diff --git a/README.md b/README.md
index aebbcb4b4..423769ae8 100644
--- a/README.md
+++ b/README.md
@@ -640,6 +640,23 @@ Configuration is managed through environment variables in `.env` file:
 - `TARGET_REPO_PATH`: Default repository path (default: `.`)
 - `LOCAL_MODEL_ENDPOINT`: Fallback endpoint for Ollama (default: `http://localhost:11434/v1`)
 
+### Custom Ignore Patterns
+
+You can specify additional directories to exclude by creating a `.cgrignore` file in your repository root:
+
+```
+# Comments start with #
+vendor
+.custom_cache
+my_build_output
+```
+
+- One directory name per line
+- Lines starting with `#` are comments
+- Blank lines are ignored
+- Patterns are exact directory name matches (not globs)
+- Patterns from `.cgrignore` are merged with `--exclude` flags and auto-detected directories
+
 ### Key Dependencies
 
 <!-- SECTION:dependencies -->
diff --git a/codebase_rag/config.py b/codebase_rag/config.py
index 20a8909ae..32f0ce6a4 100644
--- a/codebase_rag/config.py
+++ b/codebase_rag/config.py
@@ -1,14 +1,17 @@
 from __future__ import annotations
 
 from dataclasses import asdict, dataclass
+from pathlib import Path
 from typing import Unpack
 
 from dotenv import load_dotenv
+from loguru import logger
 from pydantic import AnyHttpUrl
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 from . import constants as cs
 from . import exceptions as ex
+from . import logs
 from .types_defs import ModelConfigKwargs
 
 load_dotenv()
@@ -227,3 +230,28 @@ def resolve_batch_size(self, batch_size: int | None) -> int:
 
 
 settings = AppConfig()
+
+CGRIGNORE_FILENAME = ".cgrignore"
+
+
+def load_cgrignore_patterns(repo_path: Path) -> frozenset[str]:
+    ignore_file = repo_path / CGRIGNORE_FILENAME
+    if not ignore_file.is_file():
+        return frozenset()
+
+    patterns: set[str] = set()
+    try:
+        with ignore_file.open(encoding="utf-8") as f:
+            for line in f:
+                line = line.strip()
+                if not line or line.startswith("#"):
+                    continue
+                patterns.add(line)
+        if patterns:
+            logger.info(
+                logs.CGRIGNORE_LOADED.format(count=len(patterns), path=ignore_file)
+            )
+        return frozenset(patterns)
+    except OSError as e:
+        logger.warning(logs.CGRIGNORE_READ_FAILED.format(path=ignore_file, error=e))
+        return frozenset()
diff --git a/codebase_rag/constants.py b/codebase_rag/constants.py
index 64cdca39e..f0d70de9b 100644
--- a/codebase_rag/constants.py
+++ b/codebase_rag/constants.py
@@ -632,6 +632,7 @@ class DiffMarker:
 INTERACTIVE_STYLE_DIM = "dim"
 INTERACTIVE_STATUS_DETECTED = "auto-detected"
 INTERACTIVE_STATUS_CLI = "--exclude"
+INTERACTIVE_STATUS_CGRIGNORE = ".cgrignore"
 INTERACTIVE_NESTED_SINGULAR = "{count} dir"
 INTERACTIVE_NESTED_PLURAL = "{count} dirs"
 INTERACTIVE_INSTRUCTIONS_GROUPED = (
diff --git a/codebase_rag/logs.py b/codebase_rag/logs.py
index 378dfe76f..246c1f77d 100644
--- a/codebase_rag/logs.py
+++ b/codebase_rag/logs.py
@@ -89,6 +89,10 @@
 GRAMMAR_LOAD_FAILED = "Failed to load {lang} grammar: {error}"
 INITIALIZED_PARSERS = "Initialized parsers for: {languages}"
 
+# (H) Ignore pattern logs
+CGRIGNORE_LOADED = "Loaded {count} patterns from {path}"
+CGRIGNORE_READ_FAILED = "Failed to read {path}: {error}"
+
 # (H) File watcher logs
 WATCHER_ACTIVE = "File watcher is now active."
 WATCHER_SKIP_NO_QUERY = "Ingestor does not support querying, skipping real-time update."
diff --git a/codebase_rag/main.py b/codebase_rag/main.py
index 67a5593ef..5ab097588 100644
--- a/codebase_rag/main.py
+++ b/codebase_rag/main.py
@@ -27,7 +27,7 @@
 from . import constants as cs
 from . import exceptions as ex
 from . import logs as ls
-from .config import settings
+from .config import load_cgrignore_patterns, settings
 from .models import AppContext
 from .prompts import OPTIMIZATION_PROMPT, OPTIMIZATION_PROMPT_WITH_REFERENCE
 from .services import QueryProtocol
@@ -788,7 +788,9 @@ def prompt_for_included_directories(
     cli_excludes: list[str] | None = None,
 ) -> frozenset[str]:
     detected = detect_excludable_directories(repo_path)
-    pre_excluded = frozenset(cli_excludes) if cli_excludes else frozenset()
+    cgrignore_patterns = load_cgrignore_patterns(repo_path)
+    cli_patterns = frozenset(cli_excludes) if cli_excludes else frozenset()
+    pre_excluded = cli_patterns | cgrignore_patterns
 
     if not detected and not pre_excluded:
         return frozenset()
diff --git a/codebase_rag/tests/test_cgrignore.py b/codebase_rag/tests/test_cgrignore.py
new file mode 100644
index 000000000..293e04b62
--- /dev/null
+++ b/codebase_rag/tests/test_cgrignore.py
@@ -0,0 +1,152 @@
+from __future__ import annotations
+
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from codebase_rag.config import CGRIGNORE_FILENAME, load_cgrignore_patterns
+from codebase_rag.main import prompt_for_included_directories
+
+
+def test_returns_empty_when_no_file(temp_repo: Path) -> None:
+    result = load_cgrignore_patterns(temp_repo)
+    assert result == frozenset()
+
+
+def test_loads_patterns_from_file(temp_repo: Path) -> None:
+    cgrignore = temp_repo / CGRIGNORE_FILENAME
+    cgrignore.write_text("vendor\nmy_build\n")
+
+    result = load_cgrignore_patterns(temp_repo)
+
+    assert "vendor" in result
+    assert "my_build" in result
+    assert len(result) == 2
+
+
+def test_ignores_comments_and_blank_lines(temp_repo: Path) -> None:
+    cgrignore = temp_repo / CGRIGNORE_FILENAME
+    cgrignore.write_text("# Comment\n\nvendor\n  # Indented comment\n")
+
+    result = load_cgrignore_patterns(temp_repo)
+
+    assert result == frozenset({"vendor"})
+
+
+def test_strips_whitespace(temp_repo: Path) -> None:
+    cgrignore = temp_repo / CGRIGNORE_FILENAME
+    cgrignore.write_text("  vendor  \n\ttemp\t\n")
+
+    result = load_cgrignore_patterns(temp_repo)
+
+    assert "vendor" in result
+    assert "temp" in result
+
+
+def test_returns_empty_on_read_error(
+    temp_repo: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    cgrignore = temp_repo / CGRIGNORE_FILENAME
+    cgrignore.write_text("vendor")
+
+    original_open = Path.open
+
+    def mock_open(self: Path, *args, **kwargs):  # noqa: ANN002, ANN003
+        if self.name == CGRIGNORE_FILENAME:
+            raise PermissionError("Cannot read")
+        return original_open(self, *args, **kwargs)
+
+    monkeypatch.setattr(Path, "open", mock_open)
+
+    result = load_cgrignore_patterns(temp_repo)
+    assert result == frozenset()
+
+
+def test_handles_duplicates(temp_repo: Path) -> None:
+    cgrignore = temp_repo / CGRIGNORE_FILENAME
+    cgrignore.write_text("vendor\nvendor\ntemp\n")
+
+    result = load_cgrignore_patterns(temp_repo)
+
+    assert len(result) == 2
+
+
+def test_returns_empty_if_cgrignore_is_a_directory(temp_repo: Path) -> None:
+    cgrignore_path = temp_repo / CGRIGNORE_FILENAME
+    cgrignore_path.mkdir()
+
+    result = load_cgrignore_patterns(temp_repo)
+
+    assert result == frozenset()
+
+
+class TestCgrignoreIntegration:
+    @patch("codebase_rag.main.Prompt.ask")
+    @patch("codebase_rag.main.app_context")
+    def test_cgrignore_patterns_included_in_candidates(
+        self, mock_context: MagicMock, mock_ask: MagicMock, tmp_path: Path
+    ) -> None:
+        (tmp_path / ".git").mkdir()
+        cgrignore = tmp_path / CGRIGNORE_FILENAME
+        cgrignore.write_text("vendor\ncustom_cache\n")
+        mock_ask.return_value = "all"
+
+        result = prompt_for_included_directories(tmp_path)
+
+        assert ".git" in result
+        assert "vendor" in result
+        assert "custom_cache" in result
+
+    @patch("codebase_rag.main.Prompt.ask")
+    @patch("codebase_rag.main.app_context")
+    def test_cgrignore_merged_with_cli_excludes(
+        self, mock_context: MagicMock, mock_ask: MagicMock, tmp_path: Path
+    ) -> None:
+        cgrignore = tmp_path / CGRIGNORE_FILENAME
+        cgrignore.write_text("from_cgrignore\n")
+        mock_ask.return_value = "all"
+
+        result = prompt_for_included_directories(tmp_path, cli_excludes=["from_cli"])
+
+        assert "from_cgrignore" in result
+        assert "from_cli" in result
+
+    @patch("codebase_rag.main.Prompt.ask")
+    @patch("codebase_rag.main.app_context")
+    def test_cgrignore_only_returns_without_prompt_when_empty(
+        self, mock_context: MagicMock, mock_ask: MagicMock, tmp_path: Path
+    ) -> None:
+        result = prompt_for_included_directories(tmp_path)
+
+        assert result == frozenset()
+        mock_ask.assert_not_called()
+
+    @patch("codebase_rag.main.Prompt.ask")
+    @patch("codebase_rag.main.app_context")
+    def test_cgrignore_alone_triggers_prompt(
+        self, mock_context: MagicMock, mock_ask: MagicMock, tmp_path: Path
+    ) -> None:
+        cgrignore = tmp_path / CGRIGNORE_FILENAME
+        cgrignore.write_text("my_custom_dir\n")
+        mock_ask.return_value = "none"
+
+        prompt_for_included_directories(tmp_path)
+
+        mock_ask.assert_called_once()
+
+    @patch("codebase_rag.main.Prompt.ask")
+    @patch("codebase_rag.main.app_context")
+    def test_cgrignore_deduplicates_with_detected(
+        self, mock_context: MagicMock, mock_ask: MagicMock, tmp_path: Path
+    ) -> None:
+        (tmp_path / ".git").mkdir()
+        cgrignore = tmp_path / CGRIGNORE_FILENAME
+        cgrignore.write_text(".git\nvendor\n")
+        mock_ask.return_value = "all"
+
+        result = prompt_for_included_directories(tmp_path)
+
+        assert ".git" in result
+        assert "vendor" in result
+        assert len([x for x in result if x == ".git"]) == 1