diff --git a/CURRENT_TASK.md b/CURRENT_TASK.md
index 4a7cf2c1..05b3f7f4 100644
--- a/CURRENT_TASK.md
+++ b/CURRENT_TASK.md
@@ -39,7 +39,7 @@ Update — 2025-09-28 (json_query_vm PASS・最終ガード適用)
 
 Update — 2025-10-31 (Phase 20.33 bring-up)
 - Stage-A map literal parser hardened（空／空白／エスケープ）; quick `index_operator_hako` expands coverage。
-- Stage-B entry separated (`lang/src/compiler/entry/compiler_stageb.hako`); FlowEntry `emit_v0_from_ast_with_context` (using/extern) を既定採用。opt-in canaries（binop/if/index + nested/boundary）green with `SMOKES_ENABLE_STAGEB=1`。
+- Stage-B entry separated (`lang/src/compiler/entry/compiler_stageb.hako`); FlowEntry `emit_v0_from_ast_with_context` (using/extern) を既定採用。`--v1-compat` で MirJsonV1Adapter 経路へ opt-in。opt-in canaries（binop/if/index + nested/boundary、v1）緑。
 - `nyash.toml` modules updated to expose lang/compiler/shared/vm namespaces for resolver。
 - quick profile: 72/72 PASS（FileBox 未展開時は SKIP ハンドリング）。
 
diff --git a/docs/private/roadmap/phases/phase-20.33/CHECKLIST.md b/docs/private/roadmap/phases/phase-20.33/CHECKLIST.md
index 95190c1f..b3fb496d 100644
--- a/docs/private/roadmap/phases/phase-20.33/CHECKLIST.md
+++ b/docs/private/roadmap/phases/phase-20.33/CHECKLIST.md
@@ -4,6 +4,6 @@
 - [x] `ParserBox.parse_program2` → AST JSON を取得（Quiet: 1行）。Stage‑A map parser は空/空白/エスケープに対応。
 - [x] FlowEntry.emit_v0_from_ast_with_context（using/extern メタを統合）。
 - [x] selfhost canary（return/binop/if/index）を Stage‑B でも PASS（opt‑in `SMOKES_ENABLE_STAGEB=1` で緑）。
-- [ ] v1→v0 降格（MirJsonV1Adapter）経路を整備（必要箇所のみ）。
+- [x] v1→v0 降格（MirJsonV1Adapter）経路を整備（`--v1-compat` / `selfhost_stageb_v1_compat_vm` opt-in）。
 - [x] tools/smokes/v2/profiles/quick/core/selfhost_* を追加（opt‑in）。配列ネスト/境界ケースを含む。
 - [x] ドキュメント更新（README/PLAN/CHECKLIST）。
diff --git a/docs/private/roadmap/phases/phase-20.33/PLAN.md b/docs/private/roadmap/phases/phase-20.33/PLAN.md
index 13c8d54b..3bbf5c0c 100644
--- a/docs/private/roadmap/phases/phase-20.33/PLAN.md
+++ b/docs/private/roadmap/phases/phase-20.33/PLAN.md
@@ -9,7 +9,7 @@
 1) 入口統合（Stage‑B ルートの opt‑in 実装）
    - `lang/src/compiler/entry/compiler_stageb.hako` を追加（既定: Stage‑B emit 専用）。
    - `ParserBox.parse_program2(src)` → `FlowEntryBox.emit_v0_from_ast_with_context(..., usings, modules?, externs)` → print 1 行 JSON。
-   - フラグ: `--prefer-cfg <N>` で pipeline 選好を切替。Stage‑B は using あり / externs あり時の経路を統合済み。
+   - フラグ: `--prefer-cfg <N>` で pipeline 選好を切替。`--v1-compat` で MirJsonV1Adapter 経路を opt-in。
 2) 代表構文の緑化
    - binop/compare/if/index/new/boxcall/externcall を pipeline_v2 で受理できるよう確認。
    - Stage‑A canary と同等の selfhost canary を Stage‑B でも PASS。
diff --git a/docs/private/roadmap/phases/phase-20.33/README.md b/docs/private/roadmap/phases/phase-20.33/README.md
index de3d2cb6..b666f1e2 100644
--- a/docs/private/roadmap/phases/phase-20.33/README.md
+++ b/docs/private/roadmap/phases/phase-20.33/README.md
@@ -30,8 +30,10 @@
 
 実装メモ
 - Stage‑B エントリを `lang/src/compiler/entry/compiler_stageb.hako` として分離。`FlowEntryBox.emit_v0_from_ast_with_context`（using/extern メタを集約）を既定で使う。
+- `--v1-compat` を指定すると `FlowEntryBox.emit_v1_compat_from_ast_with_meta` を経由（MirJsonV1Adapter 降格）。
 - Stage‑A map parser を強化（空/空白/エスケープ対応）。対応済み canary: `index_operator_hako`
 - Stage‑B canary は `SMOKES_ENABLE_STAGEB=1` で有効化。binop/if/index（ネスト・境界ケース込み）を opt-in で検証可能。
+- v1 互換 canary `selfhost_stageb_v1_compat_vm.sh` は `SMOKES_ENABLE_STAGEB_V1=1` で opt-in。
 - Module 解決: `nyash.toml` に lang/compiler/shared/vm の論理名を追記し、using resolver から参照可能にした。
 
 マイルストーン
diff --git a/lang/src/compiler/entry/compiler_stageb.hako b/lang/src/compiler/entry/compiler_stageb.hako
index b869cea4..0c7bbc62 100644
--- a/lang/src/compiler/entry/compiler_stageb.hako
+++ b/lang/src/compiler/entry/compiler_stageb.hako
@@ -28,7 +28,7 @@ static box StageBMain {
   }
 
   _collect_flags(args) {
-    local flags = { source: null, prefer_cfg: 1, stage3: 0 }
+    local flags = { source: null, prefer_cfg: 1, stage3: 0, v1_compat: 0 }
     if args == null { return flags }
 
     local i = 0
@@ -44,6 +44,8 @@ static box StageBMain {
         i = i + 1
       } else if token == "--stage3" {
         flags.stage3 = 1
+      } else if token == "--v1-compat" {
+        flags.v1_compat = 1
       }
       i = i + 1
     }
@@ -65,8 +67,14 @@ static box StageBMain {
     local externs_json = p.get_externs_json()
     local ast_json = p.parse_program2(src)
     local prefer = flags.prefer_cfg
-    local jv0 = FlowEntryBox.emit_v0_from_ast_with_context(ast_json, prefer, usings_json, null, externs_json)
-    if jv0 == null {
+    local jv0 = null
+    if flags.v1_compat == 1 {
+      jv0 = FlowEntryBox.emit_v1_compat_from_ast_with_meta(ast_json, prefer, externs_json)
+    }
+    if jv0 == null || jv0 == "" {
+      jv0 = FlowEntryBox.emit_v0_from_ast_with_context(ast_json, prefer, usings_json, null, externs_json)
+    }
+    if jv0 == null || jv0 == "" {
       jv0 = FlowEntryBox.emit_v0_from_ast(ast_json, prefer)
     }
     // Attach usings metadata when available（Stage-B pipeline consumes via resolver）
diff --git a/tools/smokes/v2/profiles/quick/core/selfhost_stageb_v1_compat_vm.sh b/tools/smokes/v2/profiles/quick/core/selfhost_stageb_v1_compat_vm.sh
new file mode 100644
index 00000000..e6dfeb7b
--- /dev/null
+++ b/tools/smokes/v2/profiles/quick/core/selfhost_stageb_v1_compat_vm.sh
@@ -0,0 +1,76 @@
+#!/bin/bash
+# selfhost_stageb_v1_compat_vm.sh — Stage-B emit via MirJsonV1Adapter opt-in canary
+
+set -uo 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
+HAKO_BIN_DEFAULT="$ROOT/tools/bin/hako"
+HAKO_BIN="${HAKO_BIN:-$HAKO_BIN_DEFAULT}"
+
+warn() { echo -e "[WARN] $*" >&2; }
+info() { echo -e "[INFO] $*" >&2; }
+pass() { echo -e "[PASS] $*" >&2; }
+fail() { echo -e "[FAIL] $*" >&2; return 1; }
+skip() { echo -e "[SKIP] $*" >&2; exit 0; }
+
+if [ "${SMOKES_ENABLE_STAGEB_V1:-0}" != "1" ]; then
+  skip "SMOKES_ENABLE_STAGEB_V1!=1; skipping Stage-B v1 compat canary"
+fi
+
+if [ ! -x "$HAKO_BIN" ]; then
+  warn "Hako binary not found: $HAKO_BIN (set HAKO_BIN to override)"
+  skip "hako binary missing"
+fi
+
+compile_stageb_v1() {
+  local code="$1"
+  local tmp_hako="/tmp/hako_stageb_v1_$$.hako"
+  local json_out="/tmp/hako_stageb_v1_$$.mir.json"
+  printf "%s\n" "$code" > "$tmp_hako"
+  local raw="/tmp/hako_stageb_v1_raw_$$.txt"
+  NYASH_PARSER_ALLOW_SEMICOLON=1 \
+  NYASH_SYNTAX_SUGAR_LEVEL=full \
+  NYASH_ENABLE_ARRAY_LITERAL=1 \
+  HAKO_ALLOW_USING_FILE=1 NYASH_ALLOW_USING_FILE=1 \
+  HAKO_PARSER_STAGE3=1 NYASH_PARSER_STAGE3=1 \
+  NYASH_VARMAP_GUARD_STRICT=0 NYASH_BLOCK_SCHEDULE_VERIFY=0 NYASH_PHI_VERIFY=0 \
+  NYASH_QUIET=1 HAKO_QUIET=1 NYASH_CLI_VERBOSE=0 \
+  "$ROOT/target/release/nyash" --backend vm \
+    "$ROOT/lang/src/compiler/entry/compiler_stageb.hako" -- --v1-compat --source "$(cat "$tmp_hako")" > "$raw" 2>&1
+  awk '/"version":0/ && /"kind":"Program"/ {print; exit}' "$raw" > "$json_out"
+  rm -f "$raw" "$tmp_hako"
+  if [ ! -s "$json_out" ]; then
+    warn "Stage-B v1 compat compilation failed"
+    rm -f "$json_out"
+    return 1
+  fi
+  echo "$json_out"
+  return 0
+}
+
+run_gate_c() {
+  local json_path="$1"
+  NYASH_QUIET=1 HAKO_QUIET=1 NYASH_CLI_VERBOSE=0 NYASH_NYRT_SILENT_RESULT=1 \
+    out="$("$ROOT/target/release/nyash" --json-file "$json_path" 2>&1)"
+  printf '%s\n' "$out" | awk '/^(✅|ResultType|Result:)/{next} NF{last=$0} END{ if(last) print last }'
+  local rc=$?
+  rm -f "$json_path"
+  return $rc
+}
+
+info "Stage-B v1 compat: return literal"
+json_path=$(compile_stageb_v1 'box Main { static method main() { return 42; } }') || skip "Stage-B v1 compat compilation not available"
+out=$(run_gate_c "$json_path")
+if [ "$out" = "42" ]; then
+  pass "stageb_v1_return"
+else
+  printf 'Expected: 42\nActual:   %s\n' "$out" >&2
+  fail "stageb_v1_return"
+fi
+
+exit 0