diff --git a/bex/grammar_index.py b/bex/grammar_index.py
new file mode 100644
index 0000000..9561be1
--- /dev/null
+++ b/bex/grammar_index.py
@@ -0,0 +1,139 @@
+"""Grammar index for runtime lookup.
+
+Loads grammars.yml and builds a lookup index mapping
+(package, context_symbol) → GBNF grammar string.
+
+Usage:
+    from bex.grammar_index import load_grammar_index
+
+    idx = load_grammar_index("/path/to/project")
+    grammar = idx.get(file_path="src/services/Chat.kt", context_symbol="return")
+    all_grammars = idx.get_package("src/services")
+"""
+
+import os
+import re
+import yaml
+
+
+_LABEL_RE = re.compile(r"^(.+)\s+\[([^\]]+)\]$")
+
+
+class GrammarIndex:
+    """In-memory index of grammars keyed by (package, context_symbol)."""
+
+    def __init__(self, project_root, entries):
+        """
+        Args:
+            project_root: absolute path to the project root.
+            entries: list of dicts with keys: package, grammar, score, methods, algorithm.
+        """
+        self.project_root = project_root
+        self._by_package = {}       # package → [(symbol, grammar, score)]
+        self._all = entries
+
+        for e in entries:
+            label = e["package"]
+            m = _LABEL_RE.match(label)
+            if m:
+                pkg = m.group(1).rstrip("/")
+                symbol = m.group(2)
+            else:
+                pkg = label.rstrip("/")
+                symbol = ""
+
+            self._by_package.setdefault(pkg, []).append((
+                symbol,
+                e["grammar"],
+                e.get("score", 0),
+                e.get("methods", 0),
+            ))
+
+        # Sort each package's entries by score descending (best first)
+        for pkg in self._by_package:
+            self._by_package[pkg].sort(key=lambda x: -x[2])
+
+    def get(self, file_path, context_symbol=None):
+        """Get the best grammar for a file, optionally filtered by context.
+
+        Args:
+            file_path: path to the source file (absolute or relative to project_root).
+            context_symbol: if provided, match the leaf grammar whose first symbol
+                is this. If None, return the best grammar for the package.
+
+        Returns:
+            GBNF grammar string, or None if no match.
+        """
+        pkg = self._resolve_package(file_path)
+        entries = self._by_package.get(pkg, [])
+        if not entries:
+            return None
+
+        if context_symbol:
+            for sym, grammar, score, methods in entries:
+                if sym == context_symbol:
+                    return grammar
+
+        # Fall back to best grammar for the package
+        return entries[0][1] if entries else None
+
+    def get_package(self, file_path):
+        """Get all grammars for a file's package.
+
+        Returns:
+            list of (context_symbol, grammar, score, methods) tuples,
+            sorted by score descending. Empty list if no match.
+        """
+        pkg = self._resolve_package(file_path)
+        return list(self._by_package.get(pkg, []))
+
+    def get_all(self):
+        """Return all entries as a flat list."""
+        return list(self._all)
+
+    def packages(self):
+        """Return sorted list of all indexed packages."""
+        return sorted(self._by_package.keys())
+
+    def _resolve_package(self, file_path):
+        """Map a file path to its package."""
+        # Make absolute if relative
+        if not os.path.isabs(file_path):
+            file_path = os.path.join(self.project_root, file_path)
+
+        # Get directory of file relative to project root
+        try:
+            rel = os.path.relpath(os.path.dirname(file_path), self.project_root)
+        except ValueError:
+            # Different drives on Windows
+            return ""
+        if rel == ".":
+            return ""
+        return rel
+
+    def __repr__(self):
+        return f"GrammarIndex({self.project_root}, {len(self._all)} entries, {len(self._by_package)} packages)"
+
+
+def load_grammar_index(project_root):
+    """Load grammars.yml from {project_root}/.dervish/grammars.yml.
+
+    Returns GrammarIndex, or empty index if no file found.
+    """
+    yml_path = os.path.join(project_root, ".dervish", "grammars.yml")
+    if not os.path.exists(yml_path):
+        return GrammarIndex(project_root, [])
+
+    with open(yml_path) as f:
+        data = yaml.safe_load(f)
+
+    entries = []
+    if isinstance(data, dict):
+        for module, items in data.items():
+            if not isinstance(items, list):
+                continue
+            for item in items:
+                if isinstance(item, dict) and "package" in item and "grammar" in item:
+                    entries.append(item)
+
+    return GrammarIndex(project_root, entries)
diff --git a/tests/test_grammar_index.py b/tests/test_grammar_index.py
new file mode 100644
index 0000000..8154e23
--- /dev/null
+++ b/tests/test_grammar_index.py
@@ -0,0 +1,162 @@
+"""Tests for bex.grammar_index — runtime grammar lookup."""
+
+import os
+import tempfile
+
+import pytest
+import yaml
+
+from bex.grammar_index import GrammarIndex, load_grammar_index
+
+
+# ---------------------------------------------------------------------------
+# Fixtures
+# ---------------------------------------------------------------------------
+
+def _make_yml(tmp_path, entries):
+    """Write a grammars.yml with the given entries grouped by module."""
+    dervish = tmp_path / ".dervish"
+    dervish.mkdir()
+    data = {"test_module": entries}
+    with open(dervish / "grammars.yml", "w") as f:
+        yaml.dump(data, f, default_flow_style=False)
+
+
+SAMPLE_ENTRIES = [
+    {
+        "package": "src/services [return]",
+        "methods": 10,
+        "grammar": "return.ok+?.error?",
+        "score": 0.8,
+        "algorithm": "CRX",
+        "mdl": 12.0,
+    },
+    {
+        "package": "src/services [if]",
+        "methods": 5,
+        "grammar": "if.cond+.then+.else?",
+        "score": 0.6,
+        "algorithm": "CRX",
+        "mdl": 15.0,
+    },
+    {
+        "package": "src/utils [return]",
+        "methods": 8,
+        "grammar": "return.value+",
+        "score": 1.0,
+        "algorithm": "CRX",
+        "mdl": 5.0,
+    },
+]
+
+
+# ---------------------------------------------------------------------------
+# GrammarIndex construction
+# ---------------------------------------------------------------------------
+
+class TestGrammarIndex:
+
+    def test_parse_label_with_symbol(self, tmp_path):
+        idx = GrammarIndex(str(tmp_path), SAMPLE_ENTRIES)
+        pkgs = idx.packages()
+        assert "src/services" in pkgs
+        assert "src/utils" in pkgs
+
+    def test_parse_label_without_symbol(self, tmp_path):
+        entries = [{"package": "src/root", "grammar": "a.b", "score": 0.5}]
+        idx = GrammarIndex(str(tmp_path), entries)
+        assert "src/root" in idx.packages()
+
+    def test_sorted_by_score_desc(self, tmp_path):
+        idx = GrammarIndex(str(tmp_path), SAMPLE_ENTRIES)
+        entries = idx.get_package("src/services")
+        scores = [s for _, _, s, _ in entries]
+        assert scores == sorted(scores, reverse=True)
+
+    def test_repr(self, tmp_path):
+        idx = GrammarIndex(str(tmp_path), SAMPLE_ENTRIES)
+        assert "3 entries" in repr(idx)
+        assert "2 packages" in repr(idx)
+
+
+# ---------------------------------------------------------------------------
+# get()
+# ---------------------------------------------------------------------------
+
+class TestGet:
+
+    def test_get_best_no_context(self, tmp_path):
+        idx = GrammarIndex(str(tmp_path), SAMPLE_ENTRIES)
+        g = idx.get("src/services/main.kt")
+        assert g == "return.ok+?.error?"
+
+    def test_get_by_context_symbol(self, tmp_path):
+        idx = GrammarIndex(str(tmp_path), SAMPLE_ENTRIES)
+        g = idx.get("src/services/main.kt", context_symbol="if")
+        assert g == "if.cond+.then+.else?"
+
+    def test_get_missing_context_falls_back_to_best(self, tmp_path):
+        idx = GrammarIndex(str(tmp_path), SAMPLE_ENTRIES)
+        g = idx.get("src/services/main.kt", context_symbol="nonexistent")
+        assert g == "return.ok+?.error?"
+
+    def test_get_unknown_package_returns_none(self, tmp_path):
+        idx = GrammarIndex(str(tmp_path), SAMPLE_ENTRIES)
+        g = idx.get("src/unknown/file.kt")
+        assert g is None
+
+    def test_get_absolute_path(self, tmp_path):
+        idx = GrammarIndex(str(tmp_path), SAMPLE_ENTRIES)
+        abs_path = str(tmp_path / "src" / "services" / "main.kt")
+        g = idx.get(abs_path, context_symbol="return")
+        assert g == "return.ok+?.error?"
+
+
+# ---------------------------------------------------------------------------
+# get_package()
+# ---------------------------------------------------------------------------
+
+class TestGetPackage:
+
+    def test_returns_all_entries(self, tmp_path):
+        idx = GrammarIndex(str(tmp_path), SAMPLE_ENTRIES)
+        entries = idx.get_package("src/services/main.kt")
+        assert len(entries) == 2
+
+    def test_unknown_package_returns_empty(self, tmp_path):
+        idx = GrammarIndex(str(tmp_path), SAMPLE_ENTRIES)
+        entries = idx.get_package("src/unknown/file.kt")
+        assert entries == []
+
+
+# ---------------------------------------------------------------------------
+# load_grammar_index()
+# ---------------------------------------------------------------------------
+
+class TestLoadGrammarIndex:
+
+    def test_load_existing(self, tmp_path):
+        _make_yml(tmp_path, SAMPLE_ENTRIES)
+        idx = load_grammar_index(str(tmp_path))
+        assert len(idx.get_all()) == 3
+        assert "src/services" in idx.packages()
+
+    def test_load_missing_file(self, tmp_path):
+        idx = load_grammar_index(str(tmp_path))
+        assert len(idx.get_all()) == 0
+        assert idx.packages() == []
+
+    def test_roundtrip_persisted_ragsak(self):
+        """Integration test: grammars.yml generated by analyze_directory."""
+        ragsak_yml = "/home/tobi/Desktop/kesai/RAGSAK/.dervish/grammars.yml"
+        if not os.path.exists(ragsak_yml):
+            pytest.skip("RAGSAK grammars.yml not generated yet")
+        idx = load_grammar_index("/home/tobi/Desktop/kesai/RAGSAK")
+        assert len(idx.get_all()) > 0
+        # Should be able to resolve a file in the storage package
+        g = idx.get(
+            "/home/tobi/Desktop/kesai/RAGSAK/infrastructure/adapters/search/src/main/kotlin/eu/corentic/springrag/service/storage/GcsStorage.kt",
+            context_symbol="return",
+        )
+        assert g is not None
+        assert "return" in g