diff --git a/.claude/rules/comments.md b/.claude/rules/comments.md
new file mode 100644
index 0000000..77a2b11
--- /dev/null
+++ b/.claude/rules/comments.md
@@ -0,0 +1,52 @@
+---
+paths:
+  - "src/**"
+  - "test/**"
+  - "scripts/**"
+---
+
+# Comments
+
+A code comment has exactly one purpose: to help a future maintainer change
+this code correctly, by carrying the constraints the code and tests cannot
+carry themselves. A comment addressed to anyone else (a reviewer, a diff
+reader, the author's process) does not belong in the source.
+
+A comment may only state a constraint the code cannot show: an external fact
+(a library's hidden behavior, a remote API quirk, a verified real-world
+payload) or the why of a deliberately surprising choice. Never write a comment
+that:
+
+- Explains what the adjacent code plainly shows, or restates a test name, an
+  assertion, or a log line next to it.
+- Tells past-tense history, justifies a change, or talks to a reviewer ("used
+  to", "previously", "fixed", "split out from", "now", referencing a bug story
+  or issue number): the next reader sees only the final code.
+- Guards a behavior a test could pin: if the constraint is testable and
+  untested, write the test, not the comment. A comment survives alongside a
+  test only where the code locally reads as a mistake (error swallowing, odd
+  ordering); then say why, clearly.
+- Pads with redundant contrast ("X, not Y" where Y is merely not-X): state
+  the constraint once, positively.
+
+No literal em or en dashes in any tracked .ts file, comments and
+user-facing strings included (scripts/check-dashes.sh gates the staged
+content locally and in CI); use a comma, colon, or parenthetical. A fixture
+or quoted payload that genuinely needs the character uses the \u2013 or
+\u2014 escape, which the gate does not match.
+
+Every comment must pass the strip test: remove it, and if a competent reader
+could recover the content from the code, the names, the contracts of the
+functions called, and the tests, it should not exist. A causal veneer
+("so ...", "because ...") does not turn narration into a why. A comment sits
+ON the line it explains. Show, don't tell: real payloads and verified
+behaviors belong in test fixtures, and once a test pins a constraint, a
+comment restating it dies. Length is never the criterion: an
+unneeded explanation is noise at any length, and a needed one gets however
+many words it takes to explain clearly. A long comment on something simple is a
+smell that warrants checking whether the content is needed at all, never a
+violation by itself. The file's existing comment density
+is not a license: judge each comment alone, and prefer a clearer name or a
+test over any comment. When a review-refuted finding keeps resurfacing, first
+try to make the code read right; an intent pin at the flagged site is the
+last resort.
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 23f46f9..ad02576 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -13,6 +13,8 @@ jobs:
       - uses: oven-sh/setup-bun@v2
       - run: bun install --frozen-lockfile
       - run: bunx oxlint --deny-warnings
+      # same dash gate as the local hook, for pushes that bypassed it
+      - run: ./scripts/check-dashes.sh
       # db.ts opens /storage/mp4ify.db and blob-store.ts creates /storage/blobs
       # at import time, so /storage must exist and be writable
       - run: sudo mkdir -p -m 777 /storage
diff --git a/check.sh b/check.sh
index 71d49c0..0bf5101 100755
--- a/check.sh
+++ b/check.sh
@@ -13,6 +13,8 @@ else
   exit 1
 fi
 
+./scripts/check-dashes.sh
+
 # `timeout` runs inside the container, so a hung/non-exiting `bun test` (a leaked
 # handle, a runaway loop) self-kills and `--rm` cleans up, instead of leaving an
 # orphaned container pinning a CPU. -k force-kills if SIGTERM is ignored.
diff --git a/scripts/check-dashes.sh b/scripts/check-dashes.sh
new file mode 100755
index 0000000..ae56eec
--- /dev/null
+++ b/scripts/check-dashes.sh
@@ -0,0 +1,23 @@
+#!/bin/sh
+# En/em dashes are banned in .ts source (see .claude/rules/comments.md).
+# git grep --cached scans the INDEX, i.e. exactly what a commit would record
+# (the worktree can differ both ways), and the '*.ts' pathspec tracks every
+# .ts file wherever it lives, so a new top-level dir can't dodge the gate.
+# LC_ALL=C pins byte-wise PCRE: \x{2013}-style escapes error under C locales
+# while \xe2-style bytes mis-match under UTF-8 mode. The case turns a scan
+# ERROR (e.g. a git without PCRE support) into a loud failure, never a
+# silent pass.
+st=0
+dashes=$(LC_ALL=C git grep --cached -nP '\xe2\x80[\x93\x94]' -- '*.ts') || st=$?
+case $st in
+  0)
+    printf '%s\n' "$dashes"
+    echo "em/en dash found (use a comma, colon, or parenthetical instead)" >&2
+    exit 1
+    ;;
+  1) ;;
+  *)
+    echo "em/en dash scan failed to run" >&2
+    exit 1
+    ;;
+esac