos && (sourceIsAscii || pos + len <= firstNonAsciiPos))
+    return sourceText.substr(pos, len);
+  // Use `TextDecoder` for strings longer than 9 bytes.
+  // For shorter strings, the byte-by-byte loop below avoids native call overhead.
   let end = pos + len;
-  if (len > 50) return decodeStr(uint8.subarray(pos, end));
+  if (len > 9) return decodeStr(uint8.subarray(pos, end));
   // Shorter strings decode by hand to avoid native call
   let out = "",
     c;
diff --git a/tasks/ast_tools/src/generators/raw_transfer.rs b/tasks/ast_tools/src/generators/raw_transfer.rs
index 7feffc9bb8e65..b024b6239aa0e 100644
--- a/tasks/ast_tools/src/generators/raw_transfer.rs
+++ b/tasks/ast_tools/src/generators/raw_transfer.rs
@@ -143,7 +143,7 @@ fn generate_deserializers(
         import {{ comments, initComments }} from '../plugins/comments.js';
         /* END_IF */
 
-        let uint8, uint32, float64, sourceText, sourceIsAscii, sourceStartPos, sourceEndPos;
+        let uint8, uint32, float64, sourceText, sourceIsAscii, sourceStartPos, sourceEndPos, firstNonAsciiPos;
 
         let parent = null;
         let getLoc;
@@ -188,6 +188,23 @@ fn generate_deserializers(
             sourceText = sourceTextInput;
             sourceIsAscii = sourceText.length === sourceByteLen;
 
+            if (!sourceIsAscii) {{
+                // Find first non-ASCII byte in source region.
+                // `sourceText.substr()` can be used for strings ending before this position,
+                // since byte offsets equal char offsets in the all-ASCII prefix.
+                if (LINTER) {{
+                    firstNonAsciiPos = sourceByteLen;
+                    for (let i = sourceStartPos, e = sourceStartPos + sourceByteLen; i < e; i++) {{
+                        if (uint8[i] >= 128) {{ firstNonAsciiPos = i - sourceStartPos; break; }}
+                    }}
+                }} else {{
+                    firstNonAsciiPos = sourceByteLen;
+                    for (let i = 0; i < sourceByteLen; i++) {{
+                        if (uint8[i] >= 128) {{ firstNonAsciiPos = i; break; }}
+                    }}
+                }}
+            }}
+
             if (LOC) getLoc = getLocInput;
 
             return deserialize(uint32[{data_pointer_pos_32}]);
@@ -874,15 +891,17 @@ static STR_DESERIALIZER_BODY: &str = "
     pos = uint32[pos32];
 
     if (LINTER) {
-        if (sourceIsAscii && pos >= sourceStartPos) return sourceText.substr(pos - sourceStartPos, len);
+        if (pos >= sourceStartPos && (sourceIsAscii || pos - sourceStartPos + len <= firstNonAsciiPos))
+            return sourceText.substr(pos - sourceStartPos, len);
     } else {
-        if (sourceIsAscii && pos < sourceEndPos) return sourceText.substr(pos, len);
+        if (pos < sourceEndPos && (sourceIsAscii || pos + len <= firstNonAsciiPos))
+            return sourceText.substr(pos, len);
     }
 
-    // Longer strings use `TextDecoder`
-    // TODO: Find best switch-over point
+    // Use `TextDecoder` for strings longer than 9 bytes.
+    // For shorter strings, the byte-by-byte loop below avoids native call overhead.
     const end = pos + len;
-    if (len > 50) return decodeStr(uint8.subarray(pos, end));
+    if (len > 9) return decodeStr(uint8.subarray(pos, end));
 
     // Shorter strings decode by hand to avoid native call
     let out = '',