diff --git a/.github/pull_request_template.md b/.github/pull_request_template.md
index 73da9ae5..a2b82cd6 100644
--- a/.github/pull_request_template.md
+++ b/.github/pull_request_template.md
@@ -25,6 +25,7 @@ Closes #
 
 - [ ] One concern per PR; unrelated changes are split out
 - [ ] `CHANGELOG.md` has an entry under `[Unreleased]`
+- [ ] If `pyproject.toml` dependencies changed, `poetry lock` was run and the updated `poetry.lock` is committed
 - [ ] Documentation updated (`README.md`, `docs/`, `.env.example`, `CLAUDE.md`) where settings or commands changed
 - [ ] New settings default to current behaviour
 - [ ] If AI tools helped write this change, I reviewed every line and the hand-testing above is mine
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 33887d3f..2b9ed2cf 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -31,8 +31,11 @@ jobs:
           path: .venv
           key: venv-${{ runner.os }}-3.11-${{ hashFiles('**/poetry.lock') }}
 
+      - name: Verify lockfile is up to date
+        run: poetry check --lock
+
       - name: Install dependencies
-        run: poetry lock && poetry install
+        run: poetry install --no-interaction
 
       - name: Check formatting (black)
         run: poetry run black --check src tests
@@ -67,8 +70,11 @@ jobs:
           path: .venv
           key: venv-${{ runner.os }}-3.11-${{ hashFiles('**/poetry.lock') }}
 
+      - name: Verify lockfile is up to date
+        run: poetry check --lock
+
       - name: Install dependencies
-        run: poetry lock && poetry install
+        run: poetry install --no-interaction
 
       - name: Test
         run: make test
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 697f4369..8af7f05b 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -32,6 +32,9 @@ jobs:
           path: .venv
           key: venv-${{ runner.os }}-3.11-${{ hashFiles('**/poetry.lock') }}
 
+      - name: Verify lockfile is up to date
+        run: poetry check --lock
+
       - name: Install dependencies
         run: poetry install --no-interaction
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 75a7bdf6..f51c5180 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- **CI now catches lockfile drift**: the `lint` and `test` jobs ran `poetry lock && poetry install`, which regenerated `poetry.lock` in place. A `pyproject.toml` dependency change with a stale lock therefore passed every PR and failed only at release time, as it did for v1.7.0 (#196). Both jobs now verify the lock with `poetry check --lock` and install from the committed lock, so CI tests the dependency set that actually ships rather than resolving a fresh one on every run.
+
 ## [1.7.0] - 2026-09-11
 
 Released as a minor rather than a patch: the security fix below changes runtime
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 3d189471..a646eea2 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -95,6 +95,10 @@ five-layer security model. Read it before touching `src/security/` or
 
 - Tests for behaviour changes. `make test` and `make lint` pass. CI runs
   black, isort, flake8 and the test suite on every PR.
+- **If you change dependencies, run `poetry lock` and commit the updated
+  `poetry.lock` in the same PR.** CI installs from the committed lock and
+  fails the build if the lock and `pyproject.toml` disagree, so a
+  `pyproject.toml` dependency change without a matching lock update goes red.
 - A line under `[Unreleased]` in `CHANGELOG.md`, in the Keep a Changelog
   style already used there.
 - Docs updated where a setting or command changed: `README.md`,