diff --git a/proxy/mockupstream_test.go b/proxy/mockupstream_test.go
new file mode 100644
index 0000000..7abc057
--- /dev/null
+++ b/proxy/mockupstream_test.go
@@ -0,0 +1,691 @@
+package main
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+	"testing"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus/testutil"
+)
+
+// TestMockUpstream_AllScenarios verifies that all MockUpstream scenarios work correctly.
+func TestMockUpstream_AllScenarios(t *testing.T) {
+	t.Run("429-with-retry-after-then-success", func(t *testing.T) {
+		mock := NewMockUpstream("429-with-retry-after-then-success")
+		defer mock.Close()
+
+		// Configure to fail for first 2 requests, then succeed
+		mock.SetFailUntilCount(2)
+		mock.SetRetryAfter("5")
+
+		// Make 3 requests - first 2 should return 429, third should succeed
+		for i := 0; i < 3; i++ {
+			req, _ := http.NewRequest("GET", mock.URL()+"/test", nil)
+			resp, err := http.DefaultClient.Do(req)
+			if err != nil {
+				t.Fatalf("Request %d failed: %v", i, err)
+			}
+
+			if i < 2 {
+				// First 2 requests should return 429 with Retry-After
+				if resp.StatusCode != http.StatusTooManyRequests {
+					t.Errorf("Request %d: expected status 429, got %d", i, resp.StatusCode)
+				}
+				if retryAfter := resp.Header.Get("Retry-After"); retryAfter != "5" {
+					t.Errorf("Request %d: expected Retry-After=5, got %s", i, retryAfter)
+				}
+			} else {
+				// Third request should succeed
+				if resp.StatusCode != http.StatusOK {
+					t.Errorf("Request 2: expected status 200, got %d", resp.StatusCode)
+				}
+				// Verify response body is valid JSON
+				if !jsonValid(t, resp) {
+					t.Error("Request 2: response body should be valid JSON")
+				}
+			}
+			resp.Body.Close()
+		}
+
+		// Verify request count is 3
+		if count := mock.GetRequestCount(); count != 3 {
+			t.Errorf("Expected 3 requests, got %d", count)
+		}
+	})
+
+	t.Run("429-no-header-then-429", func(t *testing.T) {
+		mock := NewMockUpstream("429-no-header-then-429")
+		defer mock.Close()
+
+		// All requests should return 429 without Retry-After header
+		for i := 0; i < 3; i++ {
+			resp, err := http.Get(mock.URL() + "/test")
+			if err != nil {
+				t.Fatalf("Request %d failed: %v", i, err)
+			}
+
+			if resp.StatusCode != http.StatusTooManyRequests {
+				t.Errorf("Request %d: expected status 429, got %d", i, resp.StatusCode)
+			}
+			if retryAfter := resp.Header.Get("Retry-After"); retryAfter != "" {
+				t.Errorf("Request %d: expected no Retry-After header, got %s", i, retryAfter)
+			}
+			resp.Body.Close()
+		}
+
+		if count := mock.GetRequestCount(); count != 3 {
+			t.Errorf("Expected 3 requests, got %d", count)
+		}
+	})
+
+	t.Run("empty-json-body", func(t *testing.T) {
+		mock := NewMockUpstream("empty-json-body")
+		defer mock.Close()
+
+		resp, err := http.Get(mock.URL() + "/test")
+		if err != nil {
+			t.Fatalf("Request failed: %v", err)
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode != http.StatusOK {
+			t.Errorf("Expected status 200, got %d", resp.StatusCode)
+		}
+
+		body, _ := io.ReadAll(resp.Body)
+		if len(body) != 0 {
+			t.Errorf("Expected empty body, got %d bytes: %s", len(body), string(body))
+		}
+	})
+
+	t.Run("invalid-json-body", func(t *testing.T) {
+		mock := NewMockUpstream("invalid-json-body")
+		defer mock.Close()
+
+		resp, err := http.Get(mock.URL() + "/test")
+		if err != nil {
+			t.Fatalf("Request failed: %v", err)
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode != http.StatusOK {
+			t.Errorf("Expected status 200, got %d", resp.StatusCode)
+		}
+
+		body, _ := io.ReadAll(resp.Body)
+		if string(body) != "{invalid json" {
+			t.Errorf("Expected invalid JSON body, got %s", string(body))
+		}
+	})
+
+	t.Run("empty-streaming", func(t *testing.T) {
+		mock := NewMockUpstream("empty-streaming")
+		defer mock.Close()
+
+		resp, err := http.Get(mock.URL() + "/test")
+		if err != nil {
+			t.Fatalf("Request failed: %v", err)
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode != http.StatusOK {
+			t.Errorf("Expected status 200, got %d", resp.StatusCode)
+		}
+
+		if ct := resp.Header.Get("Content-Type"); ct != "text/event-stream" {
+			t.Errorf("Expected Content-Type text/event-stream, got %s", ct)
+		}
+
+		body, _ := io.ReadAll(resp.Body)
+		if len(body) != 0 {
+			t.Errorf("Expected empty streaming body, got %d bytes", len(body))
+		}
+	})
+
+	t.Run("422-no-retry", func(t *testing.T) {
+		mock := NewMockUpstream("422-no-retry")
+		defer mock.Close()
+
+		resp, err := http.Get(mock.URL() + "/test")
+		if err != nil {
+			t.Fatalf("Request failed: %v", err)
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode != http.StatusUnprocessableEntity {
+			t.Errorf("Expected status 422, got %d", resp.StatusCode)
+		}
+
+		// Should have JSON body with error message
+		var result map[string]string
+		body, _ := io.ReadAll(resp.Body)
+		if err := json.Unmarshal(body, &result); err != nil {
+			t.Fatalf("Failed to parse JSON: %v", err)
+		}
+		if result["error"] != "unprocessable entity" {
+			t.Errorf("Expected error='unprocessable entity', got %s", result["error"])
+		}
+	})
+
+	t.Run("500-no-retry", func(t *testing.T) {
+		mock := NewMockUpstream("500-no-retry")
+		defer mock.Close()
+
+		resp, err := http.Get(mock.URL() + "/test")
+		if err != nil {
+			t.Fatalf("Request failed: %v", err)
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode != http.StatusInternalServerError {
+			t.Errorf("Expected status 500, got %d", resp.StatusCode)
+		}
+	})
+
+	t.Run("404-no-retry", func(t *testing.T) {
+		mock := NewMockUpstream("404-no-retry")
+		defer mock.Close()
+
+		resp, err := http.Get(mock.URL() + "/test")
+		if err != nil {
+			t.Fatalf("Request failed: %v", err)
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode != http.StatusNotFound {
+			t.Errorf("Expected status 404, got %d", resp.StatusCode)
+		}
+	})
+
+	t.Run("network-error-then-success", func(t *testing.T) {
+		mock := NewMockUpstream("network-error-then-success")
+		defer mock.Close()
+
+		// Configure to fail for first request, then succeed
+		mock.SetFailUntilCount(1)
+
+		// First request will succeed but with empty body (httptest.Server limitation)
+		// The real network simulation would fail, but in httptest the handler just returns
+		resp1, err := http.Get(mock.URL() + "/test")
+		if err != nil {
+			t.Logf("First request failed (acceptable): %v", err)
+		} else {
+			// In httptest, the request succeeds but body is empty
+			resp1.Body.Close()
+			if resp1.StatusCode != http.StatusOK {
+				t.Logf("First request status: %d (may not be 200 due to early return)", resp1.StatusCode)
+			}
+		}
+
+		// Second request should succeed with full response
+		resp2, err := http.Get(mock.URL() + "/test")
+		if err != nil {
+			t.Fatalf("Second request failed: %v", err)
+		}
+		defer resp2.Body.Close()
+
+		if resp2.StatusCode != http.StatusOK {
+			t.Errorf("Second request: expected status 200, got %d", resp2.StatusCode)
+		}
+		if !jsonValid(t, resp2) {
+			t.Error("Second request: response should be valid JSON")
+		}
+
+		// Request count should be 2
+		if count := mock.GetRequestCount(); count != 2 {
+			t.Errorf("Expected 2 requests, got %d", count)
+		}
+	})
+
+	t.Run("success", func(t *testing.T) {
+		mock := NewMockUpstream("success")
+		defer mock.Close()
+
+		resp, err := http.Get(mock.URL() + "/test")
+		if err != nil {
+			t.Fatalf("Request failed: %v", err)
+		}
+
+		if resp.StatusCode != http.StatusOK {
+			t.Errorf("Expected status 200, got %d", resp.StatusCode)
+		}
+
+		// Read and verify response structure
+		var result map[string]interface{}
+		body, _ := io.ReadAll(resp.Body)
+		resp.Body.Close()
+
+		if !json.Valid(body) {
+			t.Error("Response should be valid JSON")
+		}
+
+		if err := json.Unmarshal(body, &result); err != nil {
+			t.Fatalf("Failed to parse JSON: %v", err)
+		}
+		if result["id"] != "msg_test123" {
+			t.Errorf("Expected id=msg_test123, got %s", result["id"])
+		}
+		if result["type"] != "message" {
+			t.Errorf("Expected type=message, got %s", result["type"])
+		}
+	})
+
+	t.Run("malformed-streaming", func(t *testing.T) {
+		mock := NewMockUpstream("malformed-streaming")
+		defer mock.Close()
+
+		resp, err := http.Get(mock.URL() + "/test")
+		if err != nil {
+			t.Fatalf("Request failed: %v", err)
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode != http.StatusOK {
+			t.Errorf("Expected status 200, got %d", resp.StatusCode)
+		}
+
+		if ct := resp.Header.Get("Content-Type"); ct != "text/event-stream" {
+			t.Errorf("Expected Content-Type text/event-stream, got %s", ct)
+		}
+
+		body, _ := io.ReadAll(resp.Body)
+		if string(body) != "not valid sse format\n" {
+			t.Errorf("Expected malformed SSE data, got %s", string(body))
+		}
+	})
+}
+
+// TestMockUpstream_Methods verifies all MockUpstream methods work correctly.
+func TestMockUpstream_Methods(t *testing.T) {
+	t.Run("GetRequestCount", func(t *testing.T) {
+		mock := NewMockUpstream("success")
+		defer mock.Close()
+
+		// Initially 0
+		if count := mock.GetRequestCount(); count != 0 {
+			t.Errorf("Initial count should be 0, got %d", count)
+		}
+
+		// Make 3 requests
+		for i := 0; i < 3; i++ {
+			http.Get(mock.URL() + "/test")
+		}
+
+		// Should be 3
+		if count := mock.GetRequestCount(); count != 3 {
+			t.Errorf("Expected 3 requests, got %d", count)
+		}
+	})
+
+	t.Run("SetFailUntilCount", func(t *testing.T) {
+		mock := NewMockUpstream("429-with-retry-after-then-success")
+		defer mock.Close()
+		mock.SetRetryAfter("1")
+
+		// Test with fail count of 2
+		mock.SetFailUntilCount(2)
+
+		// First request should fail
+		resp1, _ := http.Get(mock.URL() + "/test")
+		if resp1.StatusCode != http.StatusTooManyRequests {
+			t.Errorf("First request should fail with 429, got %d", resp1.StatusCode)
+		}
+		resp1.Body.Close()
+
+		// Second request should fail
+		resp2, _ := http.Get(mock.URL() + "/test")
+		if resp2.StatusCode != http.StatusTooManyRequests {
+			t.Errorf("Second request should fail with 429, got %d", resp2.StatusCode)
+		}
+		resp2.Body.Close()
+
+		// Third request should succeed
+		resp3, _ := http.Get(mock.URL() + "/test")
+		if resp3.StatusCode != http.StatusOK {
+			t.Errorf("Third request should succeed, got %d", resp3.StatusCode)
+		}
+		resp3.Body.Close()
+	})
+
+	t.Run("SetRetryAfter", func(t *testing.T) {
+		mock := NewMockUpstream("429-with-retry-after-then-success")
+		defer mock.Close()
+		mock.SetFailUntilCount(1)
+
+		// Set custom retry-after value
+		mock.SetRetryAfter("10")
+
+		resp, _ := http.Get(mock.URL() + "/test")
+		defer resp.Body.Close()
+
+		if retryAfter := resp.Header.Get("Retry-After"); retryAfter != "10" {
+			t.Errorf("Expected Retry-After=10, got %s", retryAfter)
+		}
+	})
+
+	t.Run("SetDelayMs", func(t *testing.T) {
+		mock := NewMockUpstream("success")
+		defer mock.Close()
+
+		// Set 100ms delay
+		mock.SetDelayMs(100)
+
+		start := time.Now()
+		resp, err := http.Get(mock.URL() + "/test")
+		if err != nil {
+			t.Fatalf("Request failed: %v", err)
+		}
+		resp.Body.Close()
+		elapsed := time.Since(start)
+
+		// Should take at least 100ms (with some tolerance for test execution)
+		if elapsed < 90*time.Millisecond {
+			t.Errorf("Expected delay >= 100ms, got %v", elapsed)
+		}
+		t.Logf("Delay test: %v elapsed", elapsed)
+	})
+
+	t.Run("SetCustomHandler", func(t *testing.T) {
+		mock := NewMockUpstream("success")
+		defer mock.Close()
+
+		// Set custom handler that returns 418
+		mock.SetCustomHandler(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(418)
+			w.Write([]byte("I'm a teapot"))
+		})
+
+		resp, err := http.Get(mock.URL() + "/test")
+		if err != nil {
+			t.Fatalf("Request failed: %v", err)
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode != 418 {
+			t.Errorf("Expected status 418, got %d", resp.StatusCode)
+		}
+
+		body, _ := io.ReadAll(resp.Body)
+		if string(body) != "I'm a teapot" {
+			t.Errorf("Expected 'I'm a teapot', got %s", string(body))
+		}
+	})
+
+	t.Run("ResetCounters", func(t *testing.T) {
+		mock := NewMockUpstream("success")
+		defer mock.Close()
+
+		// Make some requests
+		for i := 0; i < 3; i++ {
+			http.Get(mock.URL() + "/test")
+		}
+
+		// Verify count is 3
+		if count := mock.GetRequestCount(); count != 3 {
+			t.Errorf("Expected 3 requests, got %d", count)
+		}
+
+		// Reset counters
+		mock.ResetCounters()
+
+		// Should be 0 now
+		if count := mock.GetRequestCount(); count != 0 {
+			t.Errorf("After reset, count should be 0, got %d", count)
+		}
+
+		// Make another request
+		http.Get(mock.URL() + "/test")
+
+		// Should be 1
+		if count := mock.GetRequestCount(); count != 1 {
+			t.Errorf("After reset and 1 request, expected 1, got %d", count)
+		}
+	})
+}
+
+// TestMockUpstream_ProxyIntegration tests MockUpstream scenarios through the proxy.
+func TestMockUpstream_ProxyIntegration(t *testing.T) {
+	ConfigureTestEnv(t)
+	t.Setenv(TestMaxRetriesEnv, "2")
+
+	t.Run("success scenario through proxy", func(t *testing.T) {
+		mock := NewMockUpstream("success")
+		defer mock.Close()
+
+		handler := CreateTestProxyHandler(t, mock.URL(), 2)
+		req := CreateNonStreamingMessagesRequest()
+
+		start := time.Now()
+		resp := ExecuteProxyRequest(t, handler, req)
+		duration := time.Since(start)
+
+		// Should succeed immediately without retries
+		if resp.StatusCode != http.StatusOK {
+			t.Errorf("Expected status 200, got %d", resp.StatusCode)
+		}
+
+		// Should only call upstream once
+		if count := mock.GetRequestCount(); count != 1 {
+			t.Errorf("Expected 1 upstream call, got %d", count)
+		}
+
+		// Should complete quickly (no retry delays)
+		if duration > 500*time.Millisecond {
+			t.Errorf("Expected fast completion, got %v", duration)
+		}
+
+		t.Logf("Success scenario: %v, %d calls", duration, mock.GetRequestCount())
+	})
+
+	t.Run("429 with retry after through proxy", func(t *testing.T) {
+		mock := NewMockUpstream("429-with-retry-after-then-success")
+		defer mock.Close()
+
+		mock.SetFailUntilCount(1)
+		mock.SetRetryAfter("1")
+
+		// Reset metrics
+		retryAttempts.Reset()
+		upstreamErrors.Reset()
+
+		handler := CreateTestProxyHandler(t, mock.URL(), 2)
+		req := CreateNonStreamingMessagesRequest()
+
+		start := time.Now()
+		resp := ExecuteProxyRequest(t, handler, req)
+		duration := time.Since(start)
+
+		// Should succeed after retry
+		if resp.StatusCode != http.StatusOK {
+			t.Errorf("Expected status 200, got %d", resp.StatusCode)
+		}
+
+		// Should call upstream twice (initial + 1 retry)
+		if count := mock.GetRequestCount(); count != 2 {
+			t.Errorf("Expected 2 upstream calls, got %d", count)
+		}
+
+		// Verify retry behavior occurred via call count and timing
+		// Metrics are tested separately in response_validation_test.go
+
+		// Should take at least 1 second (Retry-After: 1)
+		if duration < 900*time.Millisecond {
+			t.Errorf("Expected >=1s delay, got %v", duration)
+		}
+
+		t.Logf("429 retry: %v, %d calls", duration, mock.GetRequestCount())
+	})
+
+	t.Run("empty json body through proxy triggers retry", func(t *testing.T) {
+		mock := NewMockUpstream("empty-json-body")
+		defer mock.Close()
+
+		// Reset metrics
+		retryAttempts.Reset()
+		upstreamErrors.Reset()
+
+		handler := CreateTestProxyHandler(t, mock.URL(), 1)
+		req := CreateNonStreamingMessagesRequest()
+
+		start := time.Now()
+		resp := ExecuteProxyRequest(t, handler, req)
+		duration := time.Since(start)
+
+		// Should return 502 after exhausting retries
+		if resp.StatusCode != http.StatusBadGateway {
+			t.Errorf("Expected status 502, got %d", resp.StatusCode)
+		}
+
+		// Should call upstream twice (initial + 1 retry)
+		if count := mock.GetRequestCount(); count != 2 {
+			t.Errorf("Expected 2 upstream calls, got %d", count)
+		}
+
+		// Verify error metric
+		errorMetric := upstreamErrors.WithLabelValues("truncated_response", "test")
+		errorCount := testutil.ToFloat64(errorMetric)
+		if errorCount != 2.0 {
+			t.Errorf("Expected upstream_errors=2, got %f", errorCount)
+		}
+
+		t.Logf("Empty JSON: %v, %d calls, errors=%f", duration, mock.GetRequestCount(), errorCount)
+	})
+
+	t.Run("invalid json body through proxy triggers retry", func(t *testing.T) {
+		mock := NewMockUpstream("invalid-json-body")
+		defer mock.Close()
+
+		// Reset metrics
+		retryAttempts.Reset()
+		upstreamErrors.Reset()
+
+		handler := CreateTestProxyHandler(t, mock.URL(), 1)
+		req := CreateNonStreamingMessagesRequest()
+
+		resp := ExecuteProxyRequest(t, handler, req)
+
+		// Should return 502 after exhausting retries
+		if resp.StatusCode != http.StatusBadGateway {
+			t.Errorf("Expected status 502, got %d", resp.StatusCode)
+		}
+
+		// Should call upstream twice (initial + 1 retry)
+		if count := mock.GetRequestCount(); count != 2 {
+			t.Errorf("Expected 2 upstream calls, got %d", count)
+		}
+
+		t.Logf("Invalid JSON: %d calls", mock.GetRequestCount())
+	})
+
+	t.Run("empty streaming through proxy triggers retry", func(t *testing.T) {
+		mock := NewMockUpstream("empty-streaming")
+		defer mock.Close()
+
+		// Reset metrics
+		retryAttempts.Reset()
+		upstreamErrors.Reset()
+
+		handler := CreateTestProxyHandler(t, mock.URL(), 1)
+		req := CreateStreamingMessagesRequest()
+
+		resp := ExecuteProxyRequest(t, handler, req)
+
+		// Should return 502 after exhausting retries
+		if resp.StatusCode != http.StatusBadGateway {
+			t.Errorf("Expected status 502, got %d", resp.StatusCode)
+		}
+
+		// Should call upstream twice (initial + 1 retry)
+		if count := mock.GetRequestCount(); count != 2 {
+			t.Errorf("Expected 2 upstream calls, got %d", count)
+		}
+
+		// Verify error metric with correct error_type
+		errorMetric := upstreamErrors.WithLabelValues("empty_streaming", "test")
+		errorCount := testutil.ToFloat64(errorMetric)
+		if errorCount != 2.0 {
+			t.Errorf("Expected upstream_errors=2 for empty_streaming, got %f", errorCount)
+		}
+
+		t.Logf("Empty streaming: %d calls, errors=%f", mock.GetRequestCount(), errorCount)
+	})
+
+	t.Run("422 no retry through proxy", func(t *testing.T) {
+		mock := NewMockUpstream("422-no-retry")
+		defer mock.Close()
+
+		handler := CreateTestProxyHandler(t, mock.URL(), 2)
+		req := CreateNonStreamingMessagesRequest()
+
+		resp := ExecuteProxyRequest(t, handler, req)
+
+		// Should return 422 without retry
+		if resp.StatusCode != http.StatusUnprocessableEntity {
+			t.Errorf("Expected status 422, got %d", resp.StatusCode)
+		}
+
+		// Should only call upstream once (no retry)
+		if count := mock.GetRequestCount(); count != 1 {
+			t.Errorf("Expected 1 upstream call (no retry), got %d", count)
+		}
+
+		t.Logf("422 no retry: %d calls", mock.GetRequestCount())
+	})
+
+	t.Run("500 no retry through proxy", func(t *testing.T) {
+		mock := NewMockUpstream("500-no-retry")
+		defer mock.Close()
+
+		handler := CreateTestProxyHandler(t, mock.URL(), 2)
+		req := CreateNonStreamingMessagesRequest()
+
+		resp := ExecuteProxyRequest(t, handler, req)
+
+		// Should return 500 without retry
+		if resp.StatusCode != http.StatusInternalServerError {
+			t.Errorf("Expected status 500, got %d", resp.StatusCode)
+		}
+
+		// Should only call upstream once (no retry)
+		if count := mock.GetRequestCount(); count != 1 {
+			t.Errorf("Expected 1 upstream call (no retry), got %d", count)
+		}
+
+		t.Logf("500 no retry: %d calls", mock.GetRequestCount())
+	})
+
+	t.Run("404 no retry through proxy", func(t *testing.T) {
+		mock := NewMockUpstream("404-no-retry")
+		defer mock.Close()
+
+		handler := CreateTestProxyHandler(t, mock.URL(), 2)
+		req := CreateNonStreamingMessagesRequest()
+
+		resp := ExecuteProxyRequest(t, handler, req)
+
+		// Should return 404 without retry
+		if resp.StatusCode != http.StatusNotFound {
+			t.Errorf("Expected status 404, got %d", resp.StatusCode)
+		}
+
+		// Should only call upstream once (no retry)
+		if count := mock.GetRequestCount(); count != 1 {
+			t.Errorf("Expected 1 upstream call (no retry), got %d", count)
+		}
+
+		t.Logf("404 no retry: %d calls", mock.GetRequestCount())
+	})
+}
+
+// jsonValid checks if a response body contains valid JSON
+func jsonValid(t *testing.T, resp *http.Response) bool {
+	t.Helper()
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Errorf("Failed to read body: %v", err)
+		return false
+	}
+	return json.Valid(body)
+}