diff --git a/crates/perry/src/commands/compile/cjs_wrap/hoist_classes.rs b/crates/perry/src/commands/compile/cjs_wrap/hoist_classes.rs
index e2deb48aa..8e45ee601 100644
--- a/crates/perry/src/commands/compile/cjs_wrap/hoist_classes.rs
+++ b/crates/perry/src/commands/compile/cjs_wrap/hoist_classes.rs
@@ -233,7 +233,21 @@ pub fn extract_top_level_class_decls(source: &str) -> (String, Vec<String>, Stri
     // `Undefined variable in update expression`. The conservative rule
     // is to leave the class inside the IIFE when *any* of its referenced
     // identifiers would lose their binding to the IIFE-local state.
-    let iife_locals = collect_top_level_let_const_var_names(source);
+    let mut iife_locals = collect_top_level_let_const_var_names(source);
+    // Issue #5251 — the cjs_wrap preamble injects `var exports`, `var module`,
+    // and a `require` function as IIFE-local bindings (see `wrap.rs`'s
+    // `cjs_preamble`). They are NOT declared in the original source, so the
+    // textual top-level scan above never sees them. A class whose body reads
+    // `exports.X` / `module.exports` / `require(...)` must therefore ALSO stay
+    // inside the IIFE — hoisting it out severs the closure over the injected
+    // `var exports`, and `exports` then resolves as an unknown global
+    // (`exports.X` lowers to the numeric `0` sentinel → `(number).test is not a
+    // function` inside class methods/constructors of CJS packages like ajv).
+    for injected in ["exports", "module", "require"] {
+        if !iife_locals.iter().any(|n| n == injected) {
+            iife_locals.push(injected.to_string());
+        }
+    }
 
     let mut i = 0usize;
     while i < bytes.len() {
diff --git a/crates/perry/src/commands/compile/cjs_wrap/mod.rs b/crates/perry/src/commands/compile/cjs_wrap/mod.rs
index c9277873d..e263c68f1 100644
--- a/crates/perry/src/commands/compile/cjs_wrap/mod.rs
+++ b/crates/perry/src/commands/compile/cjs_wrap/mod.rs
@@ -648,6 +648,55 @@ module.exports = SafeBuffer;"#;
         );
     }
 
+    #[test]
+    fn issue_5251_class_reading_exports_stays_in_iife() {
+        // #5251: a top-level class whose body reads the cjs_wrap-injected
+        // `exports` binding (`exports.X` inside a method/ctor) must NOT be
+        // hoisted out of the IIFE — hoisting severs its closure over the
+        // injected `var exports`, so `exports.X` resolves as an unknown
+        // global and lowers to the numeric `0` sentinel inside class methods.
+        let src = "\"use strict\";\nexports.TAG = \"hi\";\nclass C { greet() { return exports.TAG + \"!\"; } }\nexports.mk = function () { return new C(); };\n";
+        let wrapped = wrap_commonjs(src, &PathBuf::from("/tmp/app/node_modules/re/index.js"));
+        let iife_start = wrapped
+            .find("const _cjs = (function() {")
+            .expect("expected an IIFE wrap (no flat default class), got:\n");
+        let class_pos = wrapped
+            .find("class C ")
+            .expect("class C must survive in the wrapped output");
+        assert!(
+            class_pos > iife_start,
+            "class reading `exports` must stay INSIDE the IIFE (after its \
+             opener), not hoisted above it, got:\n{}",
+            wrapped
+        );
+        assert!(
+            perry_parser::parse_typescript(&wrapped, "re/index.js").is_ok(),
+            "wrapped module must parse, got:\n{}",
+            wrapped
+        );
+    }
+
+    #[test]
+    fn issue_5251_class_without_exports_still_hoists() {
+        // Regression guard: a top-level class that does NOT touch the injected
+        // `exports`/`module`/`require` bindings must still hoist above the
+        // IIFE (so `import { D } from "pkg"` resolves to the real class).
+        let src = "\"use strict\";\nclass D { val() { return 42; } }\nexports.mkD = function () { return new D(); };\n";
+        let wrapped = wrap_commonjs(src, &PathBuf::from("/tmp/app/node_modules/re/index.js"));
+        let iife_start = wrapped
+            .find("const _cjs = (function() {")
+            .expect("expected an IIFE wrap, got:\n");
+        let class_pos = wrapped
+            .find("class D ")
+            .expect("class D must survive in the wrapped output");
+        assert!(
+            class_pos < iife_start,
+            "a class not referencing exports/module/require must still hoist \
+             above the IIFE, got:\n{}",
+            wrapped
+        );
+    }
+
     #[test]
     fn issue_1721_blanks_adopted_alias_require_in_body() {
         // #1721: `const c = require('./common')` adopts `c` as the import