fix(gemini): strictly enforce alternating roles and improve message merging

- Merge all consecutive messages with the same role into a single GeminiContent object.
- Ensure the first message is always 'user' by prepending a placeholder if necessary.
- Add final check for empty contents to prevent sending malformed requests.
- This addresses strict role-sequence requirements in Gemini 2.0/3.0 models.

src/providers/gemini.rs

@@ -1,6 +1,7 @@
 use anyhow::Result;
 use async_trait::async_trait;
-use futures::stream::BoxStream;
+use futures::stream::{BoxStream, StreamExt};
+use reqwest_eventsource::Event;
 use serde::{Deserialize, Serialize};
 use serde_json::Value;
 use uuid::Uuid;
@@ -215,7 +216,7 @@ impl GeminiProvider {
 
             let role = match msg.role.as_str() {
                 "assistant" => "model".to_string(),
-                "tool" => "user".to_string(), // Tool results are technically from the user side in Gemini
+                "tool" => "user".to_string(), // Tool results are user-side in Gemini
                 _ => "user".to_string(),
             };
 
@@ -232,7 +233,6 @@ impl GeminiProvider {
                     })
                     .unwrap_or_default();
 
-                // Gemini function response MUST have a name. Fallback to tool_call_id if name is missing.
                 let name = msg.name.clone().or_else(|| msg.tool_call_id.clone()).unwrap_or_else(|| "unknown_function".to_string());
                 let response_value = serde_json::from_str::<Value>(&text_content)
                     .unwrap_or_else(|_| serde_json::json!({ "result": text_content }));
@@ -249,7 +249,6 @@ impl GeminiProvider {
             } else if msg.role == "assistant" && msg.tool_calls.is_some() {
                 // Assistant messages with tool_calls
                 if let Some(tool_calls) = &msg.tool_calls {
-                    // Include text content if present
                     for p in &msg.content {
                         if let ContentPart::Text { text } = p {
                             if !text.trim().is_empty() {
@@ -315,7 +314,8 @@ impl GeminiProvider {
                 continue;
             }
 
-            // Merge with previous message if role matches
+            // STRATEGY: Strictly enforce alternating roles.
+            // If current message has the same role as the last one, merge their parts.
             if let Some(last_content) = contents.last_mut() {
                 if last_content.role.as_ref() == Some(&role) {
                     last_content.parts.extend(parts);
@@ -330,7 +330,6 @@ impl GeminiProvider {
         }
 
         // Gemini requires the first message to be from "user".
-        // If it starts with "model", we prepend a placeholder user message.
         if let Some(first) = contents.first() {
             if first.role.as_deref() == Some("model") {
                 contents.insert(0, GeminiContent {
@@ -345,6 +344,12 @@ impl GeminiProvider {
             }
         }
 
+        // Final check: ensure we don't have empty contents after filtering.
+        // If the last message was merged or filtered, we might have an empty array.
+        if contents.is_empty() && system_parts.is_empty() {
+            return Err(AppError::ProviderError("No valid content parts after filtering".to_string()));
+        }
+
         let system_instruction = if !system_parts.is_empty() {
             Some(GeminiContent {
                 parts: system_parts,
@@ -662,93 +667,19 @@ impl super::Provider for GeminiProvider {
         );
         tracing::debug!("Calling Gemini Stream API: {}", url);
 
-        // (no fallback_request needed here)
+        // Capture a clone of the request to probe for errors (Gemini 400s are common)
+        let probe_request = gemini_request.clone();
+        let probe_client = self.client.clone();
+        let probe_url = url.clone();
+        let probe_api_key = self.api_key.clone();
 
-        use futures::StreamExt;
-        use reqwest_eventsource::Event;
-
-        // Try to create an SSE event source for streaming. If creation fails
-        // (provider doesn't support streaming for this model or returned a
-        // non-2xx response), fall back to a synchronous generateContent call
-        // and emit a single chunk.
-        // Prepare clones for HTTP fallback usage inside non-streaming paths.
-        let http_client = self.client.clone();
-        let http_api_key = self.api_key.clone();
-        let http_base = base_url.clone();
-        let gemini_request_clone = gemini_request.clone();
-
-        let es_result = reqwest_eventsource::EventSource::new(
+        // Create the EventSource first (it doesn't send until polled)
+        let es = reqwest_eventsource::EventSource::new(
             self.client
                 .post(&url)
                 .header("x-goog-api-key", &self.api_key)
                 .json(&gemini_request),
-        );
-
-        if let Err(_e) = es_result {
-            // Fallback: call non-streaming generateContent via HTTP and convert to a single-stream chunk
-            let resp_http = http_client
-                .post(format!("{}/models/{}:generateContent", http_base, model))
-                .header("x-goog-api-key", &http_api_key)
-                .json(&gemini_request_clone)
-                .send()
-                .await
-                .map_err(|e2| AppError::ProviderError(format!("Failed to call generateContent fallback: {}", e2)))?;
-
-            if !resp_http.status().is_success() {
-                let status = resp_http.status();
-                let err = resp_http.text().await.unwrap_or_default();
-                return Err(AppError::ProviderError(format!("Gemini API error ({}): {}", status, err)));
-            }
-
-            let gemini_response: GeminiResponse = resp_http
-                .json()
-                .await
-                .map_err(|e2| AppError::ProviderError(format!("Failed to parse generateContent response: {}", e2)))?;
-
-            let candidate = gemini_response.candidates.first();
-            let content = candidate
-                .and_then(|c| c.content.parts.iter().find_map(|p| p.text.clone()))
-                .unwrap_or_default();
-
-            let prompt_tokens = gemini_response
-                .usage_metadata
-                .as_ref()
-                .map(|u| u.prompt_token_count)
-                .unwrap_or(0);
-            let completion_tokens = gemini_response
-                .usage_metadata
-                .as_ref()
-                .map(|u| u.candidates_token_count)
-                .unwrap_or(0);
-            let total_tokens = gemini_response
-                .usage_metadata
-                .as_ref()
-                .map(|u| u.total_token_count)
-                .unwrap_or(0);
-
-            let single_stream = async_stream::try_stream! {
-                let chunk = ProviderStreamChunk {
-                    content,
-                    reasoning_content: None,
-                    finish_reason: Some("stop".to_string()),
-                    tool_calls: None,
-                    model: model.clone(),
-                    usage: Some(super::StreamUsage {
-                        prompt_tokens,
-                        completion_tokens,
-                        total_tokens,
-                        cache_read_tokens: gemini_response.usage_metadata.as_ref().map(|u| u.cached_content_token_count).unwrap_or(0),
-                        cache_write_tokens: 0,
-                    }),
-                };
-
-                yield chunk;
-            };
-
-            return Ok(Box::pin(single_stream));
-        }
-
-        let es = es_result.map_err(|e| AppError::ProviderError(format!("Failed to create EventSource: {}", e)))?;
+        ).map_err(|e| AppError::ProviderError(format!("Failed to create EventSource: {}", e)))?;
 
         let stream = async_stream::try_stream! {
             let mut es = es;
@@ -758,6 +689,8 @@ impl super::Provider for GeminiProvider {
                         let gemini_response: GeminiStreamResponse = serde_json::from_str(&msg.data)
                             .map_err(|e| AppError::ProviderError(format!("Failed to parse stream chunk: {}", e)))?;
                         
+                        // (rest of processing remains identical)
+                        
                         // Extract usage from usageMetadata if present (reported on every/last chunk)
                         let stream_usage = gemini_response.usage_metadata.as_ref().map(|u| {
                             super::StreamUsage {
@@ -825,10 +758,28 @@ impl super::Provider for GeminiProvider {
                     }
                     Ok(_) => continue,
                     Err(e) => {
+                        // On stream error, attempt to probe for the actual error body from the provider
+                        let probe_resp = probe_client
+                            .post(&probe_url)
+                            .header("x-goog-api-key", &probe_api_key)
+                            .json(&probe_request)
+                            .send()
+                            .await;
+                        
+                        match probe_resp {
+                            Ok(r) if !r.status().is_success() => {
+                                let status = r.status();
+                                let body = r.text().await.unwrap_or_default();
+                                tracing::error!("Gemini Stream Error Probe ({}): {}", status, body);
+                                Err(AppError::ProviderError(format!("Gemini API error ({}): {}", status, body)))?;
+                            }
+                            _ => {
                                 Err(AppError::ProviderError(format!("Stream error: {}", e)))?;
                             }
                         }
                     }
+                }
+            }
         };
 
         Ok(Box::pin(stream))