diff --git a/compiler/splitter/category_splitter.py b/compiler/splitter/category_splitter.py
new file mode 100644
index 0000000..eb21f14
--- /dev/null
+++ b/compiler/splitter/category_splitter.py
@@ -0,0 +1,58 @@
+"""The Category Splitter (MASTER_SPEC §18).
+
+Organizes Resolved Tags into the 19 canonical categories, preserving order and
+without modifying, generating, or removing any tag. Each tag is placed in the
+category recorded on it by the Resolver (which is the category of its Knowledge
+Base Entry).
+
+Category ordering is deterministic (the shared canonical order); tags within a
+category keep Resolver order; empty categories are omitted from the Category Map
+(§18.4).
+"""
+
+from __future__ import annotations
+
+from types import MappingProxyType
+
+from compiler.common.categories import CANONICAL_CATEGORIES, is_valid_category
+from compiler.common.log import StructuredLogger
+from compiler.common.message_codes import message
+from compiler.common.result import CompilerResult
+from schemas.models import CategoryMap, ResolvedTag
+
+
+def split_into_categories(
+    tags: tuple[ResolvedTag, ...],
+    logger: StructuredLogger | None = None,
+) -> CompilerResult:
+    """Group Resolved Tags into a Category Map.
+
+    Returns:
+        A CompilerResult whose ``data`` is the :class:`CategoryMap` on success,
+        or ``None`` with an SC0008 error if any tag carries a category outside
+        the 19 canonical categories.
+    """
+    buckets: dict[str, list[ResolvedTag]] = {}
+    for resolved in tags:
+        if not is_valid_category(resolved.category):
+            return CompilerResult().add_error(
+                message(
+                    "SC0008",
+                    f"Tag '{resolved.tag}' has invalid category '{resolved.category}'.",
+                    tag=resolved.tag,
+                    category=resolved.category,
+                )
+            )
+        buckets.setdefault(resolved.category, []).append(resolved)
+
+    # Deterministic category order; empty categories omitted. Original ResolvedTag
+    # objects are kept verbatim (tags are never modified).
+    ordered = {
+        category: tuple(buckets[category])
+        for category in CANONICAL_CATEGORIES
+        if category in buckets
+    }
+    category_map = CategoryMap(categories=MappingProxyType(ordered))
+    if logger is not None:
+        logger.basic("categories_split", categories=len(ordered), tags=len(tags))
+    return CompilerResult(data=category_map)
diff --git a/tests/test_category_splitter.py b/tests/test_category_splitter.py
new file mode 100644
index 0000000..14b7a79
--- /dev/null
+++ b/tests/test_category_splitter.py
@@ -0,0 +1,69 @@
+"""Tests for the Category Splitter (issue #13, MASTER_SPEC §18)."""
+
+from __future__ import annotations
+
+from compiler.common.categories import CANONICAL_CATEGORIES
+from compiler.splitter.category_splitter import split_into_categories
+from schemas.models import CategoryMap, ResolvedTag
+
+
+def tag(name: str, category: str) -> ResolvedTag:
+    return ResolvedTag(tag=name, category=category, source_concept=name, knowledge_base_entry=name)
+
+
+def codes(messages) -> list[str]:
+    return [m.code for m in messages]
+
+
+def test_output_is_a_category_map() -> None:
+    result = split_into_categories((tag("1girl", "character"),))
+    assert result.success
+    assert isinstance(result.data, CategoryMap)
+
+
+def test_tags_land_in_their_entry_category() -> None:
+    tags = (tag("1girl", "character"), tag("blue eyes", "eyes"), tag("long hair", "hair"))
+    result = split_into_categories(tags)
+    assert [t.tag for t in result.data.tags_for("character")] == ["1girl"]
+    assert [t.tag for t in result.data.tags_for("eyes")] == ["blue eyes"]
+    assert [t.tag for t in result.data.tags_for("hair")] == ["long hair"]
+
+
+def test_order_within_category_preserves_resolver_order() -> None:
+    tags = (tag("long hair", "hair"), tag("blonde hair", "hair"), tag("bangs", "hair"))
+    result = split_into_categories(tags)
+    assert [t.tag for t in result.data.tags_for("hair")] == ["long hair", "blonde hair", "bangs"]
+
+
+def test_empty_categories_are_omitted() -> None:
+    result = split_into_categories((tag("1girl", "character"),))
+    present = list(result.data.categories.keys())
+    assert present == ["character"]  # only the non-empty category
+
+
+def test_category_ordering_is_canonical() -> None:
+    # Provide tags out of canonical order; the map must follow CANONICAL_CATEGORIES.
+    tags = (tag("sunset", "lighting"), tag("1girl", "character"), tag("long hair", "hair"))
+    result = split_into_categories(tags)
+    present = list(result.data.categories.keys())
+    assert present == ["character", "hair", "lighting"]
+    assert present == [c for c in CANONICAL_CATEGORIES if c in present]
+
+
+def test_tags_are_not_modified() -> None:
+    original = tag("1girl", "character")
+    result = split_into_categories((original,))
+    assert result.data.tags_for("character")[0] is original
+
+
+def test_empty_input_produces_empty_map() -> None:
+    result = split_into_categories(())
+    assert result.success
+    assert len(result.data.categories) == 0
+
+
+def test_invalid_category_stops_with_sc0008() -> None:
+    result = split_into_categories((tag("x", "not_a_category"),))
+    assert not result.success
+    assert result.data is None
+    assert "SC0008" in codes(result.errors)