diff --git a/.changeset/fix-sse-chunked-encoding.md b/.changeset/fix-sse-chunked-encoding.md
new file mode 100644
index 000000000..d2369d364
--- /dev/null
+++ b/.changeset/fix-sse-chunked-encoding.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/server': patch
+---
+
+Add Transfer-Encoding: chunked header to SSE responses in WebStandardStreamableHTTPServerTransport. Prevents HTTP/2 PROTOCOL_ERROR with adapters like @hono/node-server that buffer responses and add Content-Length headers.
diff --git a/packages/server/src/server/streamableHttp.ts b/packages/server/src/server/streamableHttp.ts
index 74e689892..fe2d04ef4 100644
--- a/packages/server/src/server/streamableHttp.ts
+++ b/packages/server/src/server/streamableHttp.ts
@@ -450,6 +450,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
         const headers: Record<string, string> = {
             'Content-Type': 'text/event-stream',
             'Cache-Control': 'no-cache, no-transform',
+            'Transfer-Encoding': 'chunked',
             Connection: 'keep-alive'
         };
 
@@ -503,6 +504,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             const headers: Record<string, string> = {
                 'Content-Type': 'text/event-stream',
                 'Cache-Control': 'no-cache, no-transform',
+                'Transfer-Encoding': 'chunked',
                 Connection: 'keep-alive'
             };
 
@@ -751,6 +753,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             const headers: Record<string, string> = {
                 'Content-Type': 'text/event-stream',
                 'Cache-Control': 'no-cache',
+                'Transfer-Encoding': 'chunked',
                 Connection: 'keep-alive'
             };