diff --git a/.github/scripts/check_boot.py b/.github/scripts/check_boot.py
index d6e7122..162094a 100644
--- a/.github/scripts/check_boot.py
+++ b/.github/scripts/check_boot.py
@@ -18,7 +18,7 @@ import sys
 import termios
 import time
 
-IMAGE = sys.argv[1] if len(sys.argv) > 1 else "os.img"
+IMAGE = sys.argv[1] if len(sys.argv) > 1 else "nadir.img"
 NEEDLE = b"64-bit"
 BOOT_WAIT_S = 8
 
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 079be16..6788b3d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1,6 +1,6 @@
 # Nadir Kernel CI. Runs on every pull request and every push to main.
 #
-# Builds os.img with make, verifies the image, then boots it headless in
+# Builds nadir.img with make, verifies the image, then boots it headless in
 # QEMU and asserts the kernel's welcome message reaches the screen.
 
 name: CI
@@ -29,16 +29,20 @@ jobs:
         run: make
 
       - name: Verify image
+        # Every check prints what it saw: a silent bare `test` once hid a
+        # failure here, so each assertion reports and names its own failure.
         run: |
           set -euo pipefail
-          test -f os.img
-          size=$(stat -c %s os.img)
-          test "$size" -ge 1024
-          test $(( (size - 512) % 512 )) -eq 0
-          sig=$(dd if=os.img bs=1 skip=510 count=2 2>/dev/null | od -A n -t x1 | tr -d ' \n')
-          test "$sig" = "55aa" || { echo "missing 0xAA55 boot signature"; exit 1; }
-          grep -q "Nadir" os.img || { echo "kernel payload missing from os.img"; exit 1; }
-          echo "os.img: $size bytes, boot signature and payload present"
+          ls -l nadir.img
+          size=$(stat -c %s nadir.img)
+          echo "image size: $size bytes"
+          test "$size" -ge 1024 || { echo "FAIL: image smaller than sector+kernel minimum"; exit 1; }
+          test $(( (size - 512) % 512 )) -eq 0 || { echo "FAIL: kernel part is not sector-aligned"; exit 1; }
+          sig=$(dd if=nadir.img bs=1 skip=510 count=2 2>/dev/null | od -A n -t x1 | tr -d ' \n')
+          echo "boot signature: $sig"
+          test "$sig" = "55aa" || { echo "FAIL: missing 0xAA55 boot signature"; exit 1; }
+          grep -q "Nadir" nadir.img || { echo "FAIL: kernel payload missing from image"; exit 1; }
+          echo "OK: nadir.img valid, boot signature and payload present"
 
       - name: Boot and check welcome message in QEMU
-        run: python3 .github/scripts/check_boot.py os.img
+        run: python3 .github/scripts/check_boot.py nadir.img
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index a85bb64..f8451b8 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -36,7 +36,7 @@ Please stick to the following rules if you plan to use AI in your contributions:
 Build from the repository root. See the [README](README.md) for prerequisites and troubleshooting:
 
 ```
-make        # builds os.img (boot sector + kernel)
+make        # builds nadir.img (boot sector + kernel)
 make run    # boots it in QEMU
 ```
 
diff --git a/README.md b/README.md
index b063a60..da1e34f 100644
--- a/README.md
+++ b/README.md
@@ -48,7 +48,7 @@ I can only recommend using Linux distribution, but the Windows Subsystem for Lin
 From the repository root:
 
 ```bash
-make        # builds os.img
+make        # builds nadir.img
 make run    # boots it in QEMU
 ```
 
@@ -73,7 +73,7 @@ Notes:
 |---|---|
 | `Boot failed: not a bootable disk` | The first 512 bytes of `nadir.img` are not a valid boot sector (they must end with the `0xAA55` signature). Re-run `make` and check it completes without errors. |
 | Black screen / blinking cursor | The boot sector only speaks legacy BIOS (`int 0x10` text mode). This works out of the box with QEMU's default SeaBIOS firmware but will not boot under UEFI-only firmware (or UEFI-only real hardware) unless legacy/CSM boot is enabled. |
-| QEMU reboots in a loop after `entering protected mode` (or before any C output) | A triple fault in the boot chain: the CPU crashed with no IDT installed and reset itself. Boot with `qemu-system-x86_64 -fda os.img -d int -no-reboot` and read the fault (`v=...`) plus registers from the log. |
+| QEMU reboots in a loop after `entering protected mode` (or before any C output) | A triple fault in the boot chain: the CPU crashed with no IDT installed and reset itself. Boot with `qemu-system-x86_64 -fda nadir.img -d int -no-reboot` and read the fault (`v=...`) plus registers from the log. |
 | No window appears under WSL2 | Use the `-display curses` variant above. It should work in any terminal. |
 
 ## How booting works