diff --git a/app/services/summarize_units.py b/app/services/summarize_units.py
new file mode 100644
index 0000000..3160ba2
--- /dev/null
+++ b/app/services/summarize_units.py
@@ -0,0 +1,166 @@
+"""summarize_units — 거대문서 요약 전용 분할(map-reduce 유닛) 순수함수 (presegment PR1).
+
+plan ds-presegment-mapreduce-2 (2026-06-29 설계 합의 · PR0 실측 봉인):
+  - CAP_TOKENS = 12,000 tok/unit — greedy-pack 상한 (PR0: giant 236건 실측 캘리브레이션)
+  - TRIGGER_TOKENS = 25,000 tok — 이하는 단일콜 유지, 초과 시 map-reduce
+  - 3-way over% 게이트 (단독 CAP 초과 섹션의 토큰 비중. 헤딩 개수는 무의미 — ASME 1,494개):
+      over% == 0        → 'auto'   (TIER1: 로컬 자동 분할, PR0 실측 78%)
+      0 < over% <= 40   → 'hybrid' (패킹분 로컬 + 초과 섹션만 클로드, 8%)
+      over% > 40        → 'whole'  (TIER2: 클로드 전체 분할, 14%)
+  - 토큰 추정 = PR0 실 Qwen 토크나이저 캘리브레이션: 한글 0.529 tok/char · 기타 0.217.
+    구 휴리스틱(0.625/0.25)은 ~15% 과대라 폐기.
+
+불변식:
+  - 순수함수 — DB/네트워크/파일 접촉 0. 분할 = 요약 전용 아티팩트(문서 아님·검색/임베딩 미편입).
+  - leaf 추출 = hier_decomp.builder 재사용, leaf_hard_max=∞ 로 window-split 억제
+    (헤딩 leaf 만 — PR0 측정환경과 동일). 인접 섹션만 greedy-pack(순서 보존·중간 폐기 0
+    — 구 deep_summary 의 head/mid/tail 가운데 폐기 버그를 커버리지로 대체).
+  - 배선(deep_summary 분기·HOLD·클로드 알람)은 PR2/PR3 — 본 모듈은 계획만 산출.
+
+호출: plan_summarize_units(md_text) -> UnitPlan
+"""
+from __future__ import annotations
+
+import sys
+from dataclasses import dataclass, field
+
+from app.services.hier_decomp.builder import HierNode, build_hier_tree
+
+CAP_TOKENS = 12_000
+TRIGGER_TOKENS = 25_000
+HYBRID_MAX_OVER_PCT = 40.0
+
+# PR0 실 Qwen tokenizer 캘리브레이션 (tok/char)
+KO_TOK_PER_CHAR = 0.529
+OTHER_TOK_PER_CHAR = 0.217
+
+_HANGUL_RANGES = (
+    (0xAC00, 0xD7A3),  # 완성형 음절
+    (0x1100, 0x11FF),  # 자모
+    (0x3130, 0x318F),  # 호환 자모
+)
+
+
+def _is_hangul(ch: str) -> bool:
+    cp = ord(ch)
+    return any(lo <= cp <= hi for lo, hi in _HANGUL_RANGES)
+
+
+def estimate_tokens(text: str) -> int:
+    """PR0 캘리브레이션 기반 토큰 추정 (한글 0.529 · 기타 0.217 tok/char)."""
+    if not text:
+        return 0
+    ko = sum(1 for ch in text if _is_hangul(ch))
+    other = len(text) - ko
+    return round(ko * KO_TOK_PER_CHAR + other * OTHER_TOK_PER_CHAR)
+
+
+@dataclass
+class SummarizeUnit:
+    """map-reduce 1유닛 — 인접 leaf 섹션들의 greedy-pack (요약 전용, 문서 아님)."""
+    index: int
+    section_titles: list[str | None] = field(default_factory=list)
+    text: str = ""
+    est_tokens: int = 0
+    over_cap: bool = False  # 단독 섹션이 CAP 초과 (hybrid 시 클로드 대상)
+
+
+@dataclass
+class UnitPlan:
+    mode: str                    # 'single' | 'map_reduce'
+    tier: str | None             # map_reduce 시 'auto' | 'hybrid' | 'whole'
+    total_est_tokens: int = 0
+    over_pct: float = 0.0
+    units: list[SummarizeUnit] = field(default_factory=list)
+
+
+def extract_leaves(md_text: str) -> list[HierNode]:
+    """헤딩 leaf 만 추출 — leaf_hard_max=∞ 로 window-split 억제 (PR0 측정환경 동일)."""
+    nodes = build_hier_tree(
+        md_text,
+        leaf_target_max=sys.maxsize,
+        leaf_hard_max=sys.maxsize,
+    )
+    return [n for n in nodes if n.is_leaf]
+
+
+def greedy_pack(leaves: list[HierNode], cap: int = CAP_TOKENS) -> list[SummarizeUnit]:
+    """인접 leaf 를 순서 보존하며 est_tokens<=cap 으로 pack. 단독 초과 leaf = 전용 유닛(over_cap)."""
+    units: list[SummarizeUnit] = []
+    cur_titles: list[str | None] = []
+    cur_texts: list[str] = []
+    cur_tokens = 0
+
+    def _flush() -> None:
+        nonlocal cur_titles, cur_texts, cur_tokens
+        if cur_texts:
+            units.append(SummarizeUnit(
+                index=len(units),
+                section_titles=cur_titles,
+                text="\n\n".join(cur_texts),
+                est_tokens=cur_tokens,
+            ))
+            cur_titles, cur_texts, cur_tokens = [], [], 0
+
+    for leaf in leaves:
+        t = estimate_tokens(leaf.text)
+        if t > cap:
+            _flush()
+            units.append(SummarizeUnit(
+                index=len(units),
+                section_titles=[leaf.section_title],
+                text=leaf.text,
+                est_tokens=t,
+                over_cap=True,
+            ))
+            continue
+        if cur_tokens + t > cap:
+            _flush()
+        cur_titles.append(leaf.section_title)
+        cur_texts.append(leaf.text)
+        cur_tokens += t
+    _flush()
+    return units
+
+
+def over_pct(leaves: list[HierNode], cap: int = CAP_TOKENS) -> float:
+    """단독 CAP 초과 섹션들의 토큰 비중(%) — 3-way 게이트 입력."""
+    total = 0
+    over = 0
+    for leaf in leaves:
+        t = estimate_tokens(leaf.text)
+        total += t
+        if t > cap:
+            over += t
+    if total == 0:
+        return 0.0
+    return over * 100.0 / total
+
+
+def gate(over: float) -> str:
+    """over% → tier. 0=auto / (0,40]=hybrid / >40=whole. 클로드 결과 재검증에도 재사용."""
+    if over <= 0.0:
+        return "auto"
+    if over <= HYBRID_MAX_OVER_PCT:
+        return "hybrid"
+    return "whole"
+
+
+def plan_summarize_units(
+    md_text: str, *,
+    cap: int = CAP_TOKENS,
+    trigger: int = TRIGGER_TOKENS,
+) -> UnitPlan:
+    """문서 → 요약 실행 계획. trigger 이하=single(현행 단일콜), 초과=map_reduce(tier+units)."""
+    total = estimate_tokens(md_text)
+    if total <= trigger:
+        return UnitPlan(mode="single", tier=None, total_est_tokens=total)
+    leaves = extract_leaves(md_text)
+    pct = over_pct(leaves, cap)
+    return UnitPlan(
+        mode="map_reduce",
+        tier=gate(pct),
+        total_est_tokens=total,
+        over_pct=round(pct, 2),
+        units=greedy_pack(leaves, cap),
+    )
diff --git a/tests/summarize_units/test_units.py b/tests/summarize_units/test_units.py
new file mode 100644
index 0000000..32b94b2
--- /dev/null
+++ b/tests/summarize_units/test_units.py
@@ -0,0 +1,180 @@
+"""summarize_units 단위테스트 (presegment PR1 — 순수함수·fixture).
+
+핵심 불변식:
+  - estimate_tokens = PR0 캘리브레이션(한글 0.529 · 기타 0.217 tok/char) 정확 재현.
+  - greedy_pack: 순서 보존·인접만·cap 준수·단독 초과 leaf=over_cap 전용 유닛·텍스트 손실 0
+    (구 deep_summary head/mid/tail 가운데 폐기 버그의 반대 성질).
+  - gate 3-way: 0=auto / (0,40]=hybrid / >40=whole (경계 포함).
+  - plan_summarize_units: trigger 이하=single(현행 단일콜 유지=무회귀) / 초과=map_reduce.
+
+pytest + 단독 실행 양쪽 지원:
+  PYTHONPATH=. .venv/bin/pytest tests/summarize_units/ -q
+"""
+from __future__ import annotations
+
+from app.services.hier_decomp.builder import HierNode
+from app.services.summarize_units import (
+    CAP_TOKENS,
+    TRIGGER_TOKENS,
+    SummarizeUnit,
+    estimate_tokens,
+    extract_leaves,
+    gate,
+    greedy_pack,
+    over_pct,
+    plan_summarize_units,
+)
+
+
+def _leaf(idx: int, text: str, title: str | None = None) -> HierNode:
+    return HierNode(idx=idx, parent_idx=None, level=1, node_type=None,
+                    section_title=title, heading_path=title, text=text)
+
+
+# ---------- estimate_tokens ----------
+
+def test_estimate_tokens_korean_calibration():
+    # 한글 1000자 → 529 tok (PR0: 0.529 tok/char)
+    assert estimate_tokens("가" * 1000) == 529
+
+
+def test_estimate_tokens_english_calibration():
+    # 비한글 1000자 → 217 tok (PR0: 0.217 tok/char)
+    assert estimate_tokens("a" * 1000) == 217
+
+
+def test_estimate_tokens_mixed_and_empty():
+    assert estimate_tokens("") == 0
+    mixed = "가" * 100 + "a" * 100
+    assert estimate_tokens(mixed) == round(100 * 0.529 + 100 * 0.217)
+
+
+# ---------- greedy_pack ----------
+
+def test_greedy_pack_adjacency_and_cap():
+    # 4000tok 짜리 한글 leaf 4개 (4000/0.529 ≈ 7562자) → cap 12000 이면 [3개, 1개]... 아니
+    # 4000*3=12000 = cap 정확 경계(<=cap 허용) → [1,2,3] + [4]
+    body = "가" * 7562  # ≈ 3999~4000 tok
+    leaves = [_leaf(i, body, f"s{i}") for i in range(4)]
+    units = greedy_pack(leaves, cap=12_000)
+    assert len(units) == 2
+    assert [len(u.section_titles) for u in units] == [3, 1]
+    # 순서 보존
+    assert units[0].section_titles == ["s0", "s1", "s2"]
+    assert units[1].section_titles == ["s3"]
+    # cap 준수
+    assert all(u.est_tokens <= 12_000 for u in units)
+
+
+def test_greedy_pack_oversized_leaf_gets_own_unit():
+    small = "가" * 1000            # ≈ 529 tok
+    big = "가" * 30_000            # ≈ 15,870 tok > CAP
+    leaves = [_leaf(0, small, "a"), _leaf(1, big, "mega"), _leaf(2, small, "b")]
+    units = greedy_pack(leaves, cap=CAP_TOKENS)
+    assert len(units) == 3
+    assert units[1].over_cap and units[1].section_titles == ["mega"]
+    assert not units[0].over_cap and not units[2].over_cap
+    # 인접성: 초과 leaf 가 앞뒤 pack 을 넘나들며 합쳐지지 않음
+    assert units[0].section_titles == ["a"] and units[2].section_titles == ["b"]
+
+
+def test_greedy_pack_no_text_loss():
+    leaves = [_leaf(i, f"본문{i} " + "가" * 500, f"s{i}") for i in range(7)]
+    units = greedy_pack(leaves, cap=1_000)
+    joined = "\n\n".join(u.text for u in units)
+    for leaf in leaves:
+        assert leaf.text in joined  # 커버리지 — 중간 폐기 0
+
+
+def test_greedy_pack_empty():
+    assert greedy_pack([]) == []
+
+
+# ---------- over_pct + gate ----------
+
+def test_over_pct_and_gate_boundaries():
+    assert gate(0.0) == "auto"
+    assert gate(0.01) == "hybrid"
+    assert gate(40.0) == "hybrid"
+    assert gate(40.01) == "whole"
+    assert gate(100.0) == "whole"
+
+
+def test_over_pct_computation():
+    # leaf: 6000tok + 18000tok(초과) → over% = 18000/24000 = 75%
+    l_small = _leaf(0, "가" * round(6000 / 0.529), "a")
+    l_big = _leaf(1, "가" * round(18000 / 0.529), "b")
+    pct = over_pct([l_small, l_big], cap=CAP_TOKENS)
+    assert 74.0 < pct < 76.0
+    assert over_pct([], cap=CAP_TOKENS) == 0.0
+    assert over_pct([l_small], cap=CAP_TOKENS) == 0.0
+
+
+# ---------- plan_summarize_units (fixture md) ----------
+
+def _md_doc(sections: int, chars_per_section: int, ch: str = "가") -> str:
+    parts = []
+    for i in range(sections):
+        parts.append(f"# 제{i+1}장 섹션{i}\n\n" + ch * chars_per_section)
+    return "\n\n".join(parts)
+
+
+def test_plan_small_doc_stays_single():
+    md = _md_doc(3, 1000)  # ≈ 3×529 tok ≪ trigger
+    plan = plan_summarize_units(md)
+    assert plan.mode == "single" and plan.tier is None and plan.units == []
+    assert plan.total_est_tokens <= TRIGGER_TOKENS
+
+
+def test_plan_large_doc_auto_tier():
+    # 섹션 20개 × ≈4000tok = ≈80K tok > trigger, 전 섹션 < cap → auto
+    md = _md_doc(20, 7562)
+    plan = plan_summarize_units(md)
+    assert plan.mode == "map_reduce"
+    assert plan.tier == "auto" and plan.over_pct == 0.0
+    assert len(plan.units) >= 2
+    assert all(u.est_tokens <= CAP_TOKENS for u in plan.units)
+
+
+def test_plan_mega_section_whole_tier():
+    # 작은 섹션 2 + 초대형 1(≈53K tok — 전체의 >40%) → whole
+    md = (_md_doc(2, 7562)
+          + "\n\n# 메가섹션\n\n" + "가" * 100_000)
+    plan = plan_summarize_units(md)
+    assert plan.mode == "map_reduce"
+    assert plan.tier == "whole" and plan.over_pct > 40.0
+    assert any(u.over_cap for u in plan.units)
+
+
+def test_plan_hybrid_tier():
+    # 정상 섹션 15개(≈60K tok) + 초과 섹션 1개(≈15.9K tok) → over% ≈ 21% → hybrid
+    md = _md_doc(15, 7562) + "\n\n# 초과섹션\n\n" + "가" * 30_000
+    plan = plan_summarize_units(md)
+    assert plan.mode == "map_reduce"
+    assert plan.tier == "hybrid"
+    assert 0.0 < plan.over_pct <= 40.0
+    over_units = [u for u in plan.units if u.over_cap]
+    assert len(over_units) == 1  # hybrid 시 클로드 대상 = 이 유닛들만
+
+
+def test_plan_headingless_giant_is_whole():
+    # 헤딩 없는 거대 EN 문서 — leaf 1개 전체 초과 → over% 100 → whole (PR0: EN 책 다수)
+    md = "x" * 200_000  # ≈ 43K tok > trigger, 단일 leaf > cap
+    plan = plan_summarize_units(md)
+    assert plan.mode == "map_reduce" and plan.tier == "whole"
+
+
+def test_plan_deterministic():
+    md = _md_doc(10, 7562)
+    p1, p2 = plan_summarize_units(md), plan_summarize_units(md)
+    assert p1 == p2
+
+
+if __name__ == "__main__":
+    import sys
+    fns = [v for k, v in sorted(globals().items()) if k.startswith("test_")]
+    for fn in fns:
+        fn()
+        print(f"ok {fn.__name__}")
+    print(f"{len(fns)} passed (standalone)")
+    sys.exit(0)