From d585f74f85a6f11ad38b6a7712f0e8c0befa3a11 Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Thu, 1 Oct 2026 17:40:11 -0500
Subject: [PATCH 1/2] Honor inherited object formatters for Effect shard hashes

---
 packages/runtime/src/scr_json.c            |  9 ++++---
 tests/corpus/object-inherited-to-string.js | 28 ++++++++++++++++++++++
 tests/fixtures/effect4/data-shard-hash.ts  |  9 +++++++
 3 files changed, 43 insertions(+), 3 deletions(-)
 create mode 100644 tests/corpus/object-inherited-to-string.js
 create mode 100644 tests/fixtures/effect4/data-shard-hash.ts

diff --git a/packages/runtime/src/scr_json.c b/packages/runtime/src/scr_json.c
index be05553a8..4082d5cfe 100644
--- a/packages/runtime/src/scr_json.c
+++ b/packages/runtime/src/scr_json.c
@@ -8822,10 +8822,13 @@ ScrMap *scr_dyn_native_collection_check(const ScrDyn *value, int map, const char
 }
 
 ScrStr *scr_dyn_to_string_method(const ScrDyn *d, const ScrStr *enc, const ScrStr *what) {
-  if (d->kind == SCR_DYN_FUNC) {
-    ScrDyn *member = scr_dyn_fn_get(d, "toString", 8);
+  /* Object.create receivers must run their inherited formatter before the
+   * default object tag; Effect shard keys use it to produce distinct hashes. */
+  const bool object_method = d->kind == SCR_DYN_OBJ && scr_dyn_property_owner(d, "toString", 8);
+  if (d->kind == SCR_DYN_FUNC || object_method) {
+    ScrDyn *member = object_method ? scr_dyn_obj_read(d, "toString", 8) : scr_dyn_fn_get(d, "toString", 8);
     if (scr_exc_pending()) { scr_dyn_release(member); return NULL; }
-    if (member && member->kind != SCR_DYN_UNDEF) {
+    if (member && (object_method || member->kind != SCR_DYN_UNDEF)) {
       scr_dyn_this_push_dyn(d);
       ScrDyn *value = scr_dyn_call(member, NULL, 0, what->data);
       scr_dyn_this_pop();
diff --git a/tests/corpus/object-inherited-to-string.js b/tests/corpus/object-inherited-to-string.js
new file mode 100644
index 000000000..69d1b4fb4
--- /dev/null
+++ b/tests/corpus/object-inherited-to-string.js
@@ -0,0 +1,28 @@
+const proto = {
+  toString() { return `value:${this.label}`; }
+};
+const value = Object.create(proto);
+value.label = "first";
+const child = Object.create(value);
+child.label = "second";
+console.log(value.toString(), child.toString(), String(child));
+Object.defineProperty(proto, "toString", {
+  configurable: true,
+  get() {
+    const label = this.label;
+    return function() { return `${label}/${this.label}`; };
+  }
+});
+console.log(value.toString(), child.toString());
+const shadow = Object.create(value);
+Object.defineProperty(shadow, "toString", { value: undefined });
+try { shadow.toString(); } catch (error) { console.log(error.name, error.message); }
+Object.defineProperty(proto, "toString", {
+  configurable: true,
+  get() { throw new Error("formatter getter"); }
+});
+try { child.toString(); } catch (error) { console.log(error.name, error.message); }
+const own = Object.create(null);
+own.label = "own";
+own.toString = function() { return this.label; };
+console.log(own.toString());
diff --git a/tests/fixtures/effect4/data-shard-hash.ts b/tests/fixtures/effect4/data-shard-hash.ts
new file mode 100644
index 000000000..12856c1ae
--- /dev/null
+++ b/tests/fixtures/effect4/data-shard-hash.ts
@@ -0,0 +1,9 @@
+import * as ShardId from "effect/cluster/ShardId"
+import * as Hash from "effect/Hash"
+import * as Equal from "effect/Equal"
+
+const first = ShardId.make("default", 1)
+const second = ShardId.make("default", 2)
+console.log(first.toString(), second.toString())
+console.log(Hash.hash(first), Hash.hash(second), Hash.string("default:1"), Hash.string("default:2"))
+console.log(Equal.equals(first, second), Equal.equals(first, ShardId.make("default", 1)))

From a17fce1cf8f376dafb942442f9640ed2d108b115 Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Thu, 1 Oct 2026 17:49:10 -0500
Subject: [PATCH 2/2] Record inherited formatter corpus preflight baseline

---
 packages/compiler/test/ts7/baselines/order-parity.json | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/packages/compiler/test/ts7/baselines/order-parity.json b/packages/compiler/test/ts7/baselines/order-parity.json
index d4b408427..afb2db298 100644
--- a/packages/compiler/test/ts7/baselines/order-parity.json
+++ b/packages/compiler/test/ts7/baselines/order-parity.json
@@ -10329,6 +10329,12 @@
    ],
    "diags": []
   },
+  "<repo>/tests/corpus/object-inherited-to-string.js": {
+   "order": [
+    "<repo>/tests/corpus/object-inherited-to-string.js"
+   ],
+   "diags": []
+  },
   "<repo>/tests/corpus/object-prototype-identity.mjs": {
    "order": [
     "<repo>/tests/corpus/object-prototype-identity.mjs"