diff --git a/crates/sylpheed-export/src/video.rs b/crates/sylpheed-export/src/video.rs
index 44ca9e95..17eeeac3 100644
--- a/crates/sylpheed-export/src/video.rs
+++ b/crates/sylpheed-export/src/video.rs
@@ -157,10 +157,35 @@ pub fn transcode(disc: &Path, out: &Path, m: &Movie) -> Result<Option<Transcoded
     let argv = args(&src, &ogv, ch);
     let command = format!("ffmpeg {}", argv.join(" "));
     let size = std::fs::metadata(&src)?.len();
-    let want = format!("{command}\nsource-bytes: {size}\nsource-channels: {ch}\n");
+    // The sidecar SAYS WHAT IT IS. It sits in the modder-facing asset tree next
+    // to the `.ogv`, and MODDING rule 2's principle is that a generated file
+    // should be tellable from a hand-made one by reading it -- a bare ffmpeg
+    // line beside a video looks like something a modder should edit or delete.
+    //
+    // The header is NOT part of the cache key: `fresh` compares only the lines
+    // that describe the encode. Otherwise rewording this comment would re-encode
+    // four minutes of video to no purpose, which is a cache that punishes
+    // documentation.
+    let key = format!("{command}\nsource-bytes: {size}\nsource-channels: {ch}\n");
+    let want = format!(
+        "# Generated by sylpheed-export. NOT an asset and not hand-editable: this\n\
+         # records how {}.ogv beside it was encoded, so a re-export can skip the\n\
+         # encode when the source and the command are both unchanged. Deleting it\n\
+         # only forces one re-encode. To change the video, override the .ogv under\n\
+         # data/mods/ (MODDING rule 4) -- editing this file changes nothing.\n{key}",
+        m.stem
+    );
 
+    let cache_key = |s: &str| -> String {
+        s.lines()
+            .filter(|l| !l.starts_with('#'))
+            .collect::<Vec<_>>()
+            .join("\n")
+    };
     let fresh = ogv.exists()
-        && std::fs::read_to_string(&stamp).map(|s| s == want).unwrap_or(false);
+        && std::fs::read_to_string(&stamp)
+            .map(|s| cache_key(&s) == cache_key(&want))
+            .unwrap_or(false);
     if !fresh {
         // Encode to a temp name and rename on success. A reader that catches
         // this mid-write sees no file at all rather than a valid-looking one
@@ -181,6 +206,17 @@ pub fn transcode(disc: &Path, out: &Path, m: &Movie) -> Result<Option<Transcoded
             bail!("ffmpeg failed on {}", m.src);
         }
         std::fs::rename(&partial, &ogv)?;
+    }
+    // Refresh the sidecar whenever its TEXT differs, encode or no encode.
+    //
+    // It used to be written only inside the `!fresh` branch, which is right for
+    // the cache and wrong for the file: a change to the header alone -- the part
+    // deliberately excluded from the key -- would then never reach an existing
+    // export, because nothing that reads the header can trigger the write that
+    // updates it. The explanation would be correct in the source and absent on
+    // disc, which is the same shape as every other documented-but-unexercised
+    // thing this port has had to find the hard way.
+    if std::fs::read_to_string(&stamp).map(|s| s != want).unwrap_or(true) {
         std::fs::write(&stamp, &want)?;
     }
     Ok(Some(Transcoded {
diff --git a/docs/port/DECISIONS.md b/docs/port/DECISIONS.md
index 898038ca..c2eaa164 100644
--- a/docs/port/DECISIONS.md
+++ b/docs/port/DECISIONS.md
@@ -6161,3 +6161,62 @@ say so, or the next person tunes toward it.
 and all three were blocked by the harness, not by the capture: an overlay posed at
 t=9 that drew nothing, a `--focus=` overwritten on every menu entry, and a banded
 comparison that did not exist.
+
+## `MODDING.md` had five rules and no check. Now it has one, and all five pass
+
+`MODDING.md` opens by calling modding *a constraint on the exporter **today**,
+not a later feature*. Nothing verified it. That is the shape this port keeps
+finding — a rule stated, believed and unexercised: the black hold implemented and
+never called, `ScreenView.skipped` written and never read, `stop_bed` provided and
+never used, `--focus` parsed and overwritten on every menu entry.
+
+`tools/port/check-modding` covers all five. Every one passes today, so it is a
+**guard, not a fix**: its value is that the next thing to break one says so.
+
+| rule | check | result |
+|---|---|---|
+| 1 — one asset, one file | every referenced sprite present, none orphaned, no split names | **174 / 174**, exact |
+| 2 — recognisable names | no hex or hash-shaped filenames | none |
+| 3 — modern formats | extensions confined to json/png/ogg/ogv (+ sidecars) | clean |
+| 4 — base and overrides | `data/mods` gitignored *and* read by the exporter | both |
+| 5 — provenance | every generated JSON carries a `source` | 17 / 17 |
+
+### It is proved to fail
+
+A check that has never failed has not been shown to work — the lesson from
+`check-capture`, which once passed a file with 36 % holes punched through it. Three
+controls, each failing correctly with a non-zero exit:
+
+* a `.cmd` sidecar with its header stripped → rule 3;
+* a `bogus.bmp` in the sprite tree → rule 3;
+* one orphaned PNG → rule 1, *"174 referenced, 175 present"*.
+
+### The one thing it found: an unlabelled generated file in the asset tree
+
+The two `.cmd` encode-cache sidecars sat beside the `.ogv`s in the modder-facing
+tree with no line saying what they were — a bare ffmpeg command next to a video
+reads as something to edit or delete. They now carry a header stating that they
+are generated, are not assets, and that the way to change a video is an override
+under `data/mods/`.
+
+Two details worth keeping:
+
+* the header is **excluded from the cache key**, so rewording it does not
+  re-encode four minutes of video. A cache that punishes documentation gets
+  documented once and never again.
+* the sidecar is now refreshed whenever its **text** differs, not only when a
+  re-encode happens. It used to be written inside the `!fresh` branch — which
+  meant a header change could never reach an existing export, because nothing
+  that reads the header triggers the write that updates it. The explanation would
+  have been correct in the source and absent on disc. Confirmed: two consecutive
+  exports, 20 s and 19 s, header present, no re-encode.
+
+### And a question I asked the Decoder that I could partly answer myself
+
+Last iteration I asked whether the 0.301 % between two of their captures implies a
+**capture-path floor on every comparison in the corpus**. It does not, and I had
+the evidence already: the port matches `live-title-press-a.png` at **0.00093 %**
+full-frame and **0.000 %** across the band. A general floor could not coexist with
+either number. So the 0.301 % is specific to the attract band capture, and my
+0.01–0.2 % rows are not sitting on a hidden floor. ⚠️ What that does *not* settle
+is why those two frames differ — still theirs, and still worth an answer.
diff --git a/tools/port/check-modding b/tools/port/check-modding
new file mode 100755
index 00000000..4aae5f89
--- /dev/null
+++ b/tools/port/check-modding
@@ -0,0 +1,92 @@
+#!/usr/bin/env bash
+# Check the export against MODDING.md's five rules.
+#
+#   tools/port/check-modding
+#
+# `MODDING.md` opens by saying modding is a requirement and **a constraint on the
+# exporter today, not a later feature**. Nothing checked it. That is the shape
+# this port keeps finding: a rule stated, believed, and unexercised -- the black
+# hold implemented and never called, `ScreenView.skipped` written and never read,
+# `stop_bed` provided and never used, `--focus` parsed and overwritten.
+#
+# So this is a guard, not a fix: every rule passes as of 2026-08-30. Its value is
+# that the next thing to break one of them says so.
+#
+# ⚠️ What it CANNOT check: rule 3's "modern, editable" is enforced by extension,
+# which cannot tell a valid PNG from a renamed one, and rule 1's "one logical
+# asset" is checked as one-file-per-reference -- an exporter that split a sprite
+# and referenced both halves would pass. These are the rules' checkable shadows.
+set -euo pipefail
+cd "${PROJECT_DIR:-/work}"
+EXPORT="${EXPORT:-export}"
+fail=0
+note() { printf '  %-6s %s\n' "$1" "$2"; [ "$1" = FAIL ] && fail=1 || true; }
+
+echo "MODDING rule 1 -- one logical asset, one file"
+python3 - "$EXPORT" <<'PY'
+import json, glob, os, sys
+E = sys.argv[1]
+ref = set()
+for p in glob.glob(f"{E}/screens/*/*.json"):
+    d = json.load(open(p))
+    for e in d["elements"]:
+        for k in ("sprite", "focus_sprite"):
+            if e.get(k): ref.add(e[k])
+        for sub in ("focus", "leaf"):
+            for fe in (e.get(sub) or {}).get("elements", []):
+                if fe.get("sprite"): ref.add(fe["sprite"])
+files = {os.path.relpath(p, E) for p in glob.glob(f"{E}/sprites/**/*.png", recursive=True)}
+missing, orphan = sorted(ref - files), sorted(files - ref)
+split = [f for f in files if any(t in os.path.basename(f) for t in ("part", "seg", "chunk"))]
+print("  %-6s %d sprites referenced, %d present" % ("OK" if not (missing or orphan) else "FAIL", len(ref), len(files)))
+if missing: print("  FAIL   referenced but absent:", missing[:5])
+if orphan:  print("  FAIL   present but unreferenced:", orphan[:5])
+if split:   print("  FAIL   split-looking names:", split[:5])
+sys.exit(1 if (missing or orphan or split) else 0)
+PY
+[ $? -eq 0 ] || fail=1
+
+echo "MODDING rule 2 -- names a person recognises"
+hex=$(find "$EXPORT" -type f | grep -Ec '0x[0-9a-f]{6,}|/[0-9a-f]{8}\.' || true)
+[ "$hex" -eq 0 ] && note OK "no hex or hash-shaped filenames" || note FAIL "$hex hash-shaped names"
+
+echo "MODDING rule 3 -- modern, editable formats only"
+bad=$(find "$EXPORT" -type f | sed 's/.*\.//' | sort -u | grep -vE '^(json|png|ogg|ogv|cmd)$' || true)
+[ -z "$bad" ] && note OK "only json/png/ogg/ogv (+ .cmd sidecars)" || note FAIL "unexpected: $(echo $bad)"
+# A `.cmd` is not an asset. It is allowed only because it SAYS SO in its own
+# first line -- see video.rs. An unlabelled one reads as something to edit.
+for c in $(find "$EXPORT" -name '*.cmd'); do
+  head -1 "$c" | grep -q '^# Generated by sylpheed-export' \
+    && note OK "$(basename "$c") is self-describing" \
+    || note FAIL "$(basename "$c") has no header saying what it is"
+done
+
+echo "MODDING rule 4 -- base and overrides, never one merged pile"
+grep -q 'data/mods' .gitignore && note OK "data/mods contents are gitignored" \
+  || note FAIL "data/mods is not gitignored -- a mod is usually a game asset"
+grep -rq 'data/mods' crates/sylpheed-export/src/ && note OK "the exporter resolves overrides" \
+  || note FAIL "nothing reads data/mods"
+
+echo "MODDING rule 5 -- provenance in every generated file"
+python3 - "$EXPORT" <<'PY'
+import json, glob, os, sys
+E = sys.argv[1]; miss = []
+for p in sorted(glob.glob(f"{E}/**/*.json", recursive=True)):
+    d = json.load(open(p))
+    if os.path.basename(p) == "manifest.json":
+        # The manifest is the provenance -- it carries disc, exporter and the
+        # formats revision for the whole tree, so it has no `source` of its own.
+        if not all(k in d for k in ("disc", "exporter", "formats_rev")): miss.append(p)
+        continue
+    src = d.get("source") or {}
+    if not (isinstance(src, dict) and src): miss.append(p)
+print("  %-6s %d json files carry provenance" % ("OK" if not miss else "FAIL",
+      len(glob.glob(f"{E}/**/*.json", recursive=True)) - len(miss)))
+for m in miss[:5]: print("  FAIL  ", m)
+sys.exit(1 if miss else 0)
+PY
+[ $? -eq 0 ] || fail=1
+
+echo
+[ $fail -eq 0 ] && echo "all five rules pass" || echo "🔴 a MODDING rule is broken"
+exit $fail