diff --git a/.gitattributes b/.gitattributes
index 176a458..20a1962 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1 +1,2 @@
 * text=auto
+/tests/fixtures/*.bundle binary
diff --git a/.github/CONTRIBUTING.md b/.github/CONTRIBUTING.md
index 28cea4c..7486bbd 100644
--- a/.github/CONTRIBUTING.md
+++ b/.github/CONTRIBUTING.md
@@ -39,6 +39,35 @@ $ vendor/bin/phpunit
 
 * A script `test-git-versions.sh` is available in repository to test gitlib against many git versions.
 * The tests will be automatically run by [GitHub Actions](https://github.com/features/actions) against pull requests.
+* Tests run fully offline: no network access is required.
+
+## Test fixtures
+
+Most tests run against a fixture repository cloned from `tests/fixtures/foobar.bundle`,
+a local git bundle. Using a bundle instead of a network clone keeps the tests fast and
+fully offline.
+
+If you need a new fixture scenario (a specific merge, encoding, or signed-commit shape,
+for example), regenerate the bundle locally, entirely from the one already in the repo:
+
+```bash
+$ git clone tests/fixtures/foobar.bundle /tmp/foobar-fixture && cd /tmp/foobar-fixture
+$ for b in $(git branch -r | grep -v HEAD | sed 's#origin/##'); do
+$     git branch --track "$b" "origin/$b"
+$ done
+# ... add your commits, branches or tags ...
+$ git bundle create foobar.bundle \
+    HEAD refs/heads/master refs/heads/new-feature refs/heads/diff-features \
+    refs/heads/pagination refs/heads/path-resolving refs/tags/0.1 refs/tags/annotated
+$ cp foobar.bundle /path/to/gitlib/tests/fixtures/foobar.bundle
+```
+
+Then update the commit SHA constants in `AbstractTestCase` to match, and run
+`tests/fixtures/verify-bundle.sh`. It checks the bundle's integrity, its ref list against
+an allow-list, and its size, since GitHub renders any change to this binary file as an
+opaque diff. If your change intentionally adds a ref or grows the file, update
+`ALLOWED_REFS` or `MAX_SIZE_KB` in that script as part of the same pull request, so the
+reason for the change is explicit and reviewable rather than a silent binary diff.
 
 ## Standard code
 
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index ae4f7d7..73634aa 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -6,6 +6,16 @@ on:
   pull_request:
 
 jobs:
+  verify-fixture:
+    name: Verify Test Fixture Bundle
+    runs-on: ubuntu-24.04
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@v7
+
+      - name: Verify fixture bundle integrity, refs and size
+        run: tests/fixtures/verify-bundle.sh
+
   check-cs:
     name: Check Coding Standards
     runs-on: ubuntu-24.04
diff --git a/tests/Gitonomy/Git/Tests/AbstractTestCase.php b/tests/Gitonomy/Git/Tests/AbstractTestCase.php
index e56bc35..80cd16e 100644
--- a/tests/Gitonomy/Git/Tests/AbstractTestCase.php
+++ b/tests/Gitonomy/Git/Tests/AbstractTestCase.php
@@ -18,7 +18,11 @@
 
 abstract class AbstractTestCase extends TestCase
 {
-    public const REPOSITORY_URL = 'https://github.com/gitonomy/foobar.git';
+    /**
+     * Local git bundle used as a fixture repository: cloning from it behaves like
+     * cloning a remote repository, but requires no network access.
+     */
+    public const REPOSITORY_URL = __DIR__.'/../../../fixtures/foobar.bundle';
 
     public const NO_MESSAGE_COMMIT = '011cd0c1625190d2959ee9a8f9f822006d94b661';
     public const LONGFILE_COMMIT = '4f17752acc9b7c54ba679291bf24cb7d354f0f4f';
diff --git a/tests/fixtures/foobar.bundle b/tests/fixtures/foobar.bundle
new file mode 100644
index 0000000..bbd71de
Binary files /dev/null and b/tests/fixtures/foobar.bundle differ
diff --git a/tests/fixtures/verify-bundle.sh b/tests/fixtures/verify-bundle.sh
new file mode 100755
index 0000000..f543e21
--- /dev/null
+++ b/tests/fixtures/verify-bundle.sh
@@ -0,0 +1,53 @@
+#!/bin/bash
+#
+# Guards tests/fixtures/foobar.bundle against unexpected changes: any pull
+# request touching it should be reviewed by hand (see CONTRIBUTING.md), but
+# this catches the obvious cases automatically:
+#   - a corrupted or incomplete bundle
+#   - refs that were not in the original fixture
+#   - a file that has silently grown well past its expected size
+#
+# Run it locally with: tests/fixtures/verify-bundle.sh
+
+set -euo pipefail
+
+cd "$(dirname "${BASH_SOURCE[0]}")" > /dev/null
+
+BUNDLE="foobar.bundle"
+MAX_SIZE_KB=200
+
+ALLOWED_REFS="
+HEAD
+refs/heads/diff-features
+refs/heads/master
+refs/heads/new-feature
+refs/heads/pagination
+refs/heads/path-resolving
+refs/tags/0.1
+refs/tags/annotated
+"
+
+echo "== Verifying $BUNDLE =="
+
+VERIFY_OUTPUT="$(git bundle verify "$BUNDLE")"
+echo "$VERIFY_OUTPUT"
+
+SIZE_KB=$(( $(stat -c%s "$BUNDLE") / 1024 ))
+echo "Size: ${SIZE_KB}KB (limit: ${MAX_SIZE_KB}KB)"
+if [ "$SIZE_KB" -gt "$MAX_SIZE_KB" ]; then
+    echo "ERROR: $BUNDLE is ${SIZE_KB}KB, which exceeds the ${MAX_SIZE_KB}KB limit." >&2
+    echo "If this growth is expected, bump MAX_SIZE_KB in $0 as part of the same PR." >&2
+    exit 1
+fi
+
+ACTUAL_REFS="$(grep -oE '(refs/[^ ]+|HEAD)$' <<< "$VERIFY_OUTPUT" | sort -u)"
+UNEXPECTED_REFS="$(comm -23 <(echo "$ACTUAL_REFS") <(sort -u <<< "$ALLOWED_REFS"))"
+
+if [ -n "$UNEXPECTED_REFS" ]; then
+    echo "ERROR: $BUNDLE contains refs that are not in the allow-list:" >&2
+    echo "$UNEXPECTED_REFS" >&2
+    echo "If this is expected, update ALLOWED_REFS in $0 as part of the same PR." >&2
+    exit 1
+fi
+
+echo "OK: bundle structure and size are within expected bounds."