diff --git a/modules/api/helpers.py b/modules/api/helpers.py
index 0843f0615..78cd697d8 100644
--- a/modules/api/helpers.py
+++ b/modules/api/helpers.py
@@ -17,9 +17,15 @@ def register_upload_store(getter_fn):
 
 def validate_sampler_name(name):
     config = sd_samplers.all_samplers_map.get(name, None)
-    if config is None:
-        raise HTTPException(status_code=404, detail="Sampler not found")
-    return name
+    if config is not None:
+        return name
+    # accept case-insensitive and alias variants, returning the canonical name so the
+    # exact-match lookup in create_sampler resolves instead of silently using the model default
+    if isinstance(name, str) and name not in ('', 'None'):
+        sampler = sd_samplers.find_sampler(name)
+        if sampler is not None:
+            return sampler.name
+    raise HTTPException(status_code=404, detail="Sampler not found")
 
 
 def decode_base64_to_image(encoding, quiet=False):
diff --git a/modules/api/models.py b/modules/api/models.py
index b24a7991c..82895dfbf 100644
--- a/modules/api/models.py
+++ b/modules/api/models.py
@@ -302,7 +302,7 @@ ReqImg2Img = PydanticModelGenerator(
     StableDiffusionProcessingImg2Img,
     [
         {"key": "sampler_index", "type": Union[int, str], "default": 0},
-        {"key": "sampler_name", "type": str, "default": "UniPC"},
+        {"key": "sampler_name", "type": str, "default": "Default"},
         {"key": "hr_sampler_name", "type": str, "default": "Same as primary"},
         {"key": "init_images", "type": list, "default": None},
         {"key": "denoising_strength", "type": float, "default": 0.5},
diff --git a/modules/ui_definitions.py b/modules/ui_definitions.py
index 17c181e50..950308273 100644
--- a/modules/ui_definitions.py
+++ b/modules/ui_definitions.py
@@ -742,7 +742,7 @@ def create_settings(cmd_opts):
                 "schedulers_solver_order": OptionInfo(0, "Solver order (where", gr.Slider, {"minimum": 0, "maximum": 5, "step": 1, "visible": False}),
                 "schedulers_use_loworder": OptionInfo(True, "Use simplified solvers in final steps", gr.Checkbox, {"visible": False}),
                 "schedulers_prediction_type": OptionInfo("default", "Override model prediction type", gr.Radio, {"choices": ["default", "epsilon", "sample", "v_prediction", "flow_prediction"], "visible": False}),
-                "schedulers_sigma": OptionInfo("default", "Sigma algorithm", gr.Radio, {"choices": ["default", "karras", "exponential", "polyexponential"], "visible": False}),
+                "schedulers_sigma": OptionInfo("default", "Sigma algorithm", gr.Radio, {"choices": ["default", "karras", "betas", "exponential", "lambdas", "flowmatch"], "visible": False}),
                 "schedulers_beta_schedule": OptionInfo("default", "Beta schedule", gr.Dropdown, {"choices": ["default", "linear", "scaled_linear", "squaredcos_cap_v2", "sigmoid"], "visible": False}),
                 "schedulers_use_thresholding": OptionInfo(False, "Use dynamic thresholding", gr.Checkbox, {"visible": False}),
                 "schedulers_timestep_spacing": OptionInfo("default", "Timestep spacing", gr.Dropdown, {"choices": ["default", "linspace", "leading", "trailing"], "visible": False}),
diff --git a/test/test-generation-api.py b/test/test-generation-api.py
index 98ec18982..ca0888dbd 100644
--- a/test/test-generation-api.py
+++ b/test/test-generation-api.py
@@ -210,6 +210,36 @@ class GenerationAPITest:
             data, elapsed = self._txt2img({'sampler_name': sampler})
             self._check_generation(data, f'generate_{sampler}', elapsed)
 
+    def test_sampler_name_resolution(self, available_samplers):
+        """Sampler name resolution: a case-insensitive name resolves to the canonical sampler
+        (and is applied, not silently swapped for the model default), while an unknown name is
+        rejected rather than falling back to the default scheduler."""
+        self._category = 'samplers'
+        print("\n--- Sampler Name Resolution ---")
+
+        if self._critical_error:
+            self.skip('sampler_lenient_case', self._critical_error)
+            self.skip('sampler_unknown_rejected', self._critical_error)
+            return
+
+        canonical = next((s for s in ('Euler a', 'DPM++ 2M', 'UniPC') if s in available_samplers), None)
+        if canonical is None:
+            self.skip('sampler_lenient_case', 'no known sampler available')
+        else:
+            data, _ = self._txt2img({'sampler_name': canonical.lower()})
+            if 'error' in data:
+                self.record(False, 'sampler_lenient_case', f"lowercase '{canonical.lower()}' rejected: {data}")
+            else:
+                resolved = canonical in self._get_info(data)
+                self.record(resolved, 'sampler_lenient_case',
+                            f"'{canonical.lower()}' -> '{canonical}'" if resolved
+                            else f"generated but '{canonical}' not in info (model default used?)")
+
+        data, _ = self._txt2img({'sampler_name': 'ThisIsNotARealSampler'})
+        rejected = 'error' in data
+        self.record(rejected, 'sampler_unknown_rejected',
+                    'unknown name rejected' if rejected else 'unknown name was NOT rejected')
+
     # =========================================================================
     # Tests: Color Grading Params
     # =========================================================================
@@ -577,6 +607,7 @@ class GenerationAPITest:
         # Samplers
         available = self.test_samplers_list()
         self.test_samplers_generate(available)
+        self.test_sampler_name_resolution(available)
 
         # Grading
         self.run_grading_tests()