diff --git a/VERSION b/VERSION
index 8f0916f..a918a2a 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.5.0
+0.6.0
diff --git a/actions/release-stable-select/action.yml b/actions/release-stable-select/action.yml
new file mode 100644
index 0000000..6806f30
--- /dev/null
+++ b/actions/release-stable-select/action.yml
@@ -0,0 +1,43 @@
+# SPDX-FileCopyrightText: 2026 LibreCode coop and contributors
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+name: Select latest stable release branch
+description: Resolve exact stable<N> branches and report whether the current branch is the highest maintained stable.
+
+inputs:
+  repository:
+    description: Repository in owner/name form. Defaults to the current repository.
+    required: false
+    default: ''
+  branch:
+    description: Branch to compare. Defaults to the current GitHub ref name.
+    required: false
+    default: ''
+
+outputs:
+  current-branch:
+    description: Branch being evaluated.
+    value: ${{ steps.resolve.outputs.current_branch }}
+  current-major:
+    description: Numeric stable release line for the current branch, or empty when it is not stable<N>.
+    value: ${{ steps.resolve.outputs.current_major }}
+  latest-branch:
+    description: Highest exact stable<N> branch found in the repository.
+    value: ${{ steps.resolve.outputs.latest_branch }}
+  latest-major:
+    description: Numeric release line of the latest stable branch.
+    value: ${{ steps.resolve.outputs.latest_major }}
+  is-latest:
+    description: Whether the current branch is exactly the latest stable branch.
+    value: ${{ steps.resolve.outputs.is_latest }}
+
+runs:
+  using: composite
+  steps:
+    - id: resolve
+      name: Resolve latest stable branch
+      shell: bash
+      env:
+        INPUT_REPOSITORY: ${{ inputs.repository }}
+        INPUT_BRANCH: ${{ inputs.branch }}
+      run: python3 "$GITHUB_ACTION_PATH/resolve.py"
diff --git a/actions/release-stable-select/resolve.py b/actions/release-stable-select/resolve.py
new file mode 100644
index 0000000..fda3161
--- /dev/null
+++ b/actions/release-stable-select/resolve.py
@@ -0,0 +1,130 @@
+#!/usr/bin/env python3
+# SPDX-FileCopyrightText: 2026 LibreCode coop and contributors
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+from __future__ import annotations
+
+import os
+import re
+import subprocess
+from dataclasses import dataclass
+from pathlib import Path
+
+REPOSITORY_RE = re.compile(r"^[A-Za-z0-9_.-]+/[A-Za-z0-9_.-]+$")
+STABLE_REF_RE = re.compile(r"^refs/heads/(stable([1-9][0-9]*))$")
+STABLE_BRANCH_RE = re.compile(r"^stable([1-9][0-9]*)$")
+
+
+@dataclass(frozen=True)
+class StableState:
+    current_branch: str
+    current_major: int | None
+    latest_branch: str | None
+    latest_major: int | None
+
+    @property
+    def is_latest(self) -> bool:
+        return self.latest_branch is not None and self.current_branch == self.latest_branch
+
+
+def parse_stable_refs(output: str) -> dict[int, str]:
+    branches: dict[int, str] = {}
+    for raw_line in output.splitlines():
+        parts = raw_line.strip().split()
+        if len(parts) != 2:
+            continue
+        match = STABLE_REF_RE.fullmatch(parts[1])
+        if match is None:
+            continue
+        branch = match.group(1)
+        major = int(match.group(2))
+        branches[major] = branch
+    return branches
+
+
+def resolve_state(current_branch: str, branches: dict[int, str]) -> StableState:
+    current_match = STABLE_BRANCH_RE.fullmatch(current_branch)
+    current_major = int(current_match.group(1)) if current_match else None
+
+    if not branches:
+        return StableState(current_branch, current_major, None, None)
+
+    latest_major = max(branches)
+    return StableState(
+        current_branch=current_branch,
+        current_major=current_major,
+        latest_branch=branches[latest_major],
+        latest_major=latest_major,
+    )
+
+
+def list_remote_stable_refs(repository: str) -> str:
+    if REPOSITORY_RE.fullmatch(repository) is None:
+        raise ValueError(f"invalid repository: {repository!r}")
+
+    result = subprocess.run(
+        [
+            "git",
+            "ls-remote",
+            "--heads",
+            f"https://github.com/{repository}.git",
+            "refs/heads/stable*",
+        ],
+        check=True,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        text=True,
+    )
+    return result.stdout
+
+
+def write_output(name: str, value: str) -> None:
+    output = os.environ.get("GITHUB_OUTPUT")
+    if output:
+        with Path(output).open("a", encoding="utf-8") as handle:
+            handle.write(f"{name}={value}\n")
+
+
+def write_summary(state: StableState) -> None:
+    summary = os.environ.get("GITHUB_STEP_SUMMARY")
+    if not summary:
+        return
+
+    current_major = str(state.current_major) if state.current_major is not None else "n/a"
+    latest_branch = state.latest_branch or "none"
+    latest_major = str(state.latest_major) if state.latest_major is not None else "n/a"
+    with Path(summary).open("a", encoding="utf-8") as handle:
+        handle.write(
+            "### Stable release branch selection\n\n"
+            f"- Current branch: `{state.current_branch}`\n"
+            f"- Current release line: `{current_major}`\n"
+            f"- Latest stable branch: `{latest_branch}`\n"
+            f"- Latest release line: `{latest_major}`\n"
+            f"- Publish nightly: `{str(state.is_latest).lower()}`\n"
+        )
+
+
+def main() -> int:
+    repository = os.environ.get("INPUT_REPOSITORY") or os.environ.get("GITHUB_REPOSITORY", "")
+    branch = os.environ.get("INPUT_BRANCH") or os.environ.get("GITHUB_REF_NAME", "")
+
+    refs = parse_stable_refs(list_remote_stable_refs(repository))
+    state = resolve_state(branch, refs)
+
+    print(f"Current branch: {state.current_branch}")
+    print(f"Current release line: {state.current_major if state.current_major is not None else 'n/a'}")
+    print(f"Latest stable branch: {state.latest_branch or 'none'}")
+    print(f"Latest release line: {state.latest_major if state.latest_major is not None else 'n/a'}")
+    print(f"Publish nightly: {str(state.is_latest).lower()}")
+
+    write_output("current_branch", state.current_branch)
+    write_output("current_major", "" if state.current_major is None else str(state.current_major))
+    write_output("latest_branch", state.latest_branch or "")
+    write_output("latest_major", "" if state.latest_major is None else str(state.latest_major))
+    write_output("is_latest", str(state.is_latest).lower())
+    write_summary(state)
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/tests/test_release_stable_select.py b/tests/test_release_stable_select.py
new file mode 100644
index 0000000..3ec756b
--- /dev/null
+++ b/tests/test_release_stable_select.py
@@ -0,0 +1,78 @@
+# SPDX-FileCopyrightText: 2026 LibreCode coop and contributors
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+import importlib.util
+import sys
+import unittest
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+MODULE_PATH = ROOT / "actions" / "release-stable-select" / "resolve.py"
+SPEC = importlib.util.spec_from_file_location("release_stable_select", MODULE_PATH)
+assert SPEC is not None and SPEC.loader is not None
+MODULE = importlib.util.module_from_spec(SPEC)
+sys.modules[SPEC.name] = MODULE
+SPEC.loader.exec_module(MODULE)
+
+parse_stable_refs = MODULE.parse_stable_refs
+resolve_state = MODULE.resolve_state
+
+
+class ReleaseStableSelectTest(unittest.TestCase):
+    def test_selects_highest_numeric_stable_branch(self) -> None:
+        refs = parse_stable_refs(
+            "a\trefs/heads/stable9\n"
+            "b\trefs/heads/stable35\n"
+            "c\trefs/heads/stable100\n"
+            "d\trefs/heads/stable34\n"
+        )
+
+        state = resolve_state("stable35", refs)
+
+        self.assertEqual(100, state.latest_major)
+        self.assertEqual("stable100", state.latest_branch)
+        self.assertEqual(35, state.current_major)
+        self.assertFalse(state.is_latest)
+
+    def test_current_latest_branch_is_publishable(self) -> None:
+        refs = parse_stable_refs(
+            "a\trefs/heads/stable33\n"
+            "b\trefs/heads/stable34\n"
+            "c\trefs/heads/stable35\n"
+        )
+
+        state = resolve_state("stable35", refs)
+
+        self.assertEqual(35, state.latest_major)
+        self.assertEqual("stable35", state.latest_branch)
+        self.assertTrue(state.is_latest)
+
+    def test_ignores_non_exact_stable_names(self) -> None:
+        refs = parse_stable_refs(
+            "a\trefs/heads/stable35\n"
+            "b\trefs/heads/stable36-test\n"
+            "c\trefs/heads/stable-next\n"
+            "d\trefs/heads/stable035\n"
+            "e\trefs/heads/notstable99\n"
+        )
+
+        self.assertEqual({35: "stable35"}, refs)
+
+    def test_non_stable_current_branch_never_matches(self) -> None:
+        state = resolve_state("main", {35: "stable35"})
+
+        self.assertIsNone(state.current_major)
+        self.assertEqual("stable35", state.latest_branch)
+        self.assertFalse(state.is_latest)
+
+    def test_no_stable_branch_fails_closed(self) -> None:
+        state = resolve_state("stable35", {})
+
+        self.assertEqual(35, state.current_major)
+        self.assertIsNone(state.latest_branch)
+        self.assertIsNone(state.latest_major)
+        self.assertFalse(state.is_latest)
+
+
+if __name__ == "__main__":
+    unittest.main()