diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 0ae8ea932e..7e9bb01cad 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -20,17 +20,11 @@ jobs:
     - name: Install greadlink
       if: startsWith(runner.os, 'macOS')
       run: brew install coreutils
-    - name: Install parallel
-      if: startsWith(runner.os, 'macOS')
-      run: |
-        brew install parallel
-        which parallel  # Verify installation
     - name: Test code
       run: test/run
       env:
-        # Ubuntu runners ship GNU parallel, causing --tap + --jobs count mismatches.
-        # Force single-threaded on Linux; let macOS use parallel (installed above).
-        TEST_JOBS: ${{ runner.os == 'Linux' && '1' || 'detect' }}
+        # --tap + --jobs causes TAP plan count mismatches on both Linux and macOS.
+        TEST_JOBS: 1
 
   build-docs:
     runs-on: ubuntu-latest
diff --git a/completion/available/herdr.completion.bash b/completion/available/herdr.completion.bash
new file mode 100644
index 0000000000..09d467d0a0
--- /dev/null
+++ b/completion/available/herdr.completion.bash
@@ -0,0 +1,13 @@
+# shellcheck shell=bash
+about-completion "Herdr Terminal Multiplexer tab completion"
+
+if ! _binary_exists herdr; then
+	_log_error "herdr not found in PATH — completion not loaded. (${BASH_SOURCE[0]})"
+	return 1
+else
+	if ! _bash-it-completion-helper-sufficient herdr; then
+		_log_warning "The completion is set for 'herdr' externally. Activation will be skipped. (${BASH_SOURCE[0]})"
+	else
+		eval "$(herdr completion bash)"
+	fi
+fi
diff --git a/completion/available/op.completion.bash b/completion/available/op.completion.bash
new file mode 100644
index 0000000000..6e500024c0
--- /dev/null
+++ b/completion/available/op.completion.bash
@@ -0,0 +1,13 @@
+# shellcheck shell=bash
+about-completion "1Password CLI tab completion"
+
+if ! _binary_exists op; then
+	_log_error "op not found in PATH — completion not loaded. (${BASH_SOURCE[0]})"
+	return 1
+else
+	if ! _bash-it-completion-helper-sufficient op; then
+		_log_warning "The completion is set for 'op' externally. Activation will be skipped. (${BASH_SOURCE[0]})"
+	else
+		eval "$(op completion bash)"
+	fi
+fi
diff --git a/docs/test.rst b/docs/test.rst
index 2923eb7af7..f5ba0cd411 100644
--- a/docs/test.rst
+++ b/docs/test.rst
@@ -26,17 +26,46 @@ To execute the unit tests, please run the ``run`` script:
 
 The ``run`` script will automatically install if it is not already present, and will then run all tests found under the ``test`` directory, including subdirectories.
 
-To run only a subset of the tests, you can provide the name of the test subdirectory that you want to run, e.g. like this for the tests in the ``test/themes`` directory:
+To run only a subset of the tests, you can provide a directory or a specific test file:
 
 .. code-block:: bash
 
-   # If you are in the root `.bash_it` directory:
+   # Run all tests in a directory:
    test/run test/themes
 
-By default, the tests run in single-threaded mode.
-If you want to speed up the test execution, you can install the `GNU ``parallel`` tool <https://www.gnu.org/software/parallel/>`_\ , which is supported by Bats.
-When using ``parallel``\ , the ``test/run`` script will use a number of threads in parallel, depending on the available CPU cores of your system.
-This can speed up test execution significantly.
+   # Run a single test file:
+   test/run test/completion/op.completion.bats
+
+   # Run multiple specific files:
+   test/run test/completion/op.completion.bats test/completion/herdr.completion.bats
+
+The tests always run in single-threaded mode (``TEST_JOBS=1``). Parallel execution
+via GNU ``parallel`` was previously supported but caused TAP plan count mismatches
+when combined with the ``--tap`` flag, producing false failures. Single-threaded
+mode is reliable and is what CI enforces.
+
+Local Runs and Isolation
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Running ``test/run`` directly on your machine works, but it inherits your shell's
+``PATH``. Any tool installed locally (e.g. ``op``, ``herdr``, ``docker``) will be
+visible to the tests, which can cause tests to behave differently than they do on CI,
+where the runner is a clean environment with only a known set of packages installed.
+
+To reproduce CI conditions exactly, use ``test/run-local``, which builds a minimal
+Docker image and runs the tests inside it:
+
+.. code-block:: bash
+
+   # Run all tests in a clean container:
+   test/run-local
+
+   # Run a subset:
+   test/run-local test/completion/op.completion.bats
+
+The image is built on the first run and cached afterwards, so subsequent runs are
+fast. The trade-off compared to running ``test/run`` directly is that the first run
+takes longer and Docker must be installed.
 
 Writing Tests
 -------------
diff --git a/test/completion/herdr.completion.bats b/test/completion/herdr.completion.bats
new file mode 100644
index 0000000000..9a32a1daf6
--- /dev/null
+++ b/test/completion/herdr.completion.bats
@@ -0,0 +1,47 @@
+# shellcheck shell=bats
+
+load "${MAIN_BASH_IT_DIR?}/test/test_helper.bash"
+
+function local_setup_file() {
+	setup_libs "helpers"
+	load "${BASH_IT?}/lib/completion.bash"
+}
+
+function local_setup() {
+	MOCK_BIN="${BATS_TEST_TMPDIR}/mock-bin"
+	mkdir -p "${MOCK_BIN}"
+	_SAVED_PATH="${PATH}"
+}
+
+function local_teardown() {
+	PATH="${_SAVED_PATH}"
+	complete -r herdr 2> /dev/null || true
+}
+
+function _mock_herdr() {
+	printf '%s\n' \
+		'#!/usr/bin/env bash' \
+		'[[ "$*" == "completion bash" ]] && echo "complete -o nospace herdr"' \
+		> "${MOCK_BIN}/herdr"
+	chmod +x "${MOCK_BIN}/herdr"
+	PATH="${MOCK_BIN}:${PATH}"
+}
+
+@test "completion herdr: fails to load when herdr is not in PATH" {
+	PATH="${MOCK_BIN}" run source "${BASH_IT?}/completion/available/herdr.completion.bash"
+	assert_failure
+}
+
+@test "completion herdr: loads successfully when herdr is available" {
+	_mock_herdr
+	run source "${BASH_IT?}/completion/available/herdr.completion.bash"
+	assert_success
+}
+
+@test "completion herdr: skips eval when completion is already managed externally" {
+	_mock_herdr
+	complete -F : herdr
+	source "${BASH_IT?}/completion/available/herdr.completion.bash"
+	run complete -p herdr
+	assert_output "complete -F : herdr"
+}
diff --git a/test/completion/op.completion.bats b/test/completion/op.completion.bats
new file mode 100644
index 0000000000..c30ab0b2ca
--- /dev/null
+++ b/test/completion/op.completion.bats
@@ -0,0 +1,47 @@
+# shellcheck shell=bats
+
+load "${MAIN_BASH_IT_DIR?}/test/test_helper.bash"
+
+function local_setup_file() {
+	setup_libs "helpers"
+	load "${BASH_IT?}/lib/completion.bash"
+}
+
+function local_setup() {
+	MOCK_BIN="${BATS_TEST_TMPDIR}/mock-bin"
+	mkdir -p "${MOCK_BIN}"
+	_SAVED_PATH="${PATH}"
+}
+
+function local_teardown() {
+	PATH="${_SAVED_PATH}"
+	complete -r op 2> /dev/null || true
+}
+
+function _mock_op() {
+	printf '%s\n' \
+		'#!/usr/bin/env bash' \
+		'[[ "$*" == "completion bash" ]] && echo "complete -o nospace op"' \
+		> "${MOCK_BIN}/op"
+	chmod +x "${MOCK_BIN}/op"
+	PATH="${MOCK_BIN}:${PATH}"
+}
+
+@test "completion op: fails to load when op is not in PATH" {
+	PATH="${MOCK_BIN}" run source "${BASH_IT?}/completion/available/op.completion.bash"
+	assert_failure
+}
+
+@test "completion op: loads successfully when op is available" {
+	_mock_op
+	run source "${BASH_IT?}/completion/available/op.completion.bash"
+	assert_success
+}
+
+@test "completion op: skips eval when completion is already managed externally" {
+	_mock_op
+	complete -F : op
+	source "${BASH_IT?}/completion/available/op.completion.bash"
+	run complete -p op
+	assert_output "complete -F : op"
+}
diff --git a/test/run-local b/test/run-local
new file mode 100755
index 0000000000..450d993d41
--- /dev/null
+++ b/test/run-local
@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+IMAGE="bash-it-local-test-runner"
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+
+# Build the image — idempotent, Docker cache makes subsequent runs instant
+docker build --tag "${IMAGE}" - <<'DOCKERFILE'
+FROM ubuntu:24.04
+RUN apt-get update -q \
+    && apt-get install -qy git locales curl iproute2 \
+    && locale-gen en_US.UTF-8 \
+    && rm -rf /var/lib/apt/lists/*
+ENV LANG=en_US.UTF-8 LC_ALL=en_US.UTF-8
+DOCKERFILE
+
+# Mirror CI exactly: TEST_JOBS=1 avoids --tap + --jobs TAP count mismatches.
+# Pass any arguments through to test/run (e.g. test/completion/op.completion.bats)
+exec docker run --rm \
+  --volume "${REPO_ROOT}:/workspace" \
+  --workdir /workspace \
+  --env TEST_JOBS=1 \
+  "${IMAGE}" \
+  bash test/run "$@"