diff --git a/.python-version b/.python-version
new file mode 100644
index 00000000..e4fba218
--- /dev/null
+++ b/.python-version
@@ -0,0 +1 @@
+3.12
diff --git a/backend/ruff.toml b/backend/ruff.toml
index 9bb4cdf0..f69ce16c 100644
--- a/backend/ruff.toml
+++ b/backend/ruff.toml
@@ -7,6 +7,10 @@
 # the repo wrap at the same width, and it's the width `ruff format` enforces (gated via
 # `ruff format --check` alongside `ruff check`).
 line-length = 100
+# The interpreter this code actually runs on, so ruff judges syntax and upgrade hints against the
+# right one. Left unset it falls back to a default that has nothing to do with us. `/.python-version`
+# is the single source of truth; tests/test_python_engine_contract.py holds this to it.
+target-version = "py312"
 
 [lint]
 # Beyond the default set, the rules that hold a LOG LINE to what E22.5 makes of it:
diff --git a/backend/tests/test_python_engine_contract.py b/backend/tests/test_python_engine_contract.py
new file mode 100644
index 00000000..49a940aa
--- /dev/null
+++ b/backend/tests/test_python_engine_contract.py
@@ -0,0 +1,66 @@
+"""The Python major is a contract, not a coincidence — the backend half (E27 S3).
+
+`0.93.2` was released to fix exactly this bug on the JavaScript side: four machines built or checked
+the app and none of them agreed on which Node they ran, the server gate went red for 73 of its last
+90 runs, and nothing told anyone. The Python side had the same shape and nobody had looked: prod
+(`Dockerfile`), the test image (`Dockerfile.test`) and CI (`check.yml`) each spelled **3.12** as an
+independent literal, there was no `.python-version`, no `requires-python`, and ruff had no
+`target-version` — while the dev machine had no 3.12 installed at all, only 3.13 and 3.14.
+
+**The contract is a chain, and it is split across two test files because no single runner can see
+every link.** This suite runs inside the test image, which mounts `backend/` and nothing else, so it
+cannot read the repo-root Dockerfile or the workflow. It therefore holds the link it CAN see, and the
+one that matters most here:
+
+    the interpreter actually running  ==  backend/pyrightconfig.json  ==  backend/ruff.toml
+
+and `frontend/src/lib/pythonEngine.contract.test.ts` — which runs on the host, with the whole tree —
+holds the rest of the chain:
+
+    /.python-version  ==  pyrightconfig.json  ==  ruff.toml  ==  Dockerfile  ==  Dockerfile.test  ==  check.yml
+
+Together they close the loop. To move the version: edit `/.python-version`, then follow the failures
+in both files. Every place they name is a place that would otherwise have been forgotten.
+"""
+
+import re
+import sys
+from pathlib import Path
+
+BACKEND = Path(__file__).resolve().parents[1]
+
+
+def _version_in(path: Path, pattern: str) -> tuple[int, int]:
+    """The (major, minor) the given file declares. A rename or a moved line lands here — re-point
+    the test, the contract did not go away with it."""
+    text = path.read_text(encoding="utf-8")
+    match = re.search(pattern, text, re.MULTILINE)
+    assert match is not None, f"{pattern!r} not found in {path.name}"
+    return int(match.group(1)), int(match.group(2))
+
+
+# pyrightconfig.json is JSONC (it carries comments), so it is read by regex rather than json.loads.
+PINNED = _version_in(BACKEND / "pyrightconfig.json", r'"pythonVersion"\s*:\s*"(\d+)\.(\d+)"')
+
+
+def test_pins_a_plausible_python_version():
+    """Guards against a typo agreeing with nothing at all."""
+    assert PINNED >= (3, 12), PINNED
+
+
+def test_this_interpreter_is_the_pinned_one():
+    """The assertion that pays. A green suite on another minor does not mean the shipped image is
+    green — it means the suite and the image have drifted and nobody noticed yet."""
+    running = sys.version_info[:2]
+    assert running == PINNED, (
+        f"these tests are running on Python {running[0]}.{running[1]}, but the app is pinned to "
+        f"{PINNED[0]}.{PINNED[1]} (backend/pyrightconfig.json, /.python-version). The suite runs in "
+        f"the test image, so this failing means backend/Dockerfile.test has drifted from the pin."
+    )
+
+
+def test_ruff_judges_the_pinned_version():
+    """Unset, ruff falls back to a default that has nothing to do with us, and its syntax and
+    upgrade advice are then aimed at some other interpreter."""
+    major, minor = _version_in(BACKEND / "ruff.toml", r'^target-version\s*=\s*"py(\d)(\d+)"')
+    assert (major, minor) == PINNED
diff --git a/frontend/src/lib/pythonEngine.contract.test.ts b/frontend/src/lib/pythonEngine.contract.test.ts
new file mode 100644
index 00000000..a0eb6829
--- /dev/null
+++ b/frontend/src/lib/pythonEngine.contract.test.ts
@@ -0,0 +1,86 @@
+import { readFileSync } from "node:fs";
+import { expect, it } from "vitest";
+
+// The Python major is a contract, not a coincidence — the host half. The twin of
+// `nodeEngine.contract.test.ts`, and written the day after that one was: `0.93.2` shipped because
+// four machines disagreed about Node, and the Python side turned out to have the identical shape
+// with nobody watching. Prod (`Dockerfile`), the test image (`backend/Dockerfile.test`) and CI
+// (`check.yml`) each spelled 3.12 as its own literal, there was no `.python-version`, and the dev
+// machine had only 3.13 and 3.14 installed — so every ad-hoc host script ran two majors ahead of
+// production.
+//
+// It lives in the FRONTEND suite for one boring reason: this is the only runner that can see the
+// whole repository. The backend suite runs inside the test image, which mounts `backend/` alone, so
+// it cannot read the repo-root Dockerfile or the workflow. It holds the other half of the chain —
+// including "the interpreter running these tests IS the pinned one", which only it can assert:
+//
+//   backend/tests/test_python_engine_contract.py :  running interpreter == pyrightconfig == ruff.toml
+//   here                                         :  .python-version == pyrightconfig == ruff.toml
+//                                                   == Dockerfile == Dockerfile.test == check.yml
+//
+// To move the version: edit `.python-version`, then follow the failures in both files.
+const readRoot = (rel: string) => readFileSync(new URL(`../../../${rel}`, import.meta.url), "utf8");
+
+function versionIn(source: string, pattern: RegExp, where: string): string {
+  const m = pattern.exec(source);
+  // A rename or a moved line lands here. Re-point the test — the contract did not go away with it.
+  expect(m, `${pattern} not found in ${where}`).not.toBeNull();
+  return `${m![1]}.${m![2]}`;
+}
+
+const pinned = versionIn(readRoot(".python-version"), /^(\d+)\.(\d+)\s*$/m, ".python-version");
+
+it("pins a plausible Python version", () => {
+  // Guards against a typo'd `.python-version` silently agreeing with nothing.
+  expect(Number(pinned.split(".")[1])).toBeGreaterThanOrEqual(12);
+  expect(pinned.split(".")[0]).toBe("3");
+});
+
+it("runs the shipped backend on the pinned version", () => {
+  // The image our users' instance actually runs.
+  expect(versionIn(readRoot("Dockerfile"), /^FROM python:(\d+)\.(\d+)-slim/m, "Dockerfile")).toBe(
+    pinned,
+  );
+});
+
+it("tests on the pinned version", () => {
+  // The image the whole pytest suite runs in — including the backend half of this contract, which
+  // asserts its own `sys.version_info` against the pin.
+  expect(
+    versionIn(
+      readRoot("backend/Dockerfile.test"),
+      /^FROM python:(\d+)\.(\d+)-slim/m,
+      "Dockerfile.test",
+    ),
+  ).toBe(pinned);
+});
+
+it("runs the server gate on the pinned version", () => {
+  // The backend job's container. Its NODE is deliberately outside the Node contract (it only runs
+  // the checkout action), but its PYTHON runs our code and our tests, so it is inside this one.
+  expect(
+    versionIn(
+      readRoot(".gitea/workflows/check.yml"),
+      /^\s*container: nikolaik\/python-nodejs:python(\d+)\.(\d+)-nodejs\d+$/m,
+      "check.yml",
+    ),
+  ).toBe(pinned);
+});
+
+it("type-checks against the pinned version", () => {
+  // A type checker aimed at another version green-lights APIs the shipped interpreter does not have
+  // — the same defect as the image split, moved to check time.
+  expect(
+    versionIn(
+      readRoot("backend/pyrightconfig.json"),
+      /"pythonVersion"\s*:\s*"(\d+)\.(\d+)"/,
+      "pyrightconfig.json",
+    ),
+  ).toBe(pinned);
+});
+
+it("lints against the pinned version", () => {
+  expect(
+    versionIn(readRoot("backend/ruff.toml"), /^target-version = "py(\d)(\d+)"$/m, "ruff.toml"),
+  ).toBe(pinned);
+});