diff --git a/CURRENT_TASK.md b/CURRENT_TASK.md
index 3cca0eaa..a8b9b70e 100644
--- a/CURRENT_TASK.md
+++ b/CURRENT_TASK.md
@@ -389,6 +389,21 @@
         - **次候補**:
           - Phase 200+: ConditionEnv 拡張（_parse_number, _atoi）
           - Phase 198+: JsonParser 残りループ個別対応
+  - [x] **Phase 200-A: ConditionEnv 拡張インフラ（型と箱のみ）** ✅ (完了: 2025-12-09)
+        - **目的**: ConditionEnv を壊さずに関数スコープ "実質定数" を扱う基盤
+        - **実施内容**:
+          - 200-A-1: CapturedVar / CapturedEnv 型導入 ✅
+          - 200-A-2: FunctionScopeCaptureAnalyzer スケルトン ✅
+          - 200-A-3: ConditionEnvBuilder v2 入口 ✅
+          - 200-A-4: ParamRole enum 追加 ✅
+          - 200-A-5: ドキュメント更新 ✅
+        - **スコープ**: Infra only / Integration pending
+        - **成果**:
+          - 型と箱の定義完了 ✅
+          - 既存ループの挙動変更なし ✅
+          - Phase 200-B への準備完了 ✅
+        - **次フェーズ**: Phase 200-B（digits 系ループへの適用）
+        - **詳細**: phase200-A-conditionenv-infra.md
 
 ---
 
diff --git a/docs/development/current/main/joinir-architecture-overview.md b/docs/development/current/main/joinir-architecture-overview.md
index 00245f96..d1c2c22e 100644
--- a/docs/development/current/main/joinir-architecture-overview.md
+++ b/docs/development/current/main/joinir-architecture-overview.md
@@ -49,10 +49,23 @@ JoinIR ラインで守るべきルールを先に書いておくよ：
    - 現状は ConditionBinding/ExitMeta/JoinFragmentMeta で役割を区別しており、将来 ParamRole enum として明示する予定。
 
 8. **LoopHeader PHI dst は予約領域（上書き禁止）**
-   - LoopHeaderPhiBuilder が生成したヘッダ PHI の dst ValueId は「現在のループ値」の SSOT として扱い、  
+   - LoopHeaderPhiBuilder が生成したヘッダ PHI の dst ValueId は「現在のループ値」の SSOT として扱い、
      BoundaryInjector や InstructionRewriter が `Copy` などで二度書きすることを禁止する。
    - merge ラインでは「ヘッダ PHI dst に対する新しい定義が出てきたら debug モードで panic する」ことで契約違反を早期検出する。
 
+9. **ParamRole の不変条件（Phase 200-A 追加）**
+   - **Condition 役のパラメータは「PHI dst にしてはいけない」**
+     - 理由: 条件専用変数はループ内で更新されない（例: `digits` in `_atoi()`）
+     - LoopHeaderPhiBuilder は Condition 役の変数に対して header PHI を生成しない
+   - **Condition 役のパラメータは「ExitLine の対象にも入れない」**
+     - 理由: 条件専用変数はループ外で使われない（ループ内でのみ参照）
+     - ExitLineReconnector は Condition 役の変数を exit_bindings から除外
+   - **ParamRole の分類**:
+     - `LoopParam`: ループ制御変数（例: `i` in `loop(i < len)`）→ header PHI + exit_bindings
+     - `Condition`: 条件専用変数（例: `digits` in `digits.indexOf(ch)`）→ condition_bindings のみ
+     - `Carrier`: 状態更新変数（例: `sum`, `count`）→ header PHI + exit_bindings
+     - `ExprResult`: ループ戻り値 → exit_phi_builder で処理
+
 ---
 
 ## 2. 主な箱と責務
@@ -262,6 +275,35 @@ JoinIR ラインで守るべきルールを先に書いておくよ：
     - `verify_joinir_contracts()`: merge_joinir_mir_blocks() の最後で全契約を一括チェック。
     - release ビルドでは完全に除去される（`#[cfg(debug_assertions)]`）。
 
+- **FunctionScopeCaptureAnalyzer / CapturedEnv（Phase 200-A 追加）**
+  - ファイル: `src/mir/loop_pattern_detection/function_scope_capture.rs`
+  - 責務:
+    - 関数スコープで宣言され、ループ内で不変な変数（"実質定数"）を検出。
+    - 例: `local digits = "0123456789"` in `JsonParser._atoi()`
+    - CapturedVar: `{ name, host_id, is_immutable }`
+    - CapturedEnv: 検出された変数のコレクション
+  - **Phase 200-A**: 型と空実装のみ（skeleton）。
+  - **Phase 200-B**: 実際の検出ロジックを実装予定（AST スキャン + 再代入チェック）。
+
+- **ParamRole enum（Phase 200-A 追加）**
+  - ファイル: `src/mir/join_ir/lowering/inline_boundary_builder.rs`
+  - 責務:
+    - JoinInlineBoundary のパラメータ役割を明示的に区別。
+    - LoopParam / Condition / Carrier / ExprResult の 4 種類。
+  - **不変条件**:
+    - **Condition 役**: PHI dst にしてはいけない（ループ内で更新されない）。
+    - **Condition 役**: ExitLine の対象にも入れない（ループ外で使われない）。
+    - 理由: 条件専用変数（例: `digits`）はループ内でのみ参照され、不変。
+  - **Phase 200-A**: enum 定義のみ。
+  - **Phase 200-B**: ルーティングロジック実装予定（CapturedEnv 統合時）。
+
+- **ConditionEnvBuilder::build_with_captures（Phase 200-A 追加）**
+  - ファイル: `src/mir/builder/control_flow/joinir/patterns/condition_env_builder.rs`
+  - 責務:
+    - 将来 CapturedEnv を受け取り、ConditionEnv に統合する v2 入口。
+  - **Phase 200-A**: 既存実装に委譲する skeleton。
+  - **Phase 200-B**: CapturedEnv の変数を condition_bindings に追加する実装予定。
+
 ### 2.4 expr result ライン（式としての戻り値）
 
 - **exit_phi_builder**
diff --git a/docs/development/current/main/phase200-A-conditionenv-infra.md b/docs/development/current/main/phase200-A-conditionenv-infra.md
new file mode 100644
index 00000000..d5395efd
--- /dev/null
+++ b/docs/development/current/main/phase200-A-conditionenv-infra.md
@@ -0,0 +1,446 @@
+# Phase 200-A: ConditionEnv 拡張インフラ（型と箱のみ）
+
+**Date**: 2025-12-09
+**Status**: Ready for Implementation
+**Prerequisite**: Phase 197 complete
+
+---
+
+## 目的
+
+**ConditionEnv を壊さずに、関数スコープの "実質定数" ローカル（例: `local digits = "0123456789"`）を安全に扱う基盤を作る。**
+
+**このフェーズでは**:
+- ✅ 型と解析箱だけを導入
+- ✅ 既存ループの挙動は変えない
+- ❌ 具体ループ（`_parse_number` / `_atoi` 等）にはまだ手を出さない
+
+**設計原則**:
+- ConditionEnv の「芯」（= boundary に載ったものだけを見る）を保持
+- 2-tier 設計（ConditionEnv / LoopBodyLocalEnv）は維持
+- 新しい箱は「関数スコープの実質定数」を捕捉するレイヤーとして追加
+
+---
+
+## アーキテクチャ概要
+
+### 現在の設計（Phase 193 まで）
+
+```
+┌─────────────────────────────────────────────────────────┐
+│ ConditionEnv                                            │
+│   - LoopParam (i, len, etc.)                           │
+│   - OuterLocal (function params: s, pos)               │
+│   - LoopBodyLocal (body-local vars: ch, digit)         │
+└─────────────────────────────────────────────────────────┘
+          ↓
+┌─────────────────────────────────────────────────────────┐
+│ JoinInlineBoundary                                      │
+│   - join_inputs / host_inputs                          │
+│   - condition_bindings                                  │
+│   - exit_bindings                                       │
+└─────────────────────────────────────────────────────────┘
+```
+
+**問題**: `digits` のような「関数スコープで宣言され、ループ内で不変なローカル」が捕捉できない
+
+### 新設計（Phase 200+）
+
+```
+┌─────────────────────────────────────────────────────────┐
+│ FunctionScopeCaptureAnalyzer                            │
+│   - 関数スコープの "実質定数" を検出                      │
+│   - CapturedEnv を生成                                   │
+└─────────────────────────────────────────────────────────┘
+          ↓
+┌─────────────────────────────────────────────────────────┐
+│ CapturedEnv                                             │
+│   - CapturedVar[] { name, host_id, is_immutable }      │
+└─────────────────────────────────────────────────────────┘
+          ↓
+┌─────────────────────────────────────────────────────────┐
+│ ConditionEnvBuilder v2                                  │
+│   - LoopParam + CapturedEnv → ConditionEnv に合成       │
+└─────────────────────────────────────────────────────────┘
+          ↓
+┌─────────────────────────────────────────────────────────┐
+│ JoinInlineBoundary (拡張)                               │
+│   - ParamRole: LoopParam / Condition / Carrier / Expr  │
+│   - CapturedEnv の変数も condition_bindings に載せる    │
+└─────────────────────────────────────────────────────────┘
+```
+
+---
+
+## Task 200-A-1: CapturedVar / CapturedEnv 型の導入
+
+### 目標
+関数スコープの「実質定数」を表現する型を定義する。
+
+### ファイル
+**新規**: `src/mir/loop_pattern_detection/function_scope_capture.rs`
+
+### 実装内容
+
+```rust
+//! Phase 200-A: Function scope capture infrastructure
+//!
+//! This module provides types for capturing function-scoped variables
+//! that are effectively immutable within a loop context.
+
+/// A variable captured from function scope for use in loop conditions.
+///
+/// Example: `local digits = "0123456789"` in JsonParser._atoi()
+#[derive(Debug, Clone)]
+pub struct CapturedVar {
+    /// Variable name (e.g., "digits", "table")
+    pub name: String,
+
+    /// MIR ValueId of the original definition
+    pub host_id: ValueId,
+
+    /// Whether this variable is never reassigned in the function
+    pub is_immutable: bool,
+}
+
+/// Environment containing function-scoped captured variables.
+///
+/// Phase 200-A: Type definition only, not yet integrated with ConditionEnv.
+#[derive(Debug, Clone, Default)]
+pub struct CapturedEnv {
+    pub vars: Vec<CapturedVar>,
+}
+
+impl CapturedEnv {
+    pub fn new() -> Self {
+        Self { vars: Vec::new() }
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.vars.is_empty()
+    }
+
+    pub fn add_var(&mut self, var: CapturedVar) {
+        self.vars.push(var);
+    }
+
+    /// Look up a captured variable by name
+    pub fn get(&self, name: &str) -> Option<&CapturedVar> {
+        self.vars.iter().find(|v| v.name == name)
+    }
+}
+```
+
+### テスト
+
+```rust
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::mir::ValueId;
+
+    #[test]
+    fn test_captured_env_empty() {
+        let env = CapturedEnv::new();
+        assert!(env.is_empty());
+        assert!(env.get("digits").is_none());
+    }
+
+    #[test]
+    fn test_captured_env_add_and_get() {
+        let mut env = CapturedEnv::new();
+        env.add_var(CapturedVar {
+            name: "digits".to_string(),
+            host_id: ValueId(42),
+            is_immutable: true,
+        });
+
+        assert!(!env.is_empty());
+        let var = env.get("digits").unwrap();
+        assert_eq!(var.name, "digits");
+        assert_eq!(var.host_id, ValueId(42));
+        assert!(var.is_immutable);
+    }
+}
+```
+
+### 成果物
+- [x] `CapturedVar` 構造体定義
+- [x] `CapturedEnv` 構造体定義
+- [x] 基本的なアクセサメソッド
+- [x] Unit tests (2件)
+
+---
+
+## Task 200-A-2: FunctionScopeCaptureAnalyzer スケルトン
+
+### 目標
+関数スコープの実質定数を検出する解析関数の枠を作る。
+
+### ファイル
+同じ `function_scope_capture.rs` に追加
+
+### 実装内容
+
+```rust
+/// Analyzes function-scoped variables that can be safely captured for loop conditions.
+///
+/// # Phase 200-A Status
+/// Currently returns empty CapturedEnv (skeleton implementation).
+/// Actual capture detection will be implemented in Phase 200-B.
+///
+/// # Future Detection Criteria (Phase 200-B+)
+/// - Variable is declared before the loop
+/// - Variable is never reassigned within the function
+/// - Variable is referenced in loop condition or body
+pub fn analyze_captured_vars(
+    _fn_body: &[Stmt],
+    _loop_ast: &Stmt,
+    _scope: &LoopScopeShape,
+) -> CapturedEnv {
+    // Phase 200-A: Skeleton implementation
+    // TODO(Phase 200-B): Implement actual capture detection
+    //
+    // Detection algorithm:
+    // 1. Find all `local` declarations before the loop
+    // 2. Check if each is never reassigned (is_immutable = true)
+    // 3. Check if referenced in loop condition/body
+    // 4. Exclude loop parameters and body-local variables
+
+    CapturedEnv::new()
+}
+```
+
+### 設計メモ
+
+**Phase 200-B で実装する検出アルゴリズム**:
+1. ループの前にある `local` 宣言を全て収集
+2. 関数全体で再代入されていないかチェック（`is_immutable`）
+3. ループ条件/本体で参照されているかチェック
+4. LoopParam / LoopBodyLocal は除外（既に ConditionEnv で扱われている）
+
+### 成果物
+- [x] `analyze_captured_vars` 関数シグネチャ
+- [x] ドキュメントコメント（将来の検出基準を記載）
+- [x] 空実装（Phase 200-B で中身を実装）
+
+---
+
+## Task 200-A-3: ConditionEnvBuilder v2 入口
+
+### 目標
+将来 CapturedEnv を受け取るフックだけ用意する。
+
+### ファイル
+`src/mir/join_ir/lowering/condition_env_builder.rs` または既存の ConditionEnv 関連ファイル
+
+### 実装内容
+
+```rust
+/// Build ConditionEnv with optional captured variables.
+///
+/// # Phase 200-A Status
+/// Currently ignores `captured` parameter and calls existing implementation.
+/// Integration with CapturedEnv will be implemented in Phase 200-B.
+///
+/// # Future Behavior (Phase 200-B+)
+/// - Add captured variables to condition_bindings
+/// - Generate Copy instructions for captured vars in entry block
+/// - Track captured vars separately from loop params
+pub fn build_condition_env_with_captures(
+    loop_params: &[ParamInfo],
+    _captured: &CapturedEnv,
+    boundary: &mut JoinInlineBoundaryBuilder,
+) -> ConditionEnv {
+    // Phase 200-A: Delegate to existing implementation
+    // TODO(Phase 200-B): Integrate captured vars into ConditionEnv
+    //
+    // Integration steps:
+    // 1. For each captured var, add to boundary.condition_bindings
+    // 2. Mark as ParamRole::Condition (not Carrier or LoopParam)
+    // 3. Ensure captured vars are NOT included in exit_bindings
+
+    build_condition_env_from_params(loop_params, boundary)
+}
+```
+
+### 成果物
+- [x] `build_condition_env_with_captures` 関数（既存実装に委譲）
+- [x] ドキュメントコメント（Phase 200-B の統合手順を記載）
+
+---
+
+## Task 200-A-4: ParamRole enum 追加
+
+### 目標
+JoinInlineBoundary のパラメータ役割を明示的に区別する。
+
+### ファイル
+`src/mir/join_ir/lowering/inline_boundary.rs` または関連ファイル
+
+### 実装内容
+
+```rust
+/// Role of a parameter in JoinIR lowering.
+///
+/// # Invariants
+/// - **LoopParam**: Participates in header PHI, updated in loop body
+/// - **Condition**: Used in condition only, NOT in header PHI, NOT in ExitLine
+/// - **Carrier**: Updated in loop body, participates in header PHI and ExitLine
+/// - **ExprResult**: Return value of the loop expression, handled by exit_phi_builder
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ParamRole {
+    /// Loop iteration variable (e.g., `i` in `loop(i < len)`)
+    LoopParam,
+
+    /// Condition-only parameter (e.g., `digits` in `digits.indexOf(ch)`)
+    /// NOT included in header PHI or ExitLine
+    Condition,
+
+    /// State carried across iterations (e.g., `sum`, `count`)
+    Carrier,
+
+    /// Expression result returned by the loop
+    ExprResult,
+}
+
+impl JoinInlineBoundaryBuilder {
+    /// Add a parameter with explicit role.
+    ///
+    /// # Phase 200-A Status
+    /// Currently stores role but does not use it for routing.
+    /// Role-based routing will be implemented in Phase 200-B.
+    pub fn add_param_with_role(&mut self, name: &str, host_id: ValueId, role: ParamRole) {
+        // Phase 200-A: Store role for future use
+        // TODO(Phase 200-B): Route based on role
+        //
+        // Routing rules:
+        // - LoopParam: join_inputs + host_inputs
+        // - Condition: condition_bindings only (no PHI, no ExitLine)
+        // - Carrier: join_inputs + host_inputs + exit_bindings
+        // - ExprResult: handled by exit_phi_builder
+
+        match role {
+            ParamRole::LoopParam | ParamRole::Carrier => {
+                // Existing behavior: add to join_inputs
+                self.add_input(name, host_id);
+            }
+            ParamRole::Condition => {
+                // Phase 200-A: Just log for now
+                // TODO(Phase 200-B): Add to condition_bindings without PHI
+            }
+            ParamRole::ExprResult => {
+                // Handled separately by set_expr_result
+            }
+        }
+    }
+}
+```
+
+### 不変条件（joinir-architecture-overview.md に追記）
+
+```markdown
+9. **ParamRole の不変条件**
+   - **Condition 役のパラメータは「PHI dst にしてはいけない」**
+     - 理由: 条件専用変数はループ内で更新されない
+     - 例: `digits` は header PHI に参加しない
+   - **Condition 役のパラメータは「ExitLine の対象にも入れない」**
+     - 理由: 条件専用変数はループ外で使われない（ループ内でのみ参照）
+     - 例: `digits.indexOf(ch)` の結果は exit_bindings に載らない
+```
+
+### 成果物
+- [x] `ParamRole` enum 定義
+- [x] `add_param_with_role` メソッド（スケルトン）
+- [x] 不変条件のドキュメント
+
+---
+
+## Task 200-A-5: ドキュメント更新
+
+### 1. joinir-architecture-overview.md
+
+**Section 2.3 (Boundary / Carrier ライン) に追記**:
+
+```markdown
+- **FunctionScopeCaptureAnalyzer / CapturedEnv** (Phase 200-A)
+  - ファイル: `src/mir/loop_pattern_detection/function_scope_capture.rs`
+  - 責務:
+    - 関数スコープで宣言され、ループ内で不変な変数を検出
+    - CapturedVar: { name, host_id, is_immutable }
+    - CapturedEnv: 検出された変数のコレクション
+  - **Phase 200-A**: 型と空実装のみ（中身は Phase 200-B で実装）
+
+- **ParamRole enum** (Phase 200-A)
+  - ファイル: `src/mir/join_ir/lowering/inline_boundary.rs`
+  - 責務:
+    - JoinInlineBoundary のパラメータ役割を明示的に区別
+    - LoopParam / Condition / Carrier / ExprResult
+  - **Phase 200-A**: enum 定義のみ（ルーティングは Phase 200-B で実装）
+```
+
+**Section 1 (不変条件) に追記**:
+
+```markdown
+9. **ParamRole の不変条件**
+   - Condition 役のパラメータは「PHI dst にしてはいけない」
+   - Condition 役のパラメータは「ExitLine の対象にも入れない」
+   - 理由: 条件専用変数はループ内で更新されず、ループ外で使われない
+```
+
+### 2. CURRENT_TASK.md
+
+**Phase 200-A セクション追加**:
+
+```markdown
+  - [x] **Phase 200-A: ConditionEnv 拡張インフラ（型と箱のみ）** ✅ (完了: 2025-12-XX)
+        - **目的**: ConditionEnv を壊さずに関数スコープ "実質定数" を扱う基盤
+        - **実装内容**:
+          - 200-A-1: CapturedVar / CapturedEnv 型導入 ✅
+          - 200-A-2: FunctionScopeCaptureAnalyzer スケルトン ✅
+          - 200-A-3: ConditionEnvBuilder v2 入口 ✅
+          - 200-A-4: ParamRole enum 追加 ✅
+          - 200-A-5: ドキュメント更新 ✅
+        - **スコープ**: Infra only / Integration pending
+        - **成果**:
+          - 型と箱の定義完了 ✅
+          - 既存ループの挙動変更なし ✅
+          - Phase 200-B への準備完了 ✅
+        - **次フェーズ**: Phase 200-B（digits 系ループへの適用）
+```
+
+---
+
+## 成功基準
+
+- [x] `CapturedVar` / `CapturedEnv` 型が定義されている
+- [x] `analyze_captured_vars` 関数スケルトンが存在する
+- [x] `build_condition_env_with_captures` 入口が存在する
+- [x] `ParamRole` enum が定義されている
+- [x] 既存テストが全て PASS（退行なし）
+- [x] ドキュメント更新（overview + CURRENT_TASK）
+
+---
+
+## 設計原則（Phase 200-A）
+
+1. **Infra only**: 型と箱の定義のみ、具体ループへの適用は Phase 200-B
+2. **既存挙動維持**: 現在のループは全く同じように動作する
+3. **箱化原則**: 新しい責務は新しい箱（FunctionScopeCaptureAnalyzer）に集約
+4. **ドキュメント駆動**: 将来の実装方針をコメントとして残す
+
+---
+
+## 関連ファイル
+
+### 新規作成
+- `src/mir/loop_pattern_detection/function_scope_capture.rs`
+
+### 修正対象
+- `src/mir/loop_pattern_detection/mod.rs`（モジュール追加）
+- `src/mir/join_ir/lowering/condition_env_builder.rs`（v2 入口）
+- `src/mir/join_ir/lowering/inline_boundary.rs`（ParamRole）
+
+### ドキュメント
+- `docs/development/current/main/joinir-architecture-overview.md`
+- `CURRENT_TASK.md`
diff --git a/src/mir/builder/control_flow/joinir/patterns/condition_env_builder.rs b/src/mir/builder/control_flow/joinir/patterns/condition_env_builder.rs
index 3d1fd7c4..1096deb0 100644
--- a/src/mir/builder/control_flow/joinir/patterns/condition_env_builder.rs
+++ b/src/mir/builder/control_flow/joinir/patterns/condition_env_builder.rs
@@ -27,6 +27,8 @@
 use crate::ast::ASTNode;
 use crate::mir::join_ir::lowering::condition_env::{ConditionBinding, ConditionEnv};
 use crate::mir::join_ir::lowering::condition_to_joinir::extract_condition_variables;
+use crate::mir::join_ir::lowering::inline_boundary_builder::JoinInlineBoundaryBuilder;
+use crate::mir::loop_pattern_detection::function_scope_capture::CapturedEnv;
 use crate::mir::ValueId;
 use std::collections::BTreeMap;
 
@@ -122,6 +124,64 @@ impl ConditionEnvBuilder {
         env.insert(loop_var_name.to_string(), ValueId(0));
         env
     }
+
+    /// Build ConditionEnv with optional captured variables (Phase 200-A v2 entry point)
+    ///
+    /// # Phase 200-A Status
+    ///
+    /// Currently ignores `captured` parameter and delegates to existing implementation.
+    /// Integration with CapturedEnv will be implemented in Phase 200-B.
+    ///
+    /// # Future Behavior (Phase 200-B+)
+    ///
+    /// - Add captured variables to ConditionEnv
+    /// - Generate condition_bindings for captured vars in boundary
+    /// - Track captured vars separately from loop params
+    /// - Ensure captured vars do NOT participate in header PHI or exit_bindings
+    ///
+    /// # Arguments
+    ///
+    /// * `loop_var_name` - Loop parameter name (e.g., "i", "pos")
+    /// * `_captured` - Function-scoped captured variables (Phase 200-B+)
+    /// * `_boundary` - Boundary builder for adding condition_bindings (Phase 200-B+)
+    ///
+    /// # Returns
+    ///
+    /// ConditionEnv with loop parameter mapping (Phase 200-A: same as build_loop_param_only)
+    ///
+    /// # Example (Future Phase 200-B)
+    ///
+    /// ```ignore
+    /// let captured = analyze_captured_vars(fn_body, loop_ast, scope);
+    /// let mut boundary = JoinInlineBoundaryBuilder::new();
+    /// let env = ConditionEnvBuilder::build_with_captures(
+    ///     "pos",
+    ///     &captured,  // Contains "digits" with host ValueId(42)
+    ///     &mut boundary,
+    /// );
+    /// // Phase 200-B: env will contain "pos" → ValueId(0), "digits" → ValueId(1)
+    /// // Phase 200-B: boundary.condition_bindings will have entry for "digits"
+    /// ```
+    pub fn build_with_captures(
+        loop_var_name: &str,
+        _captured: &CapturedEnv,
+        _boundary: &mut JoinInlineBoundaryBuilder,
+    ) -> ConditionEnv {
+        // Phase 200-A: Delegate to existing implementation
+        // TODO(Phase 200-B): Integrate captured vars into ConditionEnv
+        //
+        // Integration steps:
+        // 1. Start with loop parameter in env (ValueId(0))
+        // 2. For each captured var:
+        //    a. Allocate JoinIR-local ValueId (starting from 1)
+        //    b. Add to ConditionEnv (var.name → join_id)
+        //    c. Add to boundary.condition_bindings (host_id ↔ join_id)
+        //    d. Mark as ParamRole::Condition (not Carrier or LoopParam)
+        // 3. Ensure captured vars are NOT in exit_bindings (condition-only)
+        // 4. Return populated ConditionEnv
+
+        Self::build_loop_param_only(loop_var_name)
+    }
 }
 
 #[cfg(test)]
diff --git a/src/mir/join_ir/lowering/inline_boundary_builder.rs b/src/mir/join_ir/lowering/inline_boundary_builder.rs
index 0efce242..49a72f54 100644
--- a/src/mir/join_ir/lowering/inline_boundary_builder.rs
+++ b/src/mir/join_ir/lowering/inline_boundary_builder.rs
@@ -28,6 +28,50 @@ use crate::mir::ValueId;
 use super::inline_boundary::{JoinInlineBoundary, LoopExitBinding};
 use super::condition_to_joinir::ConditionBinding;
 
+/// Role of a parameter in JoinIR lowering (Phase 200-A)
+///
+/// This enum explicitly classifies parameters to ensure correct routing
+/// during JoinIR → MIR lowering and boundary construction.
+///
+/// # Invariants
+///
+/// - **LoopParam**: Participates in header PHI, updated in loop body
+///   - Example: `i` in `loop(i < len)` - iteration variable
+///   - Routing: join_inputs + host_inputs + header PHI + exit_bindings
+///
+/// - **Condition**: Used in condition only, NOT in header PHI, NOT in ExitLine
+///   - Example: `digits` in `digits.indexOf(ch)` - function-scoped constant
+///   - Routing: condition_bindings ONLY (no PHI, no exit_bindings)
+///   - Rationale: Condition-only vars are immutable and not updated in loop
+///
+/// - **Carrier**: Updated in loop body, participates in header PHI and ExitLine
+///   - Example: `sum`, `count` in accumulation loops
+///   - Routing: join_inputs + host_inputs + header PHI + exit_bindings
+///
+/// - **ExprResult**: Return value of the loop expression
+///   - Example: Loop result in `return loop(...)`
+///   - Routing: Handled by exit_phi_builder (set_expr_result)
+///
+/// # Phase 200-A Status
+///
+/// Enum is defined but not yet used for routing. Routing implementation
+/// will be added in Phase 200-B when CapturedEnv integration is complete.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ParamRole {
+    /// Loop iteration variable (e.g., `i` in `loop(i < len)`)
+    LoopParam,
+
+    /// Condition-only parameter (e.g., `digits` in `digits.indexOf(ch)`)
+    /// NOT included in header PHI or ExitLine
+    Condition,
+
+    /// State carried across iterations (e.g., `sum`, `count`)
+    Carrier,
+
+    /// Expression result returned by the loop
+    ExprResult,
+}
+
 /// Builder for constructing JoinInlineBoundary objects
 ///
 /// Provides a fluent API to set boundary fields without direct field manipulation.
@@ -128,6 +172,70 @@ impl JoinInlineBoundaryBuilder {
     pub fn build(self) -> JoinInlineBoundary {
         self.boundary
     }
+
+    /// Add a parameter with explicit role (Phase 200-A)
+    ///
+    /// This method allows adding parameters with explicit role classification,
+    /// ensuring correct routing during JoinIR → MIR lowering.
+    ///
+    /// # Phase 200-A Status
+    ///
+    /// Currently stores parameters based on role but does not use role for advanced routing.
+    /// Full role-based routing will be implemented in Phase 200-B.
+    ///
+    /// # Arguments
+    ///
+    /// * `name` - Variable name (e.g., "i", "digits", "sum")
+    /// * `host_id` - Host function's ValueId for this variable
+    /// * `role` - Parameter role (LoopParam / Condition / Carrier / ExprResult)
+    ///
+    /// # Routing Rules (Phase 200-B+)
+    ///
+    /// - **LoopParam**: add_input (join_inputs + host_inputs)
+    /// - **Condition**: add to condition_bindings (no PHI, no exit_bindings)
+    /// - **Carrier**: add_input + exit_bindings
+    /// - **ExprResult**: set_expr_result (handled separately)
+    ///
+    /// # Example (Future Phase 200-B)
+    ///
+    /// ```ignore
+    /// builder.add_param_with_role("i", ValueId(100), ParamRole::LoopParam);
+    /// builder.add_param_with_role("digits", ValueId(42), ParamRole::Condition);
+    /// builder.add_param_with_role("sum", ValueId(101), ParamRole::Carrier);
+    /// ```
+    pub fn add_param_with_role(&mut self, _name: &str, host_id: ValueId, role: ParamRole) {
+        // Phase 200-A: Basic routing only
+        // TODO(Phase 200-B): Implement full role-based routing
+        //
+        // Routing implementation:
+        // - LoopParam: join_inputs + host_inputs
+        // - Condition: condition_bindings (with JoinIR-local ValueId allocation)
+        // - Carrier: join_inputs + host_inputs + exit_bindings
+        // - ExprResult: Handled by set_expr_result
+
+        match role {
+            ParamRole::LoopParam | ParamRole::Carrier => {
+                // Existing behavior: add to join_inputs
+                // Note: In Phase 200-A, we don't have a simple add_input method
+                // that takes a name. This is a skeleton implementation.
+                // In Phase 200-B, we'll need to allocate JoinIR-local ValueIds.
+                let join_id = ValueId(self.boundary.join_inputs.len() as u32);
+                self.boundary.join_inputs.push(join_id);
+                self.boundary.host_inputs.push(host_id);
+            }
+            ParamRole::Condition => {
+                // Phase 200-A: Log only
+                // TODO(Phase 200-B): Add to condition_bindings without PHI
+                // 1. Allocate JoinIR-local ValueId
+                // 2. Create ConditionBinding { name, host_id, join_id }
+                // 3. Add to self.boundary.condition_bindings
+            }
+            ParamRole::ExprResult => {
+                // Handled separately by set_expr_result
+                // No action needed here
+            }
+        }
+    }
 }
 
 impl Default for JoinInlineBoundaryBuilder {
@@ -255,4 +363,40 @@ mod tests {
         assert_eq!(boundary.join_inputs.len(), 3);
         assert_eq!(boundary.host_inputs.len(), 3);
     }
+
+    // Phase 200-A: ParamRole tests
+    #[test]
+    fn test_param_role_loop_param() {
+        let mut builder = JoinInlineBoundaryBuilder::new();
+        builder.add_param_with_role("i", ValueId(100), ParamRole::LoopParam);
+
+        let boundary = builder.build();
+        assert_eq!(boundary.join_inputs.len(), 1);
+        assert_eq!(boundary.host_inputs.len(), 1);
+        assert_eq!(boundary.host_inputs[0], ValueId(100));
+    }
+
+    #[test]
+    fn test_param_role_condition() {
+        let mut builder = JoinInlineBoundaryBuilder::new();
+        // Phase 200-A: Condition role is logged but not yet routed
+        builder.add_param_with_role("digits", ValueId(42), ParamRole::Condition);
+
+        let boundary = builder.build();
+        // Phase 200-A: No action for Condition role yet
+        // Phase 200-B: This will add to condition_bindings
+        assert_eq!(boundary.join_inputs.len(), 0);
+        assert_eq!(boundary.condition_bindings.len(), 0);
+    }
+
+    #[test]
+    fn test_param_role_carrier() {
+        let mut builder = JoinInlineBoundaryBuilder::new();
+        builder.add_param_with_role("sum", ValueId(101), ParamRole::Carrier);
+
+        let boundary = builder.build();
+        assert_eq!(boundary.join_inputs.len(), 1);
+        assert_eq!(boundary.host_inputs.len(), 1);
+        assert_eq!(boundary.host_inputs[0], ValueId(101));
+    }
 }
diff --git a/src/mir/loop_pattern_detection/function_scope_capture.rs b/src/mir/loop_pattern_detection/function_scope_capture.rs
new file mode 100644
index 00000000..4b185895
--- /dev/null
+++ b/src/mir/loop_pattern_detection/function_scope_capture.rs
@@ -0,0 +1,203 @@
+//! Phase 200-A: Function scope capture infrastructure
+//!
+//! This module provides types for capturing function-scoped variables
+//! that are effectively immutable within a loop context.
+//!
+//! # Example
+//!
+//! For a function like JsonParser._atoi():
+//!
+//! ```nyash
+//! method _atoi(s, pos, len) {
+//!     local digits = "0123456789"  // <-- Captured variable
+//!     local value = 0
+//!     loop(pos < len) {
+//!         local ch = s.charAt(pos)
+//!         local digit = digits.indexOf(ch)  // Uses captured 'digits'
+//!         if (digit < 0) { break }
+//!         value = value * 10 + digit
+//!         pos = pos + 1
+//!     }
+//!     return value
+//! }
+//! ```
+//!
+//! Here, `digits` is:
+//! - Declared in function scope (before the loop)
+//! - Never reassigned (effectively immutable)
+//! - Referenced in loop body (digits.indexOf(ch))
+//!
+//! Phase 200-A creates the infrastructure to capture such variables.
+//! Phase 200-B will implement the actual detection logic.
+
+use crate::mir::ValueId;
+use crate::mir::join_ir::lowering::loop_scope_shape::LoopScopeShape;
+use crate::ast::ASTNode;
+
+/// A variable captured from function scope for use in loop conditions/body.
+///
+/// Example: `local digits = "0123456789"` in JsonParser._atoi()
+///
+/// # Invariants
+///
+/// - `name`: Variable name as it appears in the source code
+/// - `host_id`: MIR ValueId of the original definition in the host function
+/// - `is_immutable`: True if the variable is never reassigned in the function
+#[derive(Debug, Clone)]
+pub struct CapturedVar {
+    /// Variable name (e.g., "digits", "table")
+    pub name: String,
+
+    /// MIR ValueId of the original definition in the host function
+    pub host_id: ValueId,
+
+    /// Whether this variable is never reassigned in the function
+    ///
+    /// Phase 200-B will implement assignment analysis to determine this.
+    /// For now, this is always set to true as a conservative default.
+    pub is_immutable: bool,
+}
+
+/// Environment containing function-scoped captured variables.
+///
+/// Phase 200-A: Type definition only, not yet integrated with ConditionEnv.
+/// Phase 200-B: Will be populated by FunctionScopeCaptureAnalyzer and
+///               integrated into ConditionEnv via ConditionEnvBuilder v2.
+#[derive(Debug, Clone, Default)]
+pub struct CapturedEnv {
+    /// List of captured variables
+    pub vars: Vec<CapturedVar>,
+}
+
+impl CapturedEnv {
+    /// Create a new empty environment
+    pub fn new() -> Self {
+        Self { vars: Vec::new() }
+    }
+
+    /// Check if the environment is empty
+    pub fn is_empty(&self) -> bool {
+        self.vars.is_empty()
+    }
+
+    /// Add a captured variable to the environment
+    pub fn add_var(&mut self, var: CapturedVar) {
+        self.vars.push(var);
+    }
+
+    /// Look up a captured variable by name
+    ///
+    /// Returns `Some(&CapturedVar)` if found, `None` otherwise.
+    pub fn get(&self, name: &str) -> Option<&CapturedVar> {
+        self.vars.iter().find(|v| v.name == name)
+    }
+}
+
+/// Analyzes function-scoped variables that can be safely captured for loop conditions/body.
+///
+/// # Phase 200-A Status
+///
+/// Currently returns empty CapturedEnv (skeleton implementation).
+/// Actual capture detection will be implemented in Phase 200-B.
+///
+/// # Future Detection Criteria (Phase 200-B+)
+///
+/// A variable is captured if ALL of the following conditions are met:
+///
+/// 1. **Declared before the loop**: Variable must be declared in function scope before the loop
+/// 2. **Never reassigned**: Variable is never reassigned within the function (is_immutable = true)
+/// 3. **Referenced in loop**: Variable is referenced in loop condition or body
+/// 4. **Not a loop parameter**: Variable is not the loop iteration variable
+/// 5. **Not a body-local**: Variable is not declared inside the loop body
+///
+/// # Example
+///
+/// ```nyash
+/// method _atoi(s, pos, len) {
+///     local digits = "0123456789"  // ✅ Captured (declared before loop, never reassigned)
+///     local value = 0               // ❌ Not captured (reassigned in loop body)
+///     loop(pos < len) {
+///         local ch = s.charAt(pos)  // ❌ Not captured (body-local)
+///         local digit = digits.indexOf(ch)
+///         value = value * 10 + digit
+///         pos = pos + 1
+///     }
+/// }
+/// ```
+///
+/// # Arguments
+///
+/// * `_fn_body` - AST nodes of the function body (for analysis)
+/// * `_loop_ast` - AST node of the loop statement
+/// * `_scope` - LoopScopeShape (for excluding loop params and body-locals)
+///
+/// # Returns
+///
+/// `CapturedEnv` containing all captured variables (empty in Phase 200-A)
+pub fn analyze_captured_vars(
+    _fn_body: &[ASTNode],
+    _loop_ast: &ASTNode,
+    _scope: &LoopScopeShape,
+) -> CapturedEnv {
+    // Phase 200-A: Skeleton implementation
+    // TODO(Phase 200-B): Implement actual capture detection
+    //
+    // Detection algorithm:
+    // 1. Find all `local` declarations before the loop in fn_body
+    // 2. For each declaration:
+    //    a. Check if it's never reassigned in the function (is_immutable = true)
+    //    b. Check if it's referenced in loop condition or body
+    //    c. Exclude if it's in scope.pinned, scope.carriers, or scope.body_locals
+    // 3. Collect matching variables into CapturedEnv
+    // 4. Return the populated environment
+
+    CapturedEnv::new()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_captured_env_empty() {
+        let env = CapturedEnv::new();
+        assert!(env.is_empty());
+        assert!(env.get("digits").is_none());
+    }
+
+    #[test]
+    fn test_captured_env_add_and_get() {
+        let mut env = CapturedEnv::new();
+        env.add_var(CapturedVar {
+            name: "digits".to_string(),
+            host_id: ValueId(42),
+            is_immutable: true,
+        });
+
+        assert!(!env.is_empty());
+        let var = env.get("digits").unwrap();
+        assert_eq!(var.name, "digits");
+        assert_eq!(var.host_id, ValueId(42));
+        assert!(var.is_immutable);
+    }
+
+    #[test]
+    fn test_captured_env_multiple_vars() {
+        let mut env = CapturedEnv::new();
+        env.add_var(CapturedVar {
+            name: "digits".to_string(),
+            host_id: ValueId(42),
+            is_immutable: true,
+        });
+        env.add_var(CapturedVar {
+            name: "table".to_string(),
+            host_id: ValueId(100),
+            is_immutable: true,
+        });
+
+        assert_eq!(env.vars.len(), 2);
+        assert!(env.get("digits").is_some());
+        assert!(env.get("table").is_some());
+        assert!(env.get("nonexistent").is_none());
+    }
+}
diff --git a/src/mir/loop_pattern_detection/mod.rs b/src/mir/loop_pattern_detection/mod.rs
index c09fae8c..4fd83a43 100644
--- a/src/mir/loop_pattern_detection/mod.rs
+++ b/src/mir/loop_pattern_detection/mod.rs
@@ -780,3 +780,6 @@ pub use trim_loop_helper::TrimLoopHelper;
 
 // Phase 33-23: Break Condition Analysis (Stage 2, Issue 6)
 pub mod break_condition_analyzer;
+
+// Phase 200-A: Function Scope Capture Infrastructure
+pub mod function_scope_capture;