diff --git a/docs/js.md b/docs/js.md
index d43df97..a3ded33 100644
--- a/docs/js.md
+++ b/docs/js.md
@@ -4,7 +4,7 @@
 
 ### Callbacks handling
 
-All callbacks in Milo have the following signature (TypeScript syntax):
+Range callbacks in Milo have the following signature (TypeScript syntax):
 
 ```typescript
 (parser: number, offset: number, length: number) => void
@@ -18,6 +18,23 @@ where the parameters have the following meaning:
 
 If length is `0`, it means the callback has no payload associated.
 
+`on_headers` instead receives parsed message metadata:
+
+```typescript
+(
+  parser: number,
+  offset: number,
+  methodOrStatus: number,
+  shouldKeepAlive: boolean,
+  shouldUpgrade: boolean,
+  hasTrailers: boolean,
+  bodyKind: number,
+  contentLength: bigint
+) => void
+```
+
+`contentLength` is a BigInt so every accepted `Content-Length` value remains exact.
+
 Callbacks are dispatched only when enabled with `setActiveCallbacks`.
 
 Callbacks are disabled by default.
diff --git a/macros/src/wasm.rs b/macros/src/wasm.rs
index 8b8cc3e..76fd324 100644
--- a/macros/src/wasm.rs
+++ b/macros/src/wasm.rs
@@ -47,7 +47,7 @@ pub fn generate_callbacks(callbacks: &[String]) -> TokenStream {
         should_upgrade: bool,
         has_trailers: bool,
         body_kind: u8,
-        content_length: f64,
+        content_length: u64,
       );
 
       #[cfg(any(debug_assertions, feature = "debug"))]
@@ -87,7 +87,7 @@ pub fn generate_callbacks(callbacks: &[String]) -> TokenStream {
               let should_upgrade = unsafe { *self.events.add(cursor + 8) } != 0;
               let has_trailers = unsafe { *self.events.add(cursor + 9) } != 0;
               let body_kind = unsafe { *self.events.add(cursor + 10) };
-              let content_length = unsafe { core::ptr::read_unaligned(self.events.add(cursor + 11) as *const u64) }.to_le() as f64;
+              let content_length = unsafe { core::ptr::read_unaligned(self.events.add(cursor + 11) as *const u64) }.to_le();
 
               if self.active_callbacks & CALLBACK_ACTIVE_ON_HEADERS != 0 {
                 unsafe {
diff --git a/parser/wasm/src/template.js b/parser/wasm/src/template.js
index 8c3d066..55a117f 100644
--- a/parser/wasm/src/template.js
+++ b/parser/wasm/src/template.js
@@ -44,6 +44,21 @@ function hasDebug () {
   return this.milo_has_debug() !== 0
 }
 
+function wrapOnHeaders (callback) {
+  return (parser, at, methodOrStatus, keepAlive, shouldUpgrade, hasTrailers, bodyKind, contentLength) => {
+    callback(
+      parser,
+      at,
+      methodOrStatus,
+      keepAlive !== 0,
+      shouldUpgrade !== 0,
+      hasTrailers !== 0,
+      bodyKind,
+      BigInt.asUintN(64, contentLength)
+    )
+  }
+}
+
 /* REPLACE: enums */
 
 /* REPLACE: getters */
@@ -77,6 +92,10 @@ export function setup (env = {}) {
     logger = log.bind(context, logOption)
   }
 
+  if (instanceEnvironment.on_headers) {
+    instanceEnvironment.on_headers = wrapOnHeaders(instanceEnvironment.on_headers)
+  }
+
   // Create the WASM instance
   /* eslint-disable-next-line no-undef */
   const instance = new WebAssembly.Instance(wasmModule, {
diff --git a/parser/wasm/test/issues.test.js b/parser/wasm/test/issues.test.js
index 3ee83c1..e0119b3 100644
--- a/parser/wasm/test/issues.test.js
+++ b/parser/wasm/test/issues.test.js
@@ -330,6 +330,39 @@ it('issue-24 - memory_deallocation', async () => {
   }
 })
 
+for (const contentLength of [9007199254740993n, 9999999999999999999n]) {
+  it(`headers_content_length_metadata_preserves_${contentLength}`, () => {
+    let received
+    const milo = setup({
+      on_headers (parser, at, methodOrStatus, keepAlive, shouldUpgrade, hasTrailers, bodyKind, value) {
+        received = { keepAlive, shouldUpgrade, hasTrailers, bodyKind, contentLength: value }
+      }
+    })
+    const parser = milo.create()
+    const message = Buffer.from(`HTTP/1.1 200 OK\r\nContent-Length: ${contentLength}\r\n\r\n`)
+    const ptr = milo.alloc(message.length)
+
+    try {
+      milo.setShouldAutodetect(parser, false)
+      milo.setShouldSuspendAfterHeaders(parser, true)
+      milo.setActiveCallbacks(parser, milo.CALLBACK_ACTIVE_ON_HEADERS)
+      new Uint8Array(milo.memory.buffer, ptr, message.length).set(message)
+
+      assert.equal(milo.parse(parser, ptr, message.length), message.length)
+      assert.deepEqual(received, {
+        keepAlive: true,
+        shouldUpgrade: false,
+        hasTrailers: false,
+        bodyKind: 0,
+        contentLength
+      })
+    } finally {
+      milo.destroy(parser)
+      milo.dealloc(ptr, message.length)
+    }
+  })
+}
+
 it('issue-25 - headers_upgrade_metadata', () => {
   const cases = [
     ...[100, 101, 103, 200, 204, 301, 304, 400, 426, 500].map(status => ({