diff --git a/ui/litellm-dashboard/src/components/add_model/ComplexityRouterConfig.test.tsx b/ui/litellm-dashboard/src/components/add_model/ComplexityRouterConfig.test.tsx
index 133a32a7951..7185e77dea8 100644
--- a/ui/litellm-dashboard/src/components/add_model/ComplexityRouterConfig.test.tsx
+++ b/ui/litellm-dashboard/src/components/add_model/ComplexityRouterConfig.test.tsx
@@ -1,5 +1,6 @@
 import { fireEvent, renderWithProviders, screen, within } from "../../../tests/test-utils";
 import userEvent from "@testing-library/user-event";
+import { useState } from "react";
 import { vi } from "vitest";
 import ComplexityRouterConfig, { ComplexityRouterConfigValue } from "./ComplexityRouterConfig";
 vi.mock(
@@ -102,6 +103,42 @@ describe("ComplexityRouterConfig", () => {
     expect(screen.queryByRole("option", { name: "xhigh" })).not.toBeInTheDocument();
   });
 
+  it("prunes tier params when a model is removed", async () => {
+    const user = userEvent.setup();
+    const onChange = vi.fn();
+    const Wrapper = () => {
+      const [value, setValue] = useState<ComplexityRouterConfigValue>({
+        ...defaultValue,
+        tiers: { ...defaultValue.tiers, REASONING: ["reasoning-model"] },
+        tier_model_params: {},
+      });
+      return (
+        <ComplexityRouterConfig
+          {...baseProps}
+          modelInfo={[{ model_group: "reasoning-model", mode: "chat", supports_reasoning: true }]}
+          value={value}
+          onChange={(nextValue) => {
+            setValue(nextValue);
+            onChange(nextValue);
+          }}
+        />
+      );
+    };
+
+    renderWithProviders(<Wrapper />);
+    await user.click(screen.getByRole("combobox", { name: "Reasoning effort for reasoning-model" }));
+    await user.click(screen.getByText("high"));
+    await user.click(screen.getByRole("combobox", { name: "Select model(s) for reasoning queries" }));
+    await user.keyboard("{Backspace}");
+
+    expect(onChange).toHaveBeenLastCalledWith(
+      expect.objectContaining({
+        tiers: expect.objectContaining({ REASONING: [] }),
+        tier_model_params: {},
+      }),
+    );
+  });
+
   it("should display the how classification works section", () => {
     renderWithProviders(<ComplexityRouterConfig {...baseProps} />);
     fireEvent.click(screen.getByText("Advanced: Classification Method"));
diff --git a/ui/litellm-dashboard/src/components/add_model/ComplexityRouterConfig.tsx b/ui/litellm-dashboard/src/components/add_model/ComplexityRouterConfig.tsx
index 9435877b039..7c8c9182226 100644
--- a/ui/litellm-dashboard/src/components/add_model/ComplexityRouterConfig.tsx
+++ b/ui/litellm-dashboard/src/components/add_model/ComplexityRouterConfig.tsx
@@ -268,7 +268,7 @@ const ComplexityRouterConfig: React.FC<ComplexityRouterConfigProps> = ({
       ? { ...existingParams, reasoning_effort: reasoningEffort }
       : Object.fromEntries(Object.entries(existingParams).filter(([key]) => key !== "reasoning_effort"));
     const updatedTierModelParams =
-      updatedParams && Object.keys(updatedParams).length > 0
+      Object.keys(updatedParams).length > 0
         ? { ...tierModelParams, [model]: updatedParams }
         : Object.fromEntries(Object.entries(tierModelParams).filter(([key]) => key !== model));
     onChange({
diff --git a/ui/litellm-dashboard/src/components/add_model/build_complexity_router_config.ts b/ui/litellm-dashboard/src/components/add_model/build_complexity_router_config.ts
index 5554c615927..85f17de3a95 100644
--- a/ui/litellm-dashboard/src/components/add_model/build_complexity_router_config.ts
+++ b/ui/litellm-dashboard/src/components/add_model/build_complexity_router_config.ts
@@ -15,7 +15,7 @@ import {
   effectiveTierLabel,
   heuristicScoringRoleFor,
 } from "./ComplexityRouterConfig";
-import { serializeTierConfig, type TierModelParamsByTier } from "./complexity_router_tiers";
+import { serializeTierConfig, type SerializedTierConfig, type TierModelParamsByTier } from "./complexity_router_tiers";
 
 /**
  * Drop an empty system_prompt so the payload carries an override only when there is one. The
@@ -104,7 +104,7 @@ export interface BuildComplexityRouterConfigParams {
 }
 
 export interface ComplexityRouterConfigPayload {
-  tiers: Record<string, unknown>;
+  tiers: SerializedTierConfig;
   default_model?: string;
   plan_mode_min_tier?: string;
   tier_labels?: ComplexityTierLabels;
diff --git a/ui/litellm-dashboard/src/components/add_model/complexity_router_tiers.test.ts b/ui/litellm-dashboard/src/components/add_model/complexity_router_tiers.test.ts
index 06ad0e72e4b..5b74bb50714 100644
--- a/ui/litellm-dashboard/src/components/add_model/complexity_router_tiers.test.ts
+++ b/ui/litellm-dashboard/src/components/add_model/complexity_router_tiers.test.ts
@@ -74,7 +74,7 @@ describe("normalizeTierModels", () => {
     const tiers = { REASONING: normalizeTierModels(stored) };
     const params = { REASONING: extractTierModelParams(stored) };
 
-    expect(serializeTierConfig(tiers, params)).toEqual({ REASONING: stored });
+    expect(serializeTierConfig(tiers, params)).toEqual({ REASONING: [stored] });
   });
 });
 
diff --git a/ui/litellm-dashboard/src/components/add_model/complexity_router_tiers.ts b/ui/litellm-dashboard/src/components/add_model/complexity_router_tiers.ts
index 7e415504fd2..ce658bcece8 100644
--- a/ui/litellm-dashboard/src/components/add_model/complexity_router_tiers.ts
+++ b/ui/litellm-dashboard/src/components/add_model/complexity_router_tiers.ts
@@ -51,7 +51,7 @@ export const tierOptions = (
 export type TierModelParams = Record<string, unknown>;
 export type SerializedTierModel = string | { model_name: string; litellm_params: TierModelParams };
 
-export type TierModelParamsByTier = Partial<Record<string, Record<string, TierModelParams>>>;
+export type TierModelParamsByTier = Partial<Record<keyof ComplexityTiers, Record<string, TierModelParams>>>;
 
 export const extractTierModelParams = (value: unknown): Record<string, TierModelParams> => {
   const entries = Array.isArray(value) ? value : [value];
@@ -75,19 +75,23 @@ export const extractTierModelParams = (value: unknown): Record<string, TierModel
 export const serializeTierModels = (
   models: string[],
   paramsByModel: Record<string, TierModelParams> | undefined,
-): SerializedTierModel | SerializedTierModel[] => {
+): SerializedTierModel[] => {
   const entries = models.map((model) => {
     const params = paramsByModel?.[model];
     return params && Object.keys(params).length > 0 ? { model_name: model, litellm_params: params } : model;
   });
-  if (entries.length === 1 && typeof entries[0] !== "string") return entries[0];
   return entries;
 };
 
+export type SerializedTierConfig = Partial<Record<keyof ComplexityTiers, SerializedTierModel[]>>;
+
 export const serializeTierConfig = (
   tiers: Partial<ComplexityTiers>,
   paramsByTier: TierModelParamsByTier | undefined,
-): Record<string, SerializedTierModel | SerializedTierModel[]> =>
+): SerializedTierConfig =>
   Object.fromEntries(
-    Object.entries(tiers).map(([tier, models]) => [tier, serializeTierModels(models ?? [], paramsByTier?.[tier])]),
-  );
+    Object.entries(tiers).map(([tier, models]) => [
+      tier,
+      serializeTierModels(models ?? [], paramsByTier?.[tier as keyof ComplexityTiers]),
+    ]),
+  ) as SerializedTierConfig;
diff --git a/ui/litellm-dashboard/src/components/edit_auto_router/build_updated_complexity_router_config.test.ts b/ui/litellm-dashboard/src/components/edit_auto_router/build_updated_complexity_router_config.test.ts
index 2be18d19e83..e38668aec33 100644
--- a/ui/litellm-dashboard/src/components/edit_auto_router/build_updated_complexity_router_config.test.ts
+++ b/ui/litellm-dashboard/src/components/edit_auto_router/build_updated_complexity_router_config.test.ts
@@ -68,7 +68,39 @@ describe("buildUpdatedComplexityRouterConfig keyword matching", () => {
       },
     };
 
-    expect(buildUpdatedComplexityRouterConfig(stored, value, undefined, hydratedState).tiers).toEqual(stored.tiers);
+    expect(buildUpdatedComplexityRouterConfig(stored, value, undefined, hydratedState).tiers).toEqual({
+      ...stored.tiers,
+      SIMPLE: [stored.tiers.SIMPLE],
+    });
+  });
+
+  it("keeps plain tier lists unchanged and widens a stored bare string tier", () => {
+    const stored = {
+      ...STORED,
+      tiers: {
+        SIMPLE: "gpt-4o-mini",
+        MEDIUM: ["gpt-4o"],
+        COMPLEX: ["claude-sonnet-4"],
+        REASONING: ["o1-preview"],
+      },
+    };
+
+    const formValue = {
+      ...FORM_VALUE,
+      tiers: {
+        SIMPLE: ["gpt-4o-mini"],
+        MEDIUM: ["gpt-4o"],
+        COMPLEX: ["claude-sonnet-4"],
+        REASONING: ["o1-preview"],
+      },
+    };
+
+    expect(buildUpdatedComplexityRouterConfig(stored, formValue, undefined, hydratedState).tiers).toEqual({
+      SIMPLE: ["gpt-4o-mini"],
+      MEDIUM: ["gpt-4o"],
+      COMPLEX: ["claude-sonnet-4"],
+      REASONING: ["o1-preview"],
+    });
   });
 
   it("persists an edited keyword rule", () => {
diff --git a/ui/litellm-dashboard/src/components/edit_auto_router/edit_auto_router_modal.tsx b/ui/litellm-dashboard/src/components/edit_auto_router/edit_auto_router_modal.tsx
index 60be7c71c1d..a728e7bcb25 100644
--- a/ui/litellm-dashboard/src/components/edit_auto_router/edit_auto_router_modal.tsx
+++ b/ui/litellm-dashboard/src/components/edit_auto_router/edit_auto_router_modal.tsx
@@ -348,10 +348,10 @@ const EditAutoRouterModal: React.FC<EditAutoRouterModalProps> = ({
         const hydratedComplexityRouterConfig: ComplexityRouterConfigValue = {
           tiers: hydratedTiers,
           tier_model_params: Object.fromEntries(
-            ["SIMPLE", "MEDIUM", "COMPLEX", "REASONING"].map((tier) => [
-              tier,
-              extractTierModelParams(parsedConfig.tiers?.[tier]),
-            ]),
+            ["SIMPLE", "MEDIUM", "COMPLEX", "REASONING"].flatMap((tier) => {
+              const params = extractTierModelParams(parsedConfig.tiers?.[tier]);
+              return Object.keys(params).length > 0 ? [[tier, params]] : [];
+            }),
           ),
           default_model: hydratePinnedDefaultModel(
             parsedConfig.default_model,