diff --git a/.changeset/fix-transport-onerror-swallowed.md b/.changeset/fix-transport-onerror-swallowed.md
new file mode 100644
index 000000000..4702f2666
--- /dev/null
+++ b/.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`.
diff --git a/packages/server/src/server/streamableHttp.ts b/packages/server/src/server/streamableHttp.ts
index b5038d018..28d79fa28 100644
--- a/packages/server/src/server/streamableHttp.ts
+++ b/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');
             }