diff --git a/.github/workflows/CLAUDE.md b/.github/workflows/CLAUDE.md
index a7a27a9..04bdfa5 100644
--- a/.github/workflows/CLAUDE.md
+++ b/.github/workflows/CLAUDE.md
@@ -8,6 +8,32 @@
   for all test steps (PostgreSQL matrix, checkouts, git config, pgtap install, etc.).
   Called by both `ci.yml` here and by `pgxntool/ci.yml` for the commit-with-no-tests path.
 
+## Branch pairing and account matching
+
+A change to either repo should normally ship with a **matching branch (same
+name) on the same account** in the other repo. CI resolves the sibling using
+both the branch name **and** the PR head's account:
+
+- **pgxntool PRs** must have a paired pgxntool-test PR (matched by branch name +
+  head account) — enforced by `ci.yml`'s `check-test-pr`. The only escape is the
+  `commit-with-no-tests` label, which a maintainer applies (guarded by
+  `protect-label.yml`); then pgxntool is tested against pgxntool-test master.
+- **pgxntool-test PRs** may stand alone (test-only changes are a normal pattern).
+  `resolve` looks for a paired pgxntool branch on the PR head's account; if none
+  exists, it falls back to pgxntool master.
+
+### Never cross-account (security)
+
+The sibling branch is matched **only on the PR head's own account**. We never
+use a same-named branch from any other account — including Postgres-Extensions —
+to pair with a fork's branch. The **sole** cross-account exception is `master`:
+when there's no paired branch, the other repo is taken from
+**`Postgres-Extensions/master` only**, never the fork's master (which may be
+stale or deliberately modified). This is why `run-tests.yml` takes explicit
+`pgxntool-owner` / `pgxntool-test-owner` inputs: the caller resolves the correct
+account for each repo (head account for the paired side, `Postgres-Extensions`
+for a master fallback) rather than assuming everything lives under one owner.
+
 ## Cross-repo reusable workflow — tradeoffs and constraints
 
 `run-tests.yml` is referenced from **pgxntool** as:
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 5cbacf7..28ba601 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,15 +21,22 @@ jobs:
     runs-on: ubuntu-latest
     outputs:
       pgxntool-ref: ${{ steps.pgxntool-ref.outputs.ref }}
+      pgxntool-owner: ${{ steps.pgxntool-ref.outputs.owner }}
 
     steps:
       - name: Resolve pgxntool branch
         id: pgxntool-ref
-        # PR-controlled values (head_ref) are passed via env, never interpolated
-        # directly into the script, to avoid shell injection.
+        # PR-controlled values (head_ref, head owner) are passed via env, never
+        # interpolated directly into the script, to avoid shell injection.
         env:
           BRANCH: ${{ github.head_ref }}
-          REPO_OWNER: ${{ github.repository_owner }}
+          # The account this PR's branch lives on: the contributor's fork for
+          # fork PRs, or Postgres-Extensions for a maintainer branch.
+          HEAD_OWNER: ${{ github.event.pull_request.head.repo.owner.login }}
+          # This PR's own pgxntool-test is checked out from the base repo at the
+          # merge SHA — used only to print an accurate BRANCHES line below.
+          TEST_OWNER: ${{ github.repository_owner }}
+          TEST_SHA: ${{ github.sha }}
         run: |
           # Reject anything that isn't a valid git branch name before using it.
           git check-ref-format --branch "$BRANCH" >/dev/null 2>&1 || {
@@ -37,20 +44,30 @@ jobs:
             exit 1
           }
 
-          # Check whether pgxntool has a branch with the same name as this PR.
-          # If it does, use it — changes in this test PR may depend on changes
-          # in a paired pgxntool branch. If not, fall back to master.
+          # Look for a paired pgxntool branch of the SAME name on the SAME
+          # account (HEAD_OWNER). A pgxntool change should ship with a matching
+          # pgxntool-test branch and vice versa; if this test PR has one, use it.
+          #
+          # SECURITY — never cross-account: we only ever match a branch on
+          # HEAD_OWNER. If there's no paired branch (e.g. a pgxntool-test-only
+          # PR), we fall back to pgxntool master from Postgres-Extensions ONLY,
+          # never the fork's master (which may be stale or modified) and never a
+          # same-named branch on any other account. master is the sole ref we
+          # take cross-account.
           #
           # --exit-code makes git ls-remote return non-zero when the ref is
           # absent, so we can branch on the result cleanly without parsing output.
           if git ls-remote --exit-code --heads \
-               "https://github.com/${REPO_OWNER}/pgxntool.git" \
+               "https://github.com/${HEAD_OWNER}/pgxntool.git" \
                "refs/heads/${BRANCH}" > /dev/null 2>&1; then
             echo "ref=${BRANCH}" >> "$GITHUB_OUTPUT"
-            echo "=== BRANCHES: pgxntool-test=${BRANCH} pgxntool=${BRANCH} ==="
+            echo "owner=${HEAD_OWNER}" >> "$GITHUB_OUTPUT"
+            # Same shape/order as run-tests.yml: pgxntool first, owner/ref for each.
+            echo "=== BRANCHES: pgxntool=${HEAD_OWNER}/${BRANCH} pgxntool-test=${TEST_OWNER}/${TEST_SHA} ==="
           else
             echo "ref=master" >> "$GITHUB_OUTPUT"
-            echo "=== BRANCHES: pgxntool-test=${BRANCH} pgxntool=master (branch not found in pgxntool) ==="
+            echo "owner=Postgres-Extensions" >> "$GITHUB_OUTPUT"
+            echo "=== BRANCHES: pgxntool=Postgres-Extensions/master pgxntool-test=${TEST_OWNER}/${TEST_SHA} (no paired pgxntool branch on ${HEAD_OWNER}) ==="
           fi
 
   test:
@@ -60,5 +77,11 @@ jobs:
     # the cross-repo reusable workflow tradeoffs and merge order constraints.
     uses: ./.github/workflows/run-tests.yml
     with:
+      # pgxntool: the paired branch on this PR's account, or Postgres-Extensions
+      # master when there's no pairing (resolved above).
+      pgxntool-owner: ${{ needs.resolve.outputs.pgxntool-owner }}
       pgxntool-branch: ${{ needs.resolve.outputs.pgxntool-ref }}
+      # pgxntool-test: this PR's own merged code, which lives on the base repo
+      # (github.repository_owner) at the merge SHA.
+      pgxntool-test-owner: ${{ github.repository_owner }}
       pgxntool-test-ref: ${{ github.sha }}
diff --git a/.github/workflows/run-tests.yml b/.github/workflows/run-tests.yml
index 19c3e6c..04c101f 100644
--- a/.github/workflows/run-tests.yml
+++ b/.github/workflows/run-tests.yml
@@ -14,6 +14,16 @@ on:
         description: 'The pgxntool branch to check out and use as the local branch name'
         type: string
         required: true
+      pgxntool-owner:
+        description: 'GitHub account that owns the pgxntool repo to check out. Defaults to this repo owner. Set to the PR head account so a fork PR pulls its paired branch from the same fork.'
+        type: string
+        required: false
+        default: ''
+      pgxntool-test-owner:
+        description: 'GitHub account that owns the pgxntool-test repo to check out. Defaults to this repo owner.'
+        type: string
+        required: false
+        default: ''
 
 jobs:
   test:
@@ -23,7 +33,9 @@ jobs:
     # Caller-controlled inputs are exposed as env vars and referenced as $VARs
     # in run: blocks below, never interpolated directly into the shell.
     env:
+      PGXNTOOL_OWNER: ${{ inputs.pgxntool-owner || github.repository_owner }}
       PGXNTOOL_BRANCH: ${{ inputs.pgxntool-branch }}
+      PGXNTOOL_TEST_OWNER: ${{ inputs.pgxntool-test-owner || github.repository_owner }}
       PGXNTOOL_TEST_REF: ${{ inputs.pgxntool-test-ref }}
     strategy:
       matrix:
@@ -39,13 +51,13 @@ jobs:
           # are easy to correlate to the right code. This is especially useful
           # when debugging cross-repo issues where the wrong branch was picked.
           # The === BRANCHES: === line is parsed by the CI monitor script.
-          echo "=== BRANCHES: pgxntool=${PGXNTOOL_BRANCH} pgxntool-test=${PGXNTOOL_TEST_REF} ==="
+          echo "=== BRANCHES: pgxntool=${PGXNTOOL_OWNER}/${PGXNTOOL_BRANCH} pgxntool-test=${PGXNTOOL_TEST_OWNER}/${PGXNTOOL_TEST_REF} ==="
 
       - name: Check out pgxntool-test
         uses: actions/checkout@v4
         with:
           persist-credentials: false
-          repository: ${{ github.repository_owner }}/pgxntool-test
+          repository: ${{ inputs.pgxntool-test-owner || github.repository_owner }}/pgxntool-test
           ref: ${{ inputs.pgxntool-test-ref }}
           path: pgxntool-test
           # REQUIRED: pgxntool-test includes BATS as a git submodule at
@@ -59,7 +71,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           persist-credentials: false
-          repository: ${{ github.repository_owner }}/pgxntool
+          repository: ${{ inputs.pgxntool-owner || github.repository_owner }}/pgxntool
           ref: ${{ inputs.pgxntool-branch }}
           path: pgxntool
           submodules: false  # pgxntool has no submodules
diff --git a/README.md b/README.md
index 94e6857..309274d 100644
--- a/README.md
+++ b/README.md
@@ -101,31 +101,33 @@ Each test file automatically runs its prerequisites if needed, so they can be ru
 
 `pgxntool` is the framework itself; `pgxntool-test` is the test harness for it. They are kept separate because `pgxntool` is embedded into extension projects via `git subtree` — you don't want test infrastructure polluting those projects. The CI is designed to coordinate changes across both repos.
 
-A pgxntool change **should almost always have a corresponding test change** in pgxntool-test. Commits touching only pgxntool (with no test changes) should be rare. It is also normal and common to have commits that only touch pgxntool-test (e.g., improving test coverage) with no corresponding pgxntool change — as long as tests pass, that's fine.
+**The norm is that a change is paired across both repos** — it comes with a matching branch (same name, same account) in the other repo. This is effectively required for `pgxntool` changes: a pgxntool change should almost always have a corresponding pgxntool-test change, and CI blocks an unpaired pgxntool PR unless a maintainer applies the `commit-with-no-tests` label (see below). A pgxntool-test-only change (with no pgxntool counterpart) is the occasional exception — for example, improving test coverage or infrastructure — and CI runs it against `pgxntool` master. Neither repo should be changed in isolation as a matter of routine.
 
 ### PR conventions
 
-When your change requires modifications to both repos, open PRs in **both repos using the same branch name**. For example, if your feature branch is named `feature/add-pgtle-support`, create that branch in both `pgxntool` and `pgxntool-test`. **Branch names must match exactly** — the CI uses the branch name to find the paired PR.
+A change normally touches both repos, so open a PR in **each repo from a branch with the same name, on the same GitHub account** (your fork). For example, if your feature branch is named `feature/add-pgtle-support`, push it to both your `pgxntool` and `pgxntool-test` forks and open a PR from each. **The branch name *and* the account must match** — CI pairs the two PRs using both. (The exception is a pgxntool-test-only change, which needs no pgxntool PR; see above.)
 
 ### How CI works
 
 **When you open a PR in `pgxntool-test`:**
-CI checks whether `pgxntool` has a branch with the same name. If it does, tests run against that pgxntool branch. If not, tests run against `pgxntool/master`. Results appear directly on your pgxntool-test PR.
+CI looks for a `pgxntool` branch with the same name **on your account** (the PR's head fork). If it exists, tests run against that branch; if not, they run against `pgxntool` master **from Postgres-Extensions**. Results appear directly on your pgxntool-test PR.
 
 **When you open a PR in `pgxntool`:**
-CI waits for the paired pgxntool-test PR's CI to complete (polling for up to 20 minutes), then checks whether it passed. pgxntool CI does not run tests itself — it relies entirely on the pgxntool-test CI results.
+CI waits for the paired pgxntool-test PR (matched by branch name **and** account) to complete (polling for up to 20 minutes), then checks whether it passed. pgxntool CI does not run tests itself — it relies entirely on the pgxntool-test CI results.
 
 - **If a paired test PR is found and its CI passes**: pgxntool CI passes. There is no test duplication.
 - **If no paired test PR is found**: pgxntool CI fails (see below).
 
+> **Fork contributors — security note:** CI only ever pairs branches within the **same account**. It will never match your fork's branch to a same-named branch on a different account (including Postgres-Extensions). When there is no paired branch, the *other* repo is always taken from **`Postgres-Extensions/master`** — never a fork's `master` — so a stale or modified fork `master` can't influence the run. `master` is the only ref ever taken cross-account.
+
 ### What to do when pgxntool CI fails with "No paired test PR found"
 
-This failure means CI couldn't find an open PR in pgxntool-test with a matching branch name. The fix is almost always:
+This failure means CI couldn't find an open pgxntool-test PR **from your account** with a matching branch name. The fix is almost always:
 
-1. **Open a PR in pgxntool-test** from a branch with the **same name** as your pgxntool branch.
+1. **Open a PR in pgxntool-test** from a branch with the **same name, on the same account (your fork)** as your pgxntool branch.
 2. Re-run the failing CI check on your pgxntool PR (or push a new commit to trigger it).
 
-**Branch names must match exactly.** If your pgxntool branch is `fix/parse-bug`, your pgxntool-test branch must also be `fix/parse-bug`.
+**The branch name and account must match exactly.** If your pgxntool branch is `fix/parse-bug` on `your-account`, your pgxntool-test branch must also be `fix/parse-bug` on `your-account`.
 
 ### The `commit-with-no-tests` label
 
@@ -143,7 +145,7 @@ To request the label:
 ### Branch protection
 
 The `check-test-pr` status check on pgxntool is a required check for merging to `master`. It only passes when either:
-- A corresponding pgxntool-test PR exists (with a matching branch name) and its tests are **passing**, or
+- A corresponding pgxntool-test PR exists (matching branch name **and** account) and its tests are **passing**, or
 - A maintainer has applied the `commit-with-no-tests` label.
 
 This ensures pgxntool changes cannot be merged without passing test coverage.