diff --git a/ui/litellm-dashboard/src/lib/autorouter_presets.test.ts b/ui/litellm-dashboard/src/lib/autorouter_presets.test.ts
index 6324b7fd611..a2d23473fb9 100644
--- a/ui/litellm-dashboard/src/lib/autorouter_presets.test.ts
+++ b/ui/litellm-dashboard/src/lib/autorouter_presets.test.ts
@@ -604,6 +604,28 @@ describe("autorouter_presets", () => {
       });
     });
 
+    // Two spellings of one model in a tier collapse to a single registered key, and one model can
+    // only hold one param set downstream. Merging keeps whatever only one spelling set instead of
+    // dropping that spelling's params wholesale.
+    it("merges rather than drops params when two spellings resolve to the same registered model", () => {
+      const config = {
+        tiers: { SIMPLE: [], MEDIUM: [], COMPLEX: [], REASONING: ["claude-sonnet-4-5", "claude-sonnet-4.5"] },
+        tier_model_configs: {
+          REASONING: [
+            { model_name: "claude-sonnet-4-5", litellm_params: { reasoning_effort: "high", temperature: 0.2 } },
+            { model_name: "claude-sonnet-4.5", litellm_params: { reasoning_effort: "low" } },
+          ],
+        },
+        classifier_type: "heuristic" as const,
+      };
+      const prefill = buildPresetPrefill(config, groupsOnly(["claude-sonnet-4.5"]));
+      // temperature survives from the spelling that would otherwise have been overwritten;
+      // reasoning_effort, set by both, resolves last-wins.
+      expect(prefill.complexityRouterConfig.tier_model_params).toEqual({
+        REASONING: { "claude-sonnet-4.5": { reasoning_effort: "low", temperature: 0.2 } },
+      });
+    });
+
     it("leaves tier_model_params undefined for a preset that carries no per-model params", () => {
       const config = {
         tiers: { SIMPLE: ["gpt-5-nano"], MEDIUM: [], COMPLEX: [], REASONING: [] },
diff --git a/ui/litellm-dashboard/src/lib/autorouter_presets.ts b/ui/litellm-dashboard/src/lib/autorouter_presets.ts
index f6e7abee2f6..a35b868db5e 100644
--- a/ui/litellm-dashboard/src/lib/autorouter_presets.ts
+++ b/ui/litellm-dashboard/src/lib/autorouter_presets.ts
@@ -12,7 +12,11 @@ import {
 } from "@/components/add_model/ComplexityRouterConfig";
 import { KeywordTierRule } from "@/components/add_model/KeywordTierRules";
 import { hydrateKeywordTierRules } from "@/components/add_model/complexity_router_keywords";
-import { TierModelParamsByTier, hydrateTierModelParams } from "@/components/add_model/complexity_router_tiers";
+import {
+  TierModelParams,
+  TierModelParamsByTier,
+  hydrateTierModelParams,
+} from "@/components/add_model/complexity_router_tiers";
 import { DEFAULT_ESCALATION_KEYWORDS } from "@/components/add_model/EscalationKeywords";
 import { DEFAULT_MATCH_THRESHOLD } from "@/components/add_model/SemanticKeywordMatching";
 import presetsRaw from "@/autorouter_presets.json";
@@ -248,12 +252,20 @@ export const buildPresetPrefill = (
   // Params key on the model name the preset spells while every tier entry is rewritten to the
   // caller's registered spelling, so the keys have to be rewritten the same way. Otherwise
   // serializeTierModelConfigs drops them for naming a model the tier no longer holds.
+  //
+  // Two spellings in one tier can resolve to the same registered model, and one model holds one
+  // param set here and in the payload, so a collision has to collapse. Merge rather than replace:
+  // params only one spelling set still survive, and a key both set resolves last-wins, matching
+  // how hydrateTierModelParams already collapses two entries spelled identically.
   const resolveParamKeys = (params: TierModelParamsByTier | undefined): TierModelParamsByTier | undefined =>
     params &&
     Object.fromEntries(
       Object.entries(params).map(([tier, byModel]) => [
         tier,
-        Object.fromEntries(Object.entries(byModel).map(([model, litellmParams]) => [resolve(model), litellmParams])),
+        Object.entries(byModel).reduce<Record<string, TierModelParams>>((byResolved, [model, litellmParams]) => {
+          const resolved = resolve(model);
+          return { ...byResolved, [resolved]: { ...byResolved[resolved], ...litellmParams } };
+        }, {}),
       ]),
     );