diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index ed9a14d8..43c9b715 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -147,6 +147,11 @@ jobs:
           path: dist-handoff.tar
 
   publish:
+    # The only job that can mint a PyPI OIDC token. It runs no project code,
+    # no build and no install: after the policy's credential-free checkout it
+    # downloads the build job's artifact, checks its digest, extracts it and
+    # uploads it. Verification and the GitHub Release run in later jobs
+    # without id-token.
     needs: build
     runs-on: ubuntu-latest
     if: >-
@@ -156,9 +161,10 @@ jobs:
     environment: pypi
     permissions:
       actions: read
-      contents: write
       id-token: write
     steps:
+      # The central action policy's fixed preamble for a privileged artifact
+      # consumer. Nothing in this job executes the checked-out source.
       - uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803  # v6
         with:
           fetch-depth: 0
@@ -225,6 +231,20 @@ jobs:
           print-hash: true
           skip-existing: true
 
+  verify:
+    # Post-publish checks run after publication, with read-only contents and no
+    # OIDC, and fail the workflow on any mismatch.
+    needs: [build, publish]
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803  # v6
+        with:
+          fetch-depth: 0
+          persist-credentials: false
+          ref: ${{ github.sha }}
+
       - name: Verify exact PyPI JSON with an advisory rendered-page audit
         env:
           RELEASE_SOURCE_SHA: ${{ needs.build.outputs.release_commit }}
@@ -245,8 +265,21 @@ jobs:
           --attempts 30
           --interval-seconds 10
 
+  github-release:
+    # Creates the source GitHub Release only after verification passes. It
+    # runs only the preinstalled GitHub CLI: no checkout and no project code.
+    needs: [build, verify]
+    runs-on: ubuntu-latest
+    if: >-
+      (github.event_name == 'push' && startsWith(github.ref, 'refs/tags/')) ||
+      (github.event_name == 'workflow_dispatch' &&
+       github.ref == 'refs/heads/main' && inputs.publish)
+    permissions:
+      contents: write
+    steps:
       - name: Create the source GitHub Release
         env:
+          GH_REPO: ${{ github.repository }}
           GH_TOKEN: ${{ github.token }}
           RELEASE_TAG: ${{ needs.build.outputs.release_tag }}
         run: |
diff --git a/tests/test_publish_workflow_isolation.py b/tests/test_publish_workflow_isolation.py
new file mode 100644
index 00000000..a880591c
--- /dev/null
+++ b/tests/test_publish_workflow_isolation.py
@@ -0,0 +1,149 @@
+"""Contract for .github/workflows/publish.yml (#240).
+
+Only the publish job can mint a PyPI OIDC token, and it runs no project code:
+after the central policy's credential-free checkout it downloads the build
+job's artifact, checks its digest, extracts it and uploads it. Post-publish
+verification runs in a later job without id-token, and the source GitHub
+Release is created by a minimal job that runs only the preinstalled GitHub
+CLI.
+"""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+from typing import Any
+
+import pytest
+import yaml
+
+ROOT = Path(__file__).resolve().parents[1]
+PUBLISH_WORKFLOW = ROOT / ".github" / "workflows" / "publish.yml"
+
+PYPI_PUBLISH_ACTION = "pypa/gh-action-pypi-publish@"
+DOWNLOAD_ARTIFACT_ACTION = "actions/download-artifact@"
+CHECKOUT_ACTION = "actions/checkout@"
+
+# Commands that would run project code, a build backend or an installer.
+PROJECT_CODE = re.compile(
+    r"(^|[\s;&|(])(python3?|pip3?|pipx|uv|uvx|twine|hatch|poetry|tox|nox|make)(\s|$)"
+    r"|python3?\s+-m\s+(build|pip)|\bscripts/|\bsetup\.py\b|\bpyproject\.toml\b",
+)
+
+
+def workflow() -> dict[str, Any]:
+    document = yaml.safe_load(PUBLISH_WORKFLOW.read_text(encoding="utf-8"))
+    assert isinstance(document, dict)
+    return document
+
+
+def jobs() -> dict[str, dict[str, Any]]:
+    value = workflow()["jobs"]
+    assert isinstance(value, dict)
+    return value
+
+
+def needs(job: dict[str, Any]) -> list[str]:
+    value = job.get("needs", [])
+    return [value] if isinstance(value, str) else list(value)
+
+
+def uses(job: dict[str, Any]) -> list[str]:
+    return [step["uses"] for step in job.get("steps", []) if "uses" in step]
+
+
+def runs(job: dict[str, Any]) -> list[str]:
+    return [step["run"] for step in job.get("steps", []) if "run" in step]
+
+
+def test_the_workflow_grants_no_token_by_default() -> None:
+    assert workflow()["permissions"] == {"contents": "read"}
+
+
+def test_only_the_publish_job_can_mint_an_oidc_token() -> None:
+    minting = {name for name, job in jobs().items() if (job.get("permissions") or {}).get("id-token") == "write"}
+    assert minting == {"publish"}
+    assert "id-token" not in workflow()["permissions"]
+
+
+def test_the_publish_job_has_exactly_the_permissions_publication_needs() -> None:
+    publish = jobs()["publish"]
+    assert publish["permissions"] == {"actions": "read", "id-token": "write"}
+    assert publish["environment"] == "pypi"
+    assert needs(publish) == ["build"]
+
+
+def test_the_publish_job_only_downloads_checks_and_uploads() -> None:
+    publish = jobs()["publish"]
+    actions = uses(publish)
+    assert [action.split("@")[0] + "@" for action in actions] == [
+        CHECKOUT_ACTION, DOWNLOAD_ARTIFACT_ACTION, PYPI_PUBLISH_ACTION,
+    ]
+    assert all(re.fullmatch(r"[^@]+@[0-9a-f]{40}", action) for action in actions)
+    # The central action policy's fixed preamble for a privileged artifact
+    # consumer: a credential-free checkout that nothing in the job executes.
+    assert publish["steps"][0] == {"uses": actions[0], "with": {
+        "fetch-depth": 0, "persist-credentials": False, "ref": "${{ github.sha }}",
+    }}
+    # The artifact is downloaded by its recorded id, with digest enforcement.
+    download = next(step for step in publish["steps"] if step.get("uses", "").startswith(DOWNLOAD_ARTIFACT_ACTION))
+    assert download["with"]["digest-mismatch"] == "error"
+    assert download["with"]["artifact-ids"] == "${{ needs.build.outputs.artifact-id }}"
+    scripts = runs(publish)
+    assert len(scripts) == 2
+    for script in scripts:
+        assert PROJECT_CODE.search(script) is None, script
+    assert "sha256sum" in scripts[0] and "EXPECTED_ARTIFACT_DIGEST" in scripts[0]
+    assert scripts[1].split() == ["mkdir", "dist", "tar", "-xf", "isolated-python-dist/dist-handoff.tar", "-C", "dist"]
+
+
+@pytest.mark.parametrize("script", [
+    "pip install .", "python -m build", "python scripts/check_release_metadata.py", "pip install --dry-run x",
+    "twine upload dist/*", "uv build", "cd x && python3 -m pip install -e .",
+])
+def test_the_project_code_pattern_catches_installs_and_builds(script: str) -> None:
+    assert PROJECT_CODE.search(script) is not None
+
+
+def test_the_pypi_publish_action_runs_only_in_the_publish_job() -> None:
+    for name, job in jobs().items():
+        if name != "publish":
+            assert not any(action.startswith(PYPI_PUBLISH_ACTION) for action in uses(job)), name
+            assert job.get("environment") is None, name
+
+
+def test_post_publish_verification_runs_after_publication_without_oidc() -> None:
+    verify = jobs()["verify"]
+    assert verify["permissions"] == {"contents": "read"}
+    assert set(needs(verify)) == {"build", "publish"}
+    checkout = next(step for step in verify["steps"] if step.get("uses", "").startswith(CHECKOUT_ACTION))
+    assert checkout["with"]["persist-credentials"] is False
+    scripts = runs(verify)
+    # The same checks, in the same order: exact PyPI JSON, then the project
+    # surface and its supported installs.
+    assert len(scripts) == 2
+    assert "scripts/check_release_metadata.py" in scripts[0] and "--pypi-version" in scripts[0]
+    assert "scripts/check_pypi_project_surface.py" in scripts[1]
+    for script in scripts:
+        assert "--attempts 30" in script and "--interval-seconds 10" in script
+
+
+def test_the_github_release_follows_verification_and_runs_no_third_party_code() -> None:
+    release = jobs()["github-release"]
+    assert release["permissions"] == {"contents": "write"}
+    assert set(needs(release)) == {"build", "verify"}
+    # Privileged, so it runs only where publication itself may run.
+    assert release["if"] == jobs()["publish"]["if"]
+    assert uses(release) == []
+    scripts = runs(release)
+    assert len(scripts) == 1
+    commands = {line.split()[0] for line in scripts[0].splitlines() if line.strip()}
+    assert commands <= {"if", "gh", "arguments=(--verify-tag", "arguments+=(--prerelease)", "fi"}
+    assert PROJECT_CODE.search(scripts[0]) is None
+    assert "gh release create" in scripts[0] and "--verify-tag" in scripts[0]
+
+
+def test_the_build_job_holds_no_publication_credential() -> None:
+    build = jobs()["build"]
+    assert "permissions" not in build
+    assert not any(action.startswith(PYPI_PUBLISH_ACTION) for action in uses(build))