diff --git a/packages/sdk/js/src/gen/client/client.gen.ts b/packages/sdk/js/src/gen/client/client.gen.ts
index 34a8d0bece..c5b4760b74 100644
--- a/packages/sdk/js/src/gen/client/client.gen.ts
+++ b/packages/sdk/js/src/gen/client/client.gen.ts
@@ -114,10 +114,29 @@ export const createClient = (config: Config = {}): Client => {
         case "arrayBuffer":
         case "blob":
         case "formData":
-        case "json":
         case "text":
           data = await response[parseAs]()
           break
+        // altimate_change start — upstream_fix: guard JSON parse against non-JSON (HTML) response bodies
+        // "json" is split out of the fall-through group above so its parse can be guarded: a 200
+        // whose body is an HTML error page from a proxy/gateway/CDN otherwise crashes with a raw
+        // "JSON Parse error: Unrecognized token '<'". The body is read OUTSIDE the guard so a
+        // network/body-read failure (socket reset, abort) keeps its own error; only an actual
+        // JSON syntax failure gets the actionable message (mirrors the v2 client).
+        case "json": {
+          const text = await response.text()
+          try {
+            data = text ? JSON.parse(text) : {}
+          } catch (cause) {
+            throw new Error(
+              `Expected a JSON response but received ${response.headers.get("content-type") || "an unknown content type"} ` +
+                `(HTTP ${response.status}). This is usually a proxy or gateway error page, not the API.`,
+              { cause },
+            )
+          }
+          break
+        }
+        // altimate_change end
         case "stream":
           return opts.responseStyle === "data"
             ? response.body
diff --git a/packages/sdk/js/src/v2/gen/client/client.gen.ts b/packages/sdk/js/src/v2/gen/client/client.gen.ts
index 627e98ec42..67915f39ac 100644
--- a/packages/sdk/js/src/v2/gen/client/client.gen.ts
+++ b/packages/sdk/js/src/v2/gen/client/client.gen.ts
@@ -169,7 +169,19 @@ export const createClient = (config: Config = {}): Client => {
           // Some servers return 200 with no Content-Length and empty body.
           // response.json() would throw; read as text and parse if non-empty.
           const text = await response.text()
-          data = text ? JSON.parse(text) : {}
+          // altimate_change start — upstream_fix: guard JSON parse against non-JSON (HTML) response bodies
+          // A 200 whose body is an HTML error page from a proxy/gateway/CDN otherwise crashes with a
+          // raw "JSON Parse error: Unrecognized token '<'". Surface an actionable error instead.
+          try {
+            data = text ? JSON.parse(text) : {}
+          } catch (cause) {
+            throw new Error(
+              `Expected a JSON response but received ${response.headers.get("content-type") || "an unknown content type"} ` +
+                `(HTTP ${response.status}). This is usually a proxy or gateway error page, not the API.`,
+              { cause },
+            )
+          }
+          // altimate_change end
           break
         }
         case "stream":