diff --git a/client/src/lib/hooks/__tests__/useConnection.test.tsx b/client/src/lib/hooks/__tests__/useConnection.test.tsx
index 875c9e387..4e4201c28 100644
--- a/client/src/lib/hooks/__tests__/useConnection.test.tsx
+++ b/client/src/lib/hooks/__tests__/useConnection.test.tsx
@@ -1387,6 +1387,131 @@ describe("useConnection", () => {
     });
   });
 
+  describe("Direct connection OAuth token Content-Type", () => {
+    beforeEach(() => {
+      jest.clearAllMocks();
+      mockSSETransport.options = undefined;
+      mockStreamableHTTPTransport.options = undefined;
+    });
+
+    test("does not mutate the shared requestInit headers with a Content-Type", async () => {
+      // Regression test for #1160. The streamable-http custom fetch must not
+      // write a capitalized `Content-Type: application/json` onto the shared
+      // `requestHeaders` object, because that object is also passed as
+      // `requestInit.headers`, which the SDK reuses for OAuth token requests.
+      // If it is mutated here, the token request's own lowercase
+      // `content-type: application/x-www-form-urlencoded` collides (by case)
+      // with it and produces a malformed comma-joined value that strict
+      // servers reject with 415.
+      const directProps = {
+        ...defaultProps,
+        transportType: "streamable-http" as const,
+        connectionType: "direct" as const,
+      };
+
+      const { result } = renderHook(() => useConnection(directProps));
+
+      await act(async () => {
+        await result.current.connect();
+      });
+
+      const customFetch = mockStreamableHTTPTransport.options?.fetch;
+      expect(customFetch).toBeDefined();
+
+      // Issue an ordinary MCP request through the wrapper, exactly as the SDK
+      // would. In the buggy version this mutated the shared requestInit
+      // headers.
+      await customFetch?.("http://test.com/mcp", {
+        method: "POST",
+        headers: new Headers({
+          "content-type": "application/json",
+          accept: "application/json, text/event-stream",
+        }),
+      });
+
+      // The connect-time headers baked into requestInit must remain free of a
+      // Content-Type so they cannot corrupt the SDK's token request.
+      expect(
+        mockStreamableHTTPTransport.options?.requestInit?.headers,
+      ).not.toHaveProperty("Content-Type");
+    });
+
+    // Mirrors the SDK's shared `createFetchWithInit`, which wraps the custom
+    // `fetch` with the connect-time `requestInit` and is what the SDK uses for
+    // OAuth token requests. See @modelcontextprotocol/sdk shared/transport.ts.
+    const normalizeHeaders = (
+      headers: HeadersInit | undefined,
+    ): Record<string, string> => {
+      if (!headers) return {};
+      if (headers instanceof Headers) return Object.fromEntries(headers);
+      if (Array.isArray(headers)) return Object.fromEntries(headers);
+      return { ...(headers as Record<string, string>) };
+    };
+
+    test("refresh_token request keeps a single application/x-www-form-urlencoded Content-Type", async () => {
+      // End-to-end evidence for #1160: replays the exact SDK token-request
+      // composition and asserts the Content-Type that ends up on the wire is
+      // not the malformed comma-joined value.
+      const directProps = {
+        ...defaultProps,
+        transportType: "streamable-http" as const,
+        connectionType: "direct" as const,
+      };
+
+      const { result } = renderHook(() => useConnection(directProps));
+
+      await act(async () => {
+        await result.current.connect();
+      });
+
+      const customFetch = mockStreamableHTTPTransport.options?.fetch;
+      const requestInit = mockStreamableHTTPTransport.options?.requestInit;
+      expect(customFetch).toBeDefined();
+
+      // 1) An ordinary MCP request runs first (as the SDK would issue it). In
+      //    the buggy version this mutated the shared requestInit headers.
+      await customFetch?.("http://test.com/mcp", {
+        method: "POST",
+        headers: new Headers({
+          "content-type": "application/json",
+          accept: "application/json, text/event-stream",
+        }),
+      });
+
+      // 2) The SDK then issues the refresh_token request through
+      //    createFetchWithInit(customFetch, requestInit).
+      const tokenInit: RequestInit = {
+        method: "POST",
+        body: "grant_type=refresh_token&refresh_token=abc",
+        headers: new Headers({
+          "content-type": "application/x-www-form-urlencoded",
+          accept: "application/json",
+        }),
+      };
+      const mergedInit = {
+        ...requestInit,
+        ...tokenInit,
+        headers: {
+          ...normalizeHeaders(requestInit?.headers),
+          ...normalizeHeaders(tokenInit.headers),
+        },
+      };
+      await customFetch?.(
+        "http://test.com/mcp-oauth/token",
+        mergedInit as RequestInit,
+      );
+
+      // Inspect the Content-Type actually put on the wire for the token request.
+      const calls = (global.fetch as jest.Mock).mock.calls;
+      const sentHeaders = new Headers(
+        calls[calls.length - 1]?.[1]?.headers as HeadersInit | undefined,
+      );
+      expect(sentHeaders.get("content-type")).toBe(
+        "application/x-www-form-urlencoded",
+      );
+    });
+  });
+
   describe("Connection URL Verification", () => {
     beforeEach(() => {
       jest.clearAllMocks();
diff --git a/client/src/lib/hooks/useConnection.ts b/client/src/lib/hooks/useConnection.ts
index 9694b891f..455a8cc36 100644
--- a/client/src/lib/hooks/useConnection.ts
+++ b/client/src/lib/hooks/useConnection.ts
@@ -610,9 +610,17 @@ export function useConnection({
                 url: string | URL | globalThis.Request,
                 init?: RequestInit,
               ) => {
-                requestHeaders["Accept"] =
-                  "text/event-stream, application/json";
-                requestHeaders["Content-Type"] = "application/json";
+                // NOTE: do not mutate the shared `requestHeaders` object here.
+                // It is also passed as `requestInit.headers`, and the SDK reuses
+                // that same object for OAuth token requests via
+                // `createFetchWithInit`. Injecting a capitalized
+                // `Content-Type: application/json` would then collide (by case)
+                // with the token request's own lowercase
+                // `content-type: application/x-www-form-urlencoded`, producing a
+                // malformed comma-joined value that a strict server rejects with
+                // 415 before OAuth handling runs (#1160). The SDK already sets
+                // the correct Accept/Content-Type on each MCP request itself, so
+                // `...init` supplies them for the actual message requests.
                 const response = await fetch(url, {
                   headers: requestHeaders,
                   ...init,