fix(gemini): keep every system message in the system instruction

pkg/llm/gemini/gemini.go

@@ -100,10 +100,14 @@ func (p *Provider) GenerateStream(ctx context.Context, memory []history.Message,
 			return agent.LLMResult{}, fmt.Errorf("gemini: %w: %s message carries %d media parts, which this API accepts only on user messages",
 				agent.ErrUnrenderablePart, m.Role, len(m.Parts))
 		}
+		// Gemini takes a single system instruction, so every system
+		// message joins it as its own part, in order. Keeping only the
+		// last one would let a trailing hint replace the system prompt.
 		if m.Role == "system" {
-			systemInstruction = &genai.Content{
-				Parts: []*genai.Part{{Text: m.Content}},
+			if systemInstruction == nil {
+				systemInstruction = &genai.Content{}
 			}
+			systemInstruction.Parts = append(systemInstruction.Parts, &genai.Part{Text: m.Content})
 			continue
 		}
 

pkg/llm/gemini/system_instruction_test.go

@@ -0,0 +1,67 @@
+package gemini
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/hung12ct/gopheragent/pkg/agent"
+	"github.com/hung12ct/gopheragent/pkg/history"
+	"google.golang.org/genai"
+)
+
+// A loop hint appended as a trailing system message used to replace the
+// system prompt outright, because only the last system message was kept.
+func TestGenerateStream_AllSystemMessagesReachSystemInstruction(t *testing.T) {
+	var body []byte
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ = io.ReadAll(r.Body)
+		w.Header().Set("Content-Type", "text/event-stream")
+		fmt.Fprint(w, "data: {\"candidates\":[{\"content\":{\"role\":\"model\",\"parts\":[{\"text\":\"ok\"}]},\"finishReason\":\"STOP\"}]}\r\n\r\n")
+	}))
+	t.Cleanup(srv.Close)
+	client, err := genai.NewClient(context.Background(), &genai.ClientConfig{
+		APIKey:      "test-key",
+		Backend:     genai.BackendGeminiAPI,
+		HTTPOptions: genai.HTTPOptions{BaseURL: srv.URL},
+	})
+	if err != nil {
+		t.Fatalf("new client: %v", err)
+	}
+	p := &Provider{client: client, model: "gemini-test"}
+
+	memory := []history.Message{
+		{Role: "system", Content: "you are a travel agent"},
+		{Role: "user", Content: "hi"},
+		{Role: "system", Content: "wrap up now"},
+	}
+	if _, err := p.GenerateStream(context.Background(), memory, nil, make(chan agent.StreamEvent, 16)); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	var req struct {
+		SystemInstruction struct {
+			Parts []struct {
+				Text string `json:"text"`
+			} `json:"parts"`
+		} `json:"systemInstruction"`
+		Contents []json.RawMessage `json:"contents"`
+	}
+	if err := json.Unmarshal(body, &req); err != nil {
+		t.Fatalf("decode request: %v", err)
+	}
+	var got []string
+	for _, part := range req.SystemInstruction.Parts {
+		got = append(got, part.Text)
+	}
+	if want := []string{"you are a travel agent", "wrap up now"}; fmt.Sprint(got) != fmt.Sprint(want) {
+		t.Fatalf("systemInstruction parts = %q, want %q", got, want)
+	}
+	if len(req.Contents) != 1 {
+		t.Fatalf("contents = %d, want only the user message", len(req.Contents))
+	}
+}