diff --git a/pyproject.toml b/pyproject.toml
index 68251cc..29fc7f2 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -15,6 +15,8 @@ dependencies = [
     "openai>=1.0.0",
     "google-generativeai>=0.3.0",
     "types-requests>=2.32",
+    "scikit-learn>=1.0",
+    "numpy>=1.22",
 ]
 
 [project.optional-dependencies]
diff --git a/requirements.txt b/requirements.txt
index 12ec92a..2a5bacb 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -7,3 +7,5 @@ types-requests>=2.32
 anthropic>=0.7.0
 openai>=1.0.0
 google-generativeai>=0.3.0
+scikit-learn>=1.0
+numpy>=1.22
diff --git a/src/search/__init__.py b/src/search/__init__.py
index 45ad7ca..870997e 100644
--- a/src/search/__init__.py
+++ b/src/search/__init__.py
@@ -1,5 +1,20 @@
 """Utilities for indexing and searching project files."""
 
 from .file_loader import load_project_files, read_file
+from .semantic_indexer import (
+    SemanticIndex,
+    build_semantic_index,
+    get_file_embedding,
+    update_file_embedding,
+    rank_files_by_query,
+)
 
-__all__ = ["load_project_files", "read_file"]
+__all__ = [
+    "load_project_files",
+    "read_file",
+    "SemanticIndex",
+    "build_semantic_index",
+    "get_file_embedding",
+    "update_file_embedding",
+    "rank_files_by_query",
+]
diff --git a/src/search/semantic_indexer.py b/src/search/semantic_indexer.py
new file mode 100644
index 0000000..2604b53
--- /dev/null
+++ b/src/search/semantic_indexer.py
@@ -0,0 +1,63 @@
+"""Simple semantic indexing of project files."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Dict
+
+import numpy as np
+from sklearn.feature_extraction.text import TfidfVectorizer
+
+from .file_loader import load_project_files, read_file
+
+
+@dataclass
+class SemanticIndex:
+    """Mapping of file paths to embedding vectors."""
+
+    embeddings: Dict[str, np.ndarray]
+    vectorizer: TfidfVectorizer
+
+    def update_file(self, path: str | Path) -> None:
+        """Update ``embeddings`` for ``path`` in-place."""
+        emb = get_file_embedding(path, self)
+        self.embeddings[str(path)] = emb
+
+    def rank(self, query: str, top_k: int = 5) -> list[tuple[str, float]]:
+        """Return ``top_k`` files ranked by similarity to ``query``."""
+        q_vec = self.vectorizer.transform([query]).toarray()[0]
+        scores = {
+            path: float(np.dot(vec, q_vec))
+            for path, vec in self.embeddings.items()
+        }
+        return sorted(scores.items(), key=lambda x: x[1], reverse=True)[:top_k]
+
+
+def build_semantic_index(base_dir: str | Path = ".") -> SemanticIndex:
+    """Return a :class:`SemanticIndex` for all project files under ``base_dir``."""
+    files = load_project_files(base_dir)
+    texts = [read_file(p) for p in files]
+    vectorizer = TfidfVectorizer()
+    matrix = vectorizer.fit_transform(texts)
+    embeddings = {
+        str(path): matrix[idx].toarray()[0]
+        for idx, path in enumerate(files)
+    }
+    return SemanticIndex(embeddings=embeddings, vectorizer=vectorizer)
+
+
+def get_file_embedding(path: str | Path, index: SemanticIndex) -> np.ndarray:
+    """Return the embedding vector for ``path`` using ``index``'s vectorizer."""
+    text = read_file(path)
+    return index.vectorizer.transform([text]).toarray()[0]
+
+
+def update_file_embedding(path: str | Path, index: SemanticIndex) -> None:
+    """Update ``index`` with the embedding for ``path``."""
+    index.update_file(path)
+
+
+def rank_files_by_query(query: str, index: SemanticIndex, top_k: int = 5) -> list[tuple[str, float]]:
+    """Rank indexed files by semantic similarity to ``query``."""
+    return index.rank(query, top_k=top_k)
diff --git a/tests/test_semantic_indexer.py b/tests/test_semantic_indexer.py
new file mode 100644
index 0000000..c23f6be
--- /dev/null
+++ b/tests/test_semantic_indexer.py
@@ -0,0 +1,43 @@
+from search.semantic_indexer import (
+    build_semantic_index,
+    get_file_embedding,
+    update_file_embedding,
+    rank_files_by_query,
+)
+import numpy as np
+
+
+def test_build_semantic_index(tmp_path):
+    (tmp_path / "a.txt").write_text("hello world")
+    (tmp_path / "b.py").write_text("print('hi')")
+    index = build_semantic_index(tmp_path)
+    assert set(index.embeddings.keys()) == {
+        str(tmp_path / "a.txt"),
+        str(tmp_path / "b.py"),
+    }
+    for emb in index.embeddings.values():
+        assert isinstance(emb, np.ndarray)
+        assert emb.ndim == 1
+
+
+def test_get_file_embedding(tmp_path):
+    (tmp_path / "a.txt").write_text("hello")
+    index = build_semantic_index(tmp_path)
+    emb1 = get_file_embedding(tmp_path / "a.txt", index)
+    emb2 = get_file_embedding(tmp_path / "a.txt", index)
+    assert np.allclose(emb1, emb2)
+
+
+def test_update_and_rank(tmp_path):
+    f1 = tmp_path / "hello.txt"
+    f2 = tmp_path / "bye.txt"
+    f1.write_text("hello world")
+    f2.write_text("goodbye world")
+    index = build_semantic_index(tmp_path)
+
+    # modify a file and update embedding
+    f1.write_text("hello there")
+    update_file_embedding(f1, index)
+
+    ranking = rank_files_by_query("hello", index, top_k=2)
+    assert ranking[0][0] == str(f1)