diff --git a/changes/jsoo-6-5-0.internal.md b/changes/jsoo-6-5-0.internal.md
new file mode 100644
index 00000000000..341f4a5e319
--- /dev/null
+++ b/changes/jsoo-6-5-0.internal.md
@@ -0,0 +1 @@
+- Upgrade js_of_ocaml and wasm_of_ocaml to 6.5.0 for a smaller, faster-building playground compiler.
diff --git a/dune-project b/dune-project
index dafd9675b9a..2611ce4bdd8 100644
--- a/dune-project
+++ b/dune-project
@@ -47,12 +47,12 @@
    (and
     (<> :os "win32")
     :with-test
-    (= 6.4.1)))
+    (= 6.5.0)))
   (wasm_of_ocaml-compiler
    (and
     (<> :os "win32")
     :with-test
-    (= 6.4.1)))))
+    (= 6.5.0)))))
 
 (package
  (name analysis)
diff --git a/packages/playground/scripts/generate_cmijs.mjs b/packages/playground/scripts/generate_cmijs.mjs
index 84cb6349fca..614f3d99b8d 100644
--- a/packages/playground/scripts/generate_cmijs.mjs
+++ b/packages/playground/scripts/generate_cmijs.mjs
@@ -54,8 +54,54 @@ function buildCmij(pkgDir, pkgName) {
   fs.mkdirSync(outputFolder, { recursive: true });
 
   const cmijFile = path.join(outputFolder, "cmij.js");
-  const inputFiles = fs.readdirSync(libOcamlFolder).filter(isCmij).join(" ");
-  exec(`js_of_ocaml build-fs -o ${cmijFile} -I ${libOcamlFolder} ${inputFiles}`);
+  const files = fs
+    .readdirSync(libOcamlFolder)
+    .filter(isCmij)
+    .map(basename => {
+      const bytes = fs.readFileSync(path.join(libOcamlFolder, basename));
+      return `  a("/static/${basename}", "${jsBytesLiteral(bytes)}");\n`;
+    });
+
+  // The same pseudo filesystem file as `js_of_ocaml build-fs` writes. The
+  // compiler bundle creates the files when it is loaded after it
+  // (jsoo_create_file), or picks them up when loaded before it (jsoo_fs_tmp).
+  // We write it ourselves because since js_of_ocaml 6.5.0, build-fs encodes
+  // the contents in base64, which compresses much worse, and decodes them
+  // with a runtime primitive that a standalone file cannot reach.
+  fs.writeFileSync(
+    cmijFile,
+    `(function (c) {
+  "use strict";
+  function a(name, content) {
+    if (c.jsoo_create_file) c.jsoo_create_file(name, content);
+    else {
+      if (!c.jsoo_fs_tmp) c.jsoo_fs_tmp = [];
+      c.jsoo_fs_tmp.push({ name, content });
+    }
+  }
+${files.join("")}})(globalThis);
+`,
+  );
+}
+
+/**
+ * The contents of a JavaScript string literal with one character per byte,
+ * which is how js_of_ocaml represents bytes.
+ *
+ * @param {Buffer} bytes
+ * @return {string}
+ */
+function jsBytesLiteral(bytes) {
+  let s = "";
+  for (let i = 0; i < bytes.length; i++) {
+    const b = bytes[i];
+    if (b === 0x22 || b === 0x5c) s += `\\${String.fromCharCode(b)}`;
+    else if (b >= 0x20 && b < 0x7f) s += String.fromCharCode(b);
+    else if (b === 0 && !(bytes[i + 1] >= 0x30 && bytes[i + 1] <= 0x39))
+      s += "\\0";
+    else s += `\\x${b.toString(16).padStart(2, "0")}`;
+  }
+  return s;
 }
 
 /**
diff --git a/rescript.opam b/rescript.opam
index 808a8041511..cea2b62ff55 100644
--- a/rescript.opam
+++ b/rescript.opam
@@ -22,8 +22,8 @@ depends: [
   "ounit2" {with-test & = "2.2.7"}
   "odoc" {with-doc}
   "ocaml-lsp-server" {with-dev-setup & >= "1.23.0"}
-  "js_of_ocaml" {os != "win32" & with-test & = "6.4.1"}
-  "wasm_of_ocaml-compiler" {os != "win32" & with-test & = "6.4.1"}
+  "js_of_ocaml" {os != "win32" & with-test & = "6.5.0"}
+  "wasm_of_ocaml-compiler" {os != "win32" & with-test & = "6.5.0"}
 ]
 build: [
   ["dune" "subst"] {dev}