diff --git a/internal/translator/kiro/claude/kiro_claude_request.go b/internal/translator/kiro/claude/kiro_claude_request.go
index 4e498c24..259ae9f5 100644
--- a/internal/translator/kiro/claude/kiro_claude_request.go
+++ b/internal/translator/kiro/claude/kiro_claude_request.go
@@ -883,8 +883,21 @@ func BuildAssistantMessageStruct(msg gjson.Result) KiroAssistantResponseMessage
 		contentBuilder.WriteString(content.String())
 	}
 
+	// CRITICAL FIX: Kiro API requires non-empty content for assistant messages
+	// This can happen with compaction requests where assistant messages have only tool_use
+	// (no text content). Without this fix, Kiro API returns "Improperly formed request" error.
+	finalContent := contentBuilder.String()
+	if strings.TrimSpace(finalContent) == "" {
+		if len(toolUses) > 0 {
+			finalContent = kirocommon.DefaultAssistantContentWithTools
+		} else {
+			finalContent = kirocommon.DefaultAssistantContent
+		}
+		log.Debugf("kiro: assistant content was empty, using default: %s", finalContent)
+	}
+
 	return KiroAssistantResponseMessage{
-		Content:  contentBuilder.String(),
+		Content:  finalContent,
 		ToolUses: toolUses,
 	}
 }
diff --git a/internal/translator/kiro/common/constants.go b/internal/translator/kiro/common/constants.go
index 2327ab59..ab000972 100644
--- a/internal/translator/kiro/common/constants.go
+++ b/internal/translator/kiro/common/constants.go
@@ -29,6 +29,14 @@ const (
 	// InlineCodeMarker is the markdown inline code marker (backtick).
 	InlineCodeMarker = "`"
 
+	// DefaultAssistantContentWithTools is the fallback content for assistant messages
+	// that have tool_use but no text content. Kiro API requires non-empty content.
+	DefaultAssistantContentWithTools = "I'll help you with that."
+
+	// DefaultAssistantContent is the fallback content for assistant messages
+	// that have no content at all. Kiro API requires non-empty content.
+	DefaultAssistantContent = "I understand."
+
 	// KiroAgenticSystemPrompt is injected only for -agentic models to prevent timeouts on large writes.
 	// AWS Kiro API has a 2-3 minute timeout for large file write operations.
 	KiroAgenticSystemPrompt = `
diff --git a/internal/translator/kiro/openai/kiro_openai_request.go b/internal/translator/kiro/openai/kiro_openai_request.go
index 25800928..9515848f 100644
--- a/internal/translator/kiro/openai/kiro_openai_request.go
+++ b/internal/translator/kiro/openai/kiro_openai_request.go
@@ -718,13 +718,10 @@ func buildAssistantMessageFromOpenAI(msg gjson.Result) KiroAssistantResponseMess
 	// This can happen with compaction requests or error recovery scenarios
 	finalContent := contentBuilder.String()
 	if strings.TrimSpace(finalContent) == "" {
-		const defaultAssistantContentWithTools = "I'll help you with that."
-		const defaultAssistantContent = "I understand."
-
 		if len(toolUses) > 0 {
-			finalContent = defaultAssistantContentWithTools
+			finalContent = kirocommon.DefaultAssistantContentWithTools
 		} else {
-			finalContent = defaultAssistantContent
+			finalContent = kirocommon.DefaultAssistantContent
 		}
 		log.Debugf("kiro-openai: assistant content was empty, using default: %s", finalContent)
 	}