diff --git a/codex-rs/core/src/model_family.rs b/codex-rs/core/src/model_family.rs
index 3ed5fa2998..ef54a95841 100644
--- a/codex-rs/core/src/model_family.rs
+++ b/codex-rs/core/src/model_family.rs
@@ -12,6 +12,7 @@ const BASE_INSTRUCTIONS: &str = include_str!("../prompt.md");
 
 const GPT_5_CODEX_INSTRUCTIONS: &str = include_str!("../gpt_5_codex_prompt.md");
 const GPT_5_1_INSTRUCTIONS: &str = include_str!("../gpt_5_1_prompt.md");
+const GPT_5_1_CODEX_MAX_INSTRUCTIONS: &str = include_str!("../gpt-5.1-codex-max_prompt.md");
 
 /// A model family is a group of models that share certain characteristics.
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
@@ -179,7 +180,7 @@ pub fn find_family_for_model(slug: &str) -> Option<ModelFamily> {
             slug, slug,
             supports_reasoning_summaries: true,
             reasoning_summary_format: ReasoningSummaryFormat::Experimental,
-            base_instructions: BASE_INSTRUCTIONS.to_string(),
+            base_instructions: GPT_5_1_CODEX_MAX_INSTRUCTIONS.to_string(),
             apply_patch_tool_type: Some(ApplyPatchToolType::Freeform),
             shell_type: ConfigShellToolType::ShellCommand,
             supports_parallel_tool_calls: true,