diff --git a/tests/test_prompt_layout_filler.py b/tests/test_prompt_layout_filler.py
new file mode 100644
index 0000000..b6dc86a
--- /dev/null
+++ b/tests/test_prompt_layout_filler.py
@@ -0,0 +1,92 @@
+import pytest
+from src.nodes.prompt_layout_filler import PromptLayoutFiller
+
+
+@pytest.fixture
+def node():
+    return PromptLayoutFiller()
+
+
+def run(node, template, **slots):
+    return node.process(template=template, **slots)
+
+
+# --- Base cases ---
+
+
+def test_single_slot(node):
+    (text,) = run(node, "character, {0}", slot_0="sitting down")
+    assert text == "character, sitting down"
+
+
+def test_two_slots(node):
+    (text,) = run(node, "character, {0}, {1}", slot_0="sitting down", slot_1="field")
+    assert text == "character, sitting down, field"
+
+
+def test_slot_with_label(node):
+    (text,) = run(
+        node,
+        "character, {0:position}, {1:background}",
+        slot_0="sitting down",
+        slot_1="field",
+    )
+    assert text == "character, sitting down, field"
+
+
+def test_label_with_spaces(node):
+    (text,) = run(node, "{0:my position}", slot_0="sitting down")
+    assert text == "sitting down"
+
+
+def test_label_with_underscores(node):
+    (text,) = run(node, "{0:my_position}", slot_0="sitting down")
+    assert text == "sitting down"
+
+
+def test_empty_slot_value(node):
+    # Connected slot but empty value
+    (text,) = run(node, "character, {0}", slot_0="")
+    assert text == "character, "
+
+
+def test_repeated_placeholder(node):
+    # Same placeholder used twice
+    (text,) = run(node, "{0} and {0}", slot_0="hello")
+    assert text == "hello and hello"
+
+
+def test_no_placeholders(node):
+    (text,) = run(node, "character, sitting down")
+    assert text == "character, sitting down"
+
+
+def test_unused_slot_connected(node):
+    # slot_1 connected but not used in the template
+    (text,) = run(node, "character, {0}", slot_0="sitting down", slot_1="field")
+    assert text == "character, sitting down"
+
+
+# --- Error raised ---
+
+
+def test_missing_slot(node):
+    with pytest.raises(ValueError, match="not connected"):
+        run(node, "character, {0}", slot_0=None)
+
+
+def test_placeholder_out_of_range(node):
+    with pytest.raises(ValueError, match="exceeds maximum"):
+        out_slot = PromptLayoutFiller.NUM_SLOTS + 1
+        run(node, f"{{{out_slot}}}")
+
+
+def test_empty_template(node):
+    with pytest.raises(ValueError, match="empty"):
+        run(node, "")
+
+
+def test_validate_valid(node):
+    # No error if template without placeholders
+    (text,) = run(node, "character, sitting down")
+    assert text == "character, sitting down"