diff --git a/ui/litellm-dashboard/src/components/add_model/add_auto_router_tab.tsx b/ui/litellm-dashboard/src/components/add_model/add_auto_router_tab.tsx
index 96894b079c4..0d26b983ba4 100644
--- a/ui/litellm-dashboard/src/components/add_model/add_auto_router_tab.tsx
+++ b/ui/litellm-dashboard/src/components/add_model/add_auto_router_tab.tsx
@@ -41,6 +41,7 @@ import {
   getPlanModeTierError,
   getSemanticConfigError,
   getTierLabelsError,
+  dryRunRejection,
 } from "./build_complexity_router_config";
 import { activeTierName, activeTierRows, getCustomTierRowsError, resolveComplexityDefaultModel } from "./tier_rows";
 import { tierRowLabel } from "./complexity_router_tiers";
@@ -405,9 +406,10 @@ const AddAutoRouterTab: React.FC<AddAutoRouterTabProps> = ({
       complexityRouterConfigPayload as unknown as Record<string, unknown>,
       requiresTeamScope ? form.getValues("team_id") : undefined,
     );
-    if (!serverVerdict.valid && serverVerdict.error) {
+    const dryRunError = dryRunRejection(serverVerdict);
+    if (dryRunError) {
       setShowValidationErrors(true);
-      toast.fromError(serverVerdict.error);
+      toast.fromError(dryRunError);
       return;
     }
 
diff --git a/ui/litellm-dashboard/src/components/add_model/build_complexity_router_config.test.ts b/ui/litellm-dashboard/src/components/add_model/build_complexity_router_config.test.ts
index 54443d40427..c3c2be0be50 100644
--- a/ui/litellm-dashboard/src/components/add_model/build_complexity_router_config.test.ts
+++ b/ui/litellm-dashboard/src/components/add_model/build_complexity_router_config.test.ts
@@ -10,6 +10,7 @@ import {
   getTierLabelsError,
   hydrateTierLabels,
   BuildComplexityRouterConfigParams,
+  dryRunRejection,
 } from "./build_complexity_router_config";
 import { CUSTOM_TIER_RESTRICTIONS, activeTierRows } from "./tier_rows";
 
@@ -958,3 +959,24 @@ describe("hydrateCustomTierSet", () => {
     expect(hydrated?.fallback_tier_id).toBe(hydrated?.tiers[0].id);
   });
 });
+
+describe("dryRunRejection", () => {
+  it("blocks the save on a rejection whose message is missing, which the write would return as a raw 400", () => {
+    // The verdict's two fields arrive independently, so a rejection carrying no message must still
+    // stop the save rather than fall through to the write endpoint.
+    expect(dryRunRejection({ valid: false })).toBe("The proxy rejected this auto-router configuration");
+    expect(dryRunRejection({ valid: false, error: null })).toBe("The proxy rejected this auto-router configuration");
+    expect(dryRunRejection({ valid: false, error: "   " })).toBe("The proxy rejected this auto-router configuration");
+  });
+
+  it("surfaces the backend's own message when it sent one", () => {
+    expect(dryRunRejection({ valid: false, error: "session_affinity cannot be combined with tier_definitions" })).toBe(
+      "session_affinity cannot be combined with tier_definitions",
+    );
+  });
+
+  it("lets a valid verdict through, including the fail-open one a transport failure returns", () => {
+    expect(dryRunRejection({ valid: true })).toBeNull();
+    expect(dryRunRejection({ valid: true, error: null })).toBeNull();
+  });
+});
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 04e6e16f81e..24a588d4931 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
@@ -124,6 +124,17 @@ export interface BuildComplexityRouterConfigParams {
   tierModelParams?: TierModelParamsByTier;
 }
 
+/**
+ * The message to surface when the dry-run rejects a save, or null to let it through.
+ *
+ * Gated on `valid` alone. The verdict's `valid` is derived from `error` server side today, but the
+ * two arrive as independent fields, so reading `error` as the gate would let a rejection whose
+ * message is missing or blank through to the write and back as a raw 400. A transport failure fails
+ * open as `{valid: true}`, which this passes, leaving the write gate authoritative.
+ */
+export const dryRunRejection = (verdict: { valid: boolean; error?: string | null }): string | null =>
+  verdict.valid ? null : verdict.error?.trim() || "The proxy rejected this auto-router configuration";
+
 export interface TierDefinitionPayload {
   name: string;
   description?: string;
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 354d85a3cc9..fe0f6f4baba 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
@@ -36,6 +36,7 @@ import {
   hydrateCustomTierSet,
   hydratePlanModeMinTier,
   hydrateTierLabels,
+  dryRunRejection,
 } from "../add_model/build_complexity_router_config";
 import { KeywordTierRule } from "../add_model/KeywordTierRules";
 import { DEFAULT_MATCH_THRESHOLD } from "../add_model/SemanticKeywordMatching";
@@ -562,9 +563,10 @@ const EditAutoRouterModal: React.FC<EditAutoRouterModalProps> = ({
         { keywordTierRules, escalationKeywords, semanticMatchingEnabled, embeddingModel, matchThreshold },
       );
       const serverVerdict = await validateAutoRouterConfig(accessToken, updatedConfig, modelData?.model_info?.team_id);
-      if (!serverVerdict.valid && serverVerdict.error) {
+      const dryRunError = dryRunRejection(serverVerdict);
+      if (dryRunError) {
         setShowValidationErrors(true);
-        toast.fromError(serverVerdict.error);
+        toast.fromError(dryRunError);
         return;
       }