diff --git a/agent.py b/agent.py
index 0d8354b..4536bf3 100644
--- a/agent.py
+++ b/agent.py
@@ -715,7 +715,7 @@ class ProductionIncidentCommander(A2AAgent[CommanderConfig, NoAuth]):
         await workspace_write_json(ctx, f"outputs/incidents/{incident_id}/diagnosis.json", diagnosis)
         await workspace_write_text(ctx, f"outputs/incidents/{incident_id}/summary.md", _diagnosis_markdown(diagnosis), "text/markdown")
         await emit_phase(ctx, "diagnosis", evidence_count=len(evidence), status="artifacts_written", started=started)
-        return diagnosis
+        return diagnosis.model_dump(mode="json")
 
     @a2a.tool(description="Create an ordered remediation plan from diagnosis artifacts. This skill never executes changes.", timeout_seconds=300, cost_class="incident-planning", grant_mode="read_write_overlay", grant_write_prefixes=("outputs/incidents/",), grant_allow_patterns=("outputs/incidents/**",))
     async def plan_remediation(