diff --git a/content-guards/README.md b/content-guards/README.md
index 6f2e944..cedb33c 100644
--- a/content-guards/README.md
+++ b/content-guards/README.md
@@ -23,6 +23,24 @@ claude plugins add jacobpevans-cc-plugins/content-guards
 - `cspell` - Spell checking
 - `gh` - GitHub CLI
 
+## Testing
+
+The markdown-validator has automated tests using [bats-core](https://github.com/bats-core/bats-core):
+
+```bash
+# Install bats-core (if not already installed)
+brew install bats-core
+
+# Run the test suite from repo root
+bats tests/content-guards/markdown-validator/validate-markdown.bats
+```
+
+Test coverage includes:
+- File type filtering (non-markdown, missing, dotfiles)
+- Config resolution (project vs fallback)
+- Cross-repo editing scenarios
+- Unbound variable regression prevention (PR #39, #40)
+
 ## License
 
 Apache-2.0
diff --git a/content-guards/scripts/validate-markdown.sh b/content-guards/scripts/validate-markdown.sh
index 73dc0ab..46e2067 100755
--- a/content-guards/scripts/validate-markdown.sh
+++ b/content-guards/scripts/validate-markdown.sh
@@ -104,9 +104,16 @@ EOF
     fi
   fi
 
-  if ! markdownlint_output=$(markdownlint-cli2 "${config_flag[@]+"${config_flag[@]}"}" "$file_path" 2>&1); then
-    errors+=("markdownlint-cli2 failed:")
-    errors+=("$markdownlint_output")
+  if [[ ${#config_flag[@]} -gt 0 ]]; then
+    if ! markdownlint_output=$(markdownlint-cli2 "${config_flag[@]}" "$file_path" 2>&1); then
+      errors+=("markdownlint-cli2 failed:")
+      errors+=("$markdownlint_output")
+    fi
+  else
+    if ! markdownlint_output=$(markdownlint-cli2 "$file_path" 2>&1); then
+      errors+=("markdownlint-cli2 failed:")
+      errors+=("$markdownlint_output")
+    fi
   fi
 fi
 
diff --git a/tests/content-guards/markdown-validator/fixtures/cross-repo-sim/feat/new-feature/README.md b/tests/content-guards/markdown-validator/fixtures/cross-repo-sim/feat/new-feature/README.md
new file mode 100644
index 0000000..fbba89d
--- /dev/null
+++ b/tests/content-guards/markdown-validator/fixtures/cross-repo-sim/feat/new-feature/README.md
@@ -0,0 +1,5 @@
+# Cross-Repo Feature
+
+This file simulates a cross-repo editing scenario.
+
+It has no ancestor `.markdownlint*` config, so the validator should use the fallback config.
diff --git a/tests/content-guards/markdown-validator/fixtures/project-with-config/.markdownlint.json b/tests/content-guards/markdown-validator/fixtures/project-with-config/.markdownlint.json
new file mode 100644
index 0000000..439efe7
--- /dev/null
+++ b/tests/content-guards/markdown-validator/fixtures/project-with-config/.markdownlint.json
@@ -0,0 +1,6 @@
+{
+  "default": true,
+  "MD013": {
+    "line_length": 120
+  }
+}
diff --git a/tests/content-guards/markdown-validator/fixtures/project-with-config/doc.md b/tests/content-guards/markdown-validator/fixtures/project-with-config/doc.md
new file mode 100644
index 0000000..2a3b60f
--- /dev/null
+++ b/tests/content-guards/markdown-validator/fixtures/project-with-config/doc.md
@@ -0,0 +1,5 @@
+# Project Documentation
+
+This markdown file lives in a directory with a `.markdownlint.json` config.
+
+It should be validated using that project config.
diff --git a/tests/content-guards/markdown-validator/fixtures/valid-basic.md b/tests/content-guards/markdown-validator/fixtures/valid-basic.md
new file mode 100644
index 0000000..d18e3a6
--- /dev/null
+++ b/tests/content-guards/markdown-validator/fixtures/valid-basic.md
@@ -0,0 +1,6 @@
+# Valid Basic Markdown
+
+This is a minimal valid markdown file for testing.
+
+- Item 1
+- Item 2
diff --git a/tests/content-guards/markdown-validator/validate-markdown.bats b/tests/content-guards/markdown-validator/validate-markdown.bats
new file mode 100644
index 0000000..27e5b10
--- /dev/null
+++ b/tests/content-guards/markdown-validator/validate-markdown.bats
@@ -0,0 +1,81 @@
+#!/usr/bin/env bats
+# Test suite for content-guards/scripts/validate-markdown.sh
+#
+# Tests the markdown validator hook behavior including:
+# - File type filtering (non-markdown, missing, dotfiles)
+# - Config resolution (project vs fallback)
+# - Cross-repo editing scenarios
+# - Unbound variable regression (PR #39, #40)
+#
+# Run with: bats tests/content-guards/markdown-validator/validate-markdown.bats
+
+setup() {
+  # Path to the script under test (relative to repo root)
+  REPO_ROOT="$(cd "$(dirname "$BATS_TEST_FILENAME")/../../.." && pwd)"
+  SCRIPT="$REPO_ROOT/content-guards/scripts/validate-markdown.sh"
+  FIXTURES="$(dirname "$BATS_TEST_FILENAME")/fixtures"
+
+  # Verify script exists
+  if [[ ! -f "$SCRIPT" ]]; then
+    echo "ERROR: Script not found at $SCRIPT" >&2
+    return 1
+  fi
+}
+
+@test "TC1: non-markdown file is skipped" {
+  run bash -c 'echo "{\"tool_input\":{\"file_path\":\"test.py\"}}" | /bin/bash "$1"' _ "$SCRIPT"
+  [ "$status" -eq 0 ]
+  [ -z "$output" ]
+}
+
+@test "TC2: missing file is skipped" {
+  run bash -c 'echo "{\"tool_input\":{\"file_path\":\"/nonexistent/file.md\"}}" | /bin/bash "$1"' _ "$SCRIPT"
+  [ "$status" -eq 0 ]
+  [ -z "$output" ]
+}
+
+@test "TC3: home dotfile is skipped" {
+  run bash -c 'echo "{\"tool_input\":{\"file_path\":\"~/.config/foo.md\"}}" | /bin/bash "$1"' _ "$SCRIPT"
+  [ "$status" -eq 0 ]
+  [ -z "$output" ]
+}
+
+@test "TC4: .claude directory file is skipped" {
+  run bash -c 'echo "{\"tool_input\":{\"file_path\":\"/path/to/.claude/foo.md\"}}" | /bin/bash "$1"' _ "$SCRIPT"
+  [ "$status" -eq 0 ]
+  [ -z "$output" ]
+}
+
+@test "TC5: empty config_flag with project config does not cause unbound variable" {
+  run bash -c 'echo "{\"tool_input\":{\"file_path\":\"'"$FIXTURES"'/project-with-config/doc.md\"}}" | /bin/bash "$1"' _ "$SCRIPT"
+  [ "$status" -eq 0 ]
+  [[ ! "$output" =~ "unbound variable" ]]
+}
+
+@test "TC6: non-empty config_flag with fallback config" {
+  # File without ancestor config should use fallback (temp config or plugin default)
+  run bash -c 'echo "{\"tool_input\":{\"file_path\":\"'"$FIXTURES"'/cross-repo-sim/feat/new-feature/README.md\"}}" | /bin/bash "$1"' _ "$SCRIPT"
+  [ "$status" -eq 0 ]
+  [[ ! "$output" =~ "unbound variable" ]]
+}
+
+@test "TC7: cross-repo editing finds config from file directory not CWD" {
+  # Simulate being in a completely different directory (like ~/git/repo1)
+  # while editing a file in ~/git/repo2/feat/branch/
+  original_dir=$(pwd)
+  cd /tmp
+
+  run bash -c 'echo "{\"tool_input\":{\"file_path\":\"'"$FIXTURES"'/project-with-config/doc.md\"}}" | /bin/bash "$1"' _ "$SCRIPT"
+
+  # Restore original directory
+  cd "$original_dir"
+
+  [ "$status" -eq 0 ]
+  [[ ! "$output" =~ "unbound variable" ]]
+}
+
+@test "TC8: empty JSON input is handled gracefully" {
+  run bash -c 'echo "{}" | /bin/bash "$1"' _ "$SCRIPT"
+  [ "$status" -eq 0 ]
+  [ -z "$output" ]
+}