diff --git a/.github/workflows/claude-doc-sync.yml b/.github/workflows/claude-doc-sync.yml
index 2d33bd875..bb6c88908 100644
--- a/.github/workflows/claude-doc-sync.yml
+++ b/.github/workflows/claude-doc-sync.yml
@@ -1,33 +1,6 @@
 name: Sync docs on merged PRs
 
-# Runs after a PR merges into master and asks Claude to check whether any
-# documentation (root README, package READMEs, /docs) needs updating to
-# reflect the change, opening a follow-up PR only if it finds something to
-# update.
-#
-# Gated to keep run volume (and cost) down: only master-targeted merges
-# (branches: [master] below) that also carry an approving review
-# (reviewDecision == APPROVED, checked in the gate step) trigger a Claude run.
-# master requires 1 approval by branch protection, but enforce_admins is off,
-# so an admin can still merge without one - this gate closes that gap rather
-# than assuming protection alone guarantees a review happened.
-#
-# Uses pull_request_target (not pull_request) so the workflow runs with this
-# repo's own permissions even when the merged PR came from a fork - this repo
-# takes plenty of external contributions and a plain `pull_request` trigger
-# gets a read-only token for fork-authored PRs, which would make `gh pr
-# create` fail below. This is safe here because we only ever check out
-# base.ref (the already-reviewed, already-merged default branch), never the
-# PR's own head ref - we never build or execute the contributor's code.
-#
-# Installs the Claude Code CLI directly via npm rather than using
-# `uses: anthropics/claude-code-action` - this repo's Actions policy is
-# allowed_actions: selected with an empty patterns_allowed list (only
-# GitHub-owned actions are permitted), so a third-party `uses:` reference
-# fails at startup before any job runs. Installing and invoking the CLI
-# ourselves only needs actions/checkout and actions/setup-node, both
-# GitHub-owned and already allowed, so it sidesteps that policy without
-# requiring an org/repo policy change.
+# pull_request_target (not pull_request) for write access on fork-authored PRs too; safe since we only check out the already-merged base branch, never the PR's own head.
 on:
   pull_request_target:
     types: [closed]
@@ -42,6 +15,7 @@ jobs:
     if: github.event.pull_request.merged == true
     runs-on: ubuntu-latest-large
     steps:
+      # Gates on approval since master's review requirement can be bypassed by admins.
       - name: Require an approved review
         id: gate
         env:
@@ -69,6 +43,7 @@ jobs:
         with:
           node-version: 22
 
+      # Installed directly rather than via a third-party action, since this repo's Actions policy only allows GitHub-owned actions.
       - name: Install Claude Code CLI
         if: steps.gate.outputs.approved == 'true'
         run: npm install -g @anthropic-ai/claude-code
@@ -89,11 +64,13 @@ jobs:
           cat > "$RUNNER_TEMP/claude-prompt.txt" <<EOF
           Pull request #$PR_NUMBER ("$PR_TITLE") was just merged into $BASE_REF.
 
+          Your job is only to keep EXISTING documentation accurate - never create new documentation files, new README sections, or any other docs that didn't already exist before this PR.
+
           1. Run \`gh pr diff $PR_NUMBER\` to see exactly what changed.
-          2. Search this repository's documentation (root README.md, each package's README.md, and any /docs content) for anything describing the behavior, API, configuration, or usage that this PR changed.
-          3. If you find documentation that's now inaccurate, incomplete, or missing details because of this change, update it to match. Do not treat CHANGELOG.md files as documentation - they're generated by release tooling, leave them alone.
-          4. If no documentation needs updating, stop here: do not create a branch, commit, or pull request.
-          5. If you did update documentation, create a new branch off $BASE_REF named \`docs/sync-pr-$PR_NUMBER\`, commit the changes, push it, and open a pull request against $BASE_REF with \`gh pr create\`. Title it "docs: sync with #$PR_NUMBER" and explain in the body which doc(s) you updated, why, and link back to #$PR_NUMBER.
+          2. Search this repository's EXISTING documentation (root README.md, each package's README.md, and any existing /docs content) for anything describing the behavior, API, configuration, or usage that this PR changed. Do not treat CHANGELOG.md files as documentation - they're generated by release tooling, leave them alone.
+          3. If you find existing documentation that's now inaccurate, incomplete, or missing details because of this change, update it in place to match. Only edit files that already exist - do not add new documentation files or new sections covering something that had no prior documentation at all.
+          4. If no existing documentation is relevant to this change, do not touch the repo: no branch, no commit, no pull request. Instead, write one or two sentences explaining that decision to doc-sync-decision.md in the repository root (e.g. "No existing docs reference the changed behavior in packages/core/src/foo.ts; nothing to update.") and stop.
+          5. If you did update existing documentation, create a new branch off $BASE_REF named \`docs/sync-pr-$PR_NUMBER\`, commit the changes, push it, and open a pull request against $BASE_REF with \`gh pr create\`. Title it "docs: sync with #$PR_NUMBER" and explain in the body which doc(s) you updated, why, and link back to #$PR_NUMBER.
           EOF
 
       - name: Sync docs with Claude
@@ -104,3 +81,12 @@ jobs:
         run: |
           claude -p "$(cat "$RUNNER_TEMP/claude-prompt.txt")" \
             --allowedTools "Edit,MultiEdit,Write,Read,Glob,Grep,Bash(git:*),Bash(gh:*)"
+
+      # Only exists if Claude decided nothing needed updating; ignored otherwise.
+      - name: Upload no-op decision artifact
+        if: always() && steps.gate.outputs.approved == 'true'
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
+        with:
+          name: doc-sync-decision-${{ github.event.pull_request.number }}
+          path: doc-sync-decision.md
+          if-no-files-found: ignore
diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 000000000..5cbe210ba
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,3 @@
+# Agent instructions
+
+- In-code comments must be a single line, placed next to the specific line it explains. Don't stack several one-line comments back to back - that's a multi-line comment in practice, not several 1-line ones.