diff --git a/CLAUDE.md b/CLAUDE.md
index 8ac7ced3ae..9402553dbd 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -8,7 +8,7 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 Perry is a native TypeScript compiler written in Rust that compiles TypeScript source code directly to native executables. It uses SWC for TypeScript parsing and LLVM for code generation.
 
-**Current Version:** 0.5.1404
+**Current Version:** 0.5.1405
 
 
 ## TypeScript Parity Status
diff --git a/Cargo.lock b/Cargo.lock
index 7d89eb61b4..ee979ff0b2 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -5547,7 +5547,7 @@ checksum = "9b4f627cb1b25917193a259e49bdad08f671f8d9708acfd5fe0a8c1455d87220"
 
 [[package]]
 name = "perry"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "base64",
@@ -5607,14 +5607,14 @@ dependencies = [
 
 [[package]]
 name = "perry-api-manifest"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "serde",
 ]
 
 [[package]]
 name = "perry-audio-miniaudio"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "cc",
  "libc",
@@ -5622,7 +5622,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "inkwell",
@@ -5639,7 +5639,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-arkts"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -5647,7 +5647,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-glance"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -5655,7 +5655,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-js"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "perry-dispatch",
@@ -5664,7 +5664,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-swiftui"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -5672,7 +5672,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-wasm"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "base64",
@@ -5684,7 +5684,7 @@ dependencies = [
 
 [[package]]
 name = "perry-codegen-wear-tiles"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -5692,7 +5692,7 @@ dependencies = [
 
 [[package]]
 name = "perry-container-compose"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "async-trait",
@@ -5721,14 +5721,14 @@ dependencies = [
 
 [[package]]
 name = "perry-container-e2e"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
 ]
 
 [[package]]
 name = "perry-diagnostics"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "serde",
  "serde_json",
@@ -5736,7 +5736,7 @@ dependencies = [
 
 [[package]]
 name = "perry-dispatch"
-version = "0.5.1404"
+version = "0.5.1405"
 
 [[package]]
 name = "perry-doc-fixture-my-bindings"
@@ -5747,7 +5747,7 @@ dependencies = [
 
 [[package]]
 name = "perry-doc-tests"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "clap",
@@ -5762,7 +5762,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-ads"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "block2",
  "objc2",
@@ -5772,7 +5772,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-argon2"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "argon2",
  "perry-ffi",
@@ -5780,7 +5780,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-axios"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
  "reqwest",
@@ -5789,7 +5789,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-bcrypt"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "bcrypt",
  "perry-ffi",
@@ -5797,7 +5797,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-better-sqlite3"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
  "rusqlite",
@@ -5805,7 +5805,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-cheerio"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
  "scraper",
@@ -5813,7 +5813,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-commander"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
  "perry-runtime",
@@ -5821,7 +5821,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-cron"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "chrono",
  "cron",
@@ -5831,7 +5831,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-dayjs"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "chrono",
  "perry-ffi",
@@ -5839,7 +5839,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-decimal"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
  "rust_decimal",
@@ -5847,7 +5847,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-dotenv"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
  "serde_json",
@@ -5855,7 +5855,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-ethers"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
  "rand 0.10.1",
@@ -5863,7 +5863,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-events"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
  "perry-runtime",
@@ -5871,14 +5871,14 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-exponential-backoff"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
 ]
 
 [[package]]
 name = "perry-ext-fastify"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "bytes",
  "http-body-util",
@@ -5896,7 +5896,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-fetch"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "bytes",
  "lazy_static",
@@ -5909,7 +5909,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-http"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "bytes",
  "h2",
@@ -5933,7 +5933,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-ioredis"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "lazy_static",
  "perry-ffi",
@@ -5943,7 +5943,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-jsonwebtoken"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "base64",
  "jsonwebtoken",
@@ -5954,7 +5954,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-lru-cache"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "lru",
  "perry-ffi",
@@ -5963,7 +5963,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-moment"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "chrono",
  "perry-ffi",
@@ -5971,7 +5971,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-mongodb"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "bson",
  "futures-util",
@@ -5983,7 +5983,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-mysql2"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "chrono",
  "perry-ffi",
@@ -5993,7 +5993,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-nanoid"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "nanoid",
  "perry-ffi",
@@ -6002,7 +6002,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-net"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "bytes",
  "perry-ffi",
@@ -6015,7 +6015,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-node-forge"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "const-oid 0.9.6",
  "der 0.7.10",
@@ -6034,7 +6034,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-nodemailer"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "lettre",
  "perry-ffi",
@@ -6044,7 +6044,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-pdf"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
  "printpdf",
@@ -6052,7 +6052,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-pg"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
  "sqlx",
@@ -6061,7 +6061,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-ratelimit"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "governor",
  "perry-ffi",
@@ -6069,7 +6069,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-sharp"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "fast_image_resize",
  "image",
@@ -6079,14 +6079,14 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-slugify"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
 ]
 
 [[package]]
 name = "perry-ext-streams"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "lazy_static",
  "perry-ffi",
@@ -6095,7 +6095,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-undici"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
  "perry-runtime",
@@ -6104,7 +6104,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-uuid"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
  "uuid",
@@ -6112,7 +6112,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-validator"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ffi",
  "regex",
@@ -6122,7 +6122,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-ws"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "futures-util",
  "lazy_static",
@@ -6135,7 +6135,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ext-zlib"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "brotli",
  "flate2",
@@ -6145,7 +6145,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ffi"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "dashmap",
  "once_cell",
@@ -6154,7 +6154,7 @@ dependencies = [
 
 [[package]]
 name = "perry-hir"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "perry-api-manifest",
@@ -6172,7 +6172,7 @@ dependencies = [
 
 [[package]]
 name = "perry-parser"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "perry-diagnostics",
@@ -6184,7 +6184,7 @@ dependencies = [
 
 [[package]]
 name = "perry-runtime"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "base64",
@@ -6226,14 +6226,14 @@ dependencies = [
 
 [[package]]
 name = "perry-runtime-static"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-runtime",
 ]
 
 [[package]]
 name = "perry-stdlib"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "aes 0.8.4",
  "aes 0.9.1",
@@ -6328,14 +6328,14 @@ dependencies = [
 
 [[package]]
 name = "perry-stdlib-static"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-stdlib",
 ]
 
 [[package]]
 name = "perry-transform"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "perry-hir",
@@ -6344,14 +6344,14 @@ dependencies = [
 
 [[package]]
 name = "perry-ui"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ui-model",
 ]
 
 [[package]]
 name = "perry-ui-android"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "base64",
  "itoa",
@@ -6368,7 +6368,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-geisterhand"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "rand 0.10.1",
  "serde",
@@ -6378,7 +6378,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-gtk4"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "base64",
  "cairo-rs 0.22.0",
@@ -6401,7 +6401,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-ios"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "base64",
  "block2",
@@ -6417,7 +6417,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-macos"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "base64",
  "block2",
@@ -6432,7 +6432,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-model"
-version = "0.5.1404"
+version = "0.5.1405"
 
 [[package]]
 name = "perry-ui-test"
@@ -6443,11 +6443,11 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-testkit"
-version = "0.5.1404"
+version = "0.5.1405"
 
 [[package]]
 name = "perry-ui-tvos"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "base64",
  "block2",
@@ -6463,7 +6463,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-visionos"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "base64",
  "block2",
@@ -6479,7 +6479,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-watchos"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "block2",
  "libc",
@@ -6492,7 +6492,7 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-windows"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "base64",
  "libc",
@@ -6509,14 +6509,14 @@ dependencies = [
 
 [[package]]
 name = "perry-ui-windows-winui"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "perry-ui-windows",
 ]
 
 [[package]]
 name = "perry-updater"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "anyhow",
  "base64",
@@ -6532,7 +6532,7 @@ dependencies = [
 
 [[package]]
 name = "perry-wasm-host"
-version = "0.5.1404"
+version = "0.5.1405"
 dependencies = [
  "wasmi",
 ]
diff --git a/Cargo.toml b/Cargo.toml
index 226aeab86e..40620a4630 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -315,7 +315,7 @@ codegen-units = 16
 codegen-units = 16
 
 [workspace.package]
-version = "0.5.1404"
+version = "0.5.1405"
 edition = "2021"
 license = "MIT"
 repository = "https://github.com/PerryTS/perry"
diff --git a/changelog.d/7164-ffi-by-index-field-count.md b/changelog.d/7164-ffi-by-index-field-count.md
new file mode 100644
index 0000000000..46cf13dac5
--- /dev/null
+++ b/changelog.d/7164-ffi-by-index-field-count.md
@@ -0,0 +1,9 @@
+### Fixed
+
+- **`js_object_set_field` by index left the written slot outside the collector's view (#7164).** `perry_ffi::alloc_object()` allocates with `field_count = 0` and `INLINE_SLOT_FLOOR` physical slots. The documented by-index writer bounds-checks `field_index` against `max(field_count, INLINE_SLOT_FLOOR)` — so it *accepts* an index at or above `field_count` — but never widened `field_count` to cover it.
+
+  `object::gc_field_slot_range` bounds the collector's view of an object's payload by `field_count`, and `heap_payload_slot_selection` short-circuits to `Empty` on an empty payload **before** consulting the per-object pointer mask. A pointer written through the by-index setter was therefore invisible to both tracing (never marked → swept while still live) and to evacuation rewriting (a stale from-space pointer left in a live slot). That is the crash in the issue: the bogus `0x9` slot the copying minor SIGSEGVs on is downstream wreckage, not the defect.
+
+  This is the by-index counterpart of the "#7154 publication order" invariant that `field_set_by_name/tail.rs` already enforces at two sites; only the by-index path was missing it. Widening before the store is safe because the existing bounds check already holds `field_index` below the physical capacity, and every physical slot is `undefined`-initialized at allocation, so the widened range can only ever expose non-pointer sentinels.
+
+  The regression test asserts the defect **deterministically**, without racing a collector: it checks the collector's own child-slot enumerator (`gc_child_slots` — the function every mark/scan/evacuation pass calls) reports the written slot at all. Before the fix it enumerated `[]`. It then runs a real copying minor gated on `copied_objects > 0`, so a run that collected nothing cannot report success.
diff --git a/crates/perry-runtime/src/gc/tests/copying/pointer_publish_7154.rs b/crates/perry-runtime/src/gc/tests/copying/pointer_publish_7154.rs
index 6fdd5f7a57..feadbd8b99 100644
--- a/crates/perry-runtime/src/gc/tests/copying/pointer_publish_7154.rs
+++ b/crates/perry-runtime/src/gc/tests/copying/pointer_publish_7154.rs
@@ -266,3 +266,100 @@ fn test_fresh_closure_capture_slots_are_initialized_7154() {
         }
     }
 }
+
+/// #7164 — `js_object_set_field` (the BY-INDEX setter on `perry-ffi`'s stable
+/// surface, re-exported verbatim from `object/field_get_set/field_ops.rs`)
+/// must widen `field_count` the same way the by-name path already does at
+/// `object/field_set_by_name/tail.rs`'s two "#7154 publication order" sites.
+///
+/// `perry_ffi::alloc_object()` calls `js_object_alloc(0, 0)`: `field_count =
+/// 0` with `INLINE_SLOT_FLOOR` (4) physical slots undefined-initialized.
+/// `js_object_set_field(obj, 0, pointer_value)` passes the bounds check
+/// (`0 < max(field_count, 4)`) and stores the pointer, but — unlike
+/// `tail.rs`'s by-name writer — never bumps `field_count`. The collector's
+/// view of the payload (`object::gc_field_slot_range`, and downstream
+/// `heap_payload_slot_selection`'s `payload.is_empty()` short-circuit) is
+/// bounded by `field_count`, so the stored pointer is outside the traced
+/// range: never marked (swept while live) and never rewritten by a copying
+/// minor (stale from-space pointer left in a live slot). Reachable through
+/// `perry-ffi`'s documented surface alone (`alloc_object` + `js_object_set_field`).
+#[test]
+fn test_ffi_index_field_set_widens_field_count_7164() {
+    let _guard = CopyingNurseryTestGuard::new(1);
+
+    // Mirrors `perry_ffi::alloc_object()` exactly: class_id=0, field_count=0.
+    let obj = crate::object::js_object_alloc(0, 0);
+    assert_eq!(
+        unsafe { (*obj).field_count },
+        0,
+        "test setup: alloc_object()'s field_count starts at 0"
+    );
+
+    let child = crate::string::js_string_from_bytes(
+        b"ffi-index-set-field-7164".as_ptr(),
+        "ffi-index-set-field-7164".len() as u32,
+    );
+
+    // The exact documented-FFI call: `perry_ffi::js_object_set_field(obj, 0,
+    // value)` forwards verbatim to this runtime symbol.
+    crate::object::js_object_set_field(
+        obj,
+        0,
+        crate::value::JSValue::from_bits(string_bits(child as usize)),
+    );
+
+    // Deterministic assertion, independent of GC timing (mirrors the two
+    // tests above): the slot the store just published must be enumerated as
+    // a child edge, or the collector's view of the payload never covers it.
+    let field0_slot = unsafe {
+        (obj as *mut u8).add(std::mem::size_of::<crate::object::ObjectHeader>()) as usize
+    };
+    let enumerated = unsafe { enumerated_child_slots(obj as usize) };
+    assert!(
+        enumerated.contains(&field0_slot),
+        "#7164: js_object_set_field(obj, 0, ..) on a field_count=0 object must \
+         widen field_count so the collector's view covers the written slot; \
+         the collector enumerated {enumerated:?} (a raw field_count=0 leaves \
+         the whole payload range empty, so the mask is never consulted)"
+    );
+    assert_eq!(
+        unsafe { (*obj).field_count },
+        1,
+        "#7164: js_object_set_field must widen field_count to cover the \
+         written index, mirroring field_set_by_name/tail.rs's publication order"
+    );
+
+    // End to end: the object is the ONLY root — the string is reachable
+    // exclusively through the field this defect leaves untraced. It must
+    // survive a copying minor, and the slot must be rewritten to the
+    // relocated address (not left pointing at reclaimed from-space memory).
+    js_shadow_slot_set(0, ptr_bits(obj as usize));
+    let trace = collect_minor_trace(GcTriggerKind::Direct);
+    assert!(
+        trace.copying_nursery.copied_objects > 0,
+        "#7164 regression test requires a COPYING minor; a non-moving \
+         collection cannot expose a missing mark/rewrite (copied_objects=0)"
+    );
+
+    let obj_after = (js_shadow_slot_get(0) & POINTER_MASK) as *mut crate::object::ObjectHeader;
+    let stored = crate::object::js_object_get_field(obj_after, 0);
+    assert!(
+        stored.is_string(),
+        "#7164: the field must still hold a live string after a copying \
+         minor (got bits {:#x}) — an untraced slot is either collected out \
+         from under a live reference or left as a stale from-space pointer",
+        stored.bits()
+    );
+    let recovered = (stored.bits() & POINTER_MASK) as usize;
+    assert!(
+        crate::arena::classify_heap_generation(recovered) != crate::arena::HeapGeneration::Unknown,
+        "#7164: the string reached only through the object field must remain \
+         a live heap pointer after the collection (recovered {recovered:#x})"
+    );
+    unsafe {
+        assert_string_bytes(
+            recovered as *const crate::StringHeader,
+            b"ffi-index-set-field-7164",
+        );
+    }
+}
diff --git a/crates/perry-runtime/src/object/field_get_set/field_ops.rs b/crates/perry-runtime/src/object/field_get_set/field_ops.rs
index edb2770da1..cc5b647048 100644
--- a/crates/perry-runtime/src/object/field_get_set/field_ops.rs
+++ b/crates/perry-runtime/src/object/field_get_set/field_ops.rs
@@ -149,6 +149,22 @@ pub extern "C" fn js_object_set_field(obj: *mut ObjectHeader, field_index: u32,
         };
         let fields_ptr = (obj as *mut u8).add(std::mem::size_of::<ObjectHeader>()) as *mut JSValue;
         let slot = fields_ptr.add(field_index as usize);
+        // #7164 publication order (same invariant as the two "#7154
+        // publication order" sites in field_set_by_name/tail.rs): widen
+        // `field_count` FIRST, before the store. `object::gc_field_slot_range`
+        // bounds the collector's view of the payload by `field_count`, so a
+        // write at an index the count does not yet cover is invisible to BOTH
+        // tracing and evacuation rewriting -- a pointer stored there is never
+        // marked (swept while live) and never rewritten (stale from-space
+        // pointer left in a live slot) by a copying minor. This is the
+        // BY-INDEX counterpart of that bug: `alloc_limit` above already bounds
+        // `field_index` below the physical capacity, and every physical slot
+        // is undefined-initialized at allocation (`object/alloc.rs`), so
+        // widening here can only ever expose non-pointer sentinels ahead of
+        // the store that is about to fill this one in.
+        if field_index >= (*obj).field_count {
+            (*obj).field_count = field_index + 1;
+        }
         crate::gc::runtime_store_jsvalue_slot(
             obj as usize,
             slot as usize,