diff --git a/lang/src/compiler/entry/compiler_stageb.hako b/lang/src/compiler/entry/compiler_stageb.hako
index 5a15bed2..9ab81bb4 100644
--- a/lang/src/compiler/entry/compiler_stageb.hako
+++ b/lang/src/compiler/entry/compiler_stageb.hako
@@ -4,6 +4,15 @@ using lang.compiler.parser.box as ParserBox
 using lang.compiler.pipeline_v2.flow_entry as FlowEntryBox
 
 static box StageBMain {
+  _fallback_enabled() {
+    // Default ON; set HAKO_STAGEB_ALLOW_FALLBACK=0 (or NYASH_STAGEB_ALLOW_FALLBACK=0) to disable
+    local v = env.local.get("HAKO_STAGEB_ALLOW_FALLBACK")
+    if v == null { v = env.local.get("NYASH_STAGEB_ALLOW_FALLBACK") }
+    if v == null { return 1 }
+    local s = "" + v
+    if s == "0" or s == "false" or s == "off" { return 0 }
+    return 1
+  }
   _fallback_program() {
     return "{\"version\":0,\"kind\":\"Program\",\"body\":[{\"type\":\"Return\",\"expr\":{\"type\":\"Int\",\"value\":0}}]}"
   }
@@ -79,7 +88,12 @@ static box StageBMain {
       jv0 = FlowEntryBox.emit_v0_from_ast(ast_json, prefer)
     }
     if jv0 == null || jv0 == "" {
-      jv0 = me._fallback_program()
+      if me._fallback_enabled() == 1 {
+        jv0 = me._fallback_program()
+      } else {
+        // Return empty to surface failure at caller; TTL: enable stricter failure once all paths are green
+        jv0 = ""
+      }
     }
     return jv0
   }
@@ -91,7 +105,7 @@ static box StageBMain {
     local stage3 = flags.stage3
     local v1_compat = flags.v1_compat
     local json = me._do_compile_stage_b(src, prefer, stage3, v1_compat)
-    if json == null || json == "" { json = me._fallback_program() }
+    if (json == null || json == "") && me._fallback_enabled() == 1 { json = me._fallback_program() }
     print(json)
     return 0
   }
diff --git a/src/runner/child_env.rs b/src/runner/child_env.rs
new file mode 100644
index 00000000..62d0f7ba
--- /dev/null
+++ b/src/runner/child_env.rs
@@ -0,0 +1,21 @@
+/*!
+ * child_env.rs — Runner helper utilities (OOB strict, quiet exit policies)
+ */
+
+pub fn pre_run_reset_oob_if_strict() {
+    if crate::config::env::oob_strict_fail() {
+        crate::runtime::observe::reset();
+    }
+}
+
+pub fn post_run_exit_if_oob_strict_triggered() -> ! {
+    if crate::config::env::oob_strict_fail() && crate::runtime::observe::oob_seen() {
+        eprintln!("[gate-c][oob-strict] Out-of-bounds observed → exit(1)");
+        std::process::exit(1);
+    }
+    // If not strict or no OOB, return to caller path; caller should exit(…) itself.
+    // This function is defined as diverging only when it actually exits above; otherwise it does nothing.
+    // To keep signature simple for callers, they should not rely on this returning.
+    std::process::exit(0)
+}
+
diff --git a/src/runner/pipe_io.rs b/src/runner/pipe_io.rs
index 63300eea..b2746997 100644
--- a/src/runner/pipe_io.rs
+++ b/src/runner/pipe_io.rs
@@ -10,6 +10,7 @@
  */
 
 use super::*;
+use crate::runner::child_env;
 
 impl NyashRunner {
     /// Try to handle `--ny-parser-pipe` / `--json-file` flow.
@@ -47,7 +48,7 @@ impl NyashRunner {
                 Ok(Some(module)) => {
                     super::json_v0_bridge::maybe_dump_mir(&module);
                     // Gate‑C(Core) strict OOB fail‑fast: reset observe flag before run
-                    if crate::config::env::oob_strict_fail() { crate::runtime::observe::reset(); }
+                    child_env::pre_run_reset_oob_if_strict();
                     let rc = self.execute_mir_module_quiet_exit(&module);
                     if crate::config::env::oob_strict_fail() && crate::runtime::observe::oob_seen() {
                         eprintln!("[gate-c][oob-strict] Out-of-bounds observed → exit(1)");
@@ -120,7 +121,7 @@ impl NyashRunner {
                     }
                 }
                 // Default: Execute via MIR interpreter (quiet) and exit with rc mirrored from return value
-                if crate::config::env::oob_strict_fail() { crate::runtime::observe::reset(); }
+                child_env::pre_run_reset_oob_if_strict();
                 let rc = self.execute_mir_module_quiet_exit(&module);
                 if crate::config::env::oob_strict_fail() && crate::runtime::observe::oob_seen() {
                     eprintln!("[gate-c][oob-strict] Out-of-bounds observed → exit(1)");
diff --git a/tools/smokes/v2/profiles/quick/core/bridge/canonicalize_fail_vm.sh b/tools/smokes/v2/profiles/quick/core/bridge/canonicalize_fail_vm.sh
index bc29f9ff..aed86652 100644
--- a/tools/smokes/v2/profiles/quick/core/bridge/canonicalize_fail_vm.sh
+++ b/tools/smokes/v2/profiles/quick/core/bridge/canonicalize_fail_vm.sh
@@ -4,13 +4,15 @@
 set -euo pipefail
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
-ROOT="$(cd "$SCRIPT_DIR/../../../../../.." && pwd)"
+if ROOT_GIT=$(git -C "$SCRIPT_DIR" rev-parse --show-toplevel 2>/dev/null); then
+  ROOT="$ROOT_GIT"
+else
+  ROOT="$(cd "$SCRIPT_DIR/../../../../../../../../.." && pwd)"
+fi
 source "$ROOT/tools/smokes/v2/lib/test_runner.sh"
 require_env || exit 2
 
-if [ "${SMOKES_ENABLE_BRIDGE_CANON:-0}" != "1" ]; then
-  test_skip "bridge_canonicalize_fail" "opt-in (set SMOKES_ENABLE_BRIDGE_CANON=1)" && exit 0
-fi
+# Default-on: bridge canonicalize fail
 
 # v1 JSON with unsupported instruction to assert stable failure message
 json_path="/tmp/ny_v1_fail_$$.json"
@@ -32,4 +34,3 @@ else
   echo "$output" >&2
   exit 1
 fi
-
diff --git a/tools/smokes/v2/profiles/quick/core/bridge/canonicalize_off_vm.sh b/tools/smokes/v2/profiles/quick/core/bridge/canonicalize_off_vm.sh
index a27e213e..0892d043 100644
--- a/tools/smokes/v2/profiles/quick/core/bridge/canonicalize_off_vm.sh
+++ b/tools/smokes/v2/profiles/quick/core/bridge/canonicalize_off_vm.sh
@@ -4,13 +4,15 @@
 set -euo pipefail
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
-ROOT="$(cd "$SCRIPT_DIR/../../../../../.." && pwd)"
+if ROOT_GIT=$(git -C "$SCRIPT_DIR" rev-parse --show-toplevel 2>/dev/null); then
+  ROOT="$ROOT_GIT"
+else
+  ROOT="$(cd "$SCRIPT_DIR/../../../../../../../../.." && pwd)"
+fi
 source "$ROOT/tools/smokes/v2/lib/test_runner.sh"
 require_env || exit 2
 
-if [ "${SMOKES_ENABLE_BRIDGE_CANON:-0}" != "1" ]; then
-  test_skip "bridge_canonicalize_off" "opt-in (set SMOKES_ENABLE_BRIDGE_CANON=1)" && exit 0
-fi
+# Default-on: bridge canonicalize off
 
 # Same v1 minimal JSON (const+ret) without toggles should still run (no mir_call involved)
 json_path="/tmp/ny_v1_const_off_$$.json"
@@ -28,4 +30,3 @@ else
   echo "[FAIL] bridge_canonicalize_off: expected rc=0, got $rc" >&2
   exit 1
 fi
-
diff --git a/tools/smokes/v2/profiles/quick/core/bridge/canonicalize_on_vm.sh b/tools/smokes/v2/profiles/quick/core/bridge/canonicalize_on_vm.sh
index 992a4cdb..3c36f52b 100644
--- a/tools/smokes/v2/profiles/quick/core/bridge/canonicalize_on_vm.sh
+++ b/tools/smokes/v2/profiles/quick/core/bridge/canonicalize_on_vm.sh
@@ -4,13 +4,15 @@
 set -euo pipefail
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
-ROOT="$(cd "$SCRIPT_DIR/../../../../../.." && pwd)"
+if ROOT_GIT=$(git -C "$SCRIPT_DIR" rev-parse --show-toplevel 2>/dev/null); then
+  ROOT="$ROOT_GIT"
+else
+  ROOT="$(cd "$SCRIPT_DIR/../../../../../../../../.." && pwd)"
+fi
 source "$ROOT/tools/smokes/v2/lib/test_runner.sh"
 require_env || exit 2
 
-if [ "${SMOKES_ENABLE_BRIDGE_CANON:-0}" != "1" ]; then
-  test_skip "bridge_canonicalize_on" "opt-in (set SMOKES_ENABLE_BRIDGE_CANON=1)" && exit 0
-fi
+# Default-on: bridge canonicalize on
 
 # Minimal v1 JSON with only const/copy/ret (no mir_call), should run regardless
 json_path="/tmp/ny_v1_const_$$.json"
@@ -29,4 +31,3 @@ else
   echo "[FAIL] bridge_canonicalize_on: expected rc=0, got $rc" >&2
   exit 1
 fi
-
diff --git a/tools/smokes/v2/profiles/quick/core/map/map_len_set_get_vm.sh b/tools/smokes/v2/profiles/quick/core/map/map_len_set_get_vm.sh
new file mode 100644
index 00000000..8ed5b17a
--- /dev/null
+++ b/tools/smokes/v2/profiles/quick/core/map/map_len_set_get_vm.sh
@@ -0,0 +1,41 @@
+#!/bin/bash
+# map_len_set_get_vm.sh — MapBox len/set/get sequence sanity
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+if ROOT_GIT=$(git -C "$SCRIPT_DIR" rev-parse --show-toplevel 2>/dev/null); then
+  ROOT="$ROOT_GIT"
+else
+  ROOT="$(cd "$SCRIPT_DIR/../../../../../../../../.." && pwd)"
+fi
+source "$ROOT/tools/smokes/v2/lib/test_runner.sh"
+require_env || exit 2
+
+code='static box Main { main() {
+  local m = new MapBox()
+  print("" + m.size())
+  m.set("a", 1)
+  print("" + m.size())
+  m.set("b", 2)
+  print("" + m.size())
+  print("" + m.get("a"))
+  print("" + m.get("b"))
+  return 0
+} }'
+
+output=$(run_nyash_vm -c "$code" --dev)
+expected=$'0\n1\n2\n1\n2'
+
+if [ "$output" = "$expected" ]; then
+  echo "[PASS] map_len_set_get_vm"
+  exit 0
+else
+  echo "[FAIL] map_len_set_get_vm" >&2
+  echo "--- expected ---" >&2
+  echo "$expected" >&2
+  echo "--- actual ---" >&2
+  echo "$output" >&2
+  exit 1
+fi
+
diff --git a/tools/smokes/v2/profiles/quick/core/string/index_substring_vm.sh b/tools/smokes/v2/profiles/quick/core/string/index_substring_vm.sh
new file mode 100644
index 00000000..1ab062b1
--- /dev/null
+++ b/tools/smokes/v2/profiles/quick/core/string/index_substring_vm.sh
@@ -0,0 +1,38 @@
+#!/bin/bash
+# index_substring_vm.sh — String.indexOf/substring boundary behavior
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+if ROOT_GIT=$(git -C "$SCRIPT_DIR" rev-parse --show-toplevel 2>/dev/null); then
+  ROOT="$ROOT_GIT"
+else
+  ROOT="$(cd "$SCRIPT_DIR/../../../../../../../../.." && pwd)"
+fi
+source "$ROOT/tools/smokes/v2/lib/test_runner.sh"
+require_env || exit 2
+
+code='static box Main { main() {
+  local s = "hello"
+  print("" + s.indexOf("l"))    // 2
+  print("" + s.indexOf("z"))    // -1
+  print(s.substring(0, 2))       // he
+  print(s.substring(2, 5))       // llo
+  print("" + s.substring(5, 5).length())  // 0
+  return 0
+} }'
+
+output=$(run_nyash_vm -c "$code" --dev)
+expected=$'2\n-1\nhe\nllo\n0'
+
+if [ "$output" = "$expected" ]; then
+  echo "[PASS] string_index_substring_vm"
+  exit 0
+else
+  echo "[FAIL] string_index_substring_vm" >&2
+  echo "--- expected ---" >&2
+  echo "$expected" >&2
+  echo "--- actual ---" >&2
+  echo "$output" >&2
+  exit 1
+fi