diff --git a/src/proxy.merge-routing.test.ts b/src/proxy.merge-routing.test.ts
new file mode 100644
index 0000000..9e19729
--- /dev/null
+++ b/src/proxy.merge-routing.test.ts
@@ -0,0 +1,125 @@
+import { describe, expect, it } from "vitest";
+import { mergeRoutingConfig, mergeTierRecord } from "./proxy.js";
+import { DEFAULT_ROUTING_CONFIG } from "./router/index.js";
+import type { Tier, TierConfig } from "./router/index.js";
+
+describe("mergeTierRecord", () => {
+  const baseTiers: Record<Tier, TierConfig> = {
+    SIMPLE: { primary: "model-a", fallback: ["model-b"] },
+    MEDIUM: { primary: "model-c", fallback: [] },
+    COMPLEX: { primary: "model-d", fallback: [] },
+    REASONING: { primary: "model-e", fallback: [] },
+  };
+
+  it("returns base when override is undefined", () => {
+    expect(mergeTierRecord(baseTiers, undefined)).toBe(baseTiers);
+  });
+
+  it("returns undefined when override is null (disables tier set)", () => {
+    expect(mergeTierRecord(baseTiers, null)).toBeUndefined();
+  });
+
+  it("shallow-merges override into base", () => {
+    const override: Record<Tier, TierConfig> = {
+      ...baseTiers,
+      SIMPLE: { primary: "custom-model", fallback: ["custom-fallback"] },
+    };
+    const result = mergeTierRecord(baseTiers, override);
+    expect(result!.SIMPLE.primary).toBe("custom-model");
+    expect(result!.MEDIUM.primary).toBe("model-c");
+  });
+
+  it("returns override when base is undefined", () => {
+    const override = baseTiers;
+    expect(mergeTierRecord(undefined, override)).toBe(override);
+  });
+});
+
+describe("mergeRoutingConfig", () => {
+  it("returns DEFAULT_ROUTING_CONFIG when no overrides provided", () => {
+    expect(mergeRoutingConfig()).toBe(DEFAULT_ROUTING_CONFIG);
+    expect(mergeRoutingConfig(undefined)).toBe(DEFAULT_ROUTING_CONFIG);
+  });
+
+  it("keeps default agenticTiers when not overridden", () => {
+    const result = mergeRoutingConfig({ overrides: DEFAULT_ROUTING_CONFIG.overrides });
+    expect(result.agenticTiers).toEqual(DEFAULT_ROUTING_CONFIG.agenticTiers);
+  });
+
+  it("disables agenticTiers when set to null", () => {
+    const result = mergeRoutingConfig({
+      agenticTiers: null as unknown as Record<Tier, TierConfig>,
+    });
+    expect(result.agenticTiers).toBeUndefined();
+  });
+
+  it("merges custom agenticTiers with defaults", () => {
+    const customSimple: TierConfig = {
+      primary: "custom/agentic-model",
+      fallback: ["custom/fallback"],
+    };
+    const result = mergeRoutingConfig({
+      agenticTiers: {
+        ...DEFAULT_ROUTING_CONFIG.agenticTiers!,
+        SIMPLE: customSimple,
+      },
+    });
+    expect(result.agenticTiers!.SIMPLE).toEqual(customSimple);
+    // Other tiers preserved from default
+    expect(result.agenticTiers!.COMPLEX).toEqual(DEFAULT_ROUTING_CONFIG.agenticTiers!.COMPLEX);
+  });
+
+  it("disables ecoTiers when set to null", () => {
+    const result = mergeRoutingConfig({
+      ecoTiers: null as unknown as Record<Tier, TierConfig>,
+    });
+    expect(result.ecoTiers).toBeUndefined();
+  });
+
+  it("merges custom ecoTiers with defaults", () => {
+    const customMedium: TierConfig = {
+      primary: "custom/eco-model",
+      fallback: [],
+    };
+    const result = mergeRoutingConfig({
+      ecoTiers: {
+        ...DEFAULT_ROUTING_CONFIG.ecoTiers!,
+        MEDIUM: customMedium,
+      },
+    });
+    expect(result.ecoTiers!.MEDIUM).toEqual(customMedium);
+    expect(result.ecoTiers!.SIMPLE).toEqual(DEFAULT_ROUTING_CONFIG.ecoTiers!.SIMPLE);
+  });
+
+  it("merges custom premiumTiers with defaults", () => {
+    const customComplex: TierConfig = {
+      primary: "custom/premium-model",
+      fallback: ["custom/premium-fallback"],
+    };
+    const result = mergeRoutingConfig({
+      premiumTiers: {
+        ...DEFAULT_ROUTING_CONFIG.premiumTiers!,
+        COMPLEX: customComplex,
+      },
+    });
+    expect(result.premiumTiers!.COMPLEX).toEqual(customComplex);
+    expect(result.premiumTiers!.SIMPLE).toEqual(DEFAULT_ROUTING_CONFIG.premiumTiers!.SIMPLE);
+  });
+
+  it("disables premiumTiers when set to null", () => {
+    const result = mergeRoutingConfig({
+      premiumTiers: null as unknown as Record<Tier, TierConfig>,
+    });
+    expect(result.premiumTiers).toBeUndefined();
+  });
+
+  it("still merges other fields correctly alongside tier overrides", () => {
+    const result = mergeRoutingConfig({
+      agenticTiers: null as unknown as Record<Tier, TierConfig>,
+      overrides: { ...DEFAULT_ROUTING_CONFIG.overrides, agenticMode: true },
+    });
+    expect(result.agenticTiers).toBeUndefined();
+    expect(result.overrides.agenticMode).toBe(true);
+    expect(result.tiers).toEqual(DEFAULT_ROUTING_CONFIG.tiers);
+  });
+});
diff --git a/src/proxy.ts b/src/proxy.ts
index abc17fd..fe34dfd 100644
--- a/src/proxy.ts
+++ b/src/proxy.ts
@@ -53,6 +53,7 @@ import {
   type RoutingConfig,
   type ModelPricing,
   type Tier,
+  type TierConfig,
 } from "./router/index.js";
 import { classifyByRules } from "./router/rules.js";
 import {
@@ -1284,7 +1285,17 @@ export function buildProxyModelList(
 /**
  * Merge partial routing config overrides with defaults.
  */
-function mergeRoutingConfig(overrides?: Partial<RoutingConfig>): RoutingConfig {
+export function mergeTierRecord(
+  base: Record<Tier, TierConfig> | undefined,
+  override: Record<Tier, TierConfig> | null | undefined,
+): Record<Tier, TierConfig> | undefined {
+  if (override === null) return undefined; // explicitly disabled
+  if (override === undefined) return base; // not provided, keep default
+  if (!base) return override; // no default, use override as-is
+  return { ...base, ...override };
+}
+
+export function mergeRoutingConfig(overrides?: Partial<RoutingConfig>): RoutingConfig {
   if (!overrides) return DEFAULT_ROUTING_CONFIG;
   return {
     ...DEFAULT_ROUTING_CONFIG,
@@ -1292,6 +1303,9 @@ function mergeRoutingConfig(overrides?: Partial<RoutingConfig>): RoutingConfig {
     classifier: { ...DEFAULT_ROUTING_CONFIG.classifier, ...overrides.classifier },
     scoring: { ...DEFAULT_ROUTING_CONFIG.scoring, ...overrides.scoring },
     tiers: { ...DEFAULT_ROUTING_CONFIG.tiers, ...overrides.tiers },
+    agenticTiers: mergeTierRecord(DEFAULT_ROUTING_CONFIG.agenticTiers, overrides.agenticTiers),
+    ecoTiers: mergeTierRecord(DEFAULT_ROUTING_CONFIG.ecoTiers, overrides.ecoTiers),
+    premiumTiers: mergeTierRecord(DEFAULT_ROUTING_CONFIG.premiumTiers, overrides.premiumTiers),
     overrides: { ...DEFAULT_ROUTING_CONFIG.overrides, ...overrides.overrides },
   };
 }
diff --git a/src/router/index.ts b/src/router/index.ts
index 0335eac..5603483 100644
--- a/src/router/index.ts
+++ b/src/router/index.ts
@@ -35,6 +35,7 @@ export { DEFAULT_ROUTING_CONFIG } from "./config.js";
 export type {
   RoutingDecision,
   Tier,
+  TierConfig,
   RoutingConfig,
   RouterOptions,
   RouterStrategy,