diff --git a/.cursor/rules/git-commit-signing.mdc b/.cursor/rules/git-commit-signing.mdc
index 4e644e97c..641dbadfd 100644
--- a/.cursor/rules/git-commit-signing.mdc
+++ b/.cursor/rules/git-commit-signing.mdc
@@ -32,4 +32,4 @@ The same applies to `git commit --amend`: pass `-s -S` (or `--amend --no-edit -s
 
 Verify with `git log -1 --show-signature` after committing; the output should show a valid signature **and** the `Signed-off-by:` trailer.
 
-See [`contrib/claude/commit.md`](../../contrib/claude/commit.md) for full commit conventions.
+See [`contrib/claude/commit.md`](../../contrib/claude/commit.md) for full commit conventions and [`.cursor/rules/git-commit-style.mdc`](git-commit-style.mdc) for message format rules (seven-rules style, no Conventional Commits).
diff --git a/.cursor/rules/git-commit-style.mdc b/.cursor/rules/git-commit-style.mdc
new file mode 100644
index 000000000..0b5abbd50
--- /dev/null
+++ b/.cursor/rules/git-commit-style.mdc
@@ -0,0 +1,60 @@
+---
+description: Git commit message style — seven rules, no Conventional Commits
+alwaysApply: true
+---
+
+# Git commit message style
+
+This repository follows the [seven rules of a great Git commit message](https://cbea.ms/git-commit/) (see [`contrib/claude/commit.md`](../../contrib/claude/commit.md)). **Do not use Conventional Commits** (`type(scope): summary`); the contrib guide explicitly prescribes the seven-rules format, and Conventional-Commits-style messages in the history are drift, not precedent.
+
+## The seven rules
+
+1. Separate subject from body with a blank line
+2. Limit the subject line to 50 characters
+3. Capitalize the subject line
+4. Do not end the subject line with a period
+5. Use the imperative mood in the subject line
+6. Wrap the body at 72 characters
+7. Use the body to explain *what* and *why* vs. *how*
+
+The subject must complete the sentence "If applied, this commit will …".
+
+## Good vs bad
+
+```text
+# GOOD — capitalized imperative subject, no type prefix, body explains why
+Promote glob source and trigger draft on adoption
+
+The pattern-analysis worker dropped two signals on every run. ...
+
+# GOOD — single-line is fine when the change is self-explanatory
+Fix typo in third-party assessment prompt
+
+# BAD — Conventional Commits prefix
+fix(cookie-banner): disconnect observer in load() error path
+
+# BAD — Conventional Commits prefix without scope
+feat: add detection for browser-extension frames
+
+# BAD — lowercased subject
+promote glob source and trigger draft on adoption
+
+# BAD — subject ends with a period
+Promote glob source and trigger draft on adoption.
+
+# BAD — past tense / non-imperative subject
+Promoted glob source and triggered draft on adoption
+```
+
+## Why no Conventional Commits
+
+- The project does not consume the `type(scope):` prefix for tooling (no changelog generator, no semantic-release, no commit-lint).
+- Mixing the two styles in `git log` hurts readability for humans skimming history.
+- The seven-rules style carries strictly more information when written well, because the subject is a real sentence rather than a typed token.
+
+If a future need for machine-readable commit types arises, raise it in a separate change that updates [`contrib/claude/commit.md`](../../contrib/claude/commit.md) first, then this rule. Do not start drifting unilaterally.
+
+## Related
+
+- [`.cursor/rules/git-commit-signing.mdc`](git-commit-signing.mdc) — every commit must be signed with `-s -S`.
+- [`contrib/claude/commit.md`](../../contrib/claude/commit.md) — full commit conventions.
diff --git a/contrib/claude/commit.md b/contrib/claude/commit.md
index 9594e03b0..e372169e6 100644
--- a/contrib/claude/commit.md
+++ b/contrib/claude/commit.md
@@ -27,6 +27,20 @@ Not every commit needs a body -- a single line is fine when the change is self-e
 Fix typo in third-party assessment prompt
 ```
 
+## No Conventional Commits
+
+This repository **does not** use Conventional Commits (`type(scope): summary`). The seven-rules style above is the only accepted format. Existing Conventional-Commits-style messages in the history are drift and must not be used as precedent.
+
+```text
+# GOOD
+Disconnect observer in cookie-banner load() error path
+
+# BAD -- Conventional Commits prefix
+fix(cookie-banner): disconnect observer in load() error path
+```
+
+The project does not consume the `type(scope):` prefix for any tooling (no changelog generator, no semantic-release, no commit-lint), so the prefix only adds noise. If a future need for machine-readable commit types arises, raise it in a separate change that updates this document first.
+
 ## Signing and Authorship
 
 All commits **must** be signed (`-s -S`):