diff --git a/packages/haystack/README.md b/packages/haystack/README.md
index 9fd22ff..f7df137 100644
--- a/packages/haystack/README.md
+++ b/packages/haystack/README.md
@@ -20,6 +20,8 @@ TypeScript/JavaScript SDK for [Haystack Order Router](https://txnlab.gitbook.io/
 | **Free** | `1b72df7e-1131-4449-8ce1-29b79dd3f51e` | 60 requests/min | Development, testing, low-volume integrations |
 | **Production** | Request from [support@txnlab.dev](mailto:support@txnlab.dev) | Higher limits | Production applications |
 
+The SDK sends the key in the `X-API-Key` header, never in the URL, so it stays out of server logs, browser history and caches.
+
 ## Installation
 
 ```bash
@@ -413,7 +415,7 @@ new RouterClient(config: ConfigParams)
 
 | Option            | Description                                                  | Type                  | Default                                |
 | ----------------- | ------------------------------------------------------------ | --------------------- | -------------------------------------- |
-| `apiKey`          | Your Haystack Router API key                                          | `string`              | **required**                           |
+| `apiKey`          | Your Haystack Router API key, sent in the `X-API-Key` header          | `string`              | **required**                           |
 | `apiBaseUrl`      | Base URL for the Haystack Router API                                  | `string`              | `https://hayrouter.txnlab.dev`            |
 | `algodUri`        | Algod node URI                                               | `string`              | `https://mainnet-api.4160.nodely.dev/` |
 | `algodToken`      | Algod node token                                             | `string`              | `''`                                   |
diff --git a/packages/haystack/src/client.ts b/packages/haystack/src/client.ts
index 5dd84ad..6648faf 100644
--- a/packages/haystack/src/client.ts
+++ b/packages/haystack/src/client.ts
@@ -1,6 +1,7 @@
 import { Algodv2, isValidAddress, type TransactionSigner } from 'algosdk'
 import { SwapComposer, type SignerFunction } from './composer'
 import {
+  API_KEY_HEADER,
   DEFAULT_ALGOD_PORT,
   DEFAULT_ALGOD_TOKEN,
   DEFAULT_ALGOD_URI,
@@ -189,7 +190,6 @@ export class RouterClient {
 
     const url = new URL(`${this.config.apiBaseUrl}/fetchQuote`)
 
-    url.searchParams.append('apiKey', this.config.apiKey)
     url.searchParams.append('algodUri', this.config.algodUri)
     url.searchParams.append('algodToken', this.config.algodToken)
     url.searchParams.append('algodPort', String(this.config.algodPort))
@@ -214,7 +214,9 @@ export class RouterClient {
       url.searchParams.append('referrerAddress', this.config.referrerAddress)
     }
 
-    return request<FetchQuoteResponse>(url.toString())
+    return request<FetchQuoteResponse>(url.toString(), {
+      headers: this.authHeaders(),
+    })
   }
 
   /**
@@ -283,7 +285,6 @@ export class RouterClient {
     const url = new URL(`${this.config.apiBaseUrl}/fetchExecuteSwapTxns`)
 
     const body: FetchSwapTxnsBody = {
-      apiKey: this.config.apiKey,
       address,
       txnPayloadJSON: quote.txnPayload,
       slippage,
@@ -293,11 +294,21 @@ export class RouterClient {
       method: 'POST',
       headers: {
         'Content-Type': 'application/json',
+        ...this.authHeaders(),
       },
       body: JSON.stringify(body),
     })
   }
 
+  /**
+   * Headers that authenticate a request with the API key. The key goes in the
+   * `X-API-Key` header rather than the URL, where it would end up in logs, browser
+   * history and CDN caches. The API refuses a secret key (`hay_sk_`) in the URL.
+   */
+  private authHeaders(): Record<string, string> {
+    return { [API_KEY_HEADER]: this.config.apiKey }
+  }
+
   /**
    * Fetch a quote and return an enhanced quote result
    *
diff --git a/packages/haystack/src/constants.ts b/packages/haystack/src/constants.ts
index df40d0e..9bc211b 100644
--- a/packages/haystack/src/constants.ts
+++ b/packages/haystack/src/constants.ts
@@ -30,6 +30,9 @@ export const DEFAULT_ALGOD_PORT = 443
 /** Default Haystack Router API base URL */
 export const DEFAULT_API_BASE_URL = 'https://hayrouter.txnlab.dev/api'
 
+/** HTTP header that carries the Haystack Router API key */
+export const API_KEY_HEADER = 'X-API-Key'
+
 /** Default fee in basis points (0.10%) */
 export const DEFAULT_FEE_BPS = 10
 
diff --git a/packages/haystack/src/logger.ts b/packages/haystack/src/logger.ts
index 59cbe45..1e6c726 100644
--- a/packages/haystack/src/logger.ts
+++ b/packages/haystack/src/logger.ts
@@ -89,7 +89,11 @@ export class Logger {
 
         for (const [key, value] of Object.entries(data)) {
           // Redact sensitive fields
-          if (key === 'apiKey' || key === 'algodToken') {
+          if (
+            key === 'apiKey' ||
+            key === 'algodToken' ||
+            key.toLowerCase() === 'x-api-key'
+          ) {
             sanitized[key] = '[REDACTED]'
             continue
           }
diff --git a/packages/haystack/src/types.ts b/packages/haystack/src/types.ts
index f42e272..9f8e376 100644
--- a/packages/haystack/src/types.ts
+++ b/packages/haystack/src/types.ts
@@ -365,8 +365,6 @@ export interface FetchSwapTxnsParams {
  * @internal
  */
 export interface FetchSwapTxnsBody {
-  /** API key for authentication */
-  readonly apiKey: string
   /** Address of the account that will sign the transactions */
   readonly address: string
   /** Encrypted transaction payload from the quote */
diff --git a/packages/haystack/tests/client.test.ts b/packages/haystack/tests/client.test.ts
index f2c3b94..ef8ac0f 100644
--- a/packages/haystack/tests/client.test.ts
+++ b/packages/haystack/tests/client.test.ts
@@ -135,6 +135,18 @@ describe('RouterClient', () => {
       expect(mockRequest).toHaveBeenCalledOnce()
     })
 
+    it('should send the API key in the X-API-Key header, not the URL', async () => {
+      mockRequest.mockResolvedValue({ quote: '1', route: [] })
+
+      await client.fetchQuote({ fromASAID: 0, toASAID: 31566704, amount: 1 })
+
+      const [url, options] = mockRequest.mock.calls?.[0] as
+        [string, RequestInit] | []
+      expect(new URL(url as string).searchParams.has('apiKey')).toBe(false)
+      expect(url).not.toContain('test-api-key')
+      expect(options?.headers).toEqual({ 'X-API-Key': 'test-api-key' })
+    })
+
     it('should include all query parameters in the request', async () => {
       const mockQuote: Partial<FetchQuoteResponse> = {
         fromASAID: 0,
@@ -761,9 +773,13 @@ describe('RouterClient', () => {
         [string, RequestInit] | []
       expect(url).toContain('fetchExecuteSwapTxns')
       expect(options?.method).toBe('POST')
-      expect(options?.headers).toEqual({ 'Content-Type': 'application/json' })
+      expect(options?.headers).toEqual({
+        'Content-Type': 'application/json',
+        'X-API-Key': 'test-api-key',
+      })
 
       const body = JSON.parse(options?.body as string)
+      expect(body).not.toHaveProperty('apiKey')
       expect(body.address).toBe(validAddress)
       expect(body.slippage).toBe(1.0)
       expect(body.txnPayloadJSON).toEqual(mockQuote.txnPayload)
diff --git a/packages/haystack/tests/logger.test.ts b/packages/haystack/tests/logger.test.ts
index 7eb5e3e..69a1478 100644
--- a/packages/haystack/tests/logger.test.ts
+++ b/packages/haystack/tests/logger.test.ts
@@ -148,6 +148,19 @@ describe('Logger', () => {
       expect(calls[0][1]).toEqual({ apiKey: '[REDACTED]', other: 'visible' })
     })
 
+    it('should redact the X-API-Key header in any case', () => {
+      Logger.setLevel('info')
+
+      Logger.info('test', {
+        headers: { 'X-API-Key': 'secret-key', 'x-api-key': 'secret-key' },
+      })
+
+      const calls = (console.log as any).mock.calls
+      expect(calls[0][1]).toEqual({
+        headers: { 'X-API-Key': '[REDACTED]', 'x-api-key': '[REDACTED]' },
+      })
+    })
+
     it('should redact algodToken field', () => {
       Logger.setLevel('info')