diff --git a/.github/workflows/docs.yaml b/.github/workflows/docs.yaml
index 7c8a80c..1c64b06 100644
--- a/.github/workflows/docs.yaml
+++ b/.github/workflows/docs.yaml
@@ -14,6 +14,11 @@ concurrency:
   group: pages
   cancel-in-progress: true
 
+env:
+  # Keep in step with test.yml's pin -- docs runs `mojo doc`, which compiles the
+  # sources and so breaks on the same stdlib changes the tests do.
+  MOJO_VERSION: "1.0.0b3.dev2026073014"
+
 jobs:
   build-deploy:
     runs-on: ubuntu-latest
@@ -27,10 +32,10 @@ jobs:
       - name: Install uv
         uses: astral-sh/setup-uv@v5
 
-      - name: Install Mojo nightly
+      - name: Install Mojo
         run: |
           uv venv
-          uv pip install mojo \
+          uv pip install "mojo==$MOJO_VERSION" \
             --index https://whl.modular.com/nightly/simple/ \
             --prerelease allow
 
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index bf0c749..5e23e57 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -4,23 +4,46 @@ on:
   push:
     branches: [main]
   pull_request:
+  # Weekly drift check against the latest nightly. Advisory only -- it runs on
+  # main, so a failure never blocks a PR; it tells us the language moved under a
+  # frozen repo. See the install step for how the two modes differ.
+  schedule:
+    - cron: "27 13 * * 1"
+  workflow_dispatch:
+
+env:
+  # The nightly this repo is known to build and pass on. CI installs exactly
+  # this for pushes and PRs, so upstream churn cannot turn a green repo red
+  # without a commit. Bump it when the weekly drift check goes green on a newer
+  # one, or after fixing whatever it caught.
+  MOJO_VERSION: "1.0.0b3.dev2026073014"
 
 jobs:
   test:
     runs-on: ubuntu-latest
     timeout-minutes: 20
+    permissions:
+      contents: read
+      issues: write
     steps:
       - uses: actions/checkout@v4
 
       - name: Install uv
         uses: astral-sh/setup-uv@v5
 
-      - name: Install Mojo nightly
+      - name: Install Mojo
         run: |
           uv venv
-          uv pip install mojo \
-            --index https://whl.modular.com/nightly/simple/ \
-            --prerelease allow
+          if [ "${{ github.event_name }}" = "schedule" ]; then
+            echo "::notice::Drift check: installing the LATEST nightly, ignoring the $MOJO_VERSION pin"
+            uv pip install mojo \
+              --index https://whl.modular.com/nightly/simple/ \
+              --prerelease allow
+          else
+            uv pip install "mojo==$MOJO_VERSION" \
+              --index https://whl.modular.com/nightly/simple/ \
+              --prerelease allow
+          fi
           .venv/bin/mojo --version
 
       - name: Format check
@@ -33,3 +56,32 @@ jobs:
 
       - name: Unit tests — parse-error positions
         run: .venv/bin/mojo run -I src test/test_errors.mojo
+
+      - name: Report nightly drift
+        if: failure() && github.event_name == 'schedule'
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          title="Mojo nightly drift: tests fail against the latest nightly"
+          installed=$(.venv/bin/mojo --version 2>/dev/null || echo "unknown")
+          # Heredoc, not a quoted string: a multi-line shell string would keep
+          # this block's indentation and markdown would render the body as code.
+          body=$(cat <<EOF
+          The weekly drift check failed against the latest Mojo nightly.
+
+          CI is pinned to $MOJO_VERSION and stays green, so no PR is blocked -- but
+          the latest nightly ($installed) no longer builds this repo.
+
+          Run: $GITHUB_SERVER_URL/$GITHUB_REPOSITORY/actions/runs/$GITHUB_RUN_ID
+
+          Fix the breakage, then bump MOJO_VERSION in .github/workflows/test.yml
+          and .github/workflows/docs.yaml to the nightly that passes.
+          EOF
+          )
+          existing=$(gh issue list --repo "$GITHUB_REPOSITORY" --state open \
+            --search "$title in:title" --json number --jq '.[0].number // empty')
+          if [ -n "$existing" ]; then
+            gh issue comment "$existing" --repo "$GITHUB_REPOSITORY" --body "$body"
+          else
+            gh issue create --repo "$GITHUB_REPOSITORY" --title "$title" --body "$body"
+          fi
diff --git a/test/test_errors.mojo b/test/test_errors.mojo
index 812c1bc..2fa63fd 100644
--- a/test/test_errors.mojo
+++ b/test/test_errors.mojo
@@ -10,7 +10,7 @@ def _assert_lc(source: String, offset: Int, line: Int, col: Int) raises:
 
 
 def _msg(e: Error) -> String:
-    return String.write(e)
+    return String(e)
 
 
 # --------------------------------------------------------------------------