fix: trim trailing whitespaces for anthropic and bedrock anthropic provider

core/providers/anthropic/chat.go

@@ -745,6 +745,20 @@ func ToAnthropicChatRequest(ctx *schemas.BifrostContext, bifrostReq *schemas.Bif
 	anthropicReq.Messages = anthropicMessages
 	anthropicReq.System = systemContent
 
+	// Trim trailing whitespace from the last assistant message text blocks
+	// ContentStr is converted to a single text ContentBlock during message conversion
+	// so we trim the text of that block instead.
+	lastMsgIndex := len(anthropicReq.Messages) - 1
+	if lastMsgIndex >= 0 && anthropicReq.Messages[lastMsgIndex].Role == AnthropicMessageRoleAssistant {
+		blocks := anthropicReq.Messages[lastMsgIndex].Content.ContentBlocks
+		for j := len(blocks) - 1; j >= 0; j-- {
+			if blocks[j].Type == AnthropicContentBlockTypeText && blocks[j].Text != nil {
+				anthropicReq.Messages[lastMsgIndex].Content.ContentBlocks[j].Text = schemas.Ptr(strings.TrimRight(*blocks[j].Text, " \n\r\t"))
+				break
+			}
+		}
+	}
+
 	// Strip request- and tool-level fields the target Anthropic-family
 	// provider does not support. Fail-closed tool validation stays in
 	// ValidateToolsForProvider; this is strip-silently for additive fields.

core/providers/anthropic/responses.go

@@ -3344,6 +3344,20 @@ func ConvertBifrostMessagesToAnthropicMessages(ctx *schemas.BifrostContext, bifr
 	// Flush any remaining pending tool calls (with tracking)
 	flushPendingToolCallsWithTracking()
 
+	// Trim trailing whitespace from the last assistant message
+	// ContentStr is converted to a single text ContentBlock during message conversion
+	// so we trim the text of that block instead.
+	lastMsgIndex := len(anthropicMessages) - 1
+	if isRequestMessage && lastMsgIndex >= 0 && anthropicMessages[lastMsgIndex].Role == AnthropicMessageRoleAssistant {
+		blocks := anthropicMessages[lastMsgIndex].Content.ContentBlocks
+		for j := len(blocks) - 1; j >= 0; j-- {
+			if blocks[j].Type == AnthropicContentBlockTypeText && blocks[j].Text != nil {
+				anthropicMessages[lastMsgIndex].Content.ContentBlocks[j].Text = schemas.Ptr(strings.TrimRight(*blocks[j].Text, " \n\r\t"))
+				break
+			}
+		}
+	}
+
 	return anthropicMessages, systemContent
 }
 
@@ -5957,4 +5971,4 @@ func generateSyntheticInputJSONDeltas(argumentsJSON string, contentIndex *int) [
 	}
 
 	return events
-}
+}

core/providers/bedrock/chat.go

@@ -3,6 +3,7 @@ package bedrock
 import (
 	"context"
 	"fmt"
+	"strings"
 	"time"
 
 	"github.com/google/uuid"
@@ -42,6 +43,19 @@ func ToBedrockChatCompletionRequest(ctx *schemas.BifrostContext, bifrostReq *sch
 		bedrockReq.System = systemMessages
 	}
 
+	// Trim trailing whitespace from the last assistant message text blocks
+	// (only for Anthropic models which use text-based prefill)
+	lastMsgIndex := len(bedrockReq.Messages) - 1
+	if schemas.IsAnthropicModel(bifrostReq.Model) && lastMsgIndex >= 0 && bedrockReq.Messages[lastMsgIndex].Role == BedrockMessageRoleAssistant {
+		blocks := bedrockReq.Messages[lastMsgIndex].Content
+		for j := len(blocks) - 1; j >= 0; j-- {
+			if blocks[j].Text != nil {
+				bedrockReq.Messages[lastMsgIndex].Content[j].Text = schemas.Ptr(strings.TrimRight(*blocks[j].Text, " \n\r\t"))
+				break
+			}
+		}
+	}
+
 	// Convert parameters and configurations
 	if err := convertChatParameters(ctx, bifrostReq, bedrockReq); err != nil {
 		return nil, fmt.Errorf("failed to convert chat parameters: %w", err)

core/providers/bedrock/responses.go

@@ -1682,6 +1682,19 @@ func ToBedrockResponsesRequest(ctx *schemas.BifrostContext, bifrostReq *schemas.
 				}
 			}
 		}
+
+		// Trim trailing whitespace from the last assistant message text blocks
+		// (only for Anthropic models which use text-based prefill)
+		lastMsgIndex := len(bedrockReq.Messages) - 1
+		if schemas.IsAnthropicModel(bifrostReq.Model) && lastMsgIndex >= 0 && bedrockReq.Messages[lastMsgIndex].Role == BedrockMessageRoleAssistant {
+			blocks := bedrockReq.Messages[lastMsgIndex].Content
+			for j := len(blocks) - 1; j >= 0; j-- {
+				if blocks[j].Text != nil {
+					bedrockReq.Messages[lastMsgIndex].Content[j].Text = schemas.Ptr(strings.TrimRight(*blocks[j].Text, " \n\r\t"))
+					break
+				}
+			}
+		}
 	}
 
 	var responsesStructuredOutputTool *BedrockTool