diff --git a/docs/specs/DEVX-166.md b/docs/specs/DEVX-166.md
new file mode 100644
index 0000000..aca4d00
--- /dev/null
+++ b/docs/specs/DEVX-166.md
@@ -0,0 +1,27 @@
+# DEVX-166: Exclude docs/plans/* from PR size check
+
+## Problem
+Planning docs in `docs/plans/` are legitimately large (700+ lines) but
+fail the PR size check (max 500 lines). This blocks PRs that only add
+planning documents.
+
+## Approach
+REQ-1: Add `docs/plans/*` to `DEFAULT_EXCLUDED_PATTERNS` in
+  `src/devx/ci/check_pr_size.py`
+REQ-2: Add test coverage for the new exclusion pattern
+
+## Test Plan
+- `make pytest-cov` passes with 100% coverage
+- `make lint-all` passes
+
+## Deploy Plan
+- Merge to master → post-merge auto-publishes new devx version
+- Infra PR #1179 picks up the fix once devx is bumped
+
+## Rollback Plan
+- Revert the merge commit
+
+## Acceptance Criteria
+- [x] REQ-1: Add `docs/plans/*` to `DEFAULT_EXCLUDED_PATTERNS` in
+  `src/devx/ci/check_pr_size.py`
+- [x] REQ-2: Add test coverage for the new exclusion pattern
diff --git a/src/devx/ci/check_pr_size.py b/src/devx/ci/check_pr_size.py
index bab11b4..79e2010 100644
--- a/src/devx/ci/check_pr_size.py
+++ b/src/devx/ci/check_pr_size.py
@@ -36,6 +36,7 @@ DEFAULT_EXCLUDED_PATTERNS = [
     "CHANGELOG.md",
     "README.md",
     "docs/index.md",
+    "docs/plans/*",
     "*.svg",
     "uv.lock",
     "poetry.lock",
diff --git a/tests/unit/test_check_pr_size.py b/tests/unit/test_check_pr_size.py
index 0fb9451..9dcfd3e 100644
--- a/tests/unit/test_check_pr_size.py
+++ b/tests/unit/test_check_pr_size.py
@@ -26,6 +26,12 @@ class TestIsExcluded:
     def test_excludes_readme(self) -> None:
         assert is_excluded("README.md", ["README.md"])
 
+    def test_excludes_plans_glob(self) -> None:
+        assert is_excluded("docs/plans/sso-bridge-full-extraction.md", ["docs/plans/*"])
+
+    def test_does_not_exclude_specs(self) -> None:
+        assert not is_excluded("docs/specs/DEVX-165.md", ["docs/plans/*"])
+
 
 class TestCheckSize:
     def test_under_limits_passes(self) -> None: