fix: SSE data: [DONE] sentinel + response_format for Claude via GitHub

- Guard data: [DONE] in github.js TransformStream with stream === true
- Inject response_format as system prompt for Claude models via GitHub executor

Note: stream.js guards skipped, createSSEStream is only called for true streaming paths.

Cherry-picked and adapted from PR #286 by @rothnic
https://github.com/decolua/9router/pull/286

Made-with: Cursor

open-sse/executors/github.js

@@ -44,6 +44,36 @@ export class GithubExecutor extends BaseExecutor {
     if (!body?.messages) return body;
 
     const sanitized = { ...body };
+    
+    // Handle response_format for Claude models via GitHub
+    // GitHub's internal translation doesn't respect response_format, so we inject it as a system prompt
+    // AND prepend a reminder to the last user message for maximum effectiveness
+    if (body.response_format && body.model?.includes('claude')) {
+      const responseFormat = body.response_format;
+      let systemInstruction = '';
+      if (responseFormat.type === 'json_schema' && responseFormat.json_schema?.schema) {
+        systemInstruction = 'CRITICAL: You must ONLY output raw JSON. Never use markdown code blocks. Never use backticks. Never wrap JSON in triple backticks. Output ONLY the raw JSON object.';
+      } else if (responseFormat.type === 'json_object') {
+        systemInstruction = 'CRITICAL: You must ONLY output raw JSON. Never use markdown code blocks. Never use backticks.';
+      }
+      if (systemInstruction) {
+        // Add to system message
+        const systemIdx = body.messages.findIndex(m => m.role === 'system');
+        if (systemIdx >= 0) {
+          body.messages[systemIdx].content = systemInstruction + '\n\n' + body.messages[systemIdx].content;
+        } else {
+          body.messages.unshift({ role: 'system', content: systemInstruction });
+        }
+        
+        // Also prepend to the last user message as a reminder
+        const lastUserIdx = body.messages.map((m, i) => m.role === 'user' ? i : -1).filter(i => i >= 0).pop();
+        if (lastUserIdx >= 0) {
+          const userMsg = body.messages[lastUserIdx];
+          const userContent = typeof userMsg.content === 'string' ? userMsg.content : JSON.stringify(userMsg.content);
+          userMsg.content = 'Respond with ONLY raw JSON (no markdown, no backticks, no code blocks): ' + userContent;
+        }
+      }
+    }
     sanitized.messages = body.messages.map(msg => {
       // assistant messages with only tool_calls have content: null — leave as-is
       if (!msg.content) return msg;
@@ -143,7 +173,7 @@ export class GithubExecutor extends BaseExecutor {
           const parsed = parseSSELine(trimmed);
           if (!parsed) continue;
 
-          if (parsed.done) {
+          if (parsed.done && stream === true) {
             controller.enqueue(new TextEncoder().encode("data: [DONE]\n\n"));
             continue;
           }