diff --git a/apps/tests/loopform_break_and_return.hako b/apps/tests/loopform_break_and_return.hako
new file mode 100644
index 00000000..44319875
--- /dev/null
+++ b/apps/tests/loopform_break_and_return.hako
@@ -0,0 +1,26 @@
+// loopform_break_and_return.hako
+// 目的: header-cond + break + early-return が混在するループをスモークする
+
+static box Main {
+  compute(limit) {
+    local i = 0
+    // Case A: header-cond (i < limit)
+    loop(i < limit) {
+      if i == 5 { return -1 }      // 早期 return（exit PHI 非対象）
+      if i >= limit - 1 { break }  // exit-break
+      i = i + 1
+    }
+    return i
+  }
+
+  main(args) {
+    // limit が小さい場合は break 経路で終了
+    local r1 = Main.compute(3)
+    print(r1)
+    // limit が大きい場合は i==5 で早期 return
+    local r2 = Main.compute(10)
+    print(r2)
+    return 0
+  }
+}
+
diff --git a/apps/tests/loopform_continue_break_scan.hako b/apps/tests/loopform_continue_break_scan.hako
new file mode 100644
index 00000000..049337ec
--- /dev/null
+++ b/apps/tests/loopform_continue_break_scan.hako
@@ -0,0 +1,40 @@
+// loopform_continue_break_scan.hako
+// 目的: Region+next_i 型の scan ループで continue + break 混在パターンをスモークする
+
+static box Main {
+  scan(s) {
+    local i = 0
+    local n = s.length()
+    // Case C-ish: constant-true + continue/backedge + break
+    loop(1 == 1) {
+      if i >= n { break }
+
+      // Region+next_i: この周回での「次の i」を 1 箇所に集約
+      local next_i = i
+      local ch = s.substring(i, i + 1)
+
+      if ch == " " {
+        next_i = i + 1
+        i = next_i
+        continue
+      }
+      if ch == "\t" {
+        next_i = i + 2
+        i = next_i
+        continue
+      }
+      // その他の文字で終了
+      break
+    }
+    return i
+  }
+
+  main(args) {
+    // 先頭に space + tab を並べたケースで scan が 3 を返すことを想定
+    local s = " \tX"
+    local r = Main.scan(s)
+    print(r)
+    return 0
+  }
+}
+
diff --git a/apps/tests/loopform_nested_region.hako b/apps/tests/loopform_nested_region.hako
new file mode 100644
index 00000000..2d16e0b8
--- /dev/null
+++ b/apps/tests/loopform_nested_region.hako
@@ -0,0 +1,39 @@
+// loopform_nested_region.hako
+// 目的: 外側 Loop + 内側 Region 型 Loop のネストをスモークする
+
+static box Main {
+  find_first_non_space(lines) {
+    local line_idx = 0
+    local n = lines.size()
+    // outer loop: 行単位
+    loop(line_idx < n) {
+      local line = lines.get(line_idx)
+      local col = 0
+
+      // inner loop: 行内の先頭非空白を探す（Region+next_col 風）
+      loop(1 == 1) {
+        if col >= line.length() { break }
+        local ch = line.substring(col, col + 1)
+        if ch == " " {
+          col = col + 1
+          continue
+        }
+        break
+      }
+
+      if col < line.length() { return line_idx }
+      line_idx = line_idx + 1
+    }
+    return -1
+  }
+
+  main(args) {
+    local lines = new ArrayBox()
+    lines.push("   ")
+    lines.push(" xx")
+    local r = Main.find_first_non_space(lines)
+    print(r)
+    return 0
+  }
+}
+
diff --git a/apps/tests/minimal_ssa_bug.hako b/apps/tests/minimal_ssa_bug.hako
new file mode 100644
index 00000000..d4c64d7e
--- /dev/null
+++ b/apps/tests/minimal_ssa_bug.hako
@@ -0,0 +1,20 @@
+// Minimal SSA Bug Reproduction
+// Pattern: Conditional reassignment with immediate usage
+// Expected: SSA violation "use of undefined value"
+
+static box MinimalSSABug {
+  main(args) {
+    local x = "test"
+
+    // Problem pattern: reassign + immediate use in nested condition
+    if x.length() > 0 {
+      x = x.substring(0, 2)            // reassign x
+      if x.length() > 0 && x.substring(0, 1) == "t" {  // immediate use
+        x = x.substring(1, 2)          // another reassign
+        print(x)
+      }
+    }
+
+    return 0
+  }
+}
diff --git a/apps/tests/minimal_ssa_bug_loop.hako b/apps/tests/minimal_ssa_bug_loop.hako
new file mode 100644
index 00000000..23973021
--- /dev/null
+++ b/apps/tests/minimal_ssa_bug_loop.hako
@@ -0,0 +1,28 @@
+// Minimal SSA Bug Reproduction with Loop
+// Pattern: Conditional reassignment inside loop with immediate usage
+// Expected: SSA violation "use of undefined value"
+
+static box MinimalSSABugLoop {
+  main(args) {
+    local src = "using foo\nusing bar\n"
+    local i = 0
+    local n = src.length()
+
+    loop(i < n) {
+      local line = src.substring(i, i + 10)
+
+      // Problem pattern: nested conditions with reassignment + immediate use
+      if line.length() > 0 {
+        line = line.substring(0, 5)     // reassign line
+        if line.length() > 0 && line.substring(0, 1) == "u" {  // immediate use
+          line = line.substring(1, 5)   // another reassign
+          print(line)
+        }
+      }
+
+      i = i + 10
+    }
+
+    return 0
+  }
+}
diff --git a/apps/tests/minimal_ssa_skip_ws.hako b/apps/tests/minimal_ssa_skip_ws.hako
new file mode 100644
index 00000000..0066403e
--- /dev/null
+++ b/apps/tests/minimal_ssa_skip_ws.hako
@@ -0,0 +1,24 @@
+// minimal_ssa_skip_ws.hako — LoopForm exit-PHI regression canary
+// 目的: loop(1 == 1) + break だけで exit PHI が壊れる既知事例を最小化
+
+static box Main {
+  skip(s) {
+    local i = 0
+    local n = s.length()
+    // 本来は loop(i < n) だが、ワークアラウンドとして loop(1 == 1) にしていた経路で
+    // exit PHI が崩れて ValueId 未定義になる不具合を再現する。
+    loop(1 == 1) {
+      if i >= n { break }
+      local ch = s.substring(i, i + 1)
+      if ch == " " { i = i + 1 } else { break }
+    }
+    return i
+  }
+
+  main(args) {
+    local s = "   abc"
+    local r = Main.skip(s)
+    print(r)
+    return 0
+  }
+}
diff --git a/apps/tests/stage1_using_minimal.hako b/apps/tests/stage1_using_minimal.hako
new file mode 100644
index 00000000..1041f697
--- /dev/null
+++ b/apps/tests/stage1_using_minimal.hako
@@ -0,0 +1 @@
+using "foo/bar.hako" as Foo
diff --git a/apps/tests/stage1_using_namespace.hako b/apps/tests/stage1_using_namespace.hako
new file mode 100644
index 00000000..6280f90b
--- /dev/null
+++ b/apps/tests/stage1_using_namespace.hako
@@ -0,0 +1,9 @@
+using std.collections
+using "path/to/module.hako" as MyModule
+
+static box Main {
+  main(args) {
+    print("Hello")
+    return 0
+  }
+}
diff --git a/apps/tests/test_else_if_scope.hako b/apps/tests/test_else_if_scope.hako
new file mode 100644
index 00000000..d418b3bf
--- /dev/null
+++ b/apps/tests/test_else_if_scope.hako
@@ -0,0 +1,23 @@
+static box Main {
+  main(args) {
+    local x = args
+    if x == null {
+      x = new ArrayBox()
+      x.push("default")
+      if x.length() > 0 {
+        local status = "initialized"
+        if x.get(0) != null {
+          status = "has_value: " + x.length()
+        }
+        print("Status: " + status)
+      }
+    } else if x.length() >= 0 {
+      print("Length: " + ("" + x.length()))
+    }
+    if x == null {
+      return 1
+    }
+    print("Final length: " + ("" + x.length()))
+    return 0
+  }
+}
diff --git a/apps/tests/test_loop_conditional_assign.hako b/apps/tests/test_loop_conditional_assign.hako
new file mode 100644
index 00000000..2ac007ab
--- /dev/null
+++ b/apps/tests/test_loop_conditional_assign.hako
@@ -0,0 +1,15 @@
+static box Main {
+  main(args) {
+    local str = "foo/bar/baz"
+    local idx = -1
+    local t = 0
+    loop(t < str.length()) {
+      if str.substring(t, t+1) == "/" {
+        idx = t
+      }
+      t = t + 1
+    }
+    print("Last slash at: " + ("" + idx))
+    return 0
+  }
+}
diff --git a/apps/tests/test_nested_scope_phi.hako b/apps/tests/test_nested_scope_phi.hako
new file mode 100644
index 00000000..1a3a90aa
--- /dev/null
+++ b/apps/tests/test_nested_scope_phi.hako
@@ -0,0 +1,29 @@
+static box Main {
+  main(args) {
+    local src = "using foo\nusing bar"
+    local n = src.length()
+    local i = 0
+    loop(i < n) {
+      local j = i
+      loop(j < n && src.substring(j, j+1) != "\n") { j = j + 1 }
+      local line = src.substring(i, j)
+
+      if line.length() > 0 {
+        local p = line
+        local idx = -1
+        local t = 0
+        loop(t < p.length()) {
+          if p.substring(t,t+1) == " " { idx = t }
+          t = t + 1
+        }
+        if idx >= 0 {
+          p = p.substring(0, idx)
+        }
+        print("Processed: " + p)
+      }
+
+      i = j + 1
+    }
+    return 0
+  }
+}
diff --git a/apps/tests/test_string_concat_phi.hako b/apps/tests/test_string_concat_phi.hako
new file mode 100644
index 00000000..8ca2d14b
--- /dev/null
+++ b/apps/tests/test_string_concat_phi.hako
@@ -0,0 +1,28 @@
+static box TestBox {
+  get_value() {
+    return "test_result"
+  }
+}
+
+static box Main {
+  main(args) {
+    local prog = TestBox.get_value()
+    {
+      local status = "null"
+      if prog != null { status = "non-null" }
+      print("Debug: " + status)
+    }
+    if prog == null {
+      print("Error: prog is null")
+      return null
+    }
+    local ps = "" + prog
+    print("Stringified length: " + ("" + ps.length()))
+    if ps.indexOf("test") < 0 || ps.indexOf("result") < 0 {
+      print("Error: unexpected output")
+      return null
+    }
+    print("Success")
+    return ps
+  }
+}
diff --git a/docs/development/analysis/minimal_ssa_bug_analysis.md b/docs/development/analysis/minimal_ssa_bug_analysis.md
new file mode 100644
index 00000000..c8ac9ba0
--- /dev/null
+++ b/docs/development/analysis/minimal_ssa_bug_analysis.md
@@ -0,0 +1,26 @@
+# Minimal SSA Bug Analysis — loop(1==1) + break exit PHI
+
+## 何が起きていたか
+- Stage‑B / FuncScanner の `skip_whitespace` 系で、`loop(1 == 1) { ... break }` という形のループが exit PHI を壊し、`ValueId` 未定義エラー（`use of undefined value` / Dominator violation）が発生していた。
+- 原因: exit PHI 入力に「存在しない predecessor（header）」由来の値を含めていた。
+  - `loop(1 == 1)` の場合、`header → exit` の CFG エッジは存在しない（exit は break 経路のみ）。
+  - それにもかかわらず、`merge_exit_with_classification` が header 値を無条件で PHI 入力に足していたため、非支配ブロックからの値参照になり破綻。
+
+## 最小再現
+- ファイル: `apps/tests/minimal_ssa_skip_ws.hako`
+- 構造: `loop(1 == 1)` の中で `if i >= n { break }` を先頭に置く形。break 以外に exit pred が無い。
+- Rustテスト: `src/tests/mir_loopform_conditional_reassign.rs::loop_constant_true_exit_phi_dominates`（今回新設）
+
+## 修正内容
+- `src/mir/phi_core/loop_snapshot_merge.rs` の `merge_exit_with_classification` で、header が exit predecessor に含まれている場合にのみ header 値を PHI 入力へ追加するようガード。
+  - CFG に存在しない predecessor 由来の値を PHI に入れないことで Dominator violation を解消。
+
+## 追加したもの
+- 最小再現ハコ: `apps/tests/minimal_ssa_skip_ws.hako`
+- Rustテスト: `src/tests/mir_loopform_conditional_reassign.rs`
+  - `loop_constant_true_exit_phi_dominates`（今回のバグ再現→修正確認用）
+  - 将来拡張用に 2 本を `#[ignore]` で占位（条件付き再代入 / body-local 変数の exit PHI 観測）
+
+## 次のステップ
+- `#[ignore]` テストを埋める: 条件付き再代入（`loop(i < n)`）や body-local 混在ケースの SSA 安定性を追加検証。
+- Stage‑B / UsingResolver の既存スモークにも `apps/tests/minimal_ssa_skip_ws.hako` を流し込んで回帰を防ぐ。
diff --git a/src/tests/mir_loopform_complex.rs b/src/tests/mir_loopform_complex.rs
new file mode 100644
index 00000000..3fb57dad
--- /dev/null
+++ b/src/tests/mir_loopform_complex.rs
@@ -0,0 +1,85 @@
+//! LoopForm / Region+next_i 複雑パターン向けスモークテスト
+//!
+//! 目的:
+//! - continue + break 混在ループ
+//! - break + early-return 混在ループ
+//! - 外側ループ + 内側 Region 型ループのネスト
+//! を MIR Verifier ベースで「構造テスト」として押さえる。
+
+use crate::mir::{MirCompiler, MirVerifier};
+use crate::parser::NyashParser;
+
+fn setup_stage3_env() {
+    std::env::set_var("NYASH_PARSER_STAGE3", "1");
+    std::env::set_var("HAKO_PARSER_STAGE3", "1");
+    std::env::set_var("NYASH_ENABLE_USING", "1");
+    std::env::set_var("HAKO_ENABLE_USING", "1");
+    std::env::set_var("NYASH_DISABLE_PLUGINS", "1");
+}
+
+fn teardown_stage3_env() {
+    std::env::remove_var("NYASH_PARSER_STAGE3");
+    std::env::remove_var("HAKO_PARSER_STAGE3");
+    std::env::remove_var("NYASH_ENABLE_USING");
+    std::env::remove_var("HAKO_ENABLE_USING");
+    std::env::remove_var("NYASH_DISABLE_PLUGINS");
+}
+
+fn compile_module(src: &str) -> crate::mir::MirCompileResult {
+    setup_stage3_env();
+    let ast = NyashParser::parse_from_string(src).expect("parse ok");
+    let mut mc = MirCompiler::with_options(false);
+    mc.compile(ast).expect("compile ok")
+}
+
+#[test]
+fn mir_loopform_continue_break_scan_verify() {
+    // Case C-ish: constant-true + continue/backedge + break（Region+next_i 型）
+    let src = include_str!("../../apps/tests/loopform_continue_break_scan.hako");
+    let compiled = compile_module(src);
+
+    let mut verifier = MirVerifier::new();
+    if let Err(errors) = verifier.verify_module(&compiled.module) {
+        for e in &errors {
+            eprintln!("[mir-verify] {}", e);
+        }
+        teardown_stage3_env();
+        panic!("mir_loopform_continue_break_scan_verify: MIR verification failed");
+    }
+    teardown_stage3_env();
+}
+
+#[test]
+fn mir_loopform_break_and_return_verify() {
+    // Case: header-cond + break + early-return 混在
+    let src = include_str!("../../apps/tests/loopform_break_and_return.hako");
+    let compiled = compile_module(src);
+
+    let mut verifier = MirVerifier::new();
+    if let Err(errors) = verifier.verify_module(&compiled.module) {
+        for e in &errors {
+            eprintln!("[mir-verify] {}", e);
+        }
+        teardown_stage3_env();
+        panic!("mir_loopform_break_and_return_verify: MIR verification failed");
+    }
+    teardown_stage3_env();
+}
+
+#[test]
+fn mir_loopform_nested_region_verify() {
+    // Case: 外側 Loop + 内側 Region 型 Loop（inner は constant-true + continue + break）
+    let src = include_str!("../../apps/tests/loopform_nested_region.hako");
+    let compiled = compile_module(src);
+
+    let mut verifier = MirVerifier::new();
+    if let Err(errors) = verifier.verify_module(&compiled.module) {
+        for e in &errors {
+            eprintln!("[mir-verify] {}", e);
+        }
+        teardown_stage3_env();
+        panic!("mir_loopform_nested_region_verify: MIR verification failed");
+    }
+    teardown_stage3_env();
+}
+
diff --git a/src/tests/mir_loopform_conditional_reassign.rs b/src/tests/mir_loopform_conditional_reassign.rs
new file mode 100644
index 00000000..067c822e
--- /dev/null
+++ b/src/tests/mir_loopform_conditional_reassign.rs
@@ -0,0 +1,171 @@
+//! LoopForm exit PHI regression tests (conditional reassign / break)
+//!
+//! 目的:
+//! - loop(1 == 1) + break 経路で exit PHI が壊れて ValueId 未定義になる既知バグを捕まえる。
+//! - 将来: 条件付き再代入や body-local 変数が混在するケースを拡張（現状は #[ignore]）。
+
+use crate::mir::{MirCompiler, MirVerifier};
+use crate::parser::NyashParser;
+
+fn setup_stage3_env() {
+    std::env::set_var("NYASH_PARSER_STAGE3", "1");
+    std::env::set_var("HAKO_PARSER_STAGE3", "1");
+    std::env::set_var("NYASH_ENABLE_USING", "1");
+    std::env::set_var("HAKO_ENABLE_USING", "1");
+    std::env::set_var("NYASH_DISABLE_PLUGINS", "1");
+}
+
+fn teardown_stage3_env() {
+    std::env::remove_var("NYASH_PARSER_STAGE3");
+    std::env::remove_var("HAKO_PARSER_STAGE3");
+    std::env::remove_var("NYASH_ENABLE_USING");
+    std::env::remove_var("HAKO_ENABLE_USING");
+    std::env::remove_var("NYASH_DISABLE_PLUGINS");
+}
+
+fn compile_module(src: &str) -> crate::mir::MirCompileResult {
+    setup_stage3_env();
+    let ast = NyashParser::parse_from_string(src).expect("parse ok");
+    let mut mc = MirCompiler::with_options(false);
+    mc.compile(ast).expect("compile ok")
+}
+
+// [LoopForm-Test] Case B: constant-true+break-only
+#[test]
+fn loop_constant_true_exit_phi_dominates() {
+    // Repro: loop(1 == 1) + break only (header is NOT an exit predecessor)
+    // This used to create exit PHIs with header inputs from non-predecessor blocks → dominator violation.
+    let src = include_str!("../../apps/tests/minimal_ssa_skip_ws.hako");
+    let compiled = compile_module(src);
+
+    let mut verifier = MirVerifier::new();
+    if let Err(errors) = verifier.verify_module(&compiled.module) {
+        for e in &errors {
+            eprintln!("[mir-verify] {}", e);
+        }
+        teardown_stage3_env();
+        panic!("loop_constant_true_exit_phi_dominates: MIR verification failed");
+    }
+    teardown_stage3_env();
+}
+
+// [LoopForm-Test] Case A: header+break
+#[test]
+fn loop_conditional_reassign_exit_phi_header_and_break() {
+    // Case A: 通常の loop(i < n) で header→exit と body→exit の両方が存在するケース。
+    // ここで PHI が壊れていないことを確認し、今回の修正が既存パターンを regress していないことを見る。
+    let src = r#"
+static box Main {
+  loop_case(s) {
+    local i = 0
+    local n = s.length()
+    loop(i < n) {
+      if i >= n { break }
+      i = i + 1
+    }
+    return i
+  }
+
+  main(args) {
+    local s = "abc"
+    local r = Main.loop_case(s)
+    print(r)
+    return 0
+  }
+}
+"#;
+    let compiled = compile_module(src);
+
+    let mut verifier = MirVerifier::new();
+    if let Err(errors) = verifier.verify_module(&compiled.module) {
+        for e in &errors {
+            eprintln!("[mir-verify] {}", e);
+        }
+        teardown_stage3_env();
+        panic!("loop_conditional_reassign_exit_phi_header_and_break: MIR verification failed");
+    }
+    teardown_stage3_env();
+}
+
+// [LoopForm-Test] Case C: body-local (BodyLocalInternal)
+#[test]
+fn loop_body_local_exit_phi_body_only() {
+    // Case C: body-local 変数が break 経路にだけ存在し、header には存在しないケース。
+    // Option C の分類により、こうした BodyLocalInternal には exit PHI が張られず、
+    // MIR 検証が通ることを確認する。
+    let src = r#"
+static box Main {
+  loop_case(s) {
+    local i = 0
+    local n = s.length()
+    loop(1 == 1) {
+      if i >= n { break }
+      local temp = s.substring(i, i + 1)
+      if temp == "x" { break } else { i = i + 1 }
+    }
+    return i
+  }
+
+  main(args) {
+    local s = "abc"
+    local r = Main.loop_case(s)
+    print(r)
+    return 0
+  }
+}
+"#;
+    let compiled = compile_module(src);
+
+    let mut verifier = MirVerifier::new();
+    if let Err(errors) = verifier.verify_module(&compiled.module) {
+        for e in &errors {
+            eprintln!("[mir-verify] {}", e);
+        }
+        teardown_stage3_env();
+        panic!("loop_body_local_exit_phi_body_only: MIR verification failed");
+    }
+    teardown_stage3_env();
+}
+
+// [LoopForm-Test] Case D: continue+break (continue_merge → header → exit)
+#[test]
+fn loop_continue_merge_header_exit() {
+    // Case D: continue 文を含むループで、continue_merge → header → exit の組み合わせを検証。
+    // continue_merge ブロックで PHI を生成し、header PHI に正しく伝播することを確認。
+    let src = r#"
+static box Main {
+  loop_case(s) {
+    local i = 0
+    local n = s.length()
+    loop(i < n) {
+      local ch = s.substring(i, i + 1)
+      if ch == " " {
+        i = i + 1
+        continue
+      }
+      if ch == "x" { break }
+      i = i + 1
+    }
+    return i
+  }
+
+  main(args) {
+    local s = "  x  "
+    local r = Main.loop_case(s)
+    print(r)
+    return 0
+  }
+}
+"#;
+    let compiled = compile_module(src);
+
+    let mut verifier = MirVerifier::new();
+    if let Err(errors) = verifier.verify_module(&compiled.module) {
+        for e in &errors {
+            eprintln!("[mir-verify] {}", e);
+        }
+        teardown_stage3_env();
+        panic!("loop_continue_merge_header_exit: MIR verification failed");
+    }
+    teardown_stage3_env();
+}
diff --git a/src/tests/mir_loopform_exit_phi.rs b/src/tests/mir_loopform_exit_phi.rs
index f1da183b..939df387 100644
--- a/src/tests/mir_loopform_exit_phi.rs
+++ b/src/tests/mir_loopform_exit_phi.rs
@@ -5,8 +5,8 @@
  * Focus: predecessor tracking and PHI input generation for break statements
  */
 
-use crate::parser::NyashParser;
 use crate::mir::{MirCompiler, MirVerifier};
+use crate::parser::NyashParser;
 
 #[test]
 fn test_loopform_exit_phi_single_break() {
@@ -33,8 +33,7 @@ static box TestExitPhi {
     println!("=== Test: Single break statement ===");
 
     // Parse
-    let ast = NyashParser::parse_from_string(src)
-        .expect("parse failed");
+    let ast = NyashParser::parse_from_string(src).expect("parse failed");
 
     // Compile
     let mut mc = MirCompiler::with_options(false);
@@ -46,8 +45,12 @@ static box TestExitPhi {
         eprintln!("Entry block: {:?}", func.entry_block);
         eprintln!("Total blocks: {}", func.blocks.len());
         for (bid, block) in &func.blocks {
-            eprintln!("  Block {:?}: {} instructions, successors={:?}",
-                     bid, block.instructions.len(), block.successors);
+            eprintln!(
+                "  Block {:?}: {} instructions, successors={:?}",
+                bid,
+                block.instructions.len(),
+                block.successors
+            );
             if *bid == crate::mir::BasicBlockId(10) {
                 eprintln!("    BB10 instructions:");
                 for inst in &block.instructions {