diff --git a/codex-rs/core/src/safety.rs b/codex-rs/core/src/safety.rs
index dec7a0087..4ed88783a 100644
--- a/codex-rs/core/src/safety.rs
+++ b/codex-rs/core/src/safety.rs
@@ -12,6 +12,11 @@ use codex_protocol::protocol::SandboxPolicy;
 use codex_sandboxing::SandboxType;
 use codex_sandboxing::get_platform_sandbox;
 
+const PATCH_REJECTED_OUTSIDE_PROJECT_REASON: &str =
+    "writing outside of the project; rejected by user approval settings";
+const PATCH_REJECTED_READ_ONLY_REASON: &str =
+    "writing is blocked by read-only sandbox; rejected by user approval settings";
+
 #[derive(Debug, PartialEq)]
 pub enum SafetyCheck {
     AutoApprove {
@@ -85,9 +90,7 @@ pub fn assess_patch_safety(
                 None => {
                     if rejects_sandbox_approval {
                         SafetyCheck::Reject {
-                            reason:
-                                "writing outside of the project; rejected by user approval settings"
-                                    .to_string(),
+                            reason: patch_rejection_reason(sandbox_policy).to_string(),
                         }
                     } else {
                         SafetyCheck::AskUser
@@ -97,14 +100,22 @@ pub fn assess_patch_safety(
         }
     } else if rejects_sandbox_approval {
         SafetyCheck::Reject {
-            reason: "writing outside of the project; rejected by user approval settings"
-                .to_string(),
+            reason: patch_rejection_reason(sandbox_policy).to_string(),
         }
     } else {
         SafetyCheck::AskUser
     }
 }
 
+fn patch_rejection_reason(sandbox_policy: &SandboxPolicy) -> &'static str {
+    match sandbox_policy {
+        SandboxPolicy::ReadOnly { .. } => PATCH_REJECTED_READ_ONLY_REASON,
+        SandboxPolicy::WorkspaceWrite { .. }
+        | SandboxPolicy::DangerFullAccess
+        | SandboxPolicy::ExternalSandbox { .. } => PATCH_REJECTED_OUTSIDE_PROJECT_REASON,
+    }
+}
+
 fn is_write_patch_constrained_to_writable_paths(
     action: &ApplyPatchAction,
     file_system_sandbox_policy: &FileSystemSandboxPolicy,
diff --git a/codex-rs/core/src/safety_tests.rs b/codex-rs/core/src/safety_tests.rs
index e7f55693d..5ebeb64e0 100644
--- a/codex-rs/core/src/safety_tests.rs
+++ b/codex-rs/core/src/safety_tests.rs
@@ -162,8 +162,36 @@ fn granular_sandbox_approval_false_rejects_out_of_root_patch() {
             WindowsSandboxLevel::Disabled,
         ),
         SafetyCheck::Reject {
-            reason: "writing outside of the project; rejected by user approval settings"
-                .to_string(),
+            reason: PATCH_REJECTED_OUTSIDE_PROJECT_REASON.to_string(),
+        },
+    );
+}
+
+#[test]
+fn read_only_policy_rejects_patch_with_read_only_reason() {
+    let tmp = TempDir::new().unwrap();
+    let cwd = tmp.path().to_path_buf();
+    let action = ApplyPatchAction::new_add_for_test(&cwd.join("inside.txt"), "".to_string());
+    let sandbox_policy = SandboxPolicy::new_read_only_policy();
+    let file_system_sandbox_policy =
+        FileSystemSandboxPolicy::from_legacy_sandbox_policy(&sandbox_policy, &cwd);
+
+    assert!(!is_write_patch_constrained_to_writable_paths(
+        &action,
+        &file_system_sandbox_policy,
+        &cwd,
+    ));
+    assert_eq!(
+        assess_patch_safety(
+            &action,
+            AskForApproval::Never,
+            &sandbox_policy,
+            &file_system_sandbox_policy,
+            &cwd,
+            WindowsSandboxLevel::Disabled,
+        ),
+        SafetyCheck::Reject {
+            reason: PATCH_REJECTED_READ_ONLY_REASON.to_string(),
         },
     );
 }