diff --git a/lib/bash/README.md b/lib/bash/README.md
index 24d4c3b..bfd17b2 100644
--- a/lib/bash/README.md
+++ b/lib/bash/README.md
@@ -25,3 +25,10 @@ Reusable Bash libraries for command wrappers and other Bash tooling.
 The Base runtime shell files and Base version helpers remain in
 `basefoundry/base`. This repository carries only sourceable reusable library
 modules.
+
+## Naming Contract
+
+Public helpers that write through caller-supplied variable or array names
+reserve the `__` prefix for library-internal state. Passing an output name that
+begins with `__` fails before the helper changes caller state. Use a regular
+Bash variable name for public output values and arrays.
diff --git a/lib/bash/arg/tests/lib_arg.bats b/lib/bash/arg/tests/lib_arg.bats
index fa15503..5a0db27 100644
--- a/lib/bash/arg/tests/lib_arg.bats
+++ b/lib/bash/arg/tests/lib_arg.bats
@@ -90,6 +90,25 @@ EOF
     [ "${positionals_name[0]}" = "item" ]
 }
 
+@test "arg_parse rejects reserved internal output names" {
+    local -A __arg_options=([sentinel]="keep")
+    local -a __arg_positionals=(sentinel)
+    local -a specs_name=("verbose|flag|--verbose|-v")
+    local stderr_file="$TEST_TMPDIR/arg-reserved-output.err"
+    local rc
+
+    if arg_parse __arg_options __arg_positionals specs_name -- -v 2>"$stderr_file"; then
+        rc=0
+    else
+        rc=$?
+    fi
+
+    [ "$rc" -eq 1 ]
+    [ "${__arg_options[sentinel]}" = "keep" ]
+    [ "${__arg_positionals[0]}" = "sentinel" ]
+    [[ "$(cat "$stderr_file")" == *"uses the reserved '__' internal namespace"* ]]
+}
+
 @test "arg_parse accepts long option equals values and repeated options" {
     local -a specs=(
         "verbose|flag|--verbose|-v"
diff --git a/lib/bash/gh/lib_gh.sh b/lib/bash/gh/lib_gh.sh
index ed0ad59..22c362a 100644
--- a/lib/bash/gh/lib_gh.sh
+++ b/lib/bash/gh/lib_gh.sh
@@ -96,7 +96,7 @@ gh_infer_repo_from_origin() {
     local __gh_infer_repo_dir="$1"
     local __gh_infer_result_name="${2:-}"
     local __gh_infer_optional=0
-    local __gh_infer_repo __gh_infer_remote_url
+    local gh_infer_parsed_repo __gh_infer_remote_url
 
     if [[ -z "$__gh_infer_repo_dir" || -z "$__gh_infer_result_name" ]]; then
         log_error -l base_bash_libs.gh "Usage: gh_infer_repo_from_origin <repo_dir> <result_variable_name> [--optional]"
@@ -110,7 +110,8 @@ gh_infer_repo_from_origin() {
     fi
 
     __gh_infer_remote_url="$(git -C "$__gh_infer_repo_dir" remote get-url origin 2>/dev/null || true)"
-    if [[ -z "$__gh_infer_remote_url" ]] || ! gh_repo_from_remote_url "$__gh_infer_remote_url" __gh_infer_repo; then
+    if [[ -z "$__gh_infer_remote_url" ]] ||
+        ! gh_repo_from_remote_url "$__gh_infer_remote_url" gh_infer_parsed_repo; then
         if ((__gh_infer_optional)); then
             printf -v "$__gh_infer_result_name" '%s' ""
             return 0
@@ -119,7 +120,7 @@ gh_infer_repo_from_origin() {
         return 1
     fi
 
-    printf -v "$__gh_infer_result_name" '%s' "$__gh_infer_repo"
+    printf -v "$__gh_infer_result_name" '%s' "$gh_infer_parsed_repo"
 }
 
 gh_repo_default_branch() {
diff --git a/lib/bash/std/lib_std.sh b/lib/bash/std/lib_std.sh
index 8fd06e6..b962590 100644
--- a/lib/bash/std/lib_std.sh
+++ b/lib/bash/std/lib_std.sh
@@ -1833,6 +1833,11 @@ __is_valid_variable_name__() {
 __std_assert_writable_output__() {
     local function_name="${1-}" output_name="${2-}" declaration attributes
 
+    if [[ "$output_name" == __* ]]; then
+        log_error -l base_bash_libs.std "$function_name: result variable '$output_name' uses the reserved '__' internal namespace."
+        return 1
+    fi
+
     declaration="$(declare -p "$output_name" 2>/dev/null || true)"
     [[ -n "$declaration" ]] || return 0
     attributes="${declaration#declare -}"
diff --git a/lib/bash/str/tests/lib_str.bats b/lib/bash/str/tests/lib_str.bats
index b0f7acf..539e433 100644
--- a/lib/bash/str/tests/lib_str.bats
+++ b/lib/bash/str/tests/lib_str.bats
@@ -216,6 +216,22 @@ EOF
     [ "$joined" = "left|right" ]
 }
 
+@test "str_lower rejects reserved internal output names" {
+    local __str_var_name="Mixed Case"
+    local stderr_file="$TEST_TMPDIR/str-reserved-output.err"
+    local rc
+
+    if str_lower __str_var_name 2>"$stderr_file"; then
+        rc=0
+    else
+        rc=$?
+    fi
+
+    [ "$rc" -eq 1 ]
+    [ "$__str_var_name" = "Mixed Case" ]
+    [[ "$(cat "$stderr_file")" == *"uses the reserved '__' internal namespace"* ]]
+}
+
 @test "str_join rejects invalid variable names" {
     local script="$TEST_TMPDIR/str-join-invalid-array.sh"