diff --git a/open-sse/services/autoCombo/__tests__/providerDiversity.test.ts b/open-sse/services/autoCombo/__tests__/providerDiversity.test.ts
new file mode 100644
index 0000000000..8fa4e2f57a
--- /dev/null
+++ b/open-sse/services/autoCombo/__tests__/providerDiversity.test.ts
@@ -0,0 +1,125 @@
+import { describe, it, beforeEach } from "node:test";
+import assert from "node:assert/strict";
+import {
+  recordProviderUsage,
+  calculateDiversityScore,
+  getProviderDiversityBoost,
+  getDiversityReport,
+  resetDiversity,
+  configureDiversity,
+} from "../providerDiversity";
+
+describe("providerDiversity", () => {
+  beforeEach(() => {
+    resetDiversity();
+  });
+
+  describe("calculateDiversityScore", () => {
+    it("returns 1.0 when no data is recorded", () => {
+      assert.equal(calculateDiversityScore(), 1.0);
+    });
+
+    it("returns 0.0 when all requests go to one provider", () => {
+      for (let i = 0; i < 20; i++) {
+        recordProviderUsage("claude");
+      }
+      assert.equal(calculateDiversityScore(), 0.0);
+    });
+
+    it("returns 1.0 for perfectly even distribution across 2 providers", () => {
+      for (let i = 0; i < 10; i++) {
+        recordProviderUsage("claude");
+        recordProviderUsage("openai");
+      }
+      assert.equal(calculateDiversityScore(), 1.0);
+    });
+
+    it("returns value between 0 and 1 for uneven distribution", () => {
+      for (let i = 0; i < 15; i++) recordProviderUsage("claude");
+      for (let i = 0; i < 5; i++) recordProviderUsage("openai");
+
+      const score = calculateDiversityScore();
+      assert.ok(score > 0, "should be > 0 (not single provider)");
+      assert.ok(score < 1, "should be < 1 (not perfectly even)");
+    });
+
+    it("higher entropy with more providers", () => {
+      // 2 providers
+      resetDiversity();
+      for (let i = 0; i < 10; i++) {
+        recordProviderUsage("claude");
+        recordProviderUsage("openai");
+      }
+      const score2 = calculateDiversityScore();
+
+      // 4 providers (same total requests)
+      resetDiversity();
+      for (let i = 0; i < 5; i++) {
+        recordProviderUsage("claude");
+        recordProviderUsage("openai");
+        recordProviderUsage("google");
+        recordProviderUsage("together");
+      }
+      const score4 = calculateDiversityScore();
+
+      // Both should be 1.0 (perfectly distributed within their pool)
+      assert.equal(score2, 1.0);
+      assert.equal(score4, 1.0);
+    });
+  });
+
+  describe("getProviderDiversityBoost", () => {
+    it("returns 0.5 when no data is recorded", () => {
+      assert.equal(getProviderDiversityBoost("claude"), 0.5);
+    });
+
+    it("returns low boost for heavily used provider", () => {
+      for (let i = 0; i < 18; i++) recordProviderUsage("claude");
+      for (let i = 0; i < 2; i++) recordProviderUsage("openai");
+
+      const claudeBoost = getProviderDiversityBoost("claude");
+      const openaiBoost = getProviderDiversityBoost("openai");
+
+      assert.ok(claudeBoost < openaiBoost, "heavily used provider should have lower boost");
+      assert.ok(claudeBoost < 0.2, "90% used provider should have very low boost");
+      assert.ok(openaiBoost > 0.8, "10% used provider should have high boost");
+    });
+
+    it("returns 1.0 for never-used provider", () => {
+      for (let i = 0; i < 10; i++) recordProviderUsage("claude");
+
+      const boost = getProviderDiversityBoost("google");
+      assert.equal(boost, 1.0);
+    });
+  });
+
+  describe("getDiversityReport", () => {
+    it("returns structured report", () => {
+      recordProviderUsage("claude");
+      recordProviderUsage("claude");
+      recordProviderUsage("openai");
+
+      const report = getDiversityReport();
+
+      assert.equal(report.totalRequests, 3);
+      assert.ok(report.score > 0);
+      assert.ok(report.score < 1);
+      assert.equal(report.providers["claude"].count, 2);
+      assert.equal(report.providers["openai"].count, 1);
+      assert.ok(Math.abs(report.providers["claude"].share - 2 / 3) < 0.01);
+    });
+  });
+
+  describe("window management", () => {
+    it("respects windowSize limit", () => {
+      configureDiversity({ windowSize: 10, ttlMs: 3_600_000 });
+
+      for (let i = 0; i < 20; i++) {
+        recordProviderUsage("claude");
+      }
+
+      const report = getDiversityReport();
+      assert.ok(report.totalRequests <= 10, "should not exceed window size");
+    });
+  });
+});
diff --git a/open-sse/services/autoCombo/providerDiversity.ts b/open-sse/services/autoCombo/providerDiversity.ts
new file mode 100644
index 0000000000..eda6076737
--- /dev/null
+++ b/open-sse/services/autoCombo/providerDiversity.ts
@@ -0,0 +1,170 @@
+/**
+ * Provider Diversity Tracking via Shannon Entropy
+ *
+ * Measures and tracks how evenly distributed requests are across providers.
+ * A system routing 90% of traffic to one provider has a catastrophic single
+ * point of failure. This module provides a diversity score [0..1] that can
+ * be used as a scoring factor in auto-combo selection.
+ *
+ * Shannon entropy normalized to [0..1]:
+ *   - 0.0 = all requests go to one provider (maximum risk)
+ *   - 1.0 = perfectly even distribution (minimum risk)
+ *
+ * @see https://en.wikipedia.org/wiki/Entropy_(information_theory)
+ */
+
+/** Rolling window entry for provider usage tracking */
+interface UsageEntry {
+  provider: string;
+  timestamp: number;
+}
+
+/** Configuration for the diversity tracker */
+export interface DiversityConfig {
+  /** Maximum entries in the rolling window (default: 200) */
+  windowSize: number;
+  /** Time-to-live in ms for entries — older entries are pruned (default: 1 hour) */
+  ttlMs: number;
+}
+
+const DEFAULT_CONFIG: DiversityConfig = {
+  windowSize: 200,
+  ttlMs: 3_600_000, // 1 hour
+};
+
+/** In-memory rolling window of recent provider usage */
+let usageWindow: UsageEntry[] = [];
+let config: DiversityConfig = { ...DEFAULT_CONFIG };
+
+/**
+ * Configure the diversity tracker.
+ */
+export function configureDiversity(userConfig: Partial<DiversityConfig>): void {
+  config = { ...DEFAULT_CONFIG, ...userConfig };
+}
+
+/**
+ * Record that a provider was used for a request.
+ * Call this after a successful request completes.
+ */
+export function recordProviderUsage(provider: string): void {
+  const now = Date.now();
+
+  usageWindow.push({ provider, timestamp: now });
+
+  // Prune by window size
+  if (usageWindow.length > config.windowSize) {
+    usageWindow = usageWindow.slice(-config.windowSize);
+  }
+
+  // Prune by TTL
+  const cutoff = now - config.ttlMs;
+  usageWindow = usageWindow.filter((e) => e.timestamp >= cutoff);
+}
+
+/**
+ * Calculate Shannon entropy normalized to [0..1] for the current usage window.
+ *
+ * @returns Normalized entropy where 0 = single provider, 1 = perfect distribution
+ */
+export function calculateDiversityScore(): number {
+  if (usageWindow.length === 0) return 1.0; // No data = assume diverse
+
+  const now = Date.now();
+  const cutoff = now - config.ttlMs;
+  const recent = usageWindow.filter((e) => e.timestamp >= cutoff);
+
+  if (recent.length === 0) return 1.0;
+
+  // Count occurrences per provider
+  const counts = new Map<string, number>();
+  for (const entry of recent) {
+    counts.set(entry.provider, (counts.get(entry.provider) || 0) + 1);
+  }
+
+  const total = recent.length;
+  const nUnique = counts.size;
+
+  if (nUnique <= 1) return 0.0;
+
+  // Shannon entropy
+  let entropy = 0;
+  for (const count of counts.values()) {
+    const p = count / total;
+    entropy -= p * Math.log2(p);
+  }
+
+  // Normalize by maximum possible entropy
+  const maxEntropy = Math.log2(nUnique);
+  return maxEntropy > 0 ? entropy / maxEntropy : 0;
+}
+
+/**
+ * Get the diversity score for a specific provider.
+ * Returns a boost value [0..1] where underrepresented providers score higher.
+ * This can be used as a per-candidate factor in auto-combo scoring.
+ *
+ * @param provider - The provider to score
+ * @returns Diversity boost where 1.0 = never used (maximum boost), 0.0 = most used
+ */
+export function getProviderDiversityBoost(provider: string): number {
+  if (usageWindow.length === 0) return 0.5; // No data = neutral
+
+  const now = Date.now();
+  const cutoff = now - config.ttlMs;
+  const recent = usageWindow.filter((e) => e.timestamp >= cutoff);
+
+  if (recent.length === 0) return 0.5;
+
+  const total = recent.length;
+  const providerCount = recent.filter((e) => e.provider === provider).length;
+
+  // Inverse usage share: providers used less get higher boost
+  const usageShare = providerCount / total;
+  return Math.max(0, 1 - usageShare);
+}
+
+/**
+ * Get a summary of the current provider distribution.
+ * Useful for dashboard display and debugging.
+ */
+export function getDiversityReport(): {
+  score: number;
+  totalRequests: number;
+  providers: Record<string, { count: number; share: number }>;
+  windowSize: number;
+  ttlMs: number;
+} {
+  const now = Date.now();
+  const cutoff = now - config.ttlMs;
+  const recent = usageWindow.filter((e) => e.timestamp >= cutoff);
+
+  const counts = new Map<string, number>();
+  for (const entry of recent) {
+    counts.set(entry.provider, (counts.get(entry.provider) || 0) + 1);
+  }
+
+  const providers: Record<string, { count: number; share: number }> = {};
+  for (const [provider, count] of counts) {
+    providers[provider] = {
+      count,
+      share: recent.length > 0 ? count / recent.length : 0,
+    };
+  }
+
+  return {
+    score: calculateDiversityScore(),
+    totalRequests: recent.length,
+    providers,
+    windowSize: config.windowSize,
+    ttlMs: config.ttlMs,
+  };
+}
+
+/**
+ * Reset the diversity tracker. Useful for testing.
+ */
+export function resetDiversity(): void {
+  usageWindow = [];
+  config = { ...DEFAULT_CONFIG };
+}