diff --git a/nodes/__init__.py b/nodes/__init__.py
index ff17a60..9f78a53 100644
--- a/nodes/__init__.py
+++ b/nodes/__init__.py
@@ -8,6 +8,7 @@ Nodes are thin interfaces; all compiler logic lives in the ``compiler`` package.
 from __future__ import annotations
 
 from .category_splitter_node import CategorySplitterNode
+from .prompt_builder_node import PromptBuilderNode
 from .resolver_node import ResolverNode
 from .scene_analyzer_node import SceneAnalyzerNode
 from .scene_validator_node import SceneValidatorNode
@@ -17,6 +18,7 @@ NODE_CLASS_MAPPINGS: dict[str, type] = {
     "SceneCompilerValidator": SceneValidatorNode,
     "SceneCompilerResolver": ResolverNode,
     "SceneCompilerCategorySplitter": CategorySplitterNode,
+    "SceneCompilerPromptBuilder": PromptBuilderNode,
 }
 
 NODE_DISPLAY_NAME_MAPPINGS: dict[str, str] = {
@@ -24,6 +26,7 @@ NODE_DISPLAY_NAME_MAPPINGS: dict[str, str] = {
     "SceneCompilerValidator": "Scene Validator",
     "SceneCompilerResolver": "Resolver",
     "SceneCompilerCategorySplitter": "Category Splitter",
+    "SceneCompilerPromptBuilder": "Prompt Builder",
 }
 
 __all__ = ["NODE_CLASS_MAPPINGS", "NODE_DISPLAY_NAME_MAPPINGS"]
diff --git a/nodes/prompt_builder_node.py b/nodes/prompt_builder_node.py
new file mode 100644
index 0000000..5394865
--- /dev/null
+++ b/nodes/prompt_builder_node.py
@@ -0,0 +1,40 @@
+"""ComfyUI node for the Prompt Builder (MASTER_SPEC §20, §19).
+
+A thin interface: it adapts the Category Map input to the Prompt Builder module
+and exposes one string output per category plus the reserved ``negative`` and
+``scene`` outputs (always empty in V1). No compiler logic; it never imports
+ComfyUI.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from compiler.builder.prompt_builder import RESERVED_OUTPUTS, build_prompts
+from compiler.common.categories import CANONICAL_CATEGORIES
+from compiler.common.config import Config
+
+# The stable set of node outputs: one per canonical category, then the reserved
+# outputs. Matches the Prompt Outputs produced by build_prompts.
+_OUTPUT_NAMES: tuple[str, ...] = (*CANONICAL_CATEGORIES, *RESERVED_OUTPUTS)
+
+
+class PromptBuilderNode:
+    """Formats a Category Map into one prompt string per category (+ reserved)."""
+
+    CATEGORY = "Scene Compiler"
+    FUNCTION = "run"
+    RETURN_TYPES = ("STRING",) * len(_OUTPUT_NAMES)
+    RETURN_NAMES = _OUTPUT_NAMES
+
+    @classmethod
+    def INPUT_TYPES(cls) -> dict[str, Any]:
+        return {
+            "required": {"category_map": ("CATEGORY_MAP",)},
+            "optional": {"config": ("COMPILER_CONFIG",)},
+        }
+
+    def run(self, category_map: Any, config: Config | None = None) -> tuple[str, ...]:
+        result = build_prompts(category_map, config or Config())
+        by_name = {output.name: output.value for output in result.data}
+        return tuple(by_name[name] for name in _OUTPUT_NAMES)
diff --git a/tests/test_prompt_builder_node.py b/tests/test_prompt_builder_node.py
new file mode 100644
index 0000000..ec755e8
--- /dev/null
+++ b/tests/test_prompt_builder_node.py
@@ -0,0 +1,55 @@
+"""Tests for the Prompt Builder ComfyUI node (issue #23)."""
+
+from __future__ import annotations
+
+from types import MappingProxyType
+
+import nodes
+from compiler.common.categories import CANONICAL_CATEGORIES
+from compiler.common.config import Config
+from nodes.prompt_builder_node import PromptBuilderNode
+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(result: tuple[str, ...]) -> dict[str, str]:
+    return dict(zip(PromptBuilderNode.RETURN_NAMES, result, strict=True))
+
+
+def test_emits_one_output_per_category_plus_reserved() -> None:
+    assert PromptBuilderNode.RETURN_NAMES == (*CANONICAL_CATEGORIES, "negative", "scene")
+    assert PromptBuilderNode.RETURN_TYPES == ("STRING",) * 21
+
+
+def test_formats_and_orders_category_strings() -> None:
+    cm = category_map(hair=[rtag("blonde hair", "hair"), rtag("long hair", "hair")])
+    result = PromptBuilderNode().run(cm)
+    by_name = outputs(result)
+    assert by_name["hair"] == "blonde hair,long hair"
+    assert by_name["character"] == ""  # empty category -> empty string
+
+
+def test_reserved_outputs_are_empty() -> None:
+    cm = category_map(character=[rtag("1girl", "character")])
+    by_name = outputs(PromptBuilderNode().run(cm))
+    assert by_name["negative"] == ""
+    assert by_name["scene"] == ""
+
+
+def test_custom_separator_from_config() -> None:
+    cm = category_map(hair=[rtag("a", "hair"), rtag("b", "hair")])
+    config = Config.from_json({"prompt_builder": {"separator": " | "}})
+    by_name = outputs(PromptBuilderNode().run(cm, config))
+    assert by_name["hair"] == "a | b"
+
+
+def test_registered_for_comfyui() -> None:
+    assert nodes.NODE_CLASS_MAPPINGS["SceneCompilerPromptBuilder"] is PromptBuilderNode
+    assert nodes.NODE_DISPLAY_NAME_MAPPINGS["SceneCompilerPromptBuilder"] == "Prompt Builder"