handle system messages correctly with anthropic provider

greatly reduces the amount of model refusals, and gets anthropic models to behave much more in line with the actual system prompt instructions

Signed-off-by: Christopher Petito <chrisjpetito@gmail.com>

Author: krissetto

pkg/model/provider/anthropic/client.go

@@ -123,6 +123,11 @@ func (c *Client) CreateChatCompletionStream(
 		Tools:     convertTools(requestTools),
 	}
 
+	// Populate proper Anthropic system prompt from input messages
+	if sys := extractSystemBlocks(messages); len(sys) > 0 {
+		params.System = sys
+	}
+
 	if len(requestTools) > 0 {
 		slog.Debug("Adding tools to Anthropic request", "tool_count", len(requestTools))
 	}
@@ -164,6 +169,11 @@ func (c *Client) CreateChatCompletion(
 		Messages:  convertMessages(messages),
 	}
 
+	// Populate proper Anthropic system prompt from input messages
+	if sys := extractSystemBlocks(messages); len(sys) > 0 {
+		params.System = sys
+	}
+
 	// Build a fresh client per request when using the gateway
 	client := c.client
 	if c.useGateway {
@@ -185,10 +195,7 @@ func convertMessages(messages []chat.Message) []anthropic.MessageParam {
 	for i := range messages {
 		msg := &messages[i]
 		if msg.Role == chat.MessageRoleSystem {
-			// Convert system message to user message with system prefix
-			if systemContent := strings.TrimSpace("System: " + msg.Content); systemContent != "System:" {
-				anthropicMessages = append(anthropicMessages, anthropic.NewAssistantMessage(anthropic.NewTextBlock(systemContent)))
-			}
+			// System messages are handled via the top-level params.System
 			continue
 		}
 		if msg.Role == chat.MessageRoleUser {
@@ -301,6 +308,30 @@ func convertMessages(messages []chat.Message) []anthropic.MessageParam {
 	return anthropicMessages
 }
 
+// extractSystemBlocks converts any system-role messages into Anthropic system text blocks
+// to be set on the top-level MessageNewParams.System field.
+func extractSystemBlocks(messages []chat.Message) []anthropic.TextBlockParam {
+	var systemBlocks []anthropic.TextBlockParam
+	for i := range messages {
+		msg := &messages[i]
+		if msg.Role != chat.MessageRoleSystem {
+			continue
+		}
+		if len(msg.MultiContent) > 0 {
+			for _, part := range msg.MultiContent {
+				if part.Type == chat.MessagePartTypeText {
+					if txt := strings.TrimSpace(part.Text); txt != "" {
+						systemBlocks = append(systemBlocks, anthropic.TextBlockParam{Text: txt})
+					}
+				}
+			}
+		} else if txt := strings.TrimSpace(msg.Content); txt != "" {
+			systemBlocks = append(systemBlocks, anthropic.TextBlockParam{Text: txt})
+		}
+	}
+	return systemBlocks
+}
+
 func convertTools(tooles []tools.Tool) []anthropic.ToolUnionParam {
 	toolParams := make([]anthropic.ToolParam, len(tooles))
 

pkg/model/provider/anthropic/client_test.go

@@ -2,6 +2,7 @@ package anthropic
 
 import (
 	"encoding/json"
+	"strings"
 	"testing"
 
 	"github.com/docker/cagent/pkg/chat"
@@ -118,3 +119,91 @@ func TestConvertMessages_AssistantToolCalls_NoText_IncludesToolUse(t *testing.T)
 		t.Fatalf("expected content block type 'tool_use', got %v", typ)
 	}
 }
+
+func TestSystemMessages_AreExtractedAndNotInMessageList(t *testing.T) {
+	msgs := []chat.Message{
+		{Role: chat.MessageRoleSystem, Content: "  system rules here  "},
+		{Role: chat.MessageRoleUser, Content: "hi"},
+	}
+
+	// System blocks should be extracted
+	sys := extractSystemBlocks(msgs)
+	if len(sys) != 1 {
+		t.Fatalf("expected 1 system block, got %d", len(sys))
+	}
+	if strings.TrimSpace(sys[0].Text) != "system rules here" {
+		t.Fatalf("unexpected system text: %q", sys[0].Text)
+	}
+
+	// System role messages must not appear in the anthropic messages list
+	out := convertMessages(msgs)
+	if len(out) != 1 {
+		t.Fatalf("expected 1 non-system message, got %d", len(out))
+	}
+}
+
+func TestSystemMessages_MultipleExtractedAndExcludedFromMessageList(t *testing.T) {
+	msgs := []chat.Message{
+		{Role: chat.MessageRoleSystem, Content: " sys A "},
+		{Role: chat.MessageRoleSystem, Content: "\n sys B \t"},
+		{Role: chat.MessageRoleUser, Content: "hello"},
+	}
+
+	sys := extractSystemBlocks(msgs)
+	if len(sys) != 2 {
+		t.Fatalf("expected 2 system blocks, got %d", len(sys))
+	}
+	if strings.TrimSpace(sys[0].Text) != "sys A" {
+		t.Fatalf("unexpected first system text: %q", sys[0].Text)
+	}
+	if strings.TrimSpace(sys[1].Text) != "sys B" {
+		t.Fatalf("unexpected second system text: %q", sys[1].Text)
+	}
+
+	out := convertMessages(msgs)
+	if len(out) != 1 {
+		t.Fatalf("expected 1 non-system message, got %d", len(out))
+	}
+}
+
+func TestSystemMessages_InterspersedExtractedAndExcluded(t *testing.T) {
+	msgs := []chat.Message{
+		{Role: chat.MessageRoleSystem, Content: " S1 "},
+		{Role: chat.MessageRoleUser, Content: "U1"},
+		{Role: chat.MessageRoleAssistant, Content: "A1"},
+		{Role: chat.MessageRoleSystem, Content: "S2"},
+		{Role: chat.MessageRoleUser, Content: " U2 "},
+	}
+
+	// All system messages should be extracted in order of appearance
+	sys := extractSystemBlocks(msgs)
+	if len(sys) != 2 {
+		t.Fatalf("expected 2 system blocks, got %d", len(sys))
+	}
+	if strings.TrimSpace(sys[0].Text) != "S1" {
+		t.Fatalf("unexpected first system text: %q", sys[0].Text)
+	}
+	if strings.TrimSpace(sys[1].Text) != "S2" {
+		t.Fatalf("unexpected second system text: %q", sys[1].Text)
+	}
+
+	// Converted messages must exclude system roles and preserve order of others
+	out := convertMessages(msgs)
+	if len(out) != 3 {
+		t.Fatalf("expected 3 non-system messages, got %d", len(out))
+	}
+	// Check roles: user, assistant, user
+	for i, expected := range []string{"user", "assistant", "user"} {
+		b, err := json.Marshal(out[i])
+		if err != nil {
+			t.Fatalf("marshal error: %v", err)
+		}
+		var m map[string]any
+		if err := json.Unmarshal(b, &m); err != nil {
+			t.Fatalf("unmarshal error: %v", err)
+		}
+		if role, _ := m["role"].(string); role != expected {
+			t.Fatalf("unexpected role at %d: got %q want %q", i, role, expected)
+		}
+	}
+}