From d80a0fa7e4e7121ef9a5e0f9e4ca2e32adadedf6 Mon Sep 17 00:00:00 2001
From: Bryan Frimin <bryan@getprobo.com>
Date: Fri, 13 Mar 2026 19:57:18 +0100
Subject: [PATCH] Reject null input in agent tool parameter validation

JSON null unmarshals into an empty string, so the presence-only
key check let {"input":null} through, running the nested agent
with a blank user message.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>
---
 pkg/agent/agent_tool.go      |  3 ++-
 pkg/agent/agent_tool_test.go | 21 +++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/pkg/agent/agent_tool.go b/pkg/agent/agent_tool.go
index 16ccad145..51345abab 100644
--- a/pkg/agent/agent_tool.go
+++ b/pkg/agent/agent_tool.go
@@ -83,7 +83,8 @@ func (t *agentTool) Execute(ctx context.Context, arguments string) (ToolResult,
 		}, nil
 	}
 
-	if _, ok := fields["input"]; !ok {
+	raw, ok := fields["input"]
+	if !ok || string(raw) == "null" {
 		return ToolResult{
 			Content: "Missing required parameters: input",
 			IsError: true,
diff --git a/pkg/agent/agent_tool_test.go b/pkg/agent/agent_tool_test.go
index 09440df1e..fe89a8574 100644
--- a/pkg/agent/agent_tool_test.go
+++ b/pkg/agent/agent_tool_test.go
@@ -215,6 +215,27 @@ func TestAgentTool_Execute(t *testing.T) {
 		},
 	)
 
+	t.Run(
+		"null input returns tool error for missing input",
+		func(t *testing.T) {
+			t.Parallel()
+
+			ag := agent.New(
+				"sub",
+				newTestClient(&mockProvider{}),
+				agent.WithModel("test-model"),
+			)
+
+			tool := ag.AsTool("sub_tool", "A sub-agent tool.")
+			result, err := tool.Execute(context.Background(), `{"input":null}`)
+
+			require.NoError(t, err)
+			assert.True(t, result.IsError)
+			assert.Contains(t, result.Content, "Missing required parameters")
+			assert.Contains(t, result.Content, "input")
+		},
+	)
+
 	t.Run(
 		"sub-agent error propagates as Go error",
 		func(t *testing.T) {