diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index 3e1efe07..97340dbf 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -3,7 +3,10 @@ name: Publish
 on:
   push:
     tags: ['v*']
-  workflow_dispatch: {}
+  # Packaging check for every PR targeting the release branch: build the
+  # sdist/wheel and publish to TestPyPI before a release tag is cut.
+  pull_request:
+    branches: [master]
 
 permissions:
   contents: write
@@ -44,7 +47,7 @@ jobs:
 
   publish-testpypi:
     needs: build
-    if: github.event_name == 'workflow_dispatch'
+    if: github.event_name == 'pull_request'
     runs-on: ubuntu-latest
     steps:
       - name: Download dist
@@ -53,11 +56,15 @@ jobs:
           name: dist
           path: dist/
 
-      - name: Publish to TestPyPI
+      - name: Publish to TestPyPI (Trusted Publishing / OIDC)
         uses: pypa/gh-action-pypi-publish@release/v1
         with:
-          repository-url: https://test.pypi.org/
-          password: ${{ secrets.TEST_PYPI_API_TOKEN }}
+          repository-url: https://test.pypi.org/legacy/
+          # No `password`: uses OIDC Trusted Publishing configured on
+          # TestPyPI for the pull_request subject claim.
+          # Concurrent PRs share the same package version and TestPyPI
+          # rejects re-uploads of existing files; skip instead of failing.
+          skip-existing: true
 
   publish-pypi:
     needs: build
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 964743fa..d424de1a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- The publish workflow now runs on every pull request targeting `master`: it
+  builds the sdist and wheel and publishes them to TestPyPI (`skip-existing`),
+  validating packaging before a release tag is cut.
+
 ## [5.0.0] - 2026-09-16
 
 ### Added
diff --git a/MANIFEST.in b/MANIFEST.in
new file mode 100644
index 00000000..e534d459
--- /dev/null
+++ b/MANIFEST.in
@@ -0,0 +1 @@
+include requirements/*.txt
diff --git a/docs/release.rst b/docs/release.rst
index 7eb35198..8a27d913 100644
--- a/docs/release.rst
+++ b/docs/release.rst
@@ -35,8 +35,14 @@ The ``publish`` workflow (``.github/workflows/publish.yml``) runs when a
    ``PYPI_API_TOKEN`` secret is supported as a fallback.
 4. Creates a GitHub Release with the matching changelog section.
 
-A manual **TestPyPI** run is available from *Actions → Publish → Run workflow*
-(uses a ``TEST_PYPI_API_TOKEN`` secret) to validate before a real release.
+On every pull request targeting ``master``, the same build step runs and the
+artifacts are published to **TestPyPI** via Trusted Publishing (OIDC — the
+publisher's subject claim must be
+``repo:FormalLanguageConstrainedPathQuerying/CFPQ_Data:pull_request``), with
+``skip-existing`` so concurrent PRs sharing a version do not collide. This is
+a packaging check that fails the PR before a release tag is cut; since a tag
+always points at a merged PR's head, it validates exactly the code that will
+be released.
 
 Prerequisites (one-time, owner action)
 --------------------------------------
diff --git a/tasks/detailed_plan.md b/tasks/detailed_plan.md
index 48f341c0..0e60697e 100644
--- a/tasks/detailed_plan.md
+++ b/tasks/detailed_plan.md
@@ -110,3 +110,39 @@ a docs defect.
   fails — no suppression.
 - Note the behavior in the "Docs build and deploy" section of
   `docs/developer.rst`.
+
+### S7: Fix sdist packaging so `python -m build` succeeds
+
+Added 2026-09-16 at the publish gate. The first real `python -m build` run
+(publish workflow on tag `v5.0.0`) failed: `setup.py` reads
+`requirements/*.txt`, but no MANIFEST.in existed, so the files were missing
+from the sdist and the sdist→wheel step raised FileNotFoundError. This is the
+first packaging build in the project's history — it was never exercised
+before.
+
+**Code:** `MANIFEST.in` (new: `include requirements/*.txt`)
+**Tests:** skip — verified with an isolated `python -m build` (clean venv,
+same as CI); sdist contains all four requirements files and the wheel builds.
+**Docs:** none
+
+### S8: Publish to TestPyPI on every PR targeting master
+
+Added 2026-09-16 at the merge gate (user request: validate publishing
+automatically before the human merge). The `publish` workflow gains a
+`pull_request: branches: [master]` trigger; the `publish-testpypi` job runs on
+PRs with `skip-existing: true` because concurrent PRs share one package
+version and TestPyPI rejects re-uploads of existing files.
+
+Final design (after the first PR run failed): TestPyPI uses **OIDC Trusted
+Publishing**, not a token — the first run proved the pypi-publish action
+silently falls back to OIDC when no token secret exists, and the user prefers
+no long-lived secrets (consistent with the PyPI setup). The publisher's
+subject claim is `repo:FormalLanguageConstrainedPathQuerying/CFPQ_Data:pull_request`
+(the documented sub for pull_request events; confirmed in the failed run's
+claims dump). The manual `workflow_dispatch` TestPyPI path was removed: its
+sub claim cannot match the PR publisher, and the PR check fully covers
+pre-release validation (a tag always points at a merged PR's head).
+
+**Code:** `.github/workflows/publish.yml`
+**Tests:** skip — workflow-only; the build step is the check itself
+**Docs:** `docs/release.rst` (Package publishing section), `CHANGELOG.md`