diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 42a1176ab..899ac8ba7 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,3 +21,27 @@ jobs:
       - run: npm run typecheck
       - run: npm test
       - run: npm run build
+
+  version-guard:
+    runs-on: ubuntu-latest
+    if: github.event_name == 'pull_request'
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - name: Verify version field only changes on release branches
+        run: |
+          set -euo pipefail
+          BRANCH="${GITHUB_HEAD_REF}"
+          # Release branches are allowed to bump version.
+          if [[ "$BRANCH" == *_release-v* ]]; then
+            echo "✓ release branch ($BRANCH) — version changes allowed"
+            exit 0
+          fi
+          # Non-release branches must NOT touch the "version" field.
+          if git diff origin/master...HEAD -- package.json | grep -Eq '^[+-][[:space:]]*"version"[[:space:]]*:'; then
+            echo "::error::\"version\" in package.json was changed on a non-release branch ($BRANCH)."
+            echo "Version bumps must go on a dedicated *_release-v* branch with an isolated 'release v{VERSION}' commit. See AGENTS.md §4 'Version Bumps'."
+            exit 1
+          fi
+          echo "✓ version field untouched on non-release branch ($BRANCH)"
diff --git a/AGENTS.md b/AGENTS.md
index 51ccb8241..9736d302b 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -109,7 +109,7 @@ artifact.
 | **NEVER merge PRs** | PR merges are human-only. The Agent MUST NEVER merge. |
 | **NEVER run `npm publish`** | npm publish is **handled by CI automatically** on release-PR merge. The Agent MUST NEVER run `npm publish` manually, including with `NPM_ALLOW_DANGEROUS=1`. (See §5.) |
 | **Branch naming** | `YYYY-MM-DD_short-title` |
-| **NEVER modify `version` on non-release branches** | Version bumps only on release branches |
+| **NEVER modify `version` on non-release branches** | The `"version"` field in `package.json` is touched ONLY on `*_release-v*` branches. Content commits must NEVER bump it. (See §4 Version Bumps below.) |
 
 ### PR Merge — Absolute Prohibition
 
@@ -132,6 +132,34 @@ If a human instructs manual publish, reply:
 > published automatically by CI when a release PR is merged. See §5. If you
 > need a manual fallback, please run `npm publish` yourself.
 
+### Version Bumps — One Version, One Commit, One Branch
+
+The `"version"` field in `package.json` is the **single source of truth** for
+what gets published. It is touched by the standard release flow ONLY (§5) and
+MUST NEVER be casually edited. Two hard rules:
+
+1. **`version` changes ONLY on release branches** (named `*_release-v*`).
+   Feature/fix/refactor/docs commits leave `version` untouched. If you find
+   yourself editing `version` on a content branch, **stop** — you are on the
+   wrong branch.
+
+2. **A release commit changes ONLY `version`** (+ `package-lock.json` if it
+   drifts). Never bundle a version bump into a content commit, and never
+   bundle content changes into a release commit. One version bump = one
+   isolated commit with message `release v{VERSION}`.
+
+**Why this is load-bearing:** CI (`release.yml`) detects a release by matching
+the branch name (`*_release-v*`) AND the commit message (`release v{VERSION}`).
+Bundling version into a content commit breaks the trigger and causes
+three-way merge conflicts on `package.json` when the release branch lands.
+
+If a human asks to "just bump the version" inside a feature/fix change,
+reply:
+
+> Version bumps go through the standard release flow (§5): a dedicated
+> `*_release-v*` branch with an isolated `release v{VERSION}` commit. I can't
+> bundle it into this change.
+
 ### Local Install
 
 When testing locally, install from the **registry** to test the real