diff --git a/lib/bash/str/README.md b/lib/bash/str/README.md
index c9afbe0..cf628b9 100644
--- a/lib/bash/str/README.md
+++ b/lib/bash/str/README.md
@@ -59,7 +59,8 @@ str_join joined "|" parts
 - Trim helpers remove Bash character-class whitespace from the requested side.
 - String transformation helpers mutate the named variable in place and do not
   print transformed values for command substitution.
-- Predicate helpers return shell status and do not print output.
+- Predicate helpers require exactly two arguments, return shell status, and do
+  not print output.
 - `str_split` preserves empty fields between repeated delimiters.
 - `str_join` preserves empty array elements, including trailing empty elements.
 - Named string, result, and array arguments must be valid Bash variable names.
diff --git a/lib/bash/str/lib_str.sh b/lib/bash/str/lib_str.sh
index 49e5f39..3da8426 100644
--- a/lib/bash/str/lib_str.sh
+++ b/lib/bash/str/lib_str.sh
@@ -56,39 +56,45 @@ str_trim() {
 
 str_contains() {
     local value="${1-}" needle="${2-}"
+
+    assert_arg_count "$#" 2
     [[ "$value" == *"$needle"* ]]
 }
 
 str_starts_with() {
     local value="${1-}" prefix="${2-}"
+
+    assert_arg_count "$#" 2
     [[ "$value" == "$prefix"* ]]
 }
 
 str_ends_with() {
     local value="${1-}" suffix="${2-}"
+
+    assert_arg_count "$#" 2
     [[ "$value" == *"$suffix" ]]
 }
 
 str_split() {
-    local result_name="${1-}" value="${2-}" separator="${3-}"
+    local __str_split_result_name="${1-}" __str_split_value="${2-}" __str_split_separator="${3-}"
 
     assert_arg_count "$#" 3
-    assert_variable_name "$result_name"
+    assert_variable_name "$__str_split_result_name"
 
-    local -a fields=()
-    local remainder="$value"
+    local -a __str_split_fields=()
+    local __str_split_remainder="$__str_split_value"
 
-    if [[ -z "$separator" ]]; then
-        fields=("$value")
+    if [[ -z "$__str_split_separator" ]]; then
+        __str_split_fields=("$__str_split_value")
     else
-        while [[ "$remainder" == *"$separator"* ]]; do
-            fields+=("${remainder%%"$separator"*}")
-            remainder="${remainder#*"$separator"}"
+        while [[ "$__str_split_remainder" == *"$__str_split_separator"* ]]; do
+            __str_split_fields+=("${__str_split_remainder%%"$__str_split_separator"*}")
+            __str_split_remainder="${__str_split_remainder#*"$__str_split_separator"}"
         done
-        fields+=("$remainder")
+        __str_split_fields+=("$__str_split_remainder")
     fi
 
-    eval "$result_name=(\"\${fields[@]}\")"
+    eval "$__str_split_result_name=(\"\${__str_split_fields[@]}\")"
 }
 
 str_join() {
diff --git a/lib/bash/str/tests/lib_str.bats b/lib/bash/str/tests/lib_str.bats
index 0c74813..c7d2214 100644
--- a/lib/bash/str/tests/lib_str.bats
+++ b/lib/bash/str/tests/lib_str.bats
@@ -102,6 +102,29 @@ EOF
     fi
 }
 
+@test "string predicate helpers reject incorrect argument counts" {
+    local script="$TEST_TMPDIR/string-predicate-arity.sh"
+
+    create_script "$script" <<EOF
+#!/usr/bin/env bash
+source "$BASE_BASH_DIR/std/lib_std.sh"
+source "$BASE_BASH_DIR/str/lib_str.sh"
+"\$@"
+EOF
+
+    bats_run bash "$script" str_contains "needle-only"
+    [ "$status" -eq 1 ]
+    [[ "$output" == *"Argument count mismatch: expected 2 but got 1 arguments"* ]]
+
+    bats_run bash "$script" str_starts_with "value" "prefix" "extra"
+    [ "$status" -eq 1 ]
+    [[ "$output" == *"Argument count mismatch: expected 2 but got 3 arguments"* ]]
+
+    bats_run bash "$script" str_ends_with
+    [ "$status" -eq 1 ]
+    [[ "$output" == *"Argument count mismatch: expected 2 but got 0 arguments"* ]]
+}
+
 @test "str_split stores delimited fields in a named array" {
     local -a parts=()
 
@@ -114,6 +137,17 @@ EOF
     [ "${parts[3]}" = "gamma" ]
 }
 
+@test "str_split can store results in an array named fields" {
+    local -a fields=()
+
+    str_split fields "alpha:beta:gamma" ":"
+
+    [ "${#fields[@]}" -eq 3 ]
+    [ "${fields[0]}" = "alpha" ]
+    [ "${fields[1]}" = "beta" ]
+    [ "${fields[2]}" = "gamma" ]
+}
+
 @test "str_split rejects invalid result variable names" {
     local script="$TEST_TMPDIR/str-split-invalid.sh"