fix(gemini): tolerate non-candidate SSE chunks

src/providers/gemini.rs

@@ -130,6 +130,26 @@ struct GeminiResponse {
     usage_metadata: Option<GeminiUsageMetadata>,
 }
 
+// Streaming responses from Gemini may include messages without `candidates` (e.g. promptFeedback).
+// Use a more permissive struct for streaming to avoid aborting the SSE prematurely.
+#[derive(Debug, Deserialize, Default)]
+#[serde(rename_all = "camelCase")]
+struct GeminiStreamResponse {
+    #[serde(default)]
+    candidates: Vec<GeminiStreamCandidate>,
+    #[serde(default)]
+    usage_metadata: Option<GeminiUsageMetadata>,
+}
+
+#[derive(Debug, Deserialize, Default)]
+#[serde(rename_all = "camelCase")]
+struct GeminiStreamCandidate {
+    #[serde(default)]
+    content: Option<GeminiContent>,
+    #[serde(default)]
+    finish_reason: Option<String>,
+}
+
 // ========== Provider Implementation ==========
 
 pub struct GeminiProvider {
@@ -547,7 +567,7 @@ impl super::Provider for GeminiProvider {
             while let Some(event) = es.next().await {
                 match event {
                     Ok(Event::Message(msg)) => {
-                        let gemini_response: GeminiResponse = serde_json::from_str(&msg.data)
+                        let gemini_response: GeminiStreamResponse = serde_json::from_str(&msg.data)
                             .map_err(|e| AppError::ProviderError(format!("Failed to parse stream chunk: {}", e)))?;
 
                         // Extract usage from usageMetadata if present (reported on every/last chunk)
@@ -561,29 +581,55 @@ impl super::Provider for GeminiProvider {
                             }
                         });
 
+                        // Some streaming events may not contain candidates (e.g. promptFeedback).
+                        // Only emit chunks when we have candidate content or tool calls.
                         if let Some(candidate) = gemini_response.candidates.first() {
-                            let content = candidate
-                                .content
-                                .parts
-                                .iter()
-                                .find_map(|p| p.text.clone())
-                                .unwrap_or_default();
+                            if let Some(content_obj) = &candidate.content {
+                                let content = content_obj
+                                    .parts
+                                    .iter()
+                                    .find_map(|p| p.text.clone())
+                                    .unwrap_or_default();
 
-                            let tool_calls = Self::extract_tool_call_deltas(&candidate.content.parts);
+                                let tool_calls = Self::extract_tool_call_deltas(&content_obj.parts);
 
-                            // Determine finish_reason
-                            let finish_reason = candidate.finish_reason.as_ref().map(|fr| {
-                                match fr.as_str() {
-                                    "STOP" => "stop".to_string(),
-                                    _ => fr.to_lowercase(),
+                                // Determine finish_reason
+                                let finish_reason = candidate.finish_reason.as_ref().map(|fr| {
+                                    match fr.as_str() {
+                                        "STOP" => "stop".to_string(),
+                                        _ => fr.to_lowercase(),
+                                    }
+                                });
+
+                                // Avoid emitting completely empty chunks unless they carry usage.
+                                if !content.is_empty() || tool_calls.is_some() || stream_usage.is_some() {
+                                    yield ProviderStreamChunk {
+                                        content,
+                                        reasoning_content: None,
+                                        finish_reason,
+                                        tool_calls,
+                                        model: model.clone(),
+                                        usage: stream_usage,
+                                    };
                                 }
-                            });
-
+                            } else if stream_usage.is_some() {
+                                // Usage-only update
+                                yield ProviderStreamChunk {
+                                    content: String::new(),
+                                    reasoning_content: None,
+                                    finish_reason: None,
+                                    tool_calls: None,
+                                    model: model.clone(),
+                                    usage: stream_usage,
+                                };
+                            }
+                        } else if stream_usage.is_some() {
+                            // No candidates but usage present
                             yield ProviderStreamChunk {
-                                content,
+                                content: String::new(),
                                 reasoning_content: None,
-                                finish_reason,
-                                tool_calls,
+                                finish_reason: None,
+                                tool_calls: None,
                                 model: model.clone(),
                                 usage: stream_usage,
                             };