diff --git a/src/llm_core.py b/src/llm_core.py
index e4d2c51..9ee4ca6 100644
--- a/src/llm_core.py
+++ b/src/llm_core.py
@@ -512,6 +512,12 @@ def _build_anthropic_payload(model, messages, temperature, max_tokens, stream=Fa
             # Convert multimodal content (image_url → image) for Anthropic
             content = _convert_openai_content_to_anthropic(m["content"])
             chat_messages.append({"role": m["role"], "content": content})
+    # Anthropic only accepts temperature in [0.0, 1.0] and 400s on anything above
+    # 1.0. Clamp here (in the Anthropic builder only) so presets/sliders that use
+    # the wider OpenAI 0.0-2.0 range — e.g. the shipped "Nietzsche" preset at 1.2
+    # — don't hard-break every Claude request. OpenAI's own path is left untouched.
+    if temperature is not None:
+        temperature = max(0.0, min(temperature, 1.0))
     payload = {
         "model": model,
         "messages": chat_messages,
diff --git a/tests/test_llm_core_anthropic_temp_clamp.py b/tests/test_llm_core_anthropic_temp_clamp.py
new file mode 100644
index 0000000..d2f81ca
--- /dev/null
+++ b/tests/test_llm_core_anthropic_temp_clamp.py
@@ -0,0 +1,40 @@
+"""Regression guard for #1615 — Anthropic temperature must be clamped to [0.0, 1.0].
+
+Anthropic's Messages API rejects temperature > 1.0 with HTTP 400. The shipped
+"Nietzsche" preset uses temperature 1.2 (static/js/presets.js) and the UI slider
+allows up to 2.0 (static/index.html), so _build_anthropic_payload must clamp into
+[0.0, 1.0]. The clamp lives only in the Anthropic builder — OpenAI keeps its
+wider 0.0-2.0 range.
+"""
+import os
+
+os.environ.setdefault("DATABASE_URL", "sqlite:///:memory:")
+
+from src.llm_core import _build_anthropic_payload
+
+
+def _temp(t):
+    payload = _build_anthropic_payload(
+        "claude-x", [{"role": "user", "content": "hi"}], t, 100
+    )
+    return payload["temperature"]
+
+
+def test_above_range_is_clamped_to_one():
+    assert _temp(1.2) == 1.0  # the shipped "Nietzsche" preset — previously 400'd
+    assert _temp(2.0) == 1.0  # UI slider max
+
+
+def test_in_range_is_unchanged():
+    assert _temp(0.0) == 0.0
+    assert _temp(0.7) == 0.7
+    assert _temp(1.0) == 1.0
+
+
+def test_below_range_is_clamped_to_zero():
+    assert _temp(-0.5) == 0.0
+
+
+def test_none_is_passed_through_unchanged():
+    # Callers may pass None; behavior is unchanged (no clamp, no crash).
+    assert _temp(None) is None