diff --git a/src/fastcontext/agent/tool/grep.py b/src/fastcontext/agent/tool/grep.py
index 4ee161f..96fd78a 100644
--- a/src/fastcontext/agent/tool/grep.py
+++ b/src/fastcontext/agent/tool/grep.py
@@ -26,7 +26,7 @@ class GrepTool(Tool):
             "output_mode": {
                 "type": "string",
                 "enum": ["content", "files_with_matches", "count"],
-                "description": 'Output mode: "content" shows matching lines (supports -A/-B/-C context, -n line numbers, head_limit), "files_with_matches" shows file paths (supports head_limit), "count" shows match counts (supports head_limit). Defaults to "files_with_matches".',
+                "description": 'Output mode: "content" shows matching lines (supports -A/-B/-C context, -n line numbers, head_limit), "files_with_matches" shows file paths (supports head_limit), "count" shows match counts (supports head_limit). Defaults to "content".',
             },
             "-B": {
                 "type": "number",
@@ -72,7 +72,7 @@ async def call(self, parameters: str, **kwargs) -> str:
         pattern = params.get("pattern")
         path = params.get("path", cwd)
         glob = params.get("glob")
-        output_mode = params.get("output_mode")
+        output_mode = params.get("output_mode", "content")
         before_context = params.get("-B")
         after_context = params.get("-A")
         context = params.get("-C", 3)
diff --git a/tests/test_grep_default_mode.py b/tests/test_grep_default_mode.py
new file mode 100644
index 0000000..6ecbf8c
--- /dev/null
+++ b/tests/test_grep_default_mode.py
@@ -0,0 +1,31 @@
+import asyncio
+import json
+import re
+import tempfile
+from pathlib import Path
+
+from fastcontext.agent.tool.grep import GrepTool
+
+
+def test_grep_defaults_to_content():
+    """With output_mode omitted, Grep must behave as 'content' (matching lines
+    with line numbers), not as files_with_matches and not as a flag-less rg
+    call that drops -n."""
+    grep = GrepTool()
+    with tempfile.TemporaryDirectory() as cwd:
+        (Path(cwd) / "haystack.txt").write_text("alpha\nMATCH here\nbeta\n", encoding="utf-8")
+
+        out = asyncio.run(grep.call(json.dumps({"pattern": "MATCH"}), cwd=cwd))
+        lines = out.splitlines()
+
+        # Content mode with -n emits "<lineno>:<text>" rows for matches.
+        assert any(re.match(r"\d+:.*MATCH", ln) for ln in lines), f"expected line-numbered match, got: {out!r}"
+        # files_with_matches would emit only the bare path with no match text.
+        assert "MATCH here" in out
+
+
+def test_grep_schema_documents_content_default():
+    schema = GrepTool().schema()
+    desc = schema["function"]["parameters"]["properties"]["output_mode"]["description"]
+    assert 'Defaults to "content"' in desc
+    assert "files_with_matches" not in desc.split("Defaults to")[1]