fix: harden cron tool contract and repeat guard

tests/cron/test_cron_tool_list.py

@@ -7,7 +7,6 @@ import pytest
 from nanobot.agent.tools.cron import CronTool
 from nanobot.cron.service import CronService
 from nanobot.cron.types import CronJob, CronJobState, CronPayload, CronSchedule
-from tests.test_openai_api import pytest_plugins
 
 
 def _make_tool(tmp_path) -> CronTool:
@@ -346,6 +345,47 @@ def test_add_job_can_disable_delivery(tmp_path) -> None:
     assert job.payload.deliver is False
 
 
+def test_cron_schema_advertises_action_specific_requirements(tmp_path) -> None:
+    tool = _make_tool(tmp_path)
+
+    assert tool.parameters["required"] == ["action"]
+    assert tool.parameters["oneOf"] == [
+        {
+            "properties": {
+                "action": {"enum": ["add"]},
+                "message": {"type": "string", "minLength": 1},
+            },
+            "required": ["action", "message"],
+        },
+        {
+            "properties": {"action": {"enum": ["list"]}},
+            "required": ["action"],
+        },
+        {
+            "properties": {"action": {"enum": ["remove"]}},
+            "required": ["action", "job_id"],
+        },
+    ]
+
+
+def test_validate_params_requires_message_only_for_add(tmp_path) -> None:
+    tool = _make_tool(tmp_path)
+
+    assert "message is required when action='add'" in tool.validate_params({"action": "add"})
+    assert tool.validate_params({"action": "list"}) == []
+    assert "job_id is required when action='remove'" in tool.validate_params({"action": "remove"})
+
+
+def test_add_job_empty_message_returns_actionable_error(tmp_path) -> None:
+    tool = _make_tool(tmp_path)
+    tool.set_context("telegram", "chat-1")
+
+    result = tool._add_job(None, "", 60, None, None, None)
+
+    assert "action='add' requires a non-empty 'message'" in result
+    assert "Retry including message=" in result
+
+
 def test_list_excludes_disabled_jobs(tmp_path) -> None:
     tool = _make_tool(tmp_path)
     job = tool._cron.add_job(