diff --git a/apps/tests/string_cp_mode_min.hako b/apps/tests/string_cp_mode_min.hako
new file mode 100644
index 00000000..36867037
--- /dev/null
+++ b/apps/tests/string_cp_mode_min.hako
@@ -0,0 +1,9 @@
+static box Main {
+    main() {
+        local s = "aé𝄞"
+        local sb = new StringBox("aé𝄞")
+
+        print(1000 + s.length())
+        print(2000 + sb.length())
+    }
+}
diff --git a/docs/reference/environment-variables.md b/docs/reference/environment-variables.md
index df1ebbde..4053b244 100644
--- a/docs/reference/environment-variables.md
+++ b/docs/reference/environment-variables.md
@@ -108,6 +108,14 @@ NYASH_USE_STAGE1_CLI=1 STAGE1_EMIT_MIR_JSON=1 \
 
 ---
 
+## String / Unicode
+
+| 変数 | デフォルト | 適用経路 | 説明 |
+| --- | --- | --- | --- |
+| `NYASH_STR_CP=1` | OFF | Any | 文字列の `length` / `indexOf` / `lastIndexOf` / `substring` をコードポイント基準にする（既定はバイト基準） |
+
+---
+
 ## PHI デバッグ関連 (Phase 277 P2 統合版)
 
 **Phase 277 P2** で PHI 関連環境変数を **8個 → 3個** に統合しました。
diff --git a/src/backend/mir_interpreter/handlers/boxes_string.rs b/src/backend/mir_interpreter/handlers/boxes_string.rs
index cd4767b7..3a2d54f3 100644
--- a/src/backend/mir_interpreter/handlers/boxes_string.rs
+++ b/src/backend/mir_interpreter/handlers/boxes_string.rs
@@ -3,6 +3,7 @@ use super::string_method_helpers::{
     parse_index_of_args, parse_last_index_of_args, parse_substring_args, ArgParsePolicy,
 };
 use crate::boxes::string_ops;
+use crate::config::env::string_codepoint_mode;
 
 pub(super) fn try_handle_string_box(
     this: &mut MirInterpreter,
@@ -20,7 +21,7 @@ pub(super) fn try_handle_string_box(
         && std::env::var("NYASH_VM_FAST").ok().as_deref() == Some("1")
     {
         if let VMValue::String(ref raw) = recv {
-            let use_cp = std::env::var("NYASH_STR_CP").ok().as_deref() == Some("1");
+            let use_cp = string_codepoint_mode();
             let n = if use_cp {
                 raw.chars().count() as i64
             } else {
@@ -54,7 +55,7 @@ pub(super) fn try_handle_string_box(
         "length" | "size" => {
             // Bench/profile fast path: return VMValue::Integer directly (avoid boxing overhead)
             if std::env::var("NYASH_VM_FAST").ok().as_deref() == Some("1") {
-                let use_cp = std::env::var("NYASH_STR_CP").ok().as_deref() == Some("1");
+                let use_cp = string_codepoint_mode();
                 let n = if use_cp {
                     sb_norm.value.chars().count() as i64
                 } else {
diff --git a/src/boxes/basic/string_box.rs b/src/boxes/basic/string_box.rs
index 3e5670fb..dd8afd20 100644
--- a/src/boxes/basic/string_box.rs
+++ b/src/boxes/basic/string_box.rs
@@ -107,7 +107,7 @@ impl StringBox {
     /// otherwise use UTF-8 byte length (legacy/default).
     pub fn length(&self) -> Box<dyn NyashBox> {
         use crate::box_trait::IntegerBox;
-        let use_cp = std::env::var("NYASH_STR_CP").ok().as_deref() == Some("1");
+        let use_cp = crate::config::env::string_codepoint_mode();
         let n = if use_cp {
             self.value.chars().count() as i64
         } else {
@@ -144,10 +144,9 @@ impl StringBox {
 
     /// Get substring from start to end (exclusive)
     pub fn substring(&self, start: usize, end: usize) -> Box<dyn NyashBox> {
-        let chars: Vec<char> = self.value.chars().collect();
-        let actual_end = end.min(chars.len());
-        let actual_start = start.min(actual_end);
-        let substring: String = chars[actual_start..actual_end].iter().collect();
+        let mode = crate::boxes::string_ops::index_mode_from_env();
+        let substring =
+            crate::boxes::string_ops::substring(&self.value, start as i64, Some(end as i64), mode);
         Box::new(StringBox::new(substring))
     }
 }
diff --git a/src/boxes/string_ops.rs b/src/boxes/string_ops.rs
index f0eac34c..647024e0 100644
--- a/src/boxes/string_ops.rs
+++ b/src/boxes/string_ops.rs
@@ -7,7 +7,7 @@ pub enum StringIndexMode {
 }
 
 pub fn index_mode_from_env() -> StringIndexMode {
-    if std::env::var("NYASH_STR_CP").ok().as_deref() == Some("1") {
+    if crate::config::env::string_codepoint_mode() {
         StringIndexMode::CodePoint
     } else {
         StringIndexMode::Byte
diff --git a/src/config/env.rs b/src/config/env.rs
index 5c29155d..939a4fdc 100644
--- a/src/config/env.rs
+++ b/src/config/env.rs
@@ -25,6 +25,7 @@
 //! | `using_flags` | `NYASH_USING_*` | Using / Namespace 設定 |
 //! | `verification_flags` | `NYASH_VERIFY_*` | Verification 設定 |
 //! | `selfhost_flags` | `NYASH_NY_COMPILER_*` | Selfhost compiler 設定 |
+//! | `string_flags` | `NYASH_STR_*` | String / Unicode 設定 |
 //!
 //! ## 新規環境変数追加の手順
 //!
@@ -79,6 +80,7 @@
 //! - `using_flags` - Using/namespace flags (10+ functions)
 //! - `verification_flags` - Verification flags (8+ functions)
 //! - `selfhost_flags` - Selfhost compiler flags (10+ functions)
+//! - `string_flags` - String/Unicode flags (1+ functions)
 //!
 //! All functions are re-exported at the top level for backward compatibility.
 
@@ -96,6 +98,7 @@ mod macro_flags;
 mod mir_flags;
 mod parser_flags;
 mod selfhost_flags;
+mod string_flags;
 mod using_flags;
 mod verification_flags;
 mod vm_backend_flags;
@@ -114,6 +117,7 @@ pub use macro_flags::*;
 pub use mir_flags::*;
 pub use parser_flags::*;
 pub use selfhost_flags::*;
+pub use string_flags::*;
 pub use using_flags::*;
 pub use verification_flags::*;
 pub use vm_backend_flags::*;
diff --git a/src/config/env/catalog.rs b/src/config/env/catalog.rs
index bd6c928a..1578d340 100644
--- a/src/config/env/catalog.rs
+++ b/src/config/env/catalog.rs
@@ -236,6 +236,13 @@ pub fn env_vars() -> Vec<EnvVarMeta> {
             applies_to: AppliesTo::BoxFactory,
             default: None,
         },
+        // Runtime (Phase 29ab)
+        EnvVarMeta {
+            name: "NYASH_STR_CP",
+            description: "String index mode: 1=codepoint, 0=byte (default)",
+            applies_to: AppliesTo::Runtime,
+            default: Some("0"),
+        },
         // Selfhost (Phase 286B)
         EnvVarMeta {
             name: "NYASH_NY_COMPILER_TIMEOUT_MS",
diff --git a/src/config/env/string_flags.rs b/src/config/env/string_flags.rs
new file mode 100644
index 00000000..4b0c315f
--- /dev/null
+++ b/src/config/env/string_flags.rs
@@ -0,0 +1,9 @@
+//! String-related environment flags.
+
+use super::env_bool;
+
+/// NYASH_STR_CP=1: use Unicode code point indexing for string operations.
+/// Default: OFF (byte indexing).
+pub fn string_codepoint_mode() -> bool {
+    env_bool("NYASH_STR_CP")
+}
diff --git a/tools/smokes/v2/profiles/integration/apps/string_cp_mode_min_vm.sh b/tools/smokes/v2/profiles/integration/apps/string_cp_mode_min_vm.sh
new file mode 100644
index 00000000..c24f0c63
--- /dev/null
+++ b/tools/smokes/v2/profiles/integration/apps/string_cp_mode_min_vm.sh
@@ -0,0 +1,45 @@
+#!/bin/bash
+# String CP/byte mode parity for primitive String and StringBox.
+
+source "$(dirname "$0")/../../../lib/test_runner.sh"
+export SMOKES_USE_PYVM=0
+require_env || exit 2
+
+INPUT="$NYASH_ROOT/apps/tests/string_cp_mode_min.hako"
+RUN_TIMEOUT_SECS=${RUN_TIMEOUT_SECS:-10}
+
+run_case() {
+    local mode="$1"
+    local expect_len="$2"
+
+    set +e
+    OUTPUT=$(timeout "$RUN_TIMEOUT_SECS" env -u NYASH_ROOT NYASH_STR_CP="$mode" "$NYASH_BIN" "$INPUT" 2>&1)
+    EXIT_CODE=$?
+    set -e
+
+    if [ "$EXIT_CODE" -eq 124 ]; then
+        test_fail "string_cp_mode_min_vm: hakorune timed out (>${RUN_TIMEOUT_SECS}s)"
+        exit 1
+    fi
+
+    OUTPUT_CLEAN="$OUTPUT"
+
+    expect_line "$((1000 + expect_len))"
+    expect_line "$((2000 + expect_len))"
+}
+
+expect_line() {
+    local line="$1"
+    if ! echo "$OUTPUT_CLEAN" | grep -Fxq "$line"; then
+        echo "[FAIL] Missing expected line: $line"
+        echo "[INFO] Output (clean):"
+        echo "$OUTPUT_CLEAN" | tail -n 20 || true
+        test_fail "string_cp_mode_min_vm: output mismatch"
+        exit 1
+    fi
+}
+
+run_case 0 7
+run_case 1 3
+
+test_pass "string_cp_mode_min_vm: NYASH_STR_CP byte/cp modes match for String and StringBox"