diff --git a/README.md b/README.md
index e23f52a..8856f48 100644
--- a/README.md
+++ b/README.md
@@ -91,6 +91,24 @@ const client = new RelayClient(relayerUrl, chainId, wallet, builderConfig);
 const proxyClient = new RelayClient(relayerUrl, chainId, wallet, builderConfig, RelayerTxType.PROXY);
 ```
 
+### With Relayer API-Key Authentication
+
+The simplest auth path — uses the `RELAYER_API_KEY` + `RELAYER_API_KEY_ADDRESS` HTTP headers documented at [docs.polymarket.com/trading/gasless](https://docs.polymarket.com/trading/gasless). Obtain the key from polymarket.com's Settings > API Keys; the address is the EOA that owns the key.
+
+```typescript
+import { RelayClient, RelayerTxType } from "@polymarket/builder-relayer-client";
+
+const apiKeyCreds = {
+  apiKey: process.env.RELAYER_API_KEY!,
+  apiKeyAddress: process.env.RELAYER_API_KEY_ADDRESS!,
+};
+
+// Pass `apiKeyCreds` as the 6th constructor argument. The 4th (`builderConfig`)
+// can stay `undefined` — when `apiKeyCreds` is set it takes precedence over
+// any HMAC `BuilderConfig` for authenticated requests.
+const client = new RelayClient(relayerUrl, chainId, wallet, undefined, RelayerTxType.SAFE, apiKeyCreds);
+```
+
 ## Examples
 
 ### Execute ERC20 Approval Transaction
diff --git a/package.json b/package.json
index b5257a1..33a18be 100644
--- a/package.json
+++ b/package.json
@@ -13,6 +13,8 @@
         "test": "make test"
     },
     "devDependencies": {
+        "@ethersproject/providers": "5.8.0",
+        "@ethersproject/wallet": "5.8.0",
         "@types/chai": "5.2.2",
         "@types/mocha": "10.0.10",
         "@types/node": "^18.7.18",
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 79f6c6b..8df1e33 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -33,6 +33,12 @@ importers:
         specifier: ^2.31.4
         version: 2.38.5(typescript@5.9.3)
     devDependencies:
+      '@ethersproject/providers':
+        specifier: 5.8.0
+        version: 5.8.0
+      '@ethersproject/wallet':
+        specifier: 5.8.0
+        version: 5.8.0
       '@types/chai':
         specifier: 5.2.2
         version: 5.2.2
diff --git a/src/client.ts b/src/client.ts
index 32b8603..235cf86 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -17,6 +17,7 @@ import {
     OperationType,
     ProxyTransaction,
     ProxyTransactionArgs,
+    RelayerApiKeyCreds,
     RelayerTransaction,
     RelayerTransactionResponse,
     RelayerTxType,
@@ -67,12 +68,15 @@ export class RelayClient {
 
     readonly builderConfig?: BuilderConfig;
 
+    readonly apiKeyCreds?: RelayerApiKeyCreds;
+
     constructor(
         relayerUrl: string,
         chainId: number,
         signer?: Wallet | JsonRpcSigner | WalletClient,
         builderConfig?: BuilderConfig,
         relayTxType?: RelayerTxType,
+        apiKeyCreds?: RelayerApiKeyCreds,
     ) {
         this.relayerUrl = relayerUrl.endsWith("/") ? relayerUrl.slice(0, -1) : relayerUrl;
         this.chainId = chainId;
@@ -82,7 +86,7 @@ export class RelayClient {
         this.relayTxType = relayTxType;
         this.contractConfig = getContractConfig(chainId);
         this.httpClient = new HttpClient();
-        
+
         if (signer != undefined) {
             this.signer = createAbstractSigner(chainId, signer);
         }
@@ -90,6 +94,10 @@ export class RelayClient {
         if (builderConfig !== undefined) {
             this.builderConfig = builderConfig;
         }
+
+        if (apiKeyCreds !== undefined) {
+            this.apiKeyCreds = apiKeyCreds;
+        }
     }
 
     public async getNonce(signerAddress: string, signerType: string): Promise<NoncePayload> {
@@ -431,16 +439,35 @@ export class RelayClient {
         method: string,
         path: string,
         body?: string
-    ): Promise<any> {        
+    ): Promise<any> {
+        // API-key auth (preferred when configured) — see
+        // https://docs.polymarket.com/trading/gasless. Empirically required
+        // for CTF `mergePositions` on post-V2-cutover SAFEs: HMAC builder
+        // auth round-trips into STATE_FAILED at gas estimation, while the
+        // API-key path is accepted by the relayer.
+        if (this.canApiKeyAuth()) {
+            return this.send(
+                path,
+                method,
+                {
+                    headers: {
+                        RELAYER_API_KEY: this.apiKeyCreds!.apiKey,
+                        RELAYER_API_KEY_ADDRESS: this.apiKeyCreds!.apiKeyAddress,
+                    },
+                    data: body,
+                }
+            );
+        }
+
         // builders auth
         if (this.canBuilderAuth()) {
             const builderHeaders = await this._generateBuilderHeaders(method, path, body);
             if (builderHeaders !== undefined) {
                 return this.send(
                     path,
-                    method, 
+                    method,
                     { headers: builderHeaders, data: body }
-                );    
+                );
             }
         }
 
@@ -475,6 +502,14 @@ export class RelayClient {
         return (this.builderConfig != undefined && this.builderConfig.isValid());
     }
 
+    private canApiKeyAuth(): boolean {
+        return (
+            this.apiKeyCreds != undefined
+            && !!this.apiKeyCreds.apiKey
+            && !!this.apiKeyCreds.apiKeyAddress
+        );
+    }
+
     private async send(
         endpoint: string,
         method: string,
diff --git a/src/types.ts b/src/types.ts
index daa09af..b2a269c 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -157,6 +157,16 @@ export interface GetDeployedResponse {
     deployed: boolean;
 }
 
+// Auth: simple API-key header path (per
+// https://docs.polymarket.com/trading/gasless). Alternative to HMAC
+// `BuilderConfig` — pass an instance of these to the `RelayClient` constructor
+// to authenticate via `RELAYER_API_KEY` + `RELAYER_API_KEY_ADDRESS` headers.
+// Obtained from polymarket.com Settings > API Keys.
+export interface RelayerApiKeyCreds {
+    apiKey: string;
+    apiKeyAddress: string;
+}
+
 // Deposit Wallet types
 
 export interface DepositWalletCall {