fix: call onerror for silently swallowed transport errors

Nested try/catch blocks in WebStandardStreamableHTTPServerTransport's
handlePostRequest were returning error responses without invoking the
onerror callback, making transport errors invisible to consumers.

Fixes: - JSON parsing errors (req.json() failures) now call onerror
- JSON-RPC message validation errors now call onerror
- writeSSEEvent failures now call onerror instead of silently
  returning false

Closes #1395

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: corvid-agent

.changeset/fix-transport-onerror-swallowed.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/server': patch
+---
+
+Call `onerror` callback for transport errors that were previously silently swallowed. Nested try/catch blocks in `handlePostRequest` for JSON parsing and JSON-RPC validation now invoke `onerror` before returning error responses. The `writeSSEEvent` method also reports errors via `onerror` instead of silently returning `false`.

packages/server/src/server/streamableHttp.ts

@@ -569,7 +569,8 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             eventData += `data: ${JSON.stringify(message)}\n\n`;
             controller.enqueue(encoder.encode(eventData));
             return true;
-        } catch {
+        } catch (error) {
+            this.onerror?.(error as Error);
             return false;
         }
     }
@@ -627,7 +628,8 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             if (options?.parsedBody === undefined) {
                 try {
                     rawMessage = await req.json();
-                } catch {
+                } catch (error) {
+                    this.onerror?.(error as Error);
                     return this.createJsonErrorResponse(400, -32_700, 'Parse error: Invalid JSON');
                 }
             } else {
@@ -641,7 +643,8 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
                 messages = Array.isArray(rawMessage)
                     ? rawMessage.map(msg => JSONRPCMessageSchema.parse(msg))
                     : [JSONRPCMessageSchema.parse(rawMessage)];
-            } catch {
+            } catch (error) {
+                this.onerror?.(error as Error);
                 return this.createJsonErrorResponse(400, -32_700, 'Parse error: Invalid JSON-RPC message');
             }