fix: handle empty Gemini responses and reasoning loops

Author: hannesrudolph

src/api/providers/gemini.ts

@@ -199,13 +199,17 @@ export class GeminiHandler extends BaseProvider implements SingleCompletionHandl
 			let lastUsageMetadata: GenerateContentResponseUsageMetadata | undefined
 			let pendingGroundingMetadata: GroundingMetadata | undefined
 			let finalResponse: { responseId?: string } | undefined
+			let finishReason: string | undefined
 
 			let toolCallCounter = 0
+			let hasContent = false
+			let hasReasoning = false
 
 			for await (const chunk of result) {
 				// Track the final structured response (per SDK pattern: candidate.finishReason)
 				if (chunk.candidates && chunk.candidates[0]?.finishReason) {
 					finalResponse = chunk as { responseId?: string }
+					finishReason = chunk.candidates[0].finishReason
 				}
 				// Process candidates and their parts to separate thoughts from content
 				if (chunk.candidates && chunk.candidates.length > 0) {
@@ -233,9 +237,11 @@ export class GeminiHandler extends BaseProvider implements SingleCompletionHandl
 							if (part.thought) {
 								// This is a thinking/reasoning part
 								if (part.text) {
+									hasReasoning = true
 									yield { type: "reasoning", text: part.text }
 								}
 							} else if (part.functionCall) {
+								hasContent = true
 								// Gemini sends complete function calls in a single chunk
 								// Emit as partial chunks for consistent handling with NativeToolCallParser
 								const callId = `${part.functionCall.name}-${toolCallCounter}`
@@ -263,6 +269,7 @@ export class GeminiHandler extends BaseProvider implements SingleCompletionHandl
 							} else {
 								// This is regular content
 								if (part.text) {
+									hasContent = true
 									yield { type: "text", text: part.text }
 								}
 							}
@@ -272,6 +279,7 @@ export class GeminiHandler extends BaseProvider implements SingleCompletionHandl
 
 				// Fallback to the original text property if no candidates structure
 				else if (chunk.text) {
+					hasContent = true
 					yield { type: "text", text: chunk.text }
 				}
 
@@ -280,6 +288,21 @@ export class GeminiHandler extends BaseProvider implements SingleCompletionHandl
 				}
 			}
 
+			// If we had reasoning but no content, emit a placeholder text to prevent "Empty assistant response" errors.
+			// This typically happens when the model hits max output tokens while reasoning.
+			if (hasReasoning && !hasContent) {
+				let message = "(Thinking complete, but no output was generated.)"
+				if (finishReason === "MAX_TOKENS") {
+					message = "(Thinking complete, but output was truncated due to token limit.)"
+				} else if (finishReason === "SAFETY") {
+					message = "(Thinking complete, but output was blocked due to safety settings.)"
+				} else if (finishReason === "RECITATION") {
+					message = "(Thinking complete, but output was blocked due to recitation check.)"
+				}
+
+				yield { type: "text", text: message }
+			}
+
 			if (finalResponse?.responseId) {
 				// Capture responseId so Task.addToApiConversationHistory can store it
 				// alongside the assistant message in api_history.json.

src/api/transform/gemini-format.ts

@@ -47,16 +47,11 @@ export function convertAnthropicContentToGemini(
 		return [{ text: content }]
 	}
 
-	return content.flatMap((block): Part | Part[] => {
+	const parts = content.flatMap((block): Part | Part[] => {
 		// Handle thoughtSignature blocks first
 		if (isThoughtSignatureContentBlock(block)) {
-			if (includeThoughtSignatures && typeof block.thoughtSignature === "string") {
-				// The Google GenAI SDK currently exposes thoughtSignature as an
-				// extension field on Part; model it structurally without widening
-				// the upstream type.
-				return { thoughtSignature: block.thoughtSignature } as Part
-			}
-			// Explicitly omit thoughtSignature when not including it.
+			// We process thought signatures globally and attach them to the relevant parts
+			// or create a placeholder part if no other content exists.
 			return []
 		}
 
@@ -135,6 +130,24 @@ export function convertAnthropicContentToGemini(
 				return []
 		}
 	})
+
+	// Post-processing: Ensure thought signature is attached if required
+	if (includeThoughtSignatures && activeThoughtSignature) {
+		const hasSignature = parts.some((p) => "thoughtSignature" in p)
+
+		if (!hasSignature) {
+			if (parts.length > 0) {
+				// Attach to the first part (usually text)
+				// We cast to any to allow adding the property
+				;(parts[0] as any).thoughtSignature = activeThoughtSignature
+			} else {
+				// Create a placeholder part if no other content exists
+				parts.push({ text: "", thoughtSignature: activeThoughtSignature } as any as Part)
+			}
+		}
+	}
+
+	return parts
 }
 
 export function convertAnthropicMessageToGemini(