diff --git a/.codex/agents/implementation-agent.toml b/.codex/agents/implementation-agent.toml
index f0c44ad..8b159b8 100644
--- a/.codex/agents/implementation-agent.toml
+++ b/.codex/agents/implementation-agent.toml
@@ -1,6 +1,7 @@
 name = "implementation-agent"
 description = "Implements FESA solver features in C++17/MSVC by following approved TDD-first implementation plans."
 sandbox_mode = "workspace-write"
+model = "gpt-5.6-terra"
 model_reasoning_effort = "extra high"
 
 developer_instructions = """