diff --git a/.claude/CLAUDE.md b/.claude/CLAUDE.md
index 8981e7d3..5e76a959 100644
--- a/.claude/CLAUDE.md
+++ b/.claude/CLAUDE.md
@@ -176,7 +176,9 @@ capsule files — accurate):
 previously described a 1024-byte-per-block budget — right by arithmetic, wrong as a rule):
 `validate_forth_blocks` enforces a **64-char × 16-line** format —
 **line length ≤ 64 chars, ≤ 16 content lines per block** (`mkcapsule.c:344-345`, `:430`) — and
-a block number in **`[2048, 5120)`** (`:408-409`). 64 × 16 = 1024, which is where the old
+a block number of **2048 or more, with no upper bound** (`:408-411`; ruled 2026-10-05,
+`docs/v4.0.0/NUCLEUS.md` §5.2 — blocks 0–2047, the VM's fast RAM, are the only ones a capsule
+may not claim; the check was `[2048, 5120)` before). 64 × 16 = 1024, which is where the old
 figure came from, but the enforcement is per-line and per-line-count: **8 lines of 128 chars
 is 1024 bytes and still fails.** Verify with `mkcapsule --lint capsules/`, not `wc -c`.
 
diff --git a/experiments/bare_metal/README.md b/experiments/bare_metal/README.md
index 81da9ff1..54c1001d 100644
--- a/experiments/bare_metal/README.md
+++ b/experiments/bare_metal/README.md
@@ -144,8 +144,10 @@ which block slot to fill.
 
 1. The first line of each logical block must be `Block NNNN` (capital B,
    single space, decimal integer).
-2. Block numbers must be unique within a single capsule file, and must fall
-   in `[2048, 5120)`.
+2. Block numbers must be unique within a single capsule file, and must be
+   2048 or more. Blocks 0–2047 are the VM's fast RAM and the only ones a
+   capsule may not claim; there is no upper bound (ruled 2026-10-05,
+   `docs/v4.0.0/NUCLEUS.md` §5.2 — previously `[2048, 5120)`).
 3. Blocks are loaded in file order and executed top-to-bottom.
 4. Each block can hold at most 16 content lines, each at most 64 characters
    long (`validate_forth_blocks` in `tools/mkcapsule.c`, corrected 2026-09-19
diff --git a/tools/mkcapsule.c b/tools/mkcapsule.c
index 1c737904..d8b419b5 100644
--- a/tools/mkcapsule.c
+++ b/tools/mkcapsule.c
@@ -405,11 +405,13 @@ static int validate_forth_blocks(const char *fpath,
             block_num     = (int)strtol((const char *)(data + start + 6), NULL, 10);
             line_in_block = 0;
 
-            /* Block number must be in user space: [2048, 5120) */
-            if (block_num < 2048 || block_num >= 5120) {
+            /* Blocks 0..2047 are the VM's fast RAM and the only ones a
+             * capsule may not claim; there is no upper bound
+             * (docs/v4.0.0/NUCLEUS.md 5.2). */
+            if (block_num < 2048) {
                 fprintf(stderr,
-                    "mkcapsule: ERROR: %s:%d: block %d out of user range "
-                    "[2048, 5120)\n",
+                    "mkcapsule: ERROR: %s:%d: block %d is in the VM's fast "
+                    "RAM (0..2047)\n",
                     fpath, file_line, block_num);
                 errors++;
             }