diff --git a/CURRENT_TASK.md b/CURRENT_TASK.md
index 7892cd60..bfa0939a 100644
--- a/CURRENT_TASK.md
+++ b/CURRENT_TASK.md
@@ -18,9 +18,12 @@ Scope: Repo root の旧リンク互換。現行の入口は `docs/development/cu
 **CorePlan migration 道筋 SSOT**
 `docs/development/current/main/design/coreplan-migration-roadmap-ssot.md` が移行タスクの Done 判定の入口。
 
-**Next implementation (Phase 29ao P21)**
-- 目的: strict/dev Pattern1 shadow adopt の意味論破壊を防ぐため、Pattern1 subset を “body is step-only” で固定する
-- 指示書: `docs/development/current/main/phases/phase-29ao/P21-PATTERN1-SUBSET-BODY-IS-STEP-ONLY-INSTRUCTIONS.md`
+**Next implementation (Phase 29ao P22)**
+- 目的: TBD
+- 指示書: TBD
+
+**2025-12-30: Phase 29ao P21 COMPLETE (Pattern1 subset step-only gate)**
+Pattern1 subset を body=step のみに引き締め、strict/dev shadow adopt の誤マッチを遮断した。
 
 **2025-12-30: Phase 29ao P20 COMPLETE (CoreLoop ExitMap composition SSOT)**
 CoreLoop の ExitMap/Cleanup/ValueJoin 合成規約を SSOT 化し、合成境界と Fail-Fast ルールを固定した（docs-only）。
diff --git a/apps/tests/phase29ao_pattern1_subset_reject_extra_stmt.hako b/apps/tests/phase29ao_pattern1_subset_reject_extra_stmt.hako
new file mode 100644
index 00000000..c5906ef1
--- /dev/null
+++ b/apps/tests/phase29ao_pattern1_subset_reject_extra_stmt.hako
@@ -0,0 +1,16 @@
+// Phase 29ao P21: Pattern1 subset reject (extra body statement)
+// Expected: return 3 (sum increments each iteration)
+
+static box Main {
+    main() {
+        local i
+        local sum
+        i = 0
+        sum = 0
+        loop(i < 3) {
+            sum = sum + 1
+            i = i + 1
+        }
+        return sum
+    }
+}
diff --git a/docs/development/current/main/10-Now.md b/docs/development/current/main/10-Now.md
index 54c9ffe3..5f4c3d46 100644
--- a/docs/development/current/main/10-Now.md
+++ b/docs/development/current/main/10-Now.md
@@ -2,12 +2,17 @@
 
 ## Current Focus: Phase 29ao（CorePlan composition）
 
-Next: Phase 29ao P21（Pattern1 subset: body is step-only）
-指示書: `docs/development/current/main/phases/phase-29ao/P21-PATTERN1-SUBSET-BODY-IS-STEP-ONLY-INSTRUCTIONS.md`
+Next: Phase 29ao P22（TBD）
+指示書: TBD
 運用ルール: integration filter で phase143_* は回さない（JoinIR 回帰は phase29ae pack のみ）
 運用ルール: phase286_pattern9_* は legacy pack (SKIP) を使う
 移行道筋 SSOT: `docs/development/current/main/design/coreplan-migration-roadmap-ssot.md`
 
+**2025-12-30: Phase 29ao P21 完了** ✅
+- 目的: Pattern1 subset を body=step のみに引き締め、strict/dev shadow adopt の誤マッチを遮断
+- 変更: `src/mir/builder/control_flow/plan/policies/pattern1_subset_policy.rs` / `src/mir/builder/control_flow/plan/facts/pattern1_simplewhile_facts.rs` / `src/mir/builder/control_flow/plan/extractors/pattern1.rs` / `apps/tests/phase29ao_pattern1_subset_reject_extra_stmt.hako` / `tools/smokes/v2/profiles/integration/joinir/phase29ao_pattern1_subset_reject_extra_stmt_vm.sh` / `tools/smokes/v2/profiles/integration/joinir/phase29ae_regression_pack_vm.sh` / `docs/development/current/main/phases/phase-29ae/README.md` / `docs/development/current/main/phases/phase-29ao/README.md` / `docs/development/current/main/10-Now.md` / `docs/development/current/main/30-Backlog.md` / `CURRENT_TASK.md`
+- 検証: `cargo build --release` / `./tools/smokes/v2/run.sh --profile quick` / `./tools/smokes/v2/profiles/integration/joinir/phase29ae_regression_pack_vm.sh`
+
 **2025-12-30: Phase 29ao P20 完了** ✅
 - 目的: CoreLoop の ExitMap/Cleanup/ValueJoin 合成規約を SSOT 化（docs-only）
 - 変更: `docs/development/current/main/design/coreloop-exitmap-composition-ssot.md` / `docs/development/current/main/design/planfrag-ssot-registry.md` / `docs/development/current/main/phases/phase-29ao/README.md` / `docs/development/current/main/10-Now.md` / `docs/development/current/main/30-Backlog.md` / `CURRENT_TASK.md`
diff --git a/docs/development/current/main/30-Backlog.md b/docs/development/current/main/30-Backlog.md
index 58ac3f30..6a9b6274 100644
--- a/docs/development/current/main/30-Backlog.md
+++ b/docs/development/current/main/30-Backlog.md
@@ -15,8 +15,8 @@ Related:
 
 - **Phase 29ao（active）: CorePlan composition from Skeleton/Feature**
   - 入口: `docs/development/current/main/phases/phase-29ao/README.md`
-  - 状況: P0–P20 ✅ 完了 / Next: P21（Pattern1 subset hardening）
-  - Next 指示書: `docs/development/current/main/phases/phase-29ao/P21-PATTERN1-SUBSET-BODY-IS-STEP-ONLY-INSTRUCTIONS.md`
+  - 状況: P0–P21 ✅ 完了 / Next: P22（TBD）
+  - Next 指示書: TBD
 
 - **Phase 29af（✅ COMPLETE）: Boundary hygiene / regression entrypoint / carrier layout SSOT**
   - 入口: `docs/development/current/main/phases/phase-29af/README.md`
diff --git a/docs/development/current/main/phases/phase-29ae/README.md b/docs/development/current/main/phases/phase-29ae/README.md
index e9cd90f6..6da9c3f7 100644
--- a/docs/development/current/main/phases/phase-29ae/README.md
+++ b/docs/development/current/main/phases/phase-29ae/README.md
@@ -8,6 +8,7 @@ Goal: JoinIR の最小回帰セットを SSOT として固定する。
 - Pattern2 (real-world): `phase263_pattern2_*`
 - Pattern3 (If‑Phi, VM): `phase118_pattern3_if_sum_vm`
 - Pattern1 (strict shadow, VM): `phase29ao_pattern1_strict_shadow_vm`
+- Pattern1 (subset reject, VM): `phase29ao_pattern1_subset_reject_extra_stmt_vm`
 - Pattern5 (Break, VM): `phase286_pattern5_break_vm`
 - Pattern6: `phase29ab_pattern6_*`
 - Pattern7: `phase29ab_pattern7_*`
diff --git a/docs/development/current/main/phases/phase-29ao/README.md b/docs/development/current/main/phases/phase-29ao/README.md
index 0b640b5b..88a447b6 100644
--- a/docs/development/current/main/phases/phase-29ao/README.md
+++ b/docs/development/current/main/phases/phase-29ao/README.md
@@ -127,7 +127,11 @@ Gate（SSOT）:
 - 指示書: `docs/development/current/main/phases/phase-29ao/P20-CORELOOP-EXITMAP-COMPOSITION-SSOT-INSTRUCTIONS.md`
 - ねらい: Loop skeleton に対する ExitMap/Cleanup/ValueJoin の合成規約を SSOT として固定
 
+## P21: Pattern1 subset body is step-only（shadow adopt safety）✅
+
+- 指示書: `docs/development/current/main/phases/phase-29ao/P21-PATTERN1-SUBSET-BODY-IS-STEP-ONLY-INSTRUCTIONS.md`
+- ねらい: Pattern1 subset を body=step のみに引き締め、strict/dev shadow adopt の誤マッチを遮断
+
 ## Next（planned）
 
-- P21: Pattern1 subset “body is step-only” hardening
-  - 指示書: `docs/development/current/main/phases/phase-29ao/P21-PATTERN1-SUBSET-BODY-IS-STEP-ONLY-INSTRUCTIONS.md`
+- P22: TBD
diff --git a/src/mir/builder/control_flow/plan/extractors/pattern1.rs b/src/mir/builder/control_flow/plan/extractors/pattern1.rs
index 915c6c43..b1851ae9 100644
--- a/src/mir/builder/control_flow/plan/extractors/pattern1.rs
+++ b/src/mir/builder/control_flow/plan/extractors/pattern1.rs
@@ -5,6 +5,7 @@ use crate::ast::{ASTNode, BinaryOperator};
 
 // Phase 282 P9a: Use common_helpers
 use super::common_helpers::has_control_flow_statement as common_has_control_flow;
+use crate::mir::builder::control_flow::plan::policies::pattern1_subset_policy::is_pattern1_step_only_body;
 
 #[derive(Debug, Clone)]
 pub(crate) struct Pattern1Parts {
@@ -200,6 +201,10 @@ pub(crate) fn extract_pattern1_plan(
         None => return Ok(None),  // No loop increment found
     };
 
+    if !is_pattern1_step_only_body(body, &parts.loop_var) {
+        return Ok(None);
+    }
+
     Ok(Some(DomainPlan::Pattern1SimpleWhile(Pattern1SimpleWhilePlan {
         loop_var: parts.loop_var,
         condition: condition.clone(),
diff --git a/src/mir/builder/control_flow/plan/facts/pattern1_simplewhile_facts.rs b/src/mir/builder/control_flow/plan/facts/pattern1_simplewhile_facts.rs
index 9d46df35..d580c30b 100644
--- a/src/mir/builder/control_flow/plan/facts/pattern1_simplewhile_facts.rs
+++ b/src/mir/builder/control_flow/plan/facts/pattern1_simplewhile_facts.rs
@@ -6,6 +6,7 @@ use crate::mir::builder::control_flow::plan::extractors::common_helpers::{
     extract_loop_increment_plan, has_break_statement, has_continue_statement, has_if_else_statement,
     has_return_statement,
 };
+use crate::mir::builder::control_flow::plan::policies::pattern1_subset_policy::is_pattern1_step_only_body;
 
 #[derive(Debug, Clone)]
 pub(in crate::mir::builder) struct Pattern1SimpleWhileFacts {
@@ -35,6 +36,10 @@ pub(in crate::mir::builder) fn try_extract_pattern1_simplewhile_facts(
         _ => return Ok(None),
     };
 
+    if !is_pattern1_step_only_body(body, &loop_var) {
+        return Ok(None);
+    }
+
     if !is_increment_step_one(&loop_increment, &loop_var) {
         return Ok(None);
     }
diff --git a/src/mir/builder/control_flow/plan/mod.rs b/src/mir/builder/control_flow/plan/mod.rs
index a215fe89..dbd8a5e6 100644
--- a/src/mir/builder/control_flow/plan/mod.rs
+++ b/src/mir/builder/control_flow/plan/mod.rs
@@ -40,6 +40,8 @@ pub(in crate::mir::builder) mod composer;
 pub(in crate::mir::builder) mod emit;
 // Phase 29ai P6: Extractors moved into plan layer
 pub(in crate::mir::builder) mod extractors;
+// Phase 29ao P21: Pattern1 subset policy (SSOT gate)
+pub(in crate::mir::builder) mod policies;
 // Phase 29ai P5: JoinIR router → single plan extraction entrypoint
 pub(in crate::mir::builder) mod single_planner;
 
diff --git a/src/mir/builder/control_flow/plan/policies/mod.rs b/src/mir/builder/control_flow/plan/policies/mod.rs
new file mode 100644
index 00000000..7988b3db
--- /dev/null
+++ b/src/mir/builder/control_flow/plan/policies/mod.rs
@@ -0,0 +1,3 @@
+//! Phase 29ao P21: Pattern1 subset policy SSOT
+
+pub(in crate::mir::builder) mod pattern1_subset_policy;
diff --git a/src/mir/builder/control_flow/plan/policies/pattern1_subset_policy.rs b/src/mir/builder/control_flow/plan/policies/pattern1_subset_policy.rs
new file mode 100644
index 00000000..b62fe37f
--- /dev/null
+++ b/src/mir/builder/control_flow/plan/policies/pattern1_subset_policy.rs
@@ -0,0 +1,42 @@
+//! Phase 29ao P21: Pattern1 subset policy (step-only body)
+
+use crate::ast::{ASTNode, BinaryOperator, LiteralValue};
+
+pub(crate) fn is_pattern1_step_only_body(body: &[ASTNode], loop_var: &str) -> bool {
+    if body.len() != 1 {
+        return false;
+    }
+
+    let ASTNode::Assignment { target, value, .. } = &body[0] else {
+        return false;
+    };
+
+    let ASTNode::Variable { name, .. } = target.as_ref() else {
+        return false;
+    };
+    if name != loop_var {
+        return false;
+    }
+
+    let ASTNode::BinaryOp {
+        operator: BinaryOperator::Add,
+        left,
+        right,
+        ..
+    } = value.as_ref()
+    else {
+        return false;
+    };
+
+    if !matches!(left.as_ref(), ASTNode::Variable { name, .. } if name == loop_var) {
+        return false;
+    }
+
+    matches!(
+        right.as_ref(),
+        ASTNode::Literal {
+            value: LiteralValue::Integer(1),
+            ..
+        }
+    )
+}
diff --git a/tools/smokes/v2/profiles/integration/joinir/phase29ae_regression_pack_vm.sh b/tools/smokes/v2/profiles/integration/joinir/phase29ae_regression_pack_vm.sh
index d032a6a3..f0b6c574 100644
--- a/tools/smokes/v2/profiles/integration/joinir/phase29ae_regression_pack_vm.sh
+++ b/tools/smokes/v2/profiles/integration/joinir/phase29ae_regression_pack_vm.sh
@@ -21,6 +21,7 @@ run_filter "pattern2" "phase29ab_pattern2_" || exit 1
 run_filter "pattern2_realworld" "phase263_pattern2_" || exit 1
 run_filter "pattern3_ifphi_vm" "phase118_pattern3_if_sum_vm" || exit 1
 run_filter "pattern1_strict_shadow_vm" "phase29ao_pattern1_strict_shadow_vm" || exit 1
+run_filter "pattern1_subset_reject_extra_stmt_vm" "phase29ao_pattern1_subset_reject_extra_stmt_vm" || exit 1
 run_filter "pattern5_break_vm" "phase286_pattern5_break_vm" || exit 1
 run_filter "pattern6" "phase29ab_pattern6_" || exit 1
 run_filter "pattern7" "phase29ab_pattern7_" || exit 1
diff --git a/tools/smokes/v2/profiles/integration/joinir/phase29ao_pattern1_subset_reject_extra_stmt_vm.sh b/tools/smokes/v2/profiles/integration/joinir/phase29ao_pattern1_subset_reject_extra_stmt_vm.sh
new file mode 100644
index 00000000..1f94f62e
--- /dev/null
+++ b/tools/smokes/v2/profiles/integration/joinir/phase29ao_pattern1_subset_reject_extra_stmt_vm.sh
@@ -0,0 +1,33 @@
+#!/bin/bash
+# phase29ao_pattern1_subset_reject_extra_stmt_vm.sh - Pattern1 subset reject extra stmt (VM)
+#
+# Purpose:
+# - Ensure Pattern1 subset enforces "step-only body" under strict/dev shadow adopt.
+#
+# Expected:
+# - Exit code 3 (sum increments to 3). If body is dropped, exit would be 0.
+
+source "$(dirname "$0")/../../../lib/test_runner.sh"
+require_env || exit 2
+
+FIXTURE="$NYASH_ROOT/apps/tests/phase29ao_pattern1_subset_reject_extra_stmt.hako"
+RUN_TIMEOUT_SECS=${RUN_TIMEOUT_SECS:-10}
+
+set +e
+OUTPUT=$(timeout "$RUN_TIMEOUT_SECS" env NYASH_DISABLE_PLUGINS=1 HAKO_JOINIR_STRICT=1 "$NYASH_BIN" --backend vm "$FIXTURE" 2>&1)
+EXIT_CODE=$?
+set -e
+
+if [ "$EXIT_CODE" -eq 124 ]; then
+    log_error "phase29ao_pattern1_subset_reject_extra_stmt_vm: hakorune timed out (>${RUN_TIMEOUT_SECS}s)"
+    exit 1
+fi
+
+if [ "$EXIT_CODE" -ne 3 ]; then
+    log_error "phase29ao_pattern1_subset_reject_extra_stmt_vm: expected exit code 3, got $EXIT_CODE"
+    echo "$OUTPUT"
+    exit 1
+fi
+
+log_success "phase29ao_pattern1_subset_reject_extra_stmt_vm: PASS (exit=3)"
+exit 0