diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index a8207a4..cdacd63 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -2,3 +2,8 @@
 **Vulnerability:** Unvalidated inputs passed to `if()` statements can cause process crashes (`condition has length > 1`) or unexpected coercion vulnerabilities.
 **Learning:** In R, optional boolean parameters that default to `NULL` should be validated using explicit runtime type validation (e.g., `if (!is.null(flag) && (!is.logical(flag) || length(flag) != 1 || is.na(flag)))`).
 **Prevention:** Always implement explicit runtime type validation for optional boolean parameters.
+
+## 2024-07-20 - Prevent Integer Overflow Coercion Vulnerability in Interactive Prompts
+**Vulnerability:** Unbounded digit classes (`^[0-9]+$`) in interactive input validation allow excessively large numeric strings that evaluate to `NA` when coerced with `as.integer()`, which could lead to downstream crashes.
+**Learning:** `readline()` input validation should strictly match the exact expected options (e.g., `^[12]$`) rather than generic digit patterns.
+**Prevention:** Always use precise regex patterns limiting the length and allowed characters when validating inputs intended for integer conversion in R.
diff --git a/R/aFIPC.R b/R/aFIPC.R
index 6254651..c04ec0c 100644
--- a/R/aFIPC.R
+++ b/R/aFIPC.R
@@ -1,3 +1,10 @@
+.is_binary_prompt_choice <- function(value) {
+  is.character(value) &&
+    length(value) == 1L &&
+    !is.na(value) &&
+    grepl("^[12]$", value)
+}
+
 #' automated fixed item parameter linking
 #'
 #' @import mirt
@@ -141,7 +148,7 @@ autoFIPC <-
       }
       for (attempt in seq_len(3)) {
         n <- readline(prompt = "Is it correct? (1: Yes 2: No) : ")
-        if (grepl("^[0-9]+$", n)) {
+        if (.is_binary_prompt_choice(n)) {
           return(as.integer(n))
         }
       }
@@ -171,7 +178,7 @@ autoFIPC <-
               readline(
                 prompt = "Do you want to use default BILOG-MG priors for oldform Data? (1: Yes 2: No) : "
               )
-            if (grepl("^[0-9]+$", n)) {
+            if (.is_binary_prompt_choice(n)) {
               return(as.integer(n))
             }
           }
@@ -390,7 +397,7 @@ autoFIPC <-
               readline(
                 prompt = "Do you want to use default BILOG-MG priors for newform Data? (1: Yes 2: No) : "
               )
-            if (grepl("^[0-9]+$", n)) {
+            if (.is_binary_prompt_choice(n)) {
               return(as.integer(n))
             }
           }
diff --git a/tests/testthat/test-prompt-validation.R b/tests/testthat/test-prompt-validation.R
new file mode 100644
index 0000000..98b00ac
--- /dev/null
+++ b/tests/testthat/test-prompt-validation.R
@@ -0,0 +1,22 @@
+test_that("binary prompt choices accept only the documented scalar inputs", {
+  expect_true(aFIPC:::.is_binary_prompt_choice("1"))
+  expect_true(aFIPC:::.is_binary_prompt_choice("2"))
+
+  invalid <- list(
+    "0",
+    "3",
+    "01",
+    "1 ",
+    " 2",
+    "999999999999999999999999999999999999999999",
+    "not-a-number",
+    NA_character_,
+    character(),
+    c("1", "2"),
+    1L
+  )
+
+  for (value in invalid) {
+    expect_false(aFIPC:::.is_binary_prompt_choice(value))
+  }
+})