diff --git a/.cursor/rules/file_length.mdc b/.cursor/rules/file_length.mdc
new file mode 100644
index 0000000..d01369c
--- /dev/null
+++ b/.cursor/rules/file_length.mdc
@@ -0,0 +1,22 @@
+---
+description: File length limits for Python files
+globs: *.py
+alwaysApply: false
+---
+## File Length Limit
+
+Python files must not exceed 500 lines. This is enforced by `make file_len_check` (part of `make ci`) and in the linter GitHub Actions workflow.
+
+This limit exists to prevent AI-generated code from becoming monolithic and forces refactoring into smaller, more modular components.
+
+If a file genuinely needs to exceed 500 lines, add it to the `exclude` list in `pyproject.toml` under `[tool.file_length]`:
+
+```toml
+[tool.file_length]
+max_lines = 500
+exclude = [
+    "onboard.py",
+]
+```
+
+Do not add files to the exclude list without a clear justification.
diff --git a/.github/workflows/linter_require_ruff.yaml b/.github/workflows/linter_require_ruff.yaml
index c9c7f89..e21aa94 100644
--- a/.github/workflows/linter_require_ruff.yaml
+++ b/.github/workflows/linter_require_ruff.yaml
@@ -38,3 +38,5 @@ jobs:
         run: uv run python scripts/check_ai_writing.py
       - name: Run import-linter
         run: make import_lint
+      - name: Run file length check
+        run: uv run python scripts/check_file_length.py
diff --git a/Makefile b/Makefile
index 6435c37..7fbdeb9 100644
--- a/Makefile
+++ b/Makefile
@@ -248,7 +248,12 @@ check_deps: install_tools ## Check for unused dependencies
 	@uv run deptry .
 	@echo "$(GREEN)✅Dependency check completed.$(RESET)"
 
-ci: ruff vulture import_lint ty docs_lint lint_links check_deps ## Run all CI checks (ruff, vulture, import_lint, ty, docs_lint, lint_links)
+file_len_check: check_uv ## Check Python files don't exceed max line count
+	@echo "$(YELLOW)🔍Checking file lengths...$(RESET)"
+	@uv run python scripts/check_file_length.py
+	@echo "$(GREEN)✅File length check completed.$(RESET)"
+
+ci: ruff vulture import_lint ty docs_lint lint_links check_deps file_len_check ## Run all CI checks (ruff, vulture, import_lint, ty, docs_lint, lint_links, file_len_check)
 	@echo "$(GREEN)✅CI checks completed.$(RESET)"
 
 ########################################################
diff --git a/pyproject.toml b/pyproject.toml
index d4a1f37..97525f2 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -95,6 +95,12 @@ exclude = [
     "onboard.py"
 ]
 
+[tool.file_length]
+max_lines = 500
+exclude = [
+    "onboard.py",
+]
+
 [tool.coverage.run]
 branch = true
 source = ["src", "common", "utils"]
diff --git a/scripts/check_file_length.py b/scripts/check_file_length.py
new file mode 100644
index 0000000..29f024a
--- /dev/null
+++ b/scripts/check_file_length.py
@@ -0,0 +1,63 @@
+from __future__ import annotations
+
+import pathlib
+import tomllib
+
+REPO_ROOT = pathlib.Path(__file__).resolve().parent.parent
+ROOT_SKIP_DIRS = {
+    ".git",
+    ".venv",
+    ".uv_cache",
+    ".uv-cache",
+    ".uv_tools",
+    ".uv-tools",
+    ".cache",
+    "node_modules",
+    ".next",
+}
+RECURSIVE_SKIP_DIRS = {"__pycache__", ".pytest_cache"}
+
+
+def load_config() -> tuple[int, set[str]]:
+    pyproject = REPO_ROOT / "pyproject.toml"
+    with open(pyproject, "rb") as f:
+        data = tomllib.load(f)
+    cfg = data.get("tool", {}).get("file_length", {})
+    max_lines = cfg.get("max_lines", 500)
+    exclude = set(cfg.get("exclude", []))
+    return max_lines, exclude
+
+
+def main() -> int:
+    max_lines, exclude = load_config()
+    violations: list[tuple[pathlib.Path, int]] = []
+
+    for path in REPO_ROOT.rglob("*.py"):
+        rel = path.relative_to(REPO_ROOT)
+        parts = rel.parts
+        if parts[0] in ROOT_SKIP_DIRS:
+            continue
+        if any(part in RECURSIVE_SKIP_DIRS for part in parts[:-1]):
+            continue
+        if rel.as_posix() in exclude:
+            continue
+        line_count = len(path.read_text(encoding="utf-8", errors="ignore").splitlines())
+        if line_count > max_lines:
+            violations.append((rel, line_count))
+
+    if violations:
+        print(f"File length check failed: {len(violations)} file(s) exceed {max_lines} lines")
+        for rel_path, count in sorted(violations):
+            print(f"  {rel_path}: {count} lines")
+        print(
+            "Refactor large files into smaller modules, "
+            "or add to [tool.file_length] exclude in pyproject.toml."
+        )
+        return 1
+
+    print(f"File length check passed (all files <= {max_lines} lines).")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())