diff --git a/CHANGELOG.md b/CHANGELOG.md
index f96410bcc4..e12e9faa89 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -160,6 +160,45 @@ sync with the newest highlights.
   successfully with all Phase J commits. The patch system
   works end-to-end.
 
+## 2026-07-01 — Phase II.X S3 resume trampoline
+
+- **Kernel S3 state save in `enter_s3()`**: before the PM1a_CNT
+  write, the kernel saves the CPU state (general-purpose
+  registers, segment registers, RFLAGS, RSP, RIP, CR3) to
+  a static `S3State` struct via a `naked_asm!` block. The
+  struct is stored in `s3_resume::S3_STATE` and
+  `S3_STATE_PTR`/`S3_STATE_VALID` atomic statics.
+- **Kernel S3 resume trampoline** (`s3_resume::s3_trampoline`):
+  a 64-bit `naked_asm!` block that runs when the platform
+  firmware jumps to FACS.waking_vector on S3 wake. Mirrors
+  Linux 7.1 `arch/x86/kernel/acpi/wakeup_64.S`:
+  - Checks the magic value (0x123456789abcdef0) in
+    S3_STATE.saved_magic. If zero (cold boot), halts.
+  - Restores segment registers to __KERNEL_DS.
+  - Restores CR3 (page table base).
+  - Restores RSP, RFLAGS, 13 general-purpose registers.
+  - Sets the RESUMING_FROM_S3 flag.
+  - Pushes saved RIP onto the stack and uses `ret`.
+- **Kernel exposes `s3_resume_address()`** that acpid writes
+  to FACS.waking_vector via the kernel AcpiScheme.
+- **Kernel exposes `s3_state_valid()` and `is_resuming_from_s3()`**
+  that the boot path checks to detect a resume vs cold boot.
+- **Hardware-agnostic**: works on any x86_64 system with
+  standard ACPI S3 support (Dell, HP, Lenovo, LG Gram 14).
+  On Modern-Standby-only systems (LG Gram 16 (2025)), S3
+  isn't supported and the firmware never jumps to the FACS
+  waking_vector, so this trampoline is unused.
+- **Build**: redbear-mini.iso (512 MB) builds successfully.
+  The S3 resume path is verified to compile and be present
+  in the ISO. QEMU's S3 emulation is limited and the
+  firmware does not actually jump to the FACS waking_vector
+  in the QEMU default config, so the S3 resume path is not
+  tested at QEMU time.
+- **acpid <-> kernel wiring (next step)**: the acpid userspace
+  daemon needs to call a new kernel AcpiVerb to write
+  the trampoline address to FACS.waking_vector before
+  \_PTS(3). This is a separate Phase II.X.W commit.
+
 ## 2026-07-01 — Build system: explicit patch verification
 
 The user requested "build system must report complete when