diff --git a/src/shared/utils/apiKeyPolicy.ts b/src/shared/utils/apiKeyPolicy.ts
index 17530ced45..52a319e4cc 100644
--- a/src/shared/utils/apiKeyPolicy.ts
+++ b/src/shared/utils/apiKeyPolicy.ts
@@ -13,11 +13,13 @@ import { getApiKeyMetadata, getComboByName, isModelAllowedForKey } from "@/lib/l
 import { resolveComboForModel } from "@/lib/db/modelComboMappings";
 import { checkBudget } from "@/domain/costRules";
 import { checkTokenLimits } from "@omniroute/open-sse/services/tokenLimitCounter.ts";
-import { errorResponse } from "@omniroute/open-sse/utils/error.ts";
+import { errorResponse, buildErrorBody } from "@omniroute/open-sse/utils/error.ts";
 import { HTTP_STATUS } from "@omniroute/open-sse/config/constants.ts";
 import * as log from "@/sse/utils/logger";
 import { checkRateLimit, RateLimitRule } from "./rateLimiter";
 import { resolveEndpointCategory } from "@/shared/constants/endpointCategories";
+import { resolveQuotaKeyScope } from "@/lib/quota/quotaKey";
+import { parseModel } from "@omniroute/open-sse/services/model.ts";
 
 // Default to no per-key request cap. API keys can still opt into explicit
 // limits via Settings/API Manager, while provider/account quota controls remain
@@ -63,6 +65,7 @@ export interface ApiKeyMetadata {
   allowedModels?: string[];
   allowedCombos?: string[];
   allowedConnections?: string[];
+  allowedQuotas?: string[];
   noLog?: boolean;
   autoResolve?: boolean;
   budget?: number;
@@ -317,9 +320,49 @@ export async function enforceApiKeyPolicy(
     }
   }
 
-  // ── Check 3: Model restriction ──
+  // ── Check 3: Quota-exclusive enforcement (Phase A3) ──
+  //
+  // When a key has allowedQuotas, its access is governed entirely by its
+  // pools' providers — normal allowedModels/allowedCombos checks are skipped.
+  if (modelStr && apiKeyInfo.allowedQuotas && apiKeyInfo.allowedQuotas.length > 0) {
+    try {
+      const scope = await resolveQuotaKeyScope(apiKeyInfo.allowedQuotas);
+      const { provider } = parseModel(modelStr);
+      if (provider === null || scope.providers.length === 0 || !scope.providers.includes(provider)) {
+        const quotaBody = buildErrorBody(
+          HTTP_STATUS.FORBIDDEN,
+          `Model "${modelStr}" is not allowed for this quota-exclusive API key`
+        );
+        quotaBody.error.code = "QUOTA_ONLY";
+        return {
+          apiKey,
+          apiKeyInfo,
+          rejection: new Response(JSON.stringify(quotaBody), {
+            status: HTTP_STATUS.FORBIDDEN,
+            headers: { "Content-Type": "application/json" },
+          }),
+        };
+      }
+      // Provider is in scope — quota key governed by pools; skip allowedModels/allowedCombos.
+      // Continue to budget / rate-limit checks below.
+    } catch (error) {
+      log.error("API_POLICY", "Quota scope check failed. Request blocked.", { error });
+      return {
+        apiKey,
+        apiKeyInfo,
+        rejection: errorResponse(
+          HTTP_STATUS.SERVICE_UNAVAILABLE,
+          "API key quota policy unavailable"
+        ),
+      };
+    }
+  }
+
+  // ── Check 4: Model restriction (skipped when allowedQuotas governs access) ──
   let requestedComboName: string | null = null;
-  if (modelStr && apiKeyInfo.allowedCombos && apiKeyInfo.allowedCombos.length > 0) {
+  const isQuotaExclusive =
+    Boolean(apiKeyInfo.allowedQuotas) && (apiKeyInfo.allowedQuotas as string[]).length > 0;
+  if (!isQuotaExclusive && modelStr && apiKeyInfo.allowedCombos && apiKeyInfo.allowedCombos.length > 0) {
     try {
       const comboAccess = await isComboAllowedForKey(apiKeyInfo.allowedCombos, modelStr);
       requestedComboName = comboAccess.comboName;
@@ -346,7 +389,8 @@ export async function enforceApiKeyPolicy(
     }
   }
 
-  const hasModelRestrictions = apiKeyInfo.allowedModels && apiKeyInfo.allowedModels.length > 0;
+  const hasModelRestrictions =
+    !isQuotaExclusive && apiKeyInfo.allowedModels && apiKeyInfo.allowedModels.length > 0;
 
   if (!requestedComboName && modelStr && hasModelRestrictions) {
     try {
diff --git a/tests/unit/apikeypolicy-quota-only.test.ts b/tests/unit/apikeypolicy-quota-only.test.ts
new file mode 100644
index 0000000000..ee4812e7d2
--- /dev/null
+++ b/tests/unit/apikeypolicy-quota-only.test.ts
@@ -0,0 +1,213 @@
+/**
+ * tests/unit/apikeypolicy-quota-only.test.ts
+ *
+ * TDD coverage for Phase A3: quota-exclusive enforcement in enforceApiKeyPolicy.
+ *
+ * Cases:
+ * 1. Key with allowedQuotas pointing to a "codex" connection → requesting a
+ *    "cx/…" (codex) model → ALLOWED (no rejection).
+ * 2. Same key → requesting "openai/gpt-4.1" → REJECTED 403 QUOTA_ONLY.
+ * 3. Key with empty allowedQuotas → existing behavior unchanged (allowed model passes).
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { pathToFileURL } from "node:url";
+
+const TEST_DATA_DIR = fs.mkdtempSync(
+  path.join(os.tmpdir(), "omniroute-apikeypolicy-quota-only-")
+);
+process.env.DATA_DIR = TEST_DATA_DIR;
+process.env.API_KEY_SECRET = process.env.API_KEY_SECRET || "quota-only-test-secret";
+
+// Import DB modules using top-level await (Node test runner supports ESM TLA)
+const coreDb = await import("../../src/lib/db/core.ts");
+const apiKeysDb = await import("../../src/lib/db/apiKeys.ts");
+const poolsDb = await import("../../src/lib/db/quotaPools.ts");
+const providersDb = await import("../../src/lib/db/providers.ts");
+const rateLimiter = await import("../../src/shared/utils/rateLimiter.ts");
+
+rateLimiter.setRateLimiterTestMode(true);
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+async function resetStorage() {
+  apiKeysDb.resetApiKeyState();
+  coreDb.resetDbInstance();
+
+  for (let attempt = 0; attempt < 10; attempt++) {
+    try {
+      if (fs.existsSync(TEST_DATA_DIR)) {
+        fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+      }
+      break;
+    } catch (error: unknown) {
+      const err = error as NodeJS.ErrnoException;
+      if ((err?.code === "EBUSY" || err?.code === "EPERM") && attempt < 9) {
+        await new Promise((resolve) => setTimeout(resolve, 50 * (attempt + 1)));
+      } else {
+        throw error;
+      }
+    }
+  }
+  fs.mkdirSync(TEST_DATA_DIR, { recursive: true });
+}
+
+async function loadPolicy(label: string) {
+  const modulePath = path.join(process.cwd(), "src/shared/utils/apiKeyPolicy.ts");
+  return import(`${pathToFileURL(modulePath).href}?case=${label}-${Date.now()}`);
+}
+
+function makeRequest(apiKey: string | null) {
+  return new Request("http://localhost/api/v1/chat/completions", {
+    method: "POST",
+    headers: apiKey ? { Authorization: `Bearer ${apiKey}` } : {},
+  });
+}
+
+async function readBody(response: Response) {
+  return response.json() as Promise<{ error: { message: string; code: string } }>;
+}
+
+// ---------------------------------------------------------------------------
+// Lifecycle
+// ---------------------------------------------------------------------------
+
+test.beforeEach(async () => {
+  await resetStorage();
+});
+
+test.after(async () => {
+  apiKeysDb.resetApiKeyState();
+  coreDb.resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+});
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+test("quota-only key requesting a model in the allowed provider is passed through", async () => {
+  // Seed a provider connection with provider "codex" (matches "cx/gpt-5.5")
+  const conn = await providersDb.createProviderConnection({
+    provider: "codex",
+    authType: "apikey",
+    name: "quota-only-codex-conn",
+    apiKey: "sk-codex-quota-only",
+  });
+  const connId = (conn as Record<string, unknown>).id as string;
+  assert.ok(connId);
+
+  // Seed a pool referencing that connection
+  const pool = poolsDb.createPool({ connectionId: connId, name: "Codex Pool A3" });
+
+  // Create an API key bound to that pool
+  const created = await apiKeysDb.createApiKey("Quota-Only Key", "machine-a3");
+  await apiKeysDb.updateApiKeyPermissions(created.id, {
+    allowedQuotas: [pool.id],
+  });
+
+  const policy = await loadPolicy("quota-only-allowed");
+
+  // "cx/gpt-5.5" → provider resolves to "codex" (cx alias) — should be allowed
+  const result = await policy.enforceApiKeyPolicy(makeRequest(created.key), "cx/gpt-5.5");
+  assert.equal(
+    result.rejection,
+    null,
+    "quota-exclusive key should pass through for a model in its pools' providers"
+  );
+});
+
+test("quota-only key requesting a model outside allowed providers is rejected 403 QUOTA_ONLY", async () => {
+  // Seed a provider connection with provider "codex"
+  const conn = await providersDb.createProviderConnection({
+    provider: "codex",
+    authType: "apikey",
+    name: "quota-only-codex-conn-reject",
+    apiKey: "sk-codex-quota-only-reject",
+  });
+  const connId = (conn as Record<string, unknown>).id as string;
+  const pool = poolsDb.createPool({ connectionId: connId, name: "Codex Pool A3 Reject" });
+
+  const created = await apiKeysDb.createApiKey("Quota-Only Key Reject", "machine-a3-reject");
+  await apiKeysDb.updateApiKeyPermissions(created.id, {
+    allowedQuotas: [pool.id],
+  });
+
+  const policy = await loadPolicy("quota-only-rejected");
+
+  // "openai/gpt-4.1" → provider "openai" is NOT in the codex pool scope → reject
+  const result = await policy.enforceApiKeyPolicy(makeRequest(created.key), "openai/gpt-4.1");
+
+  assert.ok(result.rejection, "should produce a rejection Response");
+  assert.equal(result.rejection.status, 403, "rejection should be 403 Forbidden");
+
+  const body = await readBody(result.rejection);
+  assert.equal(body.error.code, "QUOTA_ONLY", "error code should be QUOTA_ONLY");
+  assert.match(
+    body.error.message,
+    /not allowed for this quota-exclusive API key/,
+    "error message should mention quota-exclusive"
+  );
+  // Must not leak stack traces
+  assert.ok(!body.error.message.includes(" at "), "message must not contain stack trace");
+});
+
+test("quota-only key whose allowedQuotas references a non-existent pool is rejected 403 QUOTA_ONLY (fail-closed)", async () => {
+  // Create an API key bound to a pool ID that does not exist in the DB (dangling reference)
+  const created = await apiKeysDb.createApiKey("Dangling Quota Key", "machine-a3-dangling");
+  await apiKeysDb.updateApiKeyPermissions(created.id, {
+    allowedQuotas: ["non-existent-pool-id-00000000"],
+  });
+
+  const policy = await loadPolicy("quota-dangling-pool");
+
+  // resolveQuotaKeyScope will return empty providers for the dangling pool id → fail-closed
+  const result = await policy.enforceApiKeyPolicy(makeRequest(created.key), "openai/gpt-4.1");
+
+  assert.ok(result.rejection, "should produce a rejection Response for a dangling pool");
+  assert.equal(result.rejection.status, 403, "rejection should be 403 Forbidden");
+
+  const body = await readBody(result.rejection);
+  assert.equal(body.error.code, "QUOTA_ONLY", "error code should be QUOTA_ONLY");
+  assert.match(
+    body.error.message,
+    /not allowed for this quota-exclusive API key/,
+    "error message should mention quota-exclusive"
+  );
+  assert.ok(!body.error.message.includes(" at "), "message must not contain stack trace");
+});
+
+test("key with empty allowedQuotas is subject to normal model restriction checks", async () => {
+  // A key with no allowedQuotas but with allowedModels = ["openai/*"]
+  const created = await apiKeysDb.createApiKey("Normal Key", "machine-a3-normal");
+  await apiKeysDb.updateApiKeyPermissions(created.id, {
+    allowedModels: ["openai/gpt-4.1"],
+  });
+
+  const policy = await loadPolicy("quota-empty-normal");
+
+  // Allowed model should pass
+  const allowed = await policy.enforceApiKeyPolicy(makeRequest(created.key), "openai/gpt-4.1");
+  assert.equal(
+    allowed.rejection,
+    null,
+    "model in allowedModels should pass for a non-quota key"
+  );
+
+  // Disallowed model should be rejected via the normal allowedModels path
+  const blocked = await policy.enforceApiKeyPolicy(makeRequest(created.key), "anthropic/claude-3-7-sonnet");
+  assert.ok(blocked.rejection, "disallowed model should be rejected");
+  assert.equal(blocked.rejection.status, 403);
+
+  const body = await readBody(blocked.rejection);
+  assert.match(body.error.message, /not allowed for this API key/);
+  // The code for this case comes from errorConfig (403 → "insufficient_quota")
+  // rather than QUOTA_ONLY — confirming paths are separate
+  assert.notEqual(body.error.code, "QUOTA_ONLY", "normal key rejection must NOT use QUOTA_ONLY code");
+});