diff --git a/src/client/sse.test.ts b/src/client/sse.test.ts
index 4fce9976..0439518a 100644
--- a/src/client/sse.test.ts
+++ b/src/client/sse.test.ts
@@ -295,15 +295,34 @@ describe("SSEClientTransport", () => {
       expect(lastServerRequest.headers.authorization).toBe(authToken);
     });
 
-    it("passes custom headers to fetch requests", async () => {
-      const customHeaders = {
-        Authorization: "Bearer test-token",
-        "X-Custom-Header": "custom-value",
-      };
-
+    it.each([
+      {
+        description: "plain object headers",
+        headers: {
+          Authorization: "Bearer test-token",
+          "X-Custom-Header": "custom-value",
+        },
+      },
+      {
+        description: "Headers object",
+        headers: ((): HeadersInit => {
+          const h = new Headers();
+          h.set("Authorization", "Bearer test-token");
+          h.set("X-Custom-Header", "custom-value");
+          return h;
+        })(),
+      },
+      {
+        description: "array of tuples",
+        headers: ((): HeadersInit => ([
+          ["Authorization", "Bearer test-token"],
+          ["X-Custom-Header", "custom-value"],
+        ]))(),
+      },
+    ])("passes custom headers to fetch requests ($description)", async ({ headers }) => {
       transport = new SSEClientTransport(resourceBaseUrl, {
         requestInit: {
-          headers: customHeaders,
+          headers,
         },
       });
 
@@ -337,12 +356,8 @@ describe("SSEClientTransport", () => {
 
         const calledHeaders = (global.fetch as jest.Mock).mock.calls[0][1]
           .headers;
-        expect(calledHeaders.get("Authorization")).toBe(
-          customHeaders.Authorization,
-        );
-        expect(calledHeaders.get("X-Custom-Header")).toBe(
-          customHeaders["X-Custom-Header"],
-        );
+        expect(calledHeaders.get("Authorization")).toBe("Bearer test-token");
+        expect(calledHeaders.get("X-Custom-Header")).toBe("custom-value");
         expect(calledHeaders.get("content-type")).toBe("application/json");
       } finally {
         // Restore original fetch
diff --git a/src/client/sse.ts b/src/client/sse.ts
index e1c86ccd..b7a3a91d 100644
--- a/src/client/sse.ts
+++ b/src/client/sse.ts
@@ -2,6 +2,7 @@ import { EventSource, type ErrorEvent, type EventSourceInit } from "eventsource"
 import { Transport, FetchLike } from "../shared/transport.js";
 import { JSONRPCMessage, JSONRPCMessageSchema } from "../types.js";
 import { auth, AuthResult, extractResourceMetadataUrl, OAuthClientProvider, UnauthorizedError } from "./auth.js";
+import { normalizeHeaders } from "../shared/headers.js";
 
 export class SseError extends Error {
   constructor(
@@ -107,7 +108,7 @@ export class SSEClientTransport implements Transport {
   }
 
   private async _commonHeaders(): Promise<Headers> {
-    const headers: HeadersInit = {};
+    const headers: HeadersInit & Record<string, string> = {};
     if (this._authProvider) {
       const tokens = await this._authProvider.tokens();
       if (tokens) {
@@ -118,9 +119,12 @@ export class SSEClientTransport implements Transport {
       headers["mcp-protocol-version"] = this._protocolVersion;
     }
 
-    return new Headers(
-      { ...headers, ...this._requestInit?.headers }
-    );
+    const extraHeaders = normalizeHeaders(this._requestInit?.headers);
+
+    return new Headers({
+      ...headers,
+      ...extraHeaders,
+    });
   }
 
   private _startOrAuth(): Promise<void> {
diff --git a/src/client/streamableHttp.ts b/src/client/streamableHttp.ts
index 12714ea4..561cef49 100644
--- a/src/client/streamableHttp.ts
+++ b/src/client/streamableHttp.ts
@@ -2,6 +2,7 @@ import { Transport, FetchLike } from "../shared/transport.js";
 import { isInitializedNotification, isJSONRPCRequest, isJSONRPCResponse, JSONRPCMessage, JSONRPCMessageSchema } from "../types.js";
 import { auth, AuthResult, extractResourceMetadataUrl, OAuthClientProvider, UnauthorizedError } from "./auth.js";
 import { EventSourceParserStream } from "eventsource-parser/stream";
+import { normalizeHeaders } from "../shared/headers.js";
 
 // Default reconnection options for StreamableHTTP connections
 const DEFAULT_STREAMABLE_HTTP_RECONNECTION_OPTIONS: StreamableHTTPReconnectionOptions = {
@@ -185,7 +186,7 @@ export class StreamableHTTPClientTransport implements Transport {
       headers["mcp-protocol-version"] = this._protocolVersion;
     }
 
-    const extraHeaders = this._normalizeHeaders(this._requestInit?.headers);
+    const extraHeaders = normalizeHeaders(this._requestInit?.headers);
 
     return new Headers({
       ...headers,
@@ -256,20 +257,6 @@ export class StreamableHTTPClientTransport implements Transport {
 
   }
 
-    private _normalizeHeaders(headers: HeadersInit | undefined): Record<string, string> {
-    if (!headers) return {};
-
-    if (headers instanceof Headers) {
-      return Object.fromEntries(headers.entries());
-    }
-
-    if (Array.isArray(headers)) {
-      return Object.fromEntries(headers);
-    }
-
-    return { ...headers as Record<string, string> };
-  }
-
   /**
    * Schedule a reconnection attempt with exponential backoff
    *
diff --git a/src/shared/headers.ts b/src/shared/headers.ts
new file mode 100644
index 00000000..ed0df02d
--- /dev/null
+++ b/src/shared/headers.ts
@@ -0,0 +1,15 @@
+export function normalizeHeaders(
+  headers: HeadersInit | undefined
+): Record<string, string> {
+  if (!headers) return {};
+
+  if (headers instanceof Headers) {
+    return Object.fromEntries(headers.entries());
+  }
+
+  if (Array.isArray(headers)) {
+    return Object.fromEntries(headers);
+  }
+
+  return { ...headers };
+}