diff --git a/contrib/codeql/nightly/NonBinaryIsFunction.ql b/contrib/codeql/nightly/NonBinaryIsFunction.ql
new file mode 100644
index 00000000000..74943db20f3
--- /dev/null
+++ b/contrib/codeql/nightly/NonBinaryIsFunction.ql
@@ -0,0 +1,39 @@
+/**
+ * @name Finds functions that suggest a boolean return type, but do not.
+ * @description This query identifies functions that are named with the `is` prefix but return values outside
+ *             the typical boolean range of 0 and 1.
+ * @kind problem
+ * @id asymmetric-research/non-binary-is-function
+ * @precision medium
+ * @severity warning
+ * @tags correctness
+ *       maintainability
+ */
+
+import cpp
+import semmle.code.cpp.rangeanalysis.new.SimpleRangeAnalysis
+
+/**
+ * A function whose name suggests it returns a boolean value (0 or 1).
+ */
+class IsFunction extends Function {
+  IsFunction() {
+    this.getName().matches("%\\_is\\_%") and
+    this.getType() instanceof IntegralType and
+    not this.getName() = "fd_bn254_pairing_is_one_syscall"
+    // returns values in [-1, 0]
+  }
+}
+
+from IsFunction f, ReturnStmt rs, int lowerBound, int upperBound
+where
+  rs.getEnclosingFunction() = f and
+  lowerBound = lowerBound(rs.getExpr().getFullyConverted()) and
+  upperBound = upperBound(rs.getExpr().getFullyConverted()) and
+  (
+    lowerBound < 0 or
+    upperBound > 1
+  )
+select rs,
+  "The function $@ is named like an `is` function but returns a value with bounds [" + lowerBound +
+    ", " + upperBound + "].", f, f.getName()
diff --git a/contrib/codeql/test/query-tests/NonBinaryIsFunction/NonBinaryIsFunction.c b/contrib/codeql/test/query-tests/NonBinaryIsFunction/NonBinaryIsFunction.c
new file mode 100644
index 00000000000..b1c968be865
--- /dev/null
+++ b/contrib/codeql/test/query-tests/NonBinaryIsFunction/NonBinaryIsFunction.c
@@ -0,0 +1,44 @@
+// Add missing typedef and define
+#define FD_EXECUTOR_INSTR_ERR_MISSING_ACC (-33) /* An account required by the instruction is missing */
+#define FD_BANK_FLAGS_DEAD (0x00000008UL)       /* Dead.  We stopped replaying it before we could finish it (e.g. invalid block or pruned minority fork). */
+
+typedef struct
+{
+    int acct_cnt;
+    struct
+    {
+        int is_signer;
+    } accounts[10];
+} fd_instr_info_t;
+
+typedef struct
+{
+    int flags;
+} fd_bank_t;
+
+int fd_instr_acc_is_signer_idx(fd_instr_info_t const *instr,
+                               short idx)
+{
+    if (FD_UNLIKELY(idx >= instr->acct_cnt))
+    {
+        return FD_EXECUTOR_INSTR_ERR_MISSING_ACC; // $ Alert (returns -33 which is not 0 or 1)
+    }
+
+    return !!(instr->accounts[idx].is_signer);
+}
+
+static inline int
+fd_banks_is_bank_dead(fd_bank_t *bank)
+{
+    return bank->flags & FD_BANK_FLAGS_DEAD; // $ Alert (returns value in [0, 8])
+}
+
+// inspired by the function, but heavily simplified
+int fd_bn254_pairing_is_one_syscall(unsigned long in_sz)
+{
+    if (in_sz != 128UL)
+    {
+        return -1; // No alert
+    }
+    return 0;
+}
\ No newline at end of file
diff --git a/contrib/codeql/test/query-tests/NonBinaryIsFunction/NonBinaryIsFunction.expected b/contrib/codeql/test/query-tests/NonBinaryIsFunction/NonBinaryIsFunction.expected
new file mode 100644
index 00000000000..844e363cd0f
--- /dev/null
+++ b/contrib/codeql/test/query-tests/NonBinaryIsFunction/NonBinaryIsFunction.expected
@@ -0,0 +1,2 @@
+| NonBinaryIsFunction.c:24:9:24:49 | return ... | The function $@ is named like an `is` function but returns a value with bounds [-33, -33]. | NonBinaryIsFunction.c:19:5:19:30 | fd_instr_acc_is_signer_idx | fd_instr_acc_is_signer_idx |
+| NonBinaryIsFunction.c:33:5:33:44 | return ... | The function $@ is named like an `is` function but returns a value with bounds [0, 8]. | NonBinaryIsFunction.c:31:1:31:21 | fd_banks_is_bank_dead | fd_banks_is_bank_dead |
diff --git a/contrib/codeql/test/query-tests/NonBinaryIsFunction/NonBinaryIsFunction.qlref b/contrib/codeql/test/query-tests/NonBinaryIsFunction/NonBinaryIsFunction.qlref
new file mode 100644
index 00000000000..79e6810c2fe
--- /dev/null
+++ b/contrib/codeql/test/query-tests/NonBinaryIsFunction/NonBinaryIsFunction.qlref
@@ -0,0 +1,2 @@
+query: NonBinaryIsFunction.ql
+postprocess: InlineExpectationsTestQuery.ql
\ No newline at end of file