diff --git a/changelog.d/7127-in-operator-to-property-key.md b/changelog.d/7127-in-operator-to-property-key.md
new file mode 100644
index 0000000000..293eec64fd
--- /dev/null
+++ b/changelog.d/7127-in-operator-to-property-key.md
@@ -0,0 +1,7 @@
+fix(runtime): `in` operator runs `ToPropertyKey` for every key type (#6944)
+
+`key in obj` only coerced NUMBER keys through `ToPropertyKey` (the arm added for Next.js's `Number(digest.at(-2)) in RedirectStatusCode`). Per spec, `RelationalExpression in ShiftExpression` is `ToPropertyKey(lval)` for **every** key type: an object key must have its `Symbol.toPrimitive` / `toString` / `valueOf` invoked — exactly once, and even when the property is absent, because the coercion is observable. Pre-fix the object key was compared as a raw pointer and never matched, and the user coercion never ran.
+
+`js_object_has_property` now coerces every non-string, non-symbol key via `js_to_property_key` before the lookup. Strings (heap or SSO) and symbols are already property keys — the coercion is identity and allocates nothing — so they keep the pre-fix fast path verbatim. The coercion allocates and (for object keys) runs user JS, so it can GC and evacuate the receiver; the receiver is rooted across it via `RuntimeHandleScope::root_heap_word_u64` and read back through its handle, the same idiom as `js_object_get_property_key` / `js_object_set_property_key` (#6935) and the number arm's #6941 rooting.
+
+New gap test `test-files/test_gap_in_operator_to_property_key.ts`, byte-identical to node 26.5.1: object keys via `toString` / `valueOf` fallback / `Symbol.toPrimitive`, exactly-once coercion on an absent property, object→Symbol coercion matching a symbol-keyed property, primitive keys (`true` / `null` / `undefined` / `1n`), int32-vs-f64 numeric-key agreement on a plain object, and a proxy `has` trap receiving the coerced key.
diff --git a/crates/perry-runtime/src/object/field_get_set/has_property.rs b/crates/perry-runtime/src/object/field_get_set/has_property.rs
index 5e83c68688..36985b9ad8 100644
--- a/crates/perry-runtime/src/object/field_get_set/has_property.rs
+++ b/crates/perry-runtime/src/object/field_get_set/has_property.rs
@@ -191,33 +191,33 @@ pub extern "C" fn js_object_has_property(obj: f64, key: f64) -> f64 {
 
     let obj_val = JSValue::from_bits(obj.to_bits());
 
-    // `in` runs ToPropertyKey on the key. Object property names are strings, so a
-    // NUMBER key must be coerced to its string form before the lookup — `307 in
-    // {307: …}` is `"307" in {…}` and must be true. Without this the string-only
-    // lookup below never matched a numeric key against a numeric-string property,
-    // so `307 in obj` was false while `"307" in obj` was true. Next.js's
-    // `isRedirectError` does `Number(digest.at(-2)) in RedirectStatusCode` (a
-    // `{307: …, 308: …}` map), so a `redirect()` thrown from a Server Component
-    // was not recognized as a redirect — Next treated it as a real error and a
-    // concurrently-rendered sibling's `session.user` read (guarded by that same
-    // redirect on the happy path) surfaced as a fatal 500 instead of a 307.
-    // (Symbols and strings pass through unchanged; a proxy/handle receiver is
-    // handled below with the coerced key.)
+    // `in` runs ToPropertyKey on the key for EVERY key type: per spec,
+    // `RelationalExpression in ShiftExpression` is `ToPropertyKey(lval)`, so an
+    // object key must have its `Symbol.toPrimitive` / `toString` / `valueOf`
+    // invoked — exactly once, and even when the property is absent, because the
+    // coercion is observable (#6944). Only strings (heap or SSO) and symbols
+    // are already property keys; for them the coercion is identity and
+    // allocates nothing, so it is skipped. Everything else goes through
+    // `js_to_property_key` before the lookup: numbers (`307 in {307: …}` is
+    // `"307" in {…}` — Next.js's `isRedirectError` does `Number(digest.at(-2))
+    // in RedirectStatusCode`), booleans, null, undefined, BigInt, and objects.
+    // (A proxy/handle receiver is handled below with the coerced key.)
     //
-    // #6935: that coercion ALLOCATES the stringified key, so it can trigger a
-    // GC that evacuates the receiver — and `obj` / `obj_val` are raw locals
-    // captured above. (Only the number arm coerces, so no user JS runs here,
-    // but an allocation-triggered evacuation moves the receiver just the same.)
+    // #6935: that coercion ALLOCATES the stringified key, and for an object key
+    // it also runs user JS, so it can trigger a GC that evacuates the receiver
+    // — and `obj` / `obj_val` are raw locals captured above. Root the receiver
+    // across the coercion and read it back through the handle, mirroring
+    // `js_object_get_property_key` / `js_object_set_property_key`.
     let (obj, obj_val, key) = {
         let kv = JSValue::from_bits(key.to_bits());
-        if kv.is_number() {
+        if kv.is_any_string() || unsafe { crate::symbol::js_is_symbol(key) } != 0 {
+            (obj, obj_val, key)
+        } else {
             let scope = crate::gc::RuntimeHandleScope::new();
             let obj_handle = scope.root_heap_word_u64(obj.to_bits());
             let key = unsafe { crate::object::js_to_property_key(key) };
             let obj = f64::from_bits(obj_handle.get_heap_word_u64());
             (obj, JSValue::from_bits(obj.to_bits()), key)
-        } else {
-            (obj, obj_val, key)
         }
     };
     let key_val = JSValue::from_bits(key.to_bits());
diff --git a/test-files/test_gap_in_operator_to_property_key.ts b/test-files/test_gap_in_operator_to_property_key.ts
new file mode 100644
index 0000000000..c6e7cf64df
--- /dev/null
+++ b/test-files/test_gap_in_operator_to_property_key.ts
@@ -0,0 +1,95 @@
+// #6944: the `in` operator must run ToPropertyKey on its left operand for
+// EVERY key type, not just numbers. Per spec, `RelationalExpression in
+// ShiftExpression` is `ToPropertyKey(lval)`, so an object key has its
+// `Symbol.toPrimitive` / `toString` / `valueOf` invoked — exactly once, and
+// even when the property is absent (the coercion is observable). Perry only
+// coerced number keys, so an object key was compared as a raw pointer and
+// never matched.
+
+const obj: any = { here: 1 };
+
+// object key with toString
+const k: any = { toString(): string { return "here"; } };
+console.log("object toString key :", (k as any) in obj);
+console.log("string key          :", "here" in obj);
+
+// the coercion runs exactly once, even when the property is ABSENT
+let calls = 0;
+const absent: any = {
+  toString(): string {
+    calls++;
+    return "nope";
+  },
+};
+console.log("absent object key   :", (absent as any) in obj);
+console.log("coercion call count :", calls);
+
+// Symbol.toPrimitive takes priority over toString/valueOf
+const symPrim: any = {
+  [Symbol.toPrimitive](hint: string): string {
+    return "via-" + hint;
+  },
+  toString(): string {
+    return "via-toString";
+  },
+};
+const obj2: any = { "via-string": 1, "via-toString": 2 };
+console.log("Symbol.toPrimitive  :", (symPrim as any) in obj2);
+
+// OrdinaryToPrimitive(string) tries toString first; a non-primitive toString
+// result falls through to valueOf
+const valOf: any = {
+  toString(): any {
+    return {};
+  },
+  valueOf(): number {
+    return 42;
+  },
+};
+const obj3: any = { 42: "answer" };
+console.log("valueOf fallback    :", (valOf as any) in obj3);
+
+// toString wins over valueOf (string hint order)
+const both: any = {
+  toString(): string {
+    return "str";
+  },
+  valueOf(): number {
+    return 99;
+  },
+};
+const obj4: any = { str: 1, 99: 2 };
+console.log("toString > valueOf  :", (both as any) in obj4);
+
+// an object key coercing to a Symbol matches the symbol-keyed property
+const sym = Symbol("s");
+const symKey: any = {
+  toString(): any {
+    return sym;
+  },
+};
+const withSym: any = { [sym]: 1 };
+console.log("object -> symbol key:", (symKey as any) in withSym);
+
+// non-string primitives are stringified too
+const primitives: any = { true: 1, null: 2, undefined: 3, "1": 4 };
+console.log("boolean key         :", (true as any) in primitives);
+console.log("null key            :", (null as any) in primitives);
+console.log("undefined key       :", (undefined as any) in primitives);
+console.log("bigint key          :", (1n as any) in primitives);
+
+// int32-boxed and f64 numeric keys agree on a plain object
+const small = 1;
+console.log("int32 numeric key   :", (small as any) in primitives);
+console.log("f64 numeric key     :", 1.0 in primitives);
+
+// a proxy receiver observes the COERCED key in its `has` trap
+const seen: string[] = [];
+const proxy = new Proxy({ here: 1 }, {
+  has(target, prop) {
+    seen.push(String(prop));
+    return prop in target;
+  },
+});
+console.log("proxy has trap      :", (k as any) in proxy);
+console.log("proxy saw key       :", seen.join(","));