diff --git a/compiler/builder/prompt_builder.py b/compiler/builder/prompt_builder.py
new file mode 100644
index 0000000..ae94fbd
--- /dev/null
+++ b/compiler/builder/prompt_builder.py
@@ -0,0 +1,50 @@
+"""The Prompt Builder (MASTER_SPEC §19).
+
+Formats the Category Map into one plain UTF-8 Prompt Output per category, plus
+the reserved ``negative`` and ``scene`` outputs. This stage is formatting only:
+it never generates, removes, reorders, deduplicates, escapes, or optimizes tags
+(§19.1, §19.5).
+
+Version 1 always emits all 21 outputs (19 categories + 2 reserved) so the ComfyUI
+node exposes a stable set of sockets. Empty categories yield an empty string —
+never null or placeholder text (§19.4). Output names are the lowercase canonical
+category identifiers (and ``negative`` / ``scene``), corresponding one-to-one to
+the §19.2/§19.3 display names.
+"""
+
+from __future__ import annotations
+
+from compiler.common.categories import CANONICAL_CATEGORIES
+from compiler.common.config import Config
+from compiler.common.log import StructuredLogger
+from compiler.common.result import CompilerResult
+from schemas.models import CategoryMap, PromptOutput
+
+# Reserved outputs, always emitted empty in V1 (nothing populates them).
+RESERVED_OUTPUTS: tuple[str, ...] = ("negative", "scene")
+
+
+def build_prompts(
+    category_map: CategoryMap,
+    config: Config,
+    logger: StructuredLogger | None = None,
+) -> CompilerResult:
+    """Format a Category Map into the final Prompt Outputs.
+
+    Returns:
+        A CompilerResult whose ``data`` is the tuple of :class:`PromptOutput`:
+        one per canonical category (in order) followed by the reserved outputs.
+    """
+    separator = config.prompt_builder.separator
+
+    outputs: list[PromptOutput] = []
+    for category in CANONICAL_CATEGORIES:
+        value = separator.join(tag.tag for tag in category_map.tags_for(category))
+        outputs.append(PromptOutput(name=category, value=value))
+
+    for reserved in RESERVED_OUTPUTS:
+        outputs.append(PromptOutput(name=reserved, value=""))
+
+    if logger is not None:
+        logger.basic("prompts_built", outputs=len(outputs))
+    return CompilerResult(data=tuple(outputs))
diff --git a/tests/test_prompt_builder.py b/tests/test_prompt_builder.py
new file mode 100644
index 0000000..f52f014
--- /dev/null
+++ b/tests/test_prompt_builder.py
@@ -0,0 +1,88 @@
+"""Tests for the Prompt Builder (issue #14, MASTER_SPEC §19)."""
+
+from __future__ import annotations
+
+from types import MappingProxyType
+
+from compiler.builder.prompt_builder import RESERVED_OUTPUTS, build_prompts
+from compiler.common.categories import CANONICAL_CATEGORIES
+from compiler.common.config import Config
+from schemas.models import CategoryMap, ResolvedTag
+
+
+def rtag(name: str, category: str) -> ResolvedTag:
+    return ResolvedTag(tag=name, category=category, source_concept=name, knowledge_base_entry=name)
+
+
+def category_map(**by_category: list[ResolvedTag]) -> CategoryMap:
+    return CategoryMap(categories=MappingProxyType({k: tuple(v) for k, v in by_category.items()}))
+
+
+def outputs_by_name(result) -> dict[str, str]:
+    return {o.name: o.value for o in result.data}
+
+
+def default_config() -> Config:
+    return Config()
+
+
+def test_emits_one_output_per_category_plus_reserved() -> None:
+    result = build_prompts(category_map(), default_config())
+    names = [o.name for o in result.data]
+    assert names == [*CANONICAL_CATEGORIES, *RESERVED_OUTPUTS]
+    assert len(result.data) == 19 + 2
+
+
+def test_tags_joined_with_default_comma_separator() -> None:
+    cm = category_map(hair=[rtag("blonde hair", "hair"), rtag("long hair", "hair")])
+    result = build_prompts(cm, default_config())
+    assert outputs_by_name(result)["hair"] == "blonde hair,long hair"
+
+
+def test_order_is_preserved() -> None:
+    cm = category_map(hair=[rtag("c", "hair"), rtag("a", "hair"), rtag("b", "hair")])
+    result = build_prompts(cm, default_config())
+    assert outputs_by_name(result)["hair"] == "c,a,b"
+
+
+def test_empty_category_is_empty_string() -> None:
+    result = build_prompts(category_map(), default_config())
+    values = outputs_by_name(result)
+    assert values["hair"] == ""
+    assert all(isinstance(o.value, str) for o in result.data)
+
+
+def test_reserved_outputs_present_and_empty() -> None:
+    result = build_prompts(category_map(character=[rtag("1girl", "character")]), default_config())
+    values = outputs_by_name(result)
+    assert values["negative"] == ""
+    assert values["scene"] == ""
+
+
+def test_custom_separator() -> None:
+    cm = category_map(hair=[rtag("a", "hair"), rtag("b", "hair")])
+    config = Config.from_json({"prompt_builder": {"separator": " | "}})
+    result = build_prompts(cm, config)
+    assert outputs_by_name(result)["hair"] == "a | b"
+
+
+def test_no_trailing_separator_or_whitespace() -> None:
+    cm = category_map(hair=[rtag("only", "hair")])
+    result = build_prompts(cm, default_config())
+    value = outputs_by_name(result)["hair"]
+    assert value == "only"
+    assert not value.endswith(",")
+    assert value == value.strip()
+
+
+def test_no_escaping_tags_emitted_verbatim() -> None:
+    cm = category_map(objects=[rtag("cat (animal)", "objects")])
+    result = build_prompts(cm, default_config())
+    assert outputs_by_name(result)["objects"] == "cat (animal)"
+
+
+def test_output_is_deterministic() -> None:
+    cm = category_map(hair=[rtag("a", "hair"), rtag("b", "hair")])
+    a = [(o.name, o.value) for o in build_prompts(cm, default_config()).data]
+    b = [(o.name, o.value) for o in build_prompts(cm, default_config()).data]
+    assert a == b