diff --git a/nanobot/agent/tools/filesystem.py b/nanobot/agent/tools/filesystem.py
index 2c7d8955..2d3eedc2 100644
--- a/nanobot/agent/tools/filesystem.py
+++ b/nanobot/agent/tools/filesystem.py
@@ -237,6 +237,7 @@ class ReadFileTool(_FsTool):
     _scopes = {"core", "subagent", "memory"}
 
     _MAX_CHARS = 128_000
+    _MAX_FILE_SIZE_BYTES = 100 * 1024 * 1024
     _DEFAULT_LIMIT = 2000
     _MAX_PDF_PAGES = 20
 
@@ -290,6 +291,15 @@ class ReadFileTool(_FsTool):
             if not fp.is_file():
                 return ToolResult.error(f"Error: Not a file: {path}")
 
+            file_size = fp.stat().st_size
+            if file_size > self._MAX_FILE_SIZE_BYTES:
+                size_mib = file_size / (1024 * 1024)
+                max_mib = self._MAX_FILE_SIZE_BYTES // (1024 * 1024)
+                return ToolResult.error(
+                    f"Error: File too large to read ({size_mib:.1f} MiB). "
+                    f"Maximum is {max_mib} MiB."
+                )
+
             # PDF support
             if fp.suffix.lower() == ".pdf":
                 return self._read_pdf(fp, pages)
diff --git a/tests/tools/test_filesystem_tools.py b/tests/tools/test_filesystem_tools.py
index 52667030..9b917dc6 100644
--- a/tests/tools/test_filesystem_tools.py
+++ b/tests/tools/test_filesystem_tools.py
@@ -99,6 +99,22 @@ class TestReadFileTool:
         assert len(result) <= ReadFileTool._MAX_CHARS + 500  # small margin for footer
         assert "Use offset=" in result
 
+    @pytest.mark.asyncio
+    async def test_oversized_file_is_rejected_before_read(self, tool, tmp_path, monkeypatch):
+        f = tmp_path / "huge.txt"
+        with f.open("wb") as stream:
+            stream.truncate(ReadFileTool._MAX_FILE_SIZE_BYTES + 1)
+
+        def fail_read_bytes(self):
+            raise AssertionError("oversized file content should not be loaded")
+
+        monkeypatch.setattr(type(f), "read_bytes", fail_read_bytes)
+
+        result = await tool.execute(path=str(f))
+
+        assert "File too large to read" in result
+        assert "Maximum is 100 MiB" in result
+
 
 # ---------------------------------------------------------------------------
 # _find_match  (unit tests for the helper)