fix(apicompat): default stop_reason for empty-choices responses

Adversarial re-verification found one more divergence from the
double-conversion chain: an upstream 200 with empty choices (or a nil
response) left stop_reason as an empty string, while the old chain
reports end_turn. Derive the fallback from the content blocks — the
guard never fires when choices exist, since every finish_reason maps to
a non-empty stop_reason.

Also strengthen the equivalence tests: compare tool_use Content[].Input
and tool_call Function.Arguments across bridges, and add an
empty-choices stop_reason parity case.

backend/internal/pkg/apicompat/chatcompletions_anthropic_bridge.go

@@ -402,6 +402,12 @@ func ChatCompletionsResponseToAnthropic(resp *ChatCompletionsResponse, model str
 	if len(out.Content) == 0 {
 		out.Content = []AnthropicContentBlock{{Type: "text", Text: ""}}
 	}
+	// Empty choices / nil response never enter the choices branch above; the
+	// double-conversion path still reports a completed turn ("end_turn"), and
+	// stop_reason "" is invalid for strict Anthropic clients.
+	if out.StopReason == "" {
+		out.StopReason = chatFinishReasonToAnthropicStopReason("", out.Content)
+	}
 	// The double-conversion path generates a response id when the upstream
 	// omits one (ChatCompletionsResponseToResponses); clients treat it as required.
 	if out.ID == "" {

backend/internal/pkg/apicompat/chatcompletions_anthropic_bridge_test.go

@@ -398,6 +398,7 @@ func TestChatCompletionsResponseToAnthropic_EmptyChoices(t *testing.T) {
 	require.Len(t, out.Content, 1)
 	require.Equal(t, "text", out.Content[0].Type)
 	require.Equal(t, "", out.Content[0].Text)
+	require.Equal(t, "end_turn", out.StopReason, "empty choices must not produce an empty stop_reason")
 }
 
 func TestChatCompletionsResponseToAnthropic_CacheTokens(t *testing.T) {
@@ -432,6 +433,8 @@ func TestChatCompletionsResponseToAnthropic_NilResponse(t *testing.T) {
 	out := ChatCompletionsResponseToAnthropic(nil, "claude-sonnet-4-20250514")
 	require.Len(t, out.Content, 1)
 	require.Equal(t, "text", out.Content[0].Type)
+	require.Equal(t, "end_turn", out.StopReason, "nil response must not produce an empty stop_reason")
+	require.NotEmpty(t, out.ID)
 }
 
 // ---------------------------------------------------------------------------
@@ -684,6 +687,7 @@ func TestDirectBridge_NonStreamingMatchesDoubleConversion(t *testing.T) {
 		require.Equal(t, double.Content[i].Thinking, direct.Content[i].Thinking, "block %d thinking mismatch", i)
 		require.Equal(t, double.Content[i].Name, direct.Content[i].Name, "block %d name mismatch", i)
 		require.Equal(t, double.Content[i].ID, direct.Content[i].ID, "block %d id mismatch", i)
+		require.Equal(t, string(double.Content[i].Input), string(direct.Content[i].Input), "block %d input mismatch", i)
 	}
 	require.Equal(t, double.Usage.InputTokens, direct.Usage.InputTokens)
 	require.Equal(t, double.Usage.OutputTokens, direct.Usage.OutputTokens)
@@ -744,6 +748,7 @@ func TestDirectBridge_RequestMatchesDoubleConversion(t *testing.T) {
 		for j := range direct.Messages[i].ToolCalls {
 			require.Equal(t, double.Messages[i].ToolCalls[j].ID, direct.Messages[i].ToolCalls[j].ID, "msg %d tool %d id mismatch", i, j)
 			require.Equal(t, double.Messages[i].ToolCalls[j].Function.Name, direct.Messages[i].ToolCalls[j].Function.Name, "msg %d tool %d name mismatch", i, j)
+			require.Equal(t, double.Messages[i].ToolCalls[j].Function.Arguments, direct.Messages[i].ToolCalls[j].Function.Arguments, "msg %d tool %d arguments mismatch", i, j)
 		}
 	}
 }
@@ -1046,6 +1051,20 @@ func TestAnthropicToChatCompletionsRequest_ToolChoiceUndeclaredDropped(t *testin
 	require.JSONEq(t, `{"type":"function","function":{"name":"get_weather"}}`, string(out.ToolChoice))
 }
 
+func TestDirectBridge_NonStreamingMatchesDoubleConversion_EmptyChoices(t *testing.T) {
+	// An upstream 200 with empty choices must still report a valid stop_reason,
+	// matching the double-conversion chain ("end_turn").
+	resp := &ChatCompletionsResponse{ID: "chatcmpl-empty", Model: "deepseek-v4-pro"}
+
+	direct := ChatCompletionsResponseToAnthropic(resp, "claude-sonnet-4-20250514")
+
+	responsesResp := ChatCompletionsResponseToResponses(resp, "claude-sonnet-4-20250514", nil, false, nil)
+	double := ResponsesToAnthropic(responsesResp, "claude-sonnet-4-20250514")
+
+	require.Equal(t, double.StopReason, direct.StopReason)
+	require.Equal(t, "end_turn", direct.StopReason)
+}
+
 func TestChatCompletionsResponseToAnthropic_ContentFilterWithToolUse(t *testing.T) {
 	// content_filter (and unknown finish reasons) derive stop_reason from the
 	// blocks, like the double-conversion path.