diff --git a/internal/credentials/chatgpt.go b/internal/credentials/chatgpt.go
index a2a89ed..481e197 100644
--- a/internal/credentials/chatgpt.go
+++ b/internal/credentials/chatgpt.go
@@ -87,6 +87,7 @@ func NewChatGPTInjectorWithConfig(config ChatGPTOAuthConfig) *ChatGPTInjector {
 		config.HTTPClient = &http.Client{
 			Timeout: timeouts.ResponseHeader,
 			Transport: &http.Transport{
+				Proxy:                 nil, // token refresh must go directly to auth.openai.com, never through HTTP_PROXY
 				TLSClientConfig:       &tls.Config{MinVersion: tls.VersionTLS12},
 				TLSHandshakeTimeout:   timeouts.TLSHandshake,
 				ResponseHeaderTimeout: timeouts.ResponseHeader,
diff --git a/internal/credentials/gcloud.go b/internal/credentials/gcloud.go
index dd811d8..49fca23 100644
--- a/internal/credentials/gcloud.go
+++ b/internal/credentials/gcloud.go
@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"os"
 	"sync"
+	"time"
 
 	"github.com/bbrowning/paude-proxy/internal/timeouts"
 	"golang.org/x/oauth2"
@@ -64,6 +65,7 @@ func (g *GCloudInjector) init() error {
 		httpClient := &http.Client{
 			Timeout: timeouts.ResponseHeader,
 			Transport: &http.Transport{
+				Proxy:             nil, // token refresh must go directly to Google, never through HTTP_PROXY
 				DisableKeepAlives: true,
 				TLSClientConfig: &tls.Config{
 					MinVersion: tls.VersionTLS12,
@@ -108,10 +110,17 @@ func (g *GCloudInjector) Inject(req *http.Request) InjectResult {
 	}
 
 	if !token.Valid() {
-		log.Printf("WARN gcloud token is invalid after refresh")
+		log.Printf("WARN gcloud token is invalid after refresh (length=%d)", len(token.AccessToken))
 		return InjectFailed
 	}
 
+	if token.AccessToken == SyntheticToken || len(token.AccessToken) < 20 {
+		log.Printf("ERROR gcloud token looks like a dummy/synthetic token (length=%d) — will cause upstream 401. Check if proxy's own HTTP traffic is routing through itself (HTTP_PROXY env var set on proxy container?)", len(token.AccessToken))
+		return InjectFailed
+	}
+
+	log.Printf("GCLOUD_TOKEN_DEBUG length=%d type=%q expiry=%s", len(token.AccessToken), token.TokenType, token.Expiry.UTC().Format(time.RFC3339))
+
 	req.Header.Set("Authorization", "Bearer "+token.AccessToken)
 	return InjectOK
 }
diff --git a/internal/credentials/gcloud_test.go b/internal/credentials/gcloud_test.go
index bf328ba..6c7657d 100644
--- a/internal/credentials/gcloud_test.go
+++ b/internal/credentials/gcloud_test.go
@@ -60,3 +60,44 @@ func TestGCloudInjectorFromJSON_NilHeader(t *testing.T) {
 		t.Error("request with nil Header should not succeed")
 	}
 }
+
+func TestGCloudInjector_NeverReturnsInjectAuthRequired(t *testing.T) {
+	tests := []struct {
+		name     string
+		injector *GCloudInjector
+		req      *http.Request
+	}{
+		{
+			name:     "nil request",
+			injector: NewGCloudInjector("/nonexistent/path"),
+			req:      nil,
+		},
+		{
+			name:     "init failure from bad path",
+			injector: NewGCloudInjector("/nonexistent/path"),
+			req:      &http.Request{Header: make(http.Header)},
+		},
+		{
+			name:     "init failure from invalid JSON",
+			injector: NewGCloudInjectorFromJSON([]byte("not valid json")),
+			req:      &http.Request{Header: make(http.Header)},
+		},
+		{
+			name:     "init failure from empty JSON",
+			injector: NewGCloudInjectorFromJSON([]byte("{}")),
+			req:      &http.Request{Header: make(http.Header)},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := tt.injector.Inject(tt.req)
+			if result == InjectAuthRequired {
+				t.Error("GCloudInjector must never return InjectAuthRequired — only ChatGPTInjector should")
+			}
+			if result != InjectFailed {
+				t.Errorf("expected InjectFailed, got %d", result)
+			}
+		})
+	}
+}
diff --git a/internal/credentials/store.go b/internal/credentials/store.go
index efe01e8..6924437 100644
--- a/internal/credentials/store.go
+++ b/internal/credentials/store.go
@@ -7,6 +7,11 @@ import (
 	"sync"
 )
 
+// SyntheticToken is the dummy access token returned by the token vendor to
+// agents. The GCloudInjector checks for this value to detect accidental
+// self-proxying (token refresh routing through the proxy's own token vendor).
+const SyntheticToken = "paude-proxy-managed"
+
 // InjectResult describes the outcome of a credential injection attempt.
 type InjectResult int
 
diff --git a/internal/credentials/token_vending.go b/internal/credentials/token_vending.go
index 02895ac..f8099b2 100644
--- a/internal/credentials/token_vending.go
+++ b/internal/credentials/token_vending.go
@@ -150,7 +150,7 @@ func (tv *TokenVendor) HandleTokenExchange(req *http.Request) *http.Response {
 	var resp *tokenResponse
 	if tv.googleEnabled && IsTokenExchange(req) {
 		resp = &tokenResponse{
-			AccessToken: "paude-proxy-managed",
+			AccessToken: SyntheticToken,
 			ExpiresIn:   3600,
 			TokenType:   "Bearer",
 		}
diff --git a/internal/proxy/integration_test.go b/internal/proxy/integration_test.go
index 493534e..a264466 100644
--- a/internal/proxy/integration_test.go
+++ b/internal/proxy/integration_test.go
@@ -868,6 +868,9 @@ func TestIntegration_CredentialInjectionFailure_Returns502(t *testing.T) {
 	if resp.StatusCode != http.StatusBadGateway {
 		t.Errorf("expected 502 Bad Gateway, got %d", resp.StatusCode)
 	}
+	if resp.StatusCode == http.StatusUnauthorized {
+		t.Errorf("credential injection failure must never produce 401 Unauthorized")
+	}
 
 	body, _ := io.ReadAll(resp.Body)
 	if got := string(body); got != "Proxy credential injection failed" {
@@ -1070,3 +1073,71 @@ func TestIntegration_ChatGPTLoginSanitizeReject_HTTPS(t *testing.T) {
 		t.Errorf("status = %d, want 400", resp.StatusCode)
 	}
 }
+
+func TestIntegration_UpstreamErrorPassesThrough(t *testing.T) {
+	skipIntegration(t)
+
+	ca, err := GenerateCA()
+	if err != nil {
+		t.Fatalf("generate CA: %v", err)
+	}
+
+	tests := []struct {
+		name       string
+		statusCode int
+		body       string
+	}{
+		{"429 rate limit", http.StatusTooManyRequests, `{"error":{"code":429,"message":"Rate limit exceeded"}}`},
+		{"500 internal error", http.StatusInternalServerError, `{"error":{"code":500,"message":"Internal server error"}}`},
+		{"503 service unavailable", http.StatusServiceUnavailable, `{"error":{"code":503,"message":"Service unavailable"}}`},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			upstream := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				w.Header().Set("Content-Type", "application/json")
+				w.WriteHeader(tt.statusCode)
+				_, _ = w.Write([]byte(tt.body))
+			}))
+			defer upstream.Close()
+
+			upstreamURL, _ := url.Parse(upstream.URL)
+			upstreamHostname := upstreamURL.Hostname()
+
+			df := filter.NewDomainFilter(upstreamHostname)
+
+			store := credentials.NewStore()
+			store.AddRoute(credentials.Route{
+				ExactDomain: upstreamHostname,
+				Injector:    &credentials.BearerInjector{Token: "test-secret-key"},
+			})
+
+			upstreamCAs := upstreamCertPool(t, upstream)
+			upstreamCert := upstream.TLS.Certificates[0]
+			upstreamCA, _ := x509.ParseCertificate(upstreamCert.Certificate[0])
+
+			proxyAddr, cleanup := startTestProxy(t, ca, df, store, nil, upstreamCAs)
+			defer cleanup()
+
+			client := httpClientViaProxy(t, proxyAddr, ca.Certificate, upstreamCA)
+
+			resp, err := client.Get(upstream.URL + "/test")
+			if err != nil {
+				t.Fatalf("request failed: %v", err)
+			}
+			defer resp.Body.Close()
+
+			if resp.StatusCode != tt.statusCode {
+				t.Errorf("expected upstream status %d to pass through, got %d", tt.statusCode, resp.StatusCode)
+			}
+			if resp.StatusCode == http.StatusUnauthorized {
+				t.Errorf("upstream %d must never be converted to 401", tt.statusCode)
+			}
+
+			body, _ := io.ReadAll(resp.Body)
+			if string(body) != tt.body {
+				t.Errorf("response body not passed through unchanged\ngot:  %s\nwant: %s", body, tt.body)
+			}
+		})
+	}
+}
diff --git a/internal/proxy/proxy.go b/internal/proxy/proxy.go
index ccf49cd..252f66f 100644
--- a/internal/proxy/proxy.go
+++ b/internal/proxy/proxy.go
@@ -296,6 +296,10 @@ func (cf *ClientFilter) String() string {
 // Intentionally vague to avoid revealing why the request was blocked.
 const rejectMsg = "Request blocked by proxy policy"
 
+// credInjectedFlag is stored in ctx.UserData after successful credential
+// injection so the OnResponse handler can log upstream errors for those requests.
+type credInjectedFlag struct{}
+
 // Config holds proxy configuration.
 type Config struct {
 	ListenAddr    string
@@ -482,6 +486,8 @@ func New(cfg Config) *http.Server {
 			// Inject credentials for API requests
 			if cfg.CredStore != nil {
 				switch cfg.CredStore.InjectCredentials(req) {
+				case credentials.InjectOK:
+					ctx.UserData = credInjectedFlag{}
 				case credentials.InjectAuthRequired:
 					return req, goproxy.NewResponse(req,
 						goproxy.ContentTypeText,
@@ -505,6 +511,25 @@ func New(cfg Config) *http.Server {
 		},
 	)
 
+	// Log upstream error responses for credential-injected requests.
+	// This distinguishes proxy-generated errors from upstream errors.
+	proxy.OnResponse().DoFunc(
+		func(resp *http.Response, ctx *goproxy.ProxyCtx) *http.Response {
+			if resp != nil && ctx.UserData != nil && resp.StatusCode >= 400 {
+				var host, method, path string
+				if ctx.Req != nil {
+					method = ctx.Req.Method
+					if ctx.Req.URL != nil {
+						host = ctx.Req.URL.Host
+						path = ctx.Req.URL.Path
+					}
+				}
+				log.Printf("UPSTREAM_ERROR host=%s status=%d method=%s path=%s", host, resp.StatusCode, method, path)
+			}
+			return resp
+		},
+	)
+
 	return &http.Server{
 		Addr:              cfg.ListenAddr,
 		Handler:           proxy,