diff --git a/pkg/builtin/builtin.go b/pkg/builtin/builtin.go
index f972d14c..f8f44ba9 100644
--- a/pkg/builtin/builtin.go
+++ b/pkg/builtin/builtin.go
@@ -59,7 +59,7 @@ var tools = map[string]types.Tool{
 	"sys.read": {
 		ToolDef: types.ToolDef{
 			Parameters: types.Parameters{
-				Description: "Reads the contents of a file",
+				Description: "Reads the contents of a file. Can only read plain text files, not binary files",
 				Arguments: types.ObjectSchema(
 					"filename", "The name of the file to read"),
 			},
diff --git a/pkg/tests/testdata/TestToolsChange/call1.golden b/pkg/tests/testdata/TestToolsChange/call1.golden
index 6c7c2d55..69ab3d03 100644
--- a/pkg/tests/testdata/TestToolsChange/call1.golden
+++ b/pkg/tests/testdata/TestToolsChange/call1.golden
@@ -22,7 +22,7 @@
       "function": {
         "toolID": "sys.read",
         "name": "read",
-        "description": "Reads the contents of a file",
+        "description": "Reads the contents of a file. Can only read plain text files, not binary files",
         "parameters": {
           "properties": {
             "filename": {
diff --git a/pkg/tests/testdata/TestToolsChange/step1.golden b/pkg/tests/testdata/TestToolsChange/step1.golden
index 1aae05d1..e26862ae 100644
--- a/pkg/tests/testdata/TestToolsChange/step1.golden
+++ b/pkg/tests/testdata/TestToolsChange/step1.golden
@@ -30,7 +30,7 @@
               "function": {
                 "toolID": "sys.read",
                 "name": "read",
-                "description": "Reads the contents of a file",
+                "description": "Reads the contents of a file. Can only read plain text files, not binary files",
                 "parameters": {
                   "properties": {
                     "filename": {