diff --git a/generate/derivation/__init__.py b/generate/derivation/__init__.py
index bcd84ca5..04ed1794 100644
--- a/generate/derivation/__init__.py
+++ b/generate/derivation/__init__.py
@@ -7,6 +7,11 @@ guard that keeps the (Phase 3b) bounded search honest.
 
 from __future__ import annotations
 
+from generate.derivation.clauses import (
+    ClauseResult,
+    clause_local_results,
+    segment_clauses,
+)
 from generate.derivation.comparatives import (
     ComparativeScalar,
     comparative_step,
@@ -25,6 +30,7 @@ from generate.derivation.verify import (
 )
 
 __all__ = [
+    "ClauseResult",
     "ComparativeScalar",
     "GroundedDerivation",
     "MULTIPLICATIVE_CUES",
@@ -34,12 +40,14 @@ __all__ = [
     "Step",
     "Target",
     "VALID_OPS",
+    "clause_local_results",
     "comparative_step",
     "extract_comparative_scalars",
     "extract_quantities",
     "extract_target",
     "search_chain",
     "search_multiplicative",
+    "segment_clauses",
     "select_self_verified",
     "self_verifies",
 ]
diff --git a/generate/derivation/clauses.py b/generate/derivation/clauses.py
new file mode 100644
index 00000000..9055ebb2
--- /dev/null
+++ b/generate/derivation/clauses.py
@@ -0,0 +1,74 @@
+"""ADR-0178 GB-1 — clause segmentation + clause-local sub-derivation.
+
+The first slice of the comprehension-guided composer: read the problem one clause
+at a time and derive each clause's *local* contribution, before GB-2 combines them
+across clauses. The text's clause structure is the guidance that keeps the search
+bounded and steers grouping (quantities in a clause tend to combine locally).
+
+Segmentation is lexeme/orthographic (ADR-0165) — sentence-level via terminal
+punctuation, not a grammar template. Each clause's local sub-derivation reuses the
+MS-3 :func:`search_chain` (a small bounded search over that clause's few
+quantities); a single-quantity clause is a leaf; a zero-quantity clause is context.
+Refuse-preferring: a clause whose local op is ambiguous resolves to nothing (a
+hold), it is not guessed.
+"""
+
+from __future__ import annotations
+
+import re
+from dataclasses import dataclass
+from typing import Final
+
+from generate.derivation.extract import extract_quantities
+from generate.derivation.model import Quantity
+from generate.derivation.multistep import search_chain
+
+_SENTENCE_SPLIT: Final[re.Pattern[str]] = re.compile(r"(?<=[.?!])\s+")
+
+
+@dataclass(frozen=True, slots=True)
+class ClauseResult:
+    """A clause and its locally-derived contribution.
+
+    ``value`` is the clause's local sub-result (``None`` = unresolved/hold —
+    context clause, or an ambiguous multi-quantity clause the local search
+    refused). ``resolved`` is ``True`` iff a local value was derived.
+    """
+
+    text: str
+    quantities: tuple[Quantity, ...]
+    value: float | None
+    unit: str | None
+    resolved: bool
+
+
+def segment_clauses(problem_text: str) -> tuple[str, ...]:
+    """Split a problem into clauses (sentence-level, orthographic). Deterministic."""
+    return tuple(s.strip() for s in _SENTENCE_SPLIT.split(problem_text) if s.strip())
+
+
+def clause_local_results(problem_text: str) -> tuple[ClauseResult, ...]:
+    """Derive each clause's local contribution (GB-1). Deterministic.
+
+    - 0 quantities  -> context clause (unresolved).
+    - 1 quantity    -> leaf (its value).
+    - >= 2          -> bounded local search (:func:`search_chain`); resolves to a
+                       local value or holds (unresolved) on ambiguity.
+    """
+    out: list[ClauseResult] = []
+    for clause in segment_clauses(problem_text):
+        quantities = extract_quantities(clause)
+        if len(quantities) == 0:
+            out.append(ClauseResult(clause, (), None, None, False))
+        elif len(quantities) == 1:
+            q = quantities[0]
+            out.append(ClauseResult(clause, quantities, q.value, q.unit, True))
+        else:
+            res = search_chain(clause)
+            if res is None:
+                out.append(ClauseResult(clause, quantities, None, None, False))
+            else:
+                out.append(
+                    ClauseResult(clause, quantities, res.answer, res.answer_unit, True)
+                )
+    return tuple(out)
diff --git a/tests/test_adr_0178_gb1_clauses.py b/tests/test_adr_0178_gb1_clauses.py
new file mode 100644
index 00000000..62a1023c
--- /dev/null
+++ b/tests/test_adr_0178_gb1_clauses.py
@@ -0,0 +1,79 @@
+"""ADR-0178 GB-1 — clause segmentation + clause-local sub-derivation.
+
+The first slice of the comprehension-guided composer: read the problem clause by
+clause and derive each clause's local contribution (GB-2 combines them). Covers
+segmentation, the leaf/context/local-search cases, and the refuse-preferring hold
+on ambiguous clauses.
+"""
+
+from __future__ import annotations
+
+from generate.derivation import (
+    ClauseResult,
+    clause_local_results,
+    segment_clauses,
+)
+
+
+class TestSegmentClauses:
+    def test_sentence_level_split(self) -> None:
+        clauses = segment_clauses(
+            "Sidney does 20 jacks. Brooke does three times as many. How many?"
+        )
+        assert len(clauses) == 3
+        assert clauses[0] == "Sidney does 20 jacks."
+
+    def test_collapses_whitespace_and_drops_empty(self) -> None:
+        assert segment_clauses("  A.   B.  ") == ("A.", "B.")
+
+    def test_deterministic(self) -> None:
+        t = "He has 6 boxes. Each holds 50 apples."
+        assert segment_clauses(t) == segment_clauses(t)
+
+
+class TestClauseLocalResults:
+    def test_single_clause_local_product(self) -> None:
+        # 0021: one clause, all quantities multiply locally -> 450
+        results = clause_local_results(
+            "He bench presses 15 pounds for 10 reps and does 3 sets."
+        )
+        assert len(results) == 1
+        r = results[0]
+        assert isinstance(r, ClauseResult)
+        assert r.resolved is True
+        assert r.value == 450.0
+
+    def test_single_quantity_clause_is_a_leaf(self) -> None:
+        (r,) = clause_local_results("There are 48 boxes.")
+        assert r.resolved is True
+        assert r.value == 48.0
+        assert r.unit == "boxes"
+
+    def test_zero_quantity_clause_is_context(self) -> None:
+        (r,) = clause_local_results("John is lifting weights.")
+        assert r.resolved is False
+        assert r.value is None
+        assert r.quantities == ()
+
+    def test_ambiguous_multi_quantity_clause_holds(self) -> None:
+        # two quantities, no licensed local op (no mult cue / aggregation hint)
+        # -> the local search refuses -> unresolved hold (not guessed)
+        (r,) = clause_local_results("There are 6 rows and 50 seats.")
+        assert r.resolved is False
+        assert r.value is None
+        assert len(r.quantities) == 2
+
+    def test_per_clause_structure_of_a_multi_sentence_problem(self) -> None:
+        # 0003-shape: each sentence contributes a local piece; GB-1 reports them
+        # per clause (GB-2 will chain them: 48 -> x24 -> x2). Each sentence here
+        # has a single quantity -> three leaves (48, 24, 2).
+        results = clause_local_results(
+            "There are 48 boxes. There are 24 erasers in each box. "
+            "They sell for 2 dollars each."
+        )
+        assert [r.resolved for r in results] == [True, True, True]
+        assert [r.value for r in results] == [48.0, 24.0, 2.0]
+
+    def test_deterministic(self) -> None:
+        t = "He bench presses 15 pounds for 10 reps and does 3 sets."
+        assert clause_local_results(t) == clause_local_results(t)