Investigate gas translation failure logs

common/src/main/java/io/github/chatglot/translation/provider/GasTranslationProvider.java

@@ -17,6 +17,7 @@
 import java.util.Locale;
 
 public final class GasTranslationProvider implements TranslationProvider {
+    private static final int MAX_REDIRECTS = 5;
     private final HttpClient httpClient = HttpClient.newHttpClient();
 
     @Override
@@ -38,14 +39,12 @@ public TranslationResult translate(TranslationRequest request, ChatglotConfig co
             payload.addProperty("source", normalizeLanguageCode(request.sourceLanguageHint()));
         }
 
-        HttpRequest httpRequest = HttpRequest.newBuilder(URI.create(config.gasWebAppUrl.trim()))
-            .timeout(Duration.ofSeconds(config.requestTimeoutSeconds))
-            .header("Content-Type", "application/json")
-            .POST(HttpRequest.BodyPublishers.ofString(payload.toString(), StandardCharsets.UTF_8))
-            .build();
-
         try {
-            HttpResponse<String> response = httpClient.send(httpRequest, HttpResponse.BodyHandlers.ofString(StandardCharsets.UTF_8));
+            HttpResponse<String> response = sendWithRedirects(
+                config.gasWebAppUrl.trim(),
+                payload.toString(),
+                config.requestTimeoutSeconds
+            );
             if (response.statusCode() >= 400) {
                 throw new TranslationException(
                     "GAS translation request failed ("
@@ -55,7 +54,24 @@ public TranslationResult translate(TranslationRequest request, ChatglotConfig co
                 );
             }
 
-            JsonObject root = JsonParser.parseString(response.body()).getAsJsonObject();
+            String responseBody = response.body() == null ? "" : response.body();
+            String normalizedBody = normalizeJsonBody(responseBody);
+
+            JsonObject root;
+            try {
+                root = JsonParser.parseString(normalizedBody).getAsJsonObject();
+            } catch (RuntimeException parseError) {
+                throw new TranslationException(
+                    "GAS returned non-JSON response ("
+                        + response.statusCode()
+                        + ", "
+                        + readContentType(response)
+                        + "): "
+                        + TranslationPromptBuilder.abbreviate(responseBody, 500),
+                    parseError
+                );
+            }
+
             if (root.has("ok") && !root.get("ok").getAsBoolean()) {
                 throw new TranslationException("GAS translation failed: " + extractErrorMessage(root));
             }
@@ -78,6 +94,70 @@ public TranslationResult translate(TranslationRequest request, ChatglotConfig co
         }
     }
 
+    private HttpResponse<String> sendWithRedirects(String url, String requestBody, int timeoutSeconds) throws Exception {
+        URI currentUri = URI.create(url);
+        boolean usePost = true;
+
+        for (int attempt = 0; attempt <= MAX_REDIRECTS; attempt++) {
+            HttpRequest.Builder builder = HttpRequest.newBuilder(currentUri)
+                .timeout(Duration.ofSeconds(timeoutSeconds));
+            HttpRequest request;
+            if (usePost) {
+                request = builder
+                    .header("Content-Type", "application/json")
+                    .POST(HttpRequest.BodyPublishers.ofString(requestBody, StandardCharsets.UTF_8))
+                    .build();
+            } else {
+                request = builder.GET().build();
+            }
+
+            HttpResponse<String> response = httpClient.send(request, HttpResponse.BodyHandlers.ofString(StandardCharsets.UTF_8));
+            int status = response.statusCode();
+            if (status < 300 || status >= 400) {
+                return response;
+            }
+
+            String location = response.headers().firstValue("Location").orElse("");
+            if (location.isBlank()) {
+                return response;
+            }
+
+            // GAS /exec commonly redirects POST (302) to a googleusercontent URL that should be fetched with GET.
+            if (status == 301 || status == 302 || status == 303) {
+                usePost = false;
+            }
+            currentUri = currentUri.resolve(location.trim());
+        }
+
+        throw new TranslationException("GAS translation request failed: too many redirects.");
+    }
+
+    private static String normalizeJsonBody(String body) {
+        if (body == null) {
+            return "";
+        }
+
+        String normalized = body.stripLeading();
+        if (normalized.startsWith("\uFEFF")) {
+            normalized = normalized.substring(1).stripLeading();
+        }
+
+        if (normalized.startsWith(")]}'")) {
+            int lineBreak = normalized.indexOf('\n');
+            if (lineBreak >= 0) {
+                normalized = normalized.substring(lineBreak + 1).stripLeading();
+            } else {
+                normalized = normalized.substring(4).stripLeading();
+            }
+        }
+
+        return normalized;
+    }
+
+    private static String readContentType(HttpResponse<String> response) {
+        return response.headers().firstValue("Content-Type").orElse("unknown");
+    }
+
     private static String extractErrorMessage(JsonObject root) {
         String message = getString(root, "message");
         String details = getString(root, "details");