diff --git a/.changeset/fresh-buses-auth.md b/.changeset/fresh-buses-auth.md
new file mode 100644
index 0000000000..6727dd8faa
--- /dev/null
+++ b/.changeset/fresh-buses-auth.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+Let auth provider headers override requestInit headers in client transports.
diff --git a/packages/client/src/client/sse.ts b/packages/client/src/client/sse.ts
index bf554aba29..51d96e777d 100644
--- a/packages/client/src/client/sse.ts
+++ b/packages/client/src/client/sse.ts
@@ -120,8 +120,8 @@ export class SSEClientTransport implements Transport {
         const extraHeaders = normalizeHeaders(this._requestInit?.headers);
 
         return new Headers({
-            ...headers,
-            ...extraHeaders
+            ...extraHeaders,
+            ...headers
         });
     }
 
diff --git a/packages/client/src/client/streamableHttp.ts b/packages/client/src/client/streamableHttp.ts
index 3b8ddafe5a..aca6126159 100644
--- a/packages/client/src/client/streamableHttp.ts
+++ b/packages/client/src/client/streamableHttp.ts
@@ -226,8 +226,8 @@ export class StreamableHTTPClientTransport implements Transport {
         const extraHeaders = normalizeHeaders(this._requestInit?.headers);
 
         return new Headers({
-            ...headers,
-            ...extraHeaders
+            ...extraHeaders,
+            ...headers
         });
     }
 
diff --git a/packages/client/test/client/sse.test.ts b/packages/client/test/client/sse.test.ts
index 6948d9a4e0..384416ca97 100644
--- a/packages/client/test/client/sse.test.ts
+++ b/packages/client/test/client/sse.test.ts
@@ -344,6 +344,44 @@ describe('SSEClientTransport', () => {
             }
         });
 
+        it('lets auth provider headers override custom Authorization headers', async () => {
+            const authProvider: AuthProvider = {
+                token: async () => 'fresh-token'
+            };
+
+            transport = new SSEClientTransport(resourceBaseUrl, {
+                authProvider,
+                requestInit: {
+                    headers: {
+                        Authorization: 'Bearer stale-token',
+                        'X-Custom-Header': 'custom-value'
+                    }
+                }
+            });
+
+            await transport.start();
+
+            const originalFetch = globalThis.fetch;
+            try {
+                globalThis.fetch = vi.fn().mockResolvedValue({ ok: true });
+
+                const message: JSONRPCMessage = {
+                    jsonrpc: '2.0',
+                    id: '1',
+                    method: 'test',
+                    params: {}
+                };
+
+                await transport.send(message);
+
+                const calledHeaders = (globalThis.fetch as Mock).mock.calls[0]![1].headers;
+                expect(calledHeaders.get('Authorization')).toBe('Bearer fresh-token');
+                expect(calledHeaders.get('X-Custom-Header')).toBe('custom-value');
+            } finally {
+                globalThis.fetch = originalFetch;
+            }
+        });
+
         it('passes custom headers to fetch requests (Headers class)', async () => {
             const customHeaders = new Headers({
                 Authorization: 'Bearer test-token',
diff --git a/packages/client/test/client/streamableHttp.test.ts b/packages/client/test/client/streamableHttp.test.ts
index 0edf8b75ac..08ed5678be 100644
--- a/packages/client/test/client/streamableHttp.test.ts
+++ b/packages/client/test/client/streamableHttp.test.ts
@@ -2,7 +2,7 @@ import type { JSONRPCMessage, JSONRPCRequest } from '@modelcontextprotocol/core'
 import { OAuthError, OAuthErrorCode, SdkErrorCode, SdkHttpError } from '@modelcontextprotocol/core';
 import type { Mock, Mocked } from 'vitest';
 
-import type { OAuthClientProvider } from '../../src/client/auth.js';
+import type { AuthProvider, OAuthClientProvider } from '../../src/client/auth.js';
 import { UnauthorizedError } from '../../src/client/auth.js';
 import type { ReconnectionScheduler, StartSSEOptions, StreamableHTTPReconnectionOptions } from '../../src/client/streamableHttp.js';
 import { StreamableHTTPClientTransport } from '../../src/client/streamableHttp.js';
@@ -573,6 +573,36 @@ describe('StreamableHTTPClientTransport', () => {
         expect(globalThis.fetch).toHaveBeenCalledTimes(2);
     });
 
+    it('should let auth provider headers override custom Authorization headers', async () => {
+        const authProvider: AuthProvider = {
+            token: async () => 'fresh-token'
+        };
+
+        transport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
+            authProvider,
+            requestInit: {
+                headers: {
+                    Authorization: 'Bearer stale-token',
+                    'X-Custom-Header': 'CustomValue'
+                }
+            }
+        });
+
+        let actualReqInit: RequestInit = {};
+
+        (globalThis.fetch as Mock).mockImplementation(async (_url, reqInit) => {
+            actualReqInit = reqInit;
+            return new Response(null, { status: 200, headers: { 'content-type': 'text/event-stream' } });
+        });
+
+        await transport.start();
+
+        await transport['_startOrAuthSse']({});
+
+        expect((actualReqInit.headers as Headers).get('authorization')).toBe('Bearer fresh-token');
+        expect((actualReqInit.headers as Headers).get('x-custom-header')).toBe('CustomValue');
+    });
+
     it('should always send specified custom headers (Headers class)', async () => {
         const requestInit = {
             headers: new Headers({