diff --git a/example/wasm-vt/index.html b/example/wasm-vt/index.html
index 7ed1f7b9d..6ec4e2408 100644
--- a/example/wasm-vt/index.html
+++ b/example/wasm-vt/index.html
@@ -117,6 +117,7 @@
     <script>
         let wasmInstance = null;
         let wasmMemory = null;
+        let typeLayout = null;
 
         async function loadWasm() {
             try {
@@ -136,6 +137,14 @@
                 wasmInstance = wasmModule.instance;
                 wasmMemory = wasmInstance.exports.memory;
 
+                // Load the type layout JSON from the library
+                const lenPtr = wasmInstance.exports.ghostty_wasm_alloc_usize();
+                const jsonPtr = wasmInstance.exports.ghostty_type_json(lenPtr);
+                const jsonLen = new DataView(wasmMemory.buffer).getUint32(lenPtr, true);
+                wasmInstance.exports.ghostty_wasm_free_usize(lenPtr);
+                const jsonBytes = new Uint8Array(wasmMemory.buffer, jsonPtr, jsonLen);
+                typeLayout = JSON.parse(new TextDecoder().decode(jsonBytes));
+
                 return true;
             } catch (e) {
                 console.error('Failed to load WASM:', e);
@@ -146,6 +155,24 @@
             }
         }
 
+        // Look up a field's offset and DataView setter from the type layout JSON.
+        function fieldInfo(structName, fieldName) {
+            const field = typeLayout[structName].fields[fieldName];
+            return field;
+        }
+
+        // Set a field in a DataView using the type layout JSON metadata.
+        function setField(view, structName, fieldName, value) {
+            const field = fieldInfo(structName, fieldName);
+            switch (field.type) {
+                case 'u8':  case 'bool': view.setUint8(field.offset, value); break;
+                case 'u16': view.setUint16(field.offset, value, true); break;
+                case 'u32': case 'enum': view.setUint32(field.offset, value, true); break;
+                case 'u64': view.setBigUint64(field.offset, BigInt(value), true); break;
+                default: throw new Error(`Unsupported field type: ${field.type}`);
+            }
+        }
+
         function getBuffer() {
             return wasmMemory.buffer;
         }
@@ -173,14 +200,13 @@
                 const rows = parseInt(document.getElementById('rows').value, 10);
                 const vtText = parseEscapes(document.getElementById('vtInput').value);
 
-                // GhosttyTerminalOptions: { u16 cols, u16 rows, u32(wasm) max_scrollback }
-                // On wasm32, size_t is u32. Struct layout: cols(u16) + rows(u16) + max_scrollback(u32) = 8 bytes.
-                const TERM_OPTS_SIZE = 8;
+                const TERM_OPTS_SIZE = typeLayout['GhosttyTerminalOptions'].size;
                 const optsPtr = wasmInstance.exports.ghostty_wasm_alloc_u8_array(TERM_OPTS_SIZE);
+                new Uint8Array(getBuffer(), optsPtr, TERM_OPTS_SIZE).fill(0);
                 const optsView = new DataView(getBuffer(), optsPtr, TERM_OPTS_SIZE);
-                optsView.setUint16(0, cols, true);
-                optsView.setUint16(2, rows, true);
-                optsView.setUint32(4, 0, true); // max_scrollback = 0
+                setField(optsView, 'GhosttyTerminalOptions', 'cols', cols);
+                setField(optsView, 'GhosttyTerminalOptions', 'rows', rows);
+                setField(optsView, 'GhosttyTerminalOptions', 'max_scrollback', 0);
 
                 // Allocate pointer to receive the terminal handle
                 const termPtrPtr = wasmInstance.exports.ghostty_wasm_alloc_opaque();
@@ -204,26 +230,25 @@
                 wasmInstance.exports.ghostty_wasm_free_u8_array(dataPtr, vtBytes.length);
 
                 // Create a plain-text formatter
-                // GhosttyFormatterTerminalOptions layout on wasm32 (extern struct):
-                //   size:   u32 @ 0   (= sizeof struct = 36)
-                //   emit:   u32 @ 4   (GhosttyFormatterFormat enum)
-                //   unwrap: u8  @ 8
-                //   trim:   u8  @ 9
-                //   pad:    2 bytes
-                //   extra:  GhosttyFormatterTerminalExtra @ 12 (24 bytes, all zeros)
-                // Total: 36 bytes
-                const FMT_OPTS_SIZE = 36;
+                const FMT_OPTS_SIZE = typeLayout['GhosttyFormatterTerminalOptions'].size;
                 const fmtOptsPtr = wasmInstance.exports.ghostty_wasm_alloc_u8_array(FMT_OPTS_SIZE);
                 new Uint8Array(getBuffer(), fmtOptsPtr, FMT_OPTS_SIZE).fill(0);
                 const fmtOptsView = new DataView(getBuffer(), fmtOptsPtr, FMT_OPTS_SIZE);
-                fmtOptsView.setUint32(0, FMT_OPTS_SIZE, true); // size
-                fmtOptsView.setUint32(4, GHOSTTY_FORMATTER_FORMAT_PLAIN, true); // emit
-                fmtOptsView.setUint8(8, 0);  // unwrap = false
-                fmtOptsView.setUint8(9, 1);  // trim = true
-                // extra.size @ offset 12 (GhosttyFormatterTerminalExtra = 24 bytes)
-                fmtOptsView.setUint32(12, 24, true);
-                // extra.screen.size @ offset 24 (GhosttyFormatterScreenExtra = 12 bytes)
-                fmtOptsView.setUint32(24, 12, true);
+                setField(fmtOptsView, 'GhosttyFormatterTerminalOptions', 'size', FMT_OPTS_SIZE);
+                setField(fmtOptsView, 'GhosttyFormatterTerminalOptions', 'emit', GHOSTTY_FORMATTER_FORMAT_PLAIN);
+                setField(fmtOptsView, 'GhosttyFormatterTerminalOptions', 'unwrap', 0);
+                setField(fmtOptsView, 'GhosttyFormatterTerminalOptions', 'trim', 1);
+
+                // Set the nested sized-struct `size` fields for extra and extra.screen
+                const extraOffset = fieldInfo('GhosttyFormatterTerminalOptions', 'extra').offset;
+                const extraSize = typeLayout['GhosttyFormatterTerminalExtra'].size;
+                const extraSizeField = fieldInfo('GhosttyFormatterTerminalExtra', 'size');
+                fmtOptsView.setUint32(extraOffset + extraSizeField.offset, extraSize, true);
+
+                const screenOffset = fieldInfo('GhosttyFormatterTerminalExtra', 'screen').offset;
+                const screenSize = typeLayout['GhosttyFormatterScreenExtra'].size;
+                const screenSizeField = fieldInfo('GhosttyFormatterScreenExtra', 'size');
+                fmtOptsView.setUint32(extraOffset + screenOffset + screenSizeField.offset, screenSize, true);
 
                 const fmtPtrPtr = wasmInstance.exports.ghostty_wasm_alloc_opaque();
                 const fmtResult = wasmInstance.exports.ghostty_formatter_terminal_new(