diff --git a/tests/webapp/test_llm_profiles_api.py b/tests/webapp/test_llm_profiles_api.py
index 6cc2c4a..31d5f7d 100644
--- a/tests/webapp/test_llm_profiles_api.py
+++ b/tests/webapp/test_llm_profiles_api.py
@@ -65,3 +65,75 @@ def test_update_nonexistent(client):
 def test_delete_nonexistent(client):
     resp = client.delete("/api/llm-profiles/nope")
     assert resp.status_code == 404
+
+
+# ---------------------------------------------------------------------------
+# YAML patcher tests
+# ---------------------------------------------------------------------------
+import yaml as yaml_lib
+from webapp.services.yaml_patcher import apply_profiles_to_scenario
+from webapp.models import LLMProfile
+
+
+def test_apply_judge_profile(tmp_path):
+    """Applying a judge profile patches judge_model in the YAML."""
+    scenario_file = tmp_path / "test-scenario.yaml"
+    scenario_file.write_text(
+        "scenario_name: test\nmode: offline\njudge_model: old-model\nembedding_model: emb\n"
+        "dataset: data.csv\nmetrics:\n- faithfulness\noutput_dir: outputs/test\n",
+        encoding="utf-8",
+    )
+    judge_p = LLMProfile(
+        profile_id="x", name="J", model="new-model",
+        base_url="http://x/v1", api_key="k", created_at="t", updated_at="t",
+    )
+    patched = apply_profiles_to_scenario(
+        scenario_path=str(scenario_file),
+        judge_profile=judge_p,
+        answer_profile=None,
+        dataset_profile=None,
+        _resolve_absolute=True,
+    )
+    assert "judge_model" in patched
+    data = yaml_lib.safe_load(scenario_file.read_text())
+    assert data["judge_model"] == "new-model"
+
+
+def test_apply_answer_profile(tmp_path):
+    """Applying an answer profile patches app_adapter.static_kwargs.model."""
+    scenario_file = tmp_path / "online.yaml"
+    scenario_file.write_text(
+        "scenario_name: online\nmode: online\njudge_model: j\nembedding_model: emb\n"
+        "dataset: d.csv\nmetrics:\n- faithfulness\noutput_dir: out\n"
+        "app_adapter:\n  type: python\n  callable: apps.foo:run\n"
+        "  static_kwargs:\n    model: old\n    source_chunks_path: chunks.jsonl\n",
+        encoding="utf-8",
+    )
+    answer_p = LLMProfile(
+        profile_id="y", name="A", model="new-answer-model",
+        base_url="http://x/v1", api_key="k", created_at="t", updated_at="t",
+    )
+    patched = apply_profiles_to_scenario(
+        scenario_path=str(scenario_file),
+        judge_profile=None,
+        answer_profile=answer_p,
+        dataset_profile=None,
+        _resolve_absolute=True,
+    )
+    assert "app_adapter.static_kwargs.model" in patched
+    data = yaml_lib.safe_load(scenario_file.read_text())
+    assert data["app_adapter"]["static_kwargs"]["model"] == "new-answer-model"
+
+
+def test_apply_no_profiles_returns_empty(tmp_path):
+    """When no profiles are given, no fields are patched."""
+    scenario_file = tmp_path / "noop.yaml"
+    scenario_file.write_text("scenario_name: noop\njudge_model: m\n", encoding="utf-8")
+    patched = apply_profiles_to_scenario(
+        scenario_path=str(scenario_file),
+        judge_profile=None,
+        answer_profile=None,
+        dataset_profile=None,
+        _resolve_absolute=True,
+    )
+    assert patched == []
diff --git a/webapp/services/yaml_patcher.py b/webapp/services/yaml_patcher.py
new file mode 100644
index 0000000..2dbae20
--- /dev/null
+++ b/webapp/services/yaml_patcher.py
@@ -0,0 +1,74 @@
+"""Patch LLM profile settings into scenario YAML files in-place.
+
+Only the fields that correspond to a provided (non-None) profile are touched.
+All other fields and structure are preserved as much as PyYAML allows
+(comments are lost on round-trip, which is an accepted trade-off).
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any
+
+import yaml
+
+from webapp.models import LLMProfile
+
+
+def _repo_root() -> Path:
+    return Path(__file__).resolve().parents[2]
+
+
+def _resolve_scenario_path(path_str: str) -> Path:
+    """Resolve a scenario path; absolute paths are used as-is."""
+    candidate = Path(path_str)
+    if candidate.is_absolute():
+        return candidate
+    return (_repo_root() / candidate).resolve()
+
+
+def apply_profiles_to_scenario(
+    scenario_path: str,
+    judge_profile: LLMProfile | None,
+    answer_profile: LLMProfile | None,
+    dataset_profile: LLMProfile | None,
+    _resolve_absolute: bool = False,
+) -> list[str]:
+    """Patch the YAML file at *scenario_path* with the supplied profiles.
+
+    Returns a list of dotted field names that were actually patched.
+    Setting *_resolve_absolute=True* skips repo-root resolution (used in tests).
+    """
+    if _resolve_absolute:
+        resolved = Path(scenario_path)
+    else:
+        resolved = _resolve_scenario_path(scenario_path)
+
+    if not resolved.exists():
+        raise FileNotFoundError(f"Scenario file not found: {resolved}")
+
+    data: dict[str, Any] = yaml.safe_load(resolved.read_text(encoding="utf-8")) or {}
+    patched: list[str] = []
+
+    if judge_profile is not None:
+        data["judge_model"] = judge_profile.model
+        patched.append("judge_model")
+
+    if answer_profile is not None:
+        adapter = data.get("app_adapter")
+        if isinstance(adapter, dict):
+            static_kwargs = adapter.setdefault("static_kwargs", {})
+            static_kwargs["model"] = answer_profile.model
+            patched.append("app_adapter.static_kwargs.model")
+
+    if dataset_profile is not None:
+        generation = data.get("generation")
+        if isinstance(generation, dict):
+            generation["model"] = dataset_profile.model
+            patched.append("generation.model")
+
+    resolved.write_text(
+        yaml.dump(data, allow_unicode=True, default_flow_style=False, sort_keys=False),
+        encoding="utf-8",
+    )
+    return patched