diff --git a/.changeset/oauth-header-spread-order.md b/.changeset/oauth-header-spread-order.md
new file mode 100644
index 0000000000..777bcf0447
--- /dev/null
+++ b/.changeset/oauth-header-spread-order.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+Fix the spread order in `StreamableHTTPClientTransport._commonHeaders()` and `SSEClientTransport._commonHeaders()` so SDK-derived common headers (including fresh OAuth tokens from `authProvider`) win over caller-supplied headers in `requestInit.headers`. Previously a caller-supplied `Authorization` placeholder (e.g. an env-var API key) was merged after the SDK-computed value, silently overriding OAuth-refreshed tokens and breaking the auth-refresh flow once the placeholder went stale. Closes #2208.
diff --git a/packages/client/src/client/sse.ts b/packages/client/src/client/sse.ts
index 1c81928f10..723510407b 100644
--- a/packages/client/src/client/sse.ts
+++ b/packages/client/src/client/sse.ts
@@ -168,11 +168,17 @@ export class SSEClientTransport implements Transport {
             headers['mcp-protocol-version'] = this._protocolVersion;
         }
 
+        // Order matters: caller-supplied headers (e.g. an `Authorization` placeholder
+        // for an env-var API key) are merged first, then the SDK-derived common
+        // headers spread on top. This lets OAuth-derived tokens override any
+        // stale user-supplied header (matching the order used in streamableHttp
+        // and the rest of the SDK) without requiring the caller to know which
+        // common headers the client will compute at request time. See #2208.
         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 9067fd1ec4..190b5a9cd1 100644
--- a/packages/client/src/client/streamableHttp.ts
+++ b/packages/client/src/client/streamableHttp.ts
@@ -434,11 +434,17 @@ export class StreamableHTTPClientTransport implements Transport {
             headers['mcp-protocol-version'] = this._protocolVersion;
         }
 
+        // Order matters: caller-supplied headers (e.g. an `Authorization` placeholder
+        // for an env-var API key) are merged first, then the SDK-derived common
+        // headers spread on top. This lets OAuth-derived tokens override any
+        // stale user-supplied header (matching the order used in the rest of
+        // the SDK) without requiring the caller to know which common headers
+        // the client will compute at request time. See #2208.
         const extraHeaders = normalizeHeaders(this._requestInit?.headers);
 
         return new Headers({
-            ...headers,
-            ...extraHeaders
+            ...extraHeaders,
+            ...headers
         });
     }
 
diff --git a/packages/client/test/client/streamableHttp.test.ts b/packages/client/test/client/streamableHttp.test.ts
index a36bbc0ad3..fcee3b8ab9 100644
--- a/packages/client/test/client/streamableHttp.test.ts
+++ b/packages/client/test/client/streamableHttp.test.ts
@@ -702,6 +702,42 @@ describe('StreamableHTTPClientTransport', () => {
         expect(globalThis.fetch).toHaveBeenCalledTimes(2);
     });
 
+    it('OAuth-derived Authorization overrides a stale caller-supplied Authorization', async () => {
+        // Regression test for #2208: the SDK-derived common headers must be
+        // merged on top of caller-supplied headers, so OAuth-derived tokens
+        // (or any SDK-computed value) win over a stale placeholder the
+        // caller might have set in `requestInit.headers` (e.g. an env-var
+        // API key that's no longer valid).
+        const tokens: OAuthTokens = {
+            access_token: 'oauth-access-token',
+            token_type: 'Bearer'
+        };
+        mockAuthProvider.tokens.mockResolvedValue(tokens);
+        const requestInit = {
+            headers: {
+                Authorization: 'Bearer stale-placeholder',
+                'X-Caller-Header': 'preserved'
+            }
+        };
+        transport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
+            requestInit,
+            authProvider: mockAuthProvider
+        });
+
+        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']({});
+        // OAuth-derived token wins over the stale placeholder.
+        expect((actualReqInit.headers as Headers).get('authorization')).toBe('Bearer oauth-access-token');
+        // Caller-supplied non-auth headers still pass through.
+        expect((actualReqInit.headers as Headers).get('x-caller-header')).toBe('preserved');
+    });
+
     it('should always send specified custom headers (Headers class)', async () => {
         const requestInit = {
             headers: new Headers({