diff --git a/backend/internal/pkg/apicompat/chatcompletions_responses_bridge.go b/backend/internal/pkg/apicompat/chatcompletions_responses_bridge.go
index 8fb82ef4..09b680c7 100644
--- a/backend/internal/pkg/apicompat/chatcompletions_responses_bridge.go
+++ b/backend/internal/pkg/apicompat/chatcompletions_responses_bridge.go
@@ -89,7 +89,7 @@ func responsesInputToChatMessages(instructions string, inputRaw json.RawMessage)
 			return nil, fmt.Errorf("parse responses input item: %w", err)
 		}
 
-		role := rawString(item["role"])
+		role := chatCompletionsBridgeRole(rawString(item["role"]))
 		itemType := rawString(item["type"])
 		switch itemType {
 		case "function_call":
@@ -130,9 +130,6 @@ func responsesInputToChatMessages(instructions string, inputRaw json.RawMessage)
 			continue
 		}
 
-		if role == "" {
-			role = "user"
-		}
 		content := item["content"]
 		if len(bytesTrimSpace(content)) == 0 {
 			if text := rawString(item["text"]); text != "" {
@@ -152,6 +149,17 @@ func responsesInputToChatMessages(instructions string, inputRaw json.RawMessage)
 	return messages, nil
 }
 
+func chatCompletionsBridgeRole(role string) string {
+	trimmed := strings.TrimSpace(role)
+	if trimmed == "" {
+		return "user"
+	}
+	if strings.EqualFold(trimmed, "developer") {
+		return "system"
+	}
+	return role
+}
+
 func responsesContentToChatContent(raw json.RawMessage, role string) (json.RawMessage, error) {
 	raw = bytesTrimSpace(raw)
 	if len(raw) == 0 || string(raw) == "null" {
diff --git a/backend/internal/pkg/apicompat/chatcompletions_responses_bridge_test.go b/backend/internal/pkg/apicompat/chatcompletions_responses_bridge_test.go
new file mode 100644
index 00000000..3e55e23a
--- /dev/null
+++ b/backend/internal/pkg/apicompat/chatcompletions_responses_bridge_test.go
@@ -0,0 +1,82 @@
+package apicompat
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestResponsesInputToChatMessages_DeveloperRoleMapsToSystem(t *testing.T) {
+	messages, err := responsesInputToChatMessages("", json.RawMessage(`[{"role":"developer","content":"follow project instructions"}]`))
+	require.NoError(t, err)
+	require.Len(t, messages, 1)
+
+	assert.Equal(t, "system", messages[0].Role)
+	assert.JSONEq(t, `"follow project instructions"`, string(messages[0].Content))
+}
+
+func TestResponsesInputToChatMessages_KeepsChatCompletionRoles(t *testing.T) {
+	input := json.RawMessage(`[
+		{"role":"system","content":"system message"},
+		{"role":"user","content":"user message"},
+		{"role":"assistant","content":"assistant message"},
+		{"role":"tool","content":"tool message"}
+	]`)
+
+	messages, err := responsesInputToChatMessages("", input)
+	require.NoError(t, err)
+	require.Len(t, messages, 4)
+
+	assert.Equal(t, []string{"system", "user", "assistant", "tool"}, chatMessageRoles(messages))
+}
+
+func TestResponsesInputToChatMessages_EmptyRoleFallsBackToUser(t *testing.T) {
+	messages, err := responsesInputToChatMessages("", json.RawMessage(`[{"role":"","content":"hello"}]`))
+	require.NoError(t, err)
+	require.Len(t, messages, 1)
+
+	assert.Equal(t, "user", messages[0].Role)
+}
+
+func TestResponsesInputToChatMessages_DeveloperRoleTrimAndCaseInsensitive(t *testing.T) {
+	input := json.RawMessage(`[
+		{"role":" Developer ","content":"one"},
+		{"role":"\tDEVELOPER\n","content":"two"}
+	]`)
+
+	messages, err := responsesInputToChatMessages("", input)
+	require.NoError(t, err)
+	require.Len(t, messages, 2)
+
+	assert.Equal(t, []string{"system", "system"}, chatMessageRoles(messages))
+}
+
+func TestResponsesToChatCompletionsRequest_InstructionsAndInputDeveloperRole(t *testing.T) {
+	req := &ResponsesRequest{
+		Model:        "gpt-4o",
+		Instructions: "Use concise answers.",
+		Input: json.RawMessage(`[
+			{"role":"developer","content":[{"type":"input_text","text":"Prefer JSON."}]},
+			{"role":"user","content":"Hello"}
+		]`),
+	}
+
+	out, err := ResponsesToChatCompletionsRequest(req)
+	require.NoError(t, err)
+	require.Len(t, out.Messages, 3)
+
+	assert.Equal(t, []string{"system", "system", "user"}, chatMessageRoles(out.Messages))
+	assert.JSONEq(t, `"Use concise answers."`, string(out.Messages[0].Content))
+	assert.JSONEq(t, `"Prefer JSON."`, string(out.Messages[1].Content))
+	assert.JSONEq(t, `"Hello"`, string(out.Messages[2].Content))
+}
+
+func chatMessageRoles(messages []ChatMessage) []string {
+	roles := make([]string, 0, len(messages))
+	for _, message := range messages {
+		roles = append(roles, message.Role)
+	}
+	return roles
+}