diff --git a/.github/scripts/check_semver.py b/.github/scripts/check_semver.py
new file mode 100755
index 0000000..3d30b49
--- /dev/null
+++ b/.github/scripts/check_semver.py
@@ -0,0 +1,214 @@
+#!/usr/bin/env python3
+"""PR semver gate for the datamaxi public API.
+
+Python analog of the Rust sibling's ``cargo-semver-checks`` job. It diffs the
+public API of the ``datamaxi`` package between the PR base commit and the PR
+head (working tree) with ``griffe check``, then fails when the version bump in
+``datamaxi/__version__.py`` is smaller than the detected change requires.
+
+Policy (0.x-aware, and future-proof for >=1.0):
+  * pre-1.0 (base major == 0): breaking changes must land a **minor** bump
+    (e.g. 0.29.0 -> 0.30.0), per the pre-1.0 SemVer convention.
+  * >=1.0: breaking changes must land a **major** bump.
+  * a version that goes backwards always fails.
+  * no breaking changes + non-decreasing version always passes (we do not force
+    a bump for additive-only changes, which griffe check does not surface).
+
+griffe is static (AST-based), so it needs neither the optional deps
+(``httpx``/``websockets``) nor prod credentials/network. griffe checks out the
+base ref through an internal git worktree, so the workflow must use
+``fetch-depth: 0``.
+
+Usage:
+    python .github/scripts/check_semver.py <base-ref>
+"""
+
+from __future__ import annotations
+
+import re
+import subprocess
+import sys
+
+PACKAGE = "datamaxi"
+VERSION_FILE = "datamaxi/__version__.py"
+
+_VERSION_RE = re.compile(r"""__version__\s*=\s*["']([^"']+)["']""")
+# Markers that mean griffe itself blew up (bad ref, load failure, ...) rather
+# than reporting genuine API breakages.
+_ERROR_MARKERS = ("Traceback (most recent call last)", "GitError", "fatal:")
+
+LEVELS = {"none": 0, "patch": 1, "minor": 2, "major": 3}
+
+
+def fail(msg: str) -> None:
+    print(f"::error::{msg}" if _in_gha() else f"ERROR: {msg}")
+    sys.exit(1)
+
+
+def tool_error(msg: str) -> None:
+    print(f"::error::{msg}" if _in_gha() else f"TOOL ERROR: {msg}")
+    sys.exit(2)
+
+
+def _in_gha() -> bool:
+    import os
+
+    return os.environ.get("GITHUB_ACTIONS") == "true"
+
+
+def read_version_at(ref: str | None) -> str | None:
+    """Return the ``__version__`` string at ``ref`` (None == working tree)."""
+    if ref is None:
+        with open(VERSION_FILE, encoding="utf-8") as fh:
+            text = fh.read()
+    else:
+        proc = subprocess.run(
+            ["git", "show", f"{ref}:{VERSION_FILE}"],
+            capture_output=True,
+            text=True,
+        )
+        if proc.returncode != 0:
+            return None
+        text = proc.stdout
+    match = _VERSION_RE.search(text)
+    return match.group(1) if match else None
+
+
+def parse_version(version: str) -> tuple[int, int, int]:
+    """Parse ``major.minor.patch`` leniently (ignores any pre-release suffix)."""
+    nums = []
+    for part in version.split(".")[:3]:
+        match = re.match(r"\d+", part)
+        nums.append(int(match.group(0)) if match else 0)
+    while len(nums) < 3:
+        nums.append(0)
+    return nums[0], nums[1], nums[2]
+
+
+def actual_bump_level(base: tuple, head: tuple) -> str:
+    if head < base:
+        return "decrease"
+    if head == base:
+        return "none"
+    if head[0] != base[0]:
+        return "major"
+    if head[1] != base[1]:
+        return "minor"
+    return "patch"
+
+
+def required_bump_level(base: tuple) -> str:
+    """Minimum bump a breaking change requires, given the base version."""
+    return "major" if base[0] >= 1 else "minor"
+
+
+def run_griffe(base_ref: str) -> tuple[bool, str]:
+    """Return (breaking?, report). griffe writes its report to stderr."""
+    cmd = [
+        sys.executable,
+        "-m",
+        "griffe",
+        "check",
+        PACKAGE,
+        "-a",
+        base_ref,
+        "-f",
+        "oneline",
+    ]
+    proc = subprocess.run(cmd, capture_output=True, text=True)
+    report = (proc.stdout + proc.stderr).strip()
+
+    if proc.returncode == 0:
+        return False, report
+    if any(marker in report for marker in _ERROR_MARKERS):
+        tool_error(f"griffe failed to run:\n{report}")
+    # Non-zero + no error marker == breaking changes were reported.
+    return True, report
+
+
+def main() -> None:
+    if len(sys.argv) != 2 or sys.argv[1] in ("-h", "--help"):
+        print(__doc__)
+        sys.exit(0 if sys.argv[1:] in (["-h"], ["--help"]) else 2)
+    base_ref = sys.argv[1]
+
+    # Validate the base ref up front so griffe git-worktree errors can't be
+    # mistaken for breaking changes.
+    if (
+        subprocess.run(
+            ["git", "rev-parse", "--verify", "--quiet", f"{base_ref}^{{commit}}"],
+            capture_output=True,
+        ).returncode
+        != 0
+    ):
+        tool_error(
+            f"base ref '{base_ref}' is not a valid commit "
+            "(the workflow needs actions/checkout with fetch-depth: 0)."
+        )
+
+    base_str = read_version_at(base_ref)
+    head_str = read_version_at(None)
+
+    if head_str is None:
+        fail(f"could not read __version__ from {VERSION_FILE} in the working tree.")
+    if base_str is None:
+        # No version file at base (very old history / brand-new package). We
+        # cannot enforce a policy without a baseline, so pass with a note.
+        print(
+            f"NOTE: no readable {VERSION_FILE} at base '{base_ref}'; "
+            "skipping semver enforcement."
+        )
+        sys.exit(0)
+
+    base = parse_version(base_str)
+    head = parse_version(head_str)
+
+    breaking, report = run_griffe(base_ref)
+    level = actual_bump_level(base, head)
+
+    print(f"base version : {base_str}  (from {base_ref[:12]})")
+    print(f"head version : {head_str}  (working tree)")
+    print(f"version bump : {level}")
+    print(f"breaking API changes detected: {'yes' if breaking else 'no'}")
+
+    if level == "decrease":
+        if report:
+            print("\ngriffe report:\n" + report)
+        fail(
+            f"version went backwards: {base_str} -> {head_str}. "
+            "The version must never decrease."
+        )
+
+    if not breaking:
+        print(
+            "\nOK: no breaking public-API changes; version is non-decreasing. "
+            "Nothing to enforce."
+        )
+        return
+
+    required = required_bump_level(base)
+    if LEVELS[level] < LEVELS[required]:
+        era = "pre-1.0" if base[0] < 1 else ">=1.0"
+        print("\ngriffe-detected breaking changes:\n" + report)
+        fail(
+            f"breaking public-API changes require at least a '{required}' version "
+            f"bump ({era} policy), but {base_str} -> {head_str} is a '{level}' bump. "
+            f"Bump {VERSION_FILE} accordingly (e.g. "
+            f"{_suggest(base, required)})."
+        )
+
+    print(
+        f"\nOK: breaking changes are covered by a '{level}' bump "
+        f"({base_str} -> {head_str}), which meets the required '{required}' bump."
+    )
+
+
+def _suggest(base: tuple, required: str) -> str:
+    major, minor, patch = base
+    if required == "major":
+        return f"{major + 1}.0.0"
+    return f"{major}.{minor + 1}.0"
+
+
+if __name__ == "__main__":
+    main()
diff --git a/.github/workflows/semver.yml b/.github/workflows/semver.yml
new file mode 100644
index 0000000..c2b72a5
--- /dev/null
+++ b/.github/workflows/semver.yml
@@ -0,0 +1,46 @@
+# Semver check — the Python analog of the Rust sibling repo's
+# `cargo-semver-checks` gate.
+#
+# On every pull request this diffs the public API of the `datamaxi` package
+# between the PR base commit and the PR head, and FAILS when the version bump in
+# `datamaxi/__version__.py` is smaller than the change requires. This keeps
+# breaking changes from shipping under a patch bump (or no bump at all), which
+# matters here because several public modules (`_endpoints.py`,
+# `_ws_endpoints.py`, `_ws_models.py`) are codegen-generated and churn often —
+# so we enforce the bump rather than block the churn.
+#
+# The repo is still 0.x, so breaking changes require a MINOR bump
+# (0.29.0 -> 0.30.0); the check script also does the right thing once the repo
+# reaches >=1.0 (breaking => MAJOR). The diff uses `griffe check`, which is
+# static (AST-based) and needs neither the optional deps (httpx/websockets) nor
+# prod credentials/network. griffe checks out the base ref through an internal
+# git worktree, hence `fetch-depth: 0`.
+
+name: Semver check
+
+on:
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  semver:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          # Full history so griffe can check out the PR base commit.
+          fetch-depth: 0
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+      - name: Set up uv
+        uses: astral-sh/setup-uv@v6
+        with:
+          enable-cache: true
+      - name: Install griffe
+        run: uv pip install --system 'griffe>=2,<3'
+      - name: Check public-API semver vs base version bump
+        run: python .github/scripts/check_semver.py "${{ github.event.pull_request.base.sha }}"