diff --git a/.devcontainer/devcontainer.json b/.devcontainer/devcontainer.json
index 7717f9af..9317c15f 100644
--- a/.devcontainer/devcontainer.json
+++ b/.devcontainer/devcontainer.json
@@ -26,7 +26,7 @@
   },
   "forwardPorts": [3000],
   "containerUser": "vscode",
-  "postCreateCommand": "npm install",
+  "postCreateCommand": "npm ci",
   "waitFor": "postCreateCommand", // otherwise automated jest tests fail
   "features": {
     "node": {
diff --git a/AGENTS.md b/AGENTS.md
index 56401424..bd2454d7 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -40,7 +40,9 @@ fail the PR.
 
 ## Build & Test
 
-- **Install**: `npm install`
+- **Install**: `npm ci` — installs the exact versions in `package-lock.json`, as
+  CI does. Use `npm install <package>` only to add or upgrade a dependency on
+  purpose; it re-resolves the semver ranges and rewrites the lockfile.
 - **Start Dev Server**: `npm run docus:start`
 - **Build**: `npm run build`
 - **Unit Tests**: `npm run test:unit` — required "Validate repository" CI check
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 32326e01..e17ccd6f 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -8,10 +8,14 @@ This document covers the development workflow and the data contribution model.
 Prerequisites: Node.js 22+ (LTS recommended) and npm.
 
 ```bash
-npm install
+npm ci
 npm run docus:start
 ```
 
+`npm ci` installs the exact, integrity-checked versions in `package-lock.json`,
+matching CI. Reach for `npm install <package>` only to add or upgrade a
+dependency, and commit the resulting lockfile change deliberately.
+
 The dev server runs at `http://localhost:3000`. In the devcontainer:
 
 ```bash
diff --git a/README.md b/README.md
index 91528030..77b6c853 100644
--- a/README.md
+++ b/README.md
@@ -34,9 +34,15 @@ Ensure you have the following installed:
 ### Install dependencies
 
 ```bash
-npm install
+npm ci
 ```
 
+`npm ci` installs exactly the versions recorded in `package-lock.json` and
+verifies their integrity hashes, which is what CI runs. Use
+`npm install <package>` only when you intend to add or upgrade a dependency;
+that re-resolves the ranges in `package.json` and rewrites the lockfile, so the
+change belongs in its own reviewed commit.
+
 ### Run the site
 
 ```bash
diff --git a/tests/dev-environment.test.mjs b/tests/dev-environment.test.mjs
index 57443cc0..2c4fbfef 100644
--- a/tests/dev-environment.test.mjs
+++ b/tests/dev-environment.test.mjs
@@ -177,6 +177,55 @@ test('every npm script invoked by devcontainer lifecycle commands exists', () =>
   );
 });
 
+// CI installs with `npm ci`, the only npm command that treats
+// package-lock.json as an instruction: exact versions, integrity hashes
+// verified, lockfile left alone. A bare `npm install` re-resolves the caret
+// ranges in package.json against the registry, runs the lifecycle scripts of
+// whatever it picked, and rewrites the lockfile in the contributor's tree — so
+// a doc that prescribes it hands every contributor an unreviewed dependency
+// graph and lets an upgrade ride into an unrelated PR. `npm install <pkg>` is
+// still the right way to add a dependency, so only the bare form is rejected.
+const BARE_NPM_INSTALL = /\bnpm install\s*(?=$|[\n`'"&|;])/;
+
+test('no developer doc or devcontainer command prescribes a bare npm install', () => {
+  const offenders = [];
+
+  for (const doc of ['README.md', 'CONTRIBUTING.md', 'AGENTS.md']) {
+    const lines = readFileSync(join(root, doc), 'utf8').split('\n');
+    for (const [index, line] of lines.entries()) {
+      if (BARE_NPM_INSTALL.test(line)) offenders.push(`${doc}:${index + 1}`);
+    }
+  }
+
+  const devcontainer = JSON.parse(stripJsonComments(devcontainerRaw));
+  for (const key of [
+    'initializeCommand',
+    'onCreateCommand',
+    'updateContentCommand',
+    'postCreateCommand',
+    'postStartCommand',
+    'postAttachCommand',
+  ]) {
+    const value = devcontainer[key];
+    const commands =
+      typeof value === 'string'
+        ? [value]
+        : Array.isArray(value)
+          ? [value.join(' ')]
+          : value && typeof value === 'object'
+            ? Object.values(value).map((entry) => String(entry))
+            : [];
+    if (commands.some((command) => BARE_NPM_INSTALL.test(command)))
+      offenders.push(`devcontainer.${key}`);
+  }
+
+  assert.deepEqual(
+    offenders,
+    [],
+    `these prescribe a bare \`npm install\`, which ignores package-lock.json; use \`npm ci\`: ${offenders.join(', ')}`,
+  );
+});
+
 // The dev port the devcontainer forwards has to be the one `just serve`
 // actually opens, or the recipe starts a server nobody outside the container
 // can reach.