diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3c7d515..6d6902f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 - Add `WithCallContext` client option that sets the `X-Call-Context` header so API calls act on behalf of a managed account (identified by company UID).
 
 ## Changes
+- Fix NAT gateway diagnostics polling treating the API's in-progress signal (HTTP 400 `The polling result for async mode is not ready yet`) as a fatal error. The poll now keeps polling while the operation is processing, returns a completed 200 result including an empty route array, and leaves every other error fatal.
 - Bump Go toolchain to 1.26.5 to pick up a `crypto/tls` fix for an Encrypted Client Hello privacy leak ([GO-2026-5856](https://pkg.go.dev/vuln/GO-2026-5856)).
 
 # 1.0.0 Release
diff --git a/errors.go b/errors.go
index e2c2597..691cebe 100644
--- a/errors.go
+++ b/errors.go
@@ -125,6 +125,20 @@ func IsServiceNotFoundError(err error) bool {
 		strings.Contains(apiErr.Message, "Could not find a service with UID")
 }
 
+// isNATGatewayDiagnosticsInProgress reports whether err is the async diagnostics
+// "still processing" signal: the non-standard HTTP 400 the NAT gateway routes
+// operation endpoint returns while the result is not yet ready. The signal is a
+// message string with no error code behind it, and other diagnostics APIs
+// report the same state differently, so this stays specific to NAT gateway.
+func isNATGatewayDiagnosticsInProgress(err error) bool {
+	apiErr, ok := err.(*ErrorResponse)
+	if !ok || apiErr.Response == nil {
+		return false
+	}
+	return apiErr.Response.StatusCode == http.StatusBadRequest &&
+		strings.Contains(apiErr.Message, "The polling result for async mode is not ready yet")
+}
+
 // ErrTransitVXCCancelLaterNotAllowed is returned when attempting to schedule Transit VXC deletion for later (only CANCEL_NOW is allowed)
 var ErrTransitVXCCancelLaterNotAllowed = errors.New("transit vxc (megaport internet) does not support scheduled deletion (cancel later), only immediate deletion (CANCEL_NOW) is allowed")
 
diff --git a/nat_gateway.go b/nat_gateway.go
index 53a5934..7714cdc 100644
--- a/nat_gateway.go
+++ b/nat_gateway.go
@@ -112,6 +112,15 @@ func NewNATGatewayService(c *Client) *NATGatewayServiceOp {
 // NATGatewayServiceOp handles communication with NAT Gateway methods of the Megaport API.
 type NATGatewayServiceOp struct {
 	Client *Client
+	// pollInitialDelay overrides diagnosticsPollInitialDelay when non-zero.
+	// Intended for tests that want to avoid real-time waits.
+	pollInitialDelay time.Duration
+	// pollInterval overrides diagnosticsPollInterval when non-zero.
+	// Intended for tests that want to avoid real-time waits.
+	pollInterval time.Duration
+	// pollTimeout overrides diagnosticsPollTimeout when non-zero.
+	// Intended for tests that want to avoid real-time waits.
+	pollTimeout time.Duration
 }
 
 // GetNATGatewayTelemetryRequest represents a request to get telemetry data for a NAT Gateway.
diff --git a/nat_gateway_diagnostics.go b/nat_gateway_diagnostics.go
index 0b829c2..66f4b85 100644
--- a/nat_gateway_diagnostics.go
+++ b/nat_gateway_diagnostics.go
@@ -26,6 +26,30 @@ const (
 	diagnosticsPollTimeout      = 60 * time.Second
 )
 
+// effectivePollInitialDelay returns the initial delay before the first poll.
+func (svc *NATGatewayServiceOp) effectivePollInitialDelay() time.Duration {
+	if svc.pollInitialDelay != 0 {
+		return svc.pollInitialDelay
+	}
+	return diagnosticsPollInitialDelay
+}
+
+// effectivePollInterval returns the interval between polls.
+func (svc *NATGatewayServiceOp) effectivePollInterval() time.Duration {
+	if svc.pollInterval != 0 {
+		return svc.pollInterval
+	}
+	return diagnosticsPollInterval
+}
+
+// effectivePollTimeout returns the SDK-managed overall poll timeout.
+func (svc *NATGatewayServiceOp) effectivePollTimeout() time.Duration {
+	if svc.pollTimeout != 0 {
+		return svc.pollTimeout
+	}
+	return diagnosticsPollTimeout
+}
+
 // ListNATGatewayIPRoutesAsync submits an IP routes diagnostics request.
 func (svc *NATGatewayServiceOp) ListNATGatewayIPRoutesAsync(ctx context.Context, productUID, ipAddress string) (string, error) {
 	if productUID == "" {
@@ -106,11 +130,14 @@ func (svc *NATGatewayServiceOp) GetNATGatewayDiagnosticsRoutes(ctx context.Conte
 }
 
 // pollDiagnosticsRoutes polls GetNATGatewayDiagnosticsRoutes until the
-// operation returns a non-empty result, the SDK-managed
-// diagnosticsPollTimeout elapses, or the caller's context is cancelled.
-// Empty responses are treated as "still processing".
+// operation completes, the SDK-managed diagnosticsPollTimeout elapses, or the
+// caller's context is cancelled. While the operation is still processing the
+// endpoint returns an HTTP 400 that isNATGatewayDiagnosticsInProgress
+// detects, and the loop keeps polling; any other error is returned to the
+// caller. A 200 response is the completed result and is returned as-is,
+// including an empty route slice.
 func (svc *NATGatewayServiceOp) pollDiagnosticsRoutes(ctx context.Context, productUID, operationID string) ([]*NATGatewayRoute, error) {
-	pollCtx, cancel := context.WithTimeout(ctx, diagnosticsPollTimeout)
+	pollCtx, cancel := context.WithTimeout(ctx, svc.effectivePollTimeout())
 	defer cancel()
 	// pollDoneErr returns ctx.Err() when the caller's context is the one that
 	// fired (cancellation or caller-imposed deadline) and
@@ -126,18 +153,25 @@ func (svc *NATGatewayServiceOp) pollDiagnosticsRoutes(ctx context.Context, produ
 	select {
 	case <-pollCtx.Done():
 		return nil, pollDoneErr()
-	case <-time.After(diagnosticsPollInitialDelay):
+	case <-time.After(svc.effectivePollInitialDelay()):
 	}
-	ticker := time.NewTicker(diagnosticsPollInterval)
+	ticker := time.NewTicker(svc.effectivePollInterval())
 	defer ticker.Stop()
 	for {
 		routes, err := svc.GetNATGatewayDiagnosticsRoutes(pollCtx, productUID, operationID)
-		if err != nil {
-			return nil, err
-		}
-		if len(routes) > 0 {
+		if err == nil {
 			return routes, nil
 		}
+		if !isNATGatewayDiagnosticsInProgress(err) {
+			// A poll-context expiry mid-request surfaces as a wrapped context
+			// error; attribute it to the deadline/cancellation via pollDoneErr
+			// so callers get a consistent error, leaving genuine API failures
+			// untouched.
+			if pollCtx.Err() != nil && (errors.Is(err, context.DeadlineExceeded) || errors.Is(err, context.Canceled)) {
+				return nil, pollDoneErr()
+			}
+			return nil, err
+		}
 		select {
 		case <-pollCtx.Done():
 			return nil, pollDoneErr()
diff --git a/nat_gateway_test.go b/nat_gateway_test.go
index 2f50fa2..4c8d555 100644
--- a/nat_gateway_test.go
+++ b/nat_gateway_test.go
@@ -1228,9 +1228,24 @@ func (suite *NATGatewayClientTestSuite) TestGetNATGatewayDiagnosticsRoutesValida
 	suite.ErrorIs(err, ErrNATGatewayDiagnosticsOperationEmpty)
 }
 
-func (suite *NATGatewayClientTestSuite) TestListNATGatewayIPRoutesPolling() {
+// fastPollNATService returns the suite's NAT Gateway service with the poll
+// cadence collapsed to millisecond delays so poll-driven tests run without
+// real-time waits.
+func (suite *NATGatewayClientTestSuite) fastPollNATService() *NATGatewayServiceOp {
+	op, ok := suite.client.NATGatewayService.(*NATGatewayServiceOp)
+	suite.Require().True(ok)
+	op.pollInitialDelay = time.Millisecond
+	op.pollInterval = time.Millisecond
+	return op
+}
+
+const diagnosticsInProgressBody = `{"message":"The polling result for async mode is not ready yet"}`
+
+// TestListNATGatewayIPRoutesPollInProgressThenComplete drives the poll through
+// the real API contract: an in-progress HTTP 400 followed by a completed 200.
+func (suite *NATGatewayClientTestSuite) TestListNATGatewayIPRoutesPollInProgressThenComplete() {
 	ctx := context.Background()
-	natSvc := suite.client.NATGatewayService
+	natSvc := suite.fastPollNATService()
 	productUID := "uid-poll"
 
 	var opCalls atomic.Int32
@@ -1240,46 +1255,124 @@ func (suite *NATGatewayClientTestSuite) TestListNATGatewayIPRoutesPolling() {
 		fmt.Fprint(w, `{"message":"ok","terms":"","data":"op-poll"}`)
 	})
 	suite.mux.HandleFunc("/v3/products/nat_gateways/"+productUID+"/diagnostics/routes/operation", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "application/json")
-		// First call returns empty (still processing); subsequent calls return data.
+		// First poll: still processing (HTTP 400). Subsequent: completed 200.
 		if opCalls.Add(1) == 1 {
-			fmt.Fprint(w, `{"message":"ok","terms":"","data":[]}`)
+			w.WriteHeader(http.StatusBadRequest)
+			fmt.Fprint(w, diagnosticsInProgressBody)
 			return
 		}
+		w.Header().Set("Content-Type", "application/json")
 		fmt.Fprint(w, `{"message":"ok","terms":"","data":[
 			{"prefix":"10.0.0.0/24","protocol":"STATIC","nextHop":{"ip":"10.0.0.1","vxc":{"id":"vxc-1","name":"v1"}}},
 			{"prefix":"192.168.0.0/16","asPath":"65000","origin":"IGP","best":true,"nextHop":{"ip":"10.0.0.2","vxc":{"id":"vxc-2","name":"v2"}}}
 		]}`)
 	})
 
-	// Bypass the long polling defaults by polling directly via the async + Get methods,
-	// so this test stays fast. The poll timeout/interval are package-level constants
-	// and not worth plumbing through a setter just for testing.
-	opID, err := natSvc.ListNATGatewayIPRoutesAsync(ctx, productUID, "")
+	routes, err := natSvc.ListNATGatewayIPRoutes(ctx, productUID, "")
 	suite.Require().NoError(err)
-	suite.Equal("op-poll", opID)
+	suite.Len(routes, 1) // only the IP route is extracted; the BGP route is dropped
+	suite.Equal("10.0.0.0/24", routes[0].Prefix)
+	suite.GreaterOrEqual(opCalls.Load(), int32(2))
+}
 
-	// Drain the empty response then the populated one.
-	routes, err := natSvc.GetNATGatewayDiagnosticsRoutes(ctx, productUID, opID)
+// TestListNATGatewayIPRoutesPollEmptyComplete verifies a 200 with an empty data
+// array is a completed result: the poll returns empty without further polling.
+func (suite *NATGatewayClientTestSuite) TestListNATGatewayIPRoutesPollEmptyComplete() {
+	ctx := context.Background()
+	natSvc := suite.fastPollNATService()
+	productUID := "uid-empty"
+
+	var opCalls atomic.Int32
+
+	suite.mux.HandleFunc("/v3/products/nat_gateways/"+productUID+"/diagnostics/routes/ip", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		fmt.Fprint(w, `{"message":"ok","terms":"","data":"op-empty"}`)
+	})
+	suite.mux.HandleFunc("/v3/products/nat_gateways/"+productUID+"/diagnostics/routes/operation", func(w http.ResponseWriter, r *http.Request) {
+		opCalls.Add(1)
+		w.Header().Set("Content-Type", "application/json")
+		fmt.Fprint(w, `{"message":"ok","terms":"","data":[]}`)
+	})
+
+	routes, err := natSvc.ListNATGatewayIPRoutes(ctx, productUID, "")
 	suite.Require().NoError(err)
 	suite.Empty(routes)
+	suite.Equal(int32(1), opCalls.Load())
+}
 
-	routes, err = natSvc.GetNATGatewayDiagnosticsRoutes(ctx, productUID, opID)
-	suite.Require().NoError(err)
-	suite.Len(routes, 2)
+// TestListNATGatewayIPRoutesPollFatalError verifies a non-in-progress error is
+// returned unchanged rather than swallowed as "still processing".
+func (suite *NATGatewayClientTestSuite) TestListNATGatewayIPRoutesPollFatalError() {
+	ctx := context.Background()
+	natSvc := suite.fastPollNATService()
+	productUID := "uid-fatal"
 
-	// Discriminator: one IP, one BGP.
-	var ipCount, bgpCount int
-	for _, r := range routes {
-		if r.IP != nil {
-			ipCount++
-		}
-		if r.BGP != nil {
-			bgpCount++
-		}
-	}
-	suite.Equal(1, ipCount)
-	suite.Equal(1, bgpCount)
+	var opCalls atomic.Int32
+
+	suite.mux.HandleFunc("/v3/products/nat_gateways/"+productUID+"/diagnostics/routes/ip", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		fmt.Fprint(w, `{"message":"ok","terms":"","data":"op-fatal"}`)
+	})
+	suite.mux.HandleFunc("/v3/products/nat_gateways/"+productUID+"/diagnostics/routes/operation", func(w http.ResponseWriter, r *http.Request) {
+		opCalls.Add(1)
+		w.WriteHeader(http.StatusBadRequest)
+		fmt.Fprint(w, `{"message":"unknown operationId"}`)
+	})
+
+	_, err := natSvc.ListNATGatewayIPRoutes(ctx, productUID, "")
+	suite.Require().Error(err)
+	suite.False(isNATGatewayDiagnosticsInProgress(err))
+	suite.Equal(int32(1), opCalls.Load())
+}
+
+// TestListNATGatewayIPRoutesPollTimeout verifies an operation that never
+// completes surfaces ErrNATGatewayDiagnosticsTimeout.
+func (suite *NATGatewayClientTestSuite) TestListNATGatewayIPRoutesPollTimeout() {
+	ctx := context.Background()
+	natSvc := suite.fastPollNATService()
+	natSvc.pollTimeout = 20 * time.Millisecond
+	productUID := "uid-timeout"
+
+	var opCalls atomic.Int32
+
+	suite.mux.HandleFunc("/v3/products/nat_gateways/"+productUID+"/diagnostics/routes/ip", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		fmt.Fprint(w, `{"message":"ok","terms":"","data":"op-timeout"}`)
+	})
+	suite.mux.HandleFunc("/v3/products/nat_gateways/"+productUID+"/diagnostics/routes/operation", func(w http.ResponseWriter, r *http.Request) {
+		opCalls.Add(1)
+		w.WriteHeader(http.StatusBadRequest)
+		fmt.Fprint(w, diagnosticsInProgressBody)
+	})
+
+	_, err := natSvc.ListNATGatewayIPRoutes(ctx, productUID, "")
+	suite.ErrorIs(err, ErrNATGatewayDiagnosticsTimeout)
+	suite.GreaterOrEqual(opCalls.Load(), int32(1)) // at least one in-progress poll was tolerated before the timeout
+}
+
+// TestListNATGatewayIPRoutesPollCallerCancelled verifies that when the caller's
+// own context is cancelled mid-poll, the poll returns that cancellation error
+// rather than the SDK-managed timeout sentinel.
+func (suite *NATGatewayClientTestSuite) TestListNATGatewayIPRoutesPollCallerCancelled() {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	natSvc := suite.fastPollNATService()
+	productUID := "uid-cancel"
+
+	suite.mux.HandleFunc("/v3/products/nat_gateways/"+productUID+"/diagnostics/routes/ip", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		fmt.Fprint(w, `{"message":"ok","terms":"","data":"op-cancel"}`)
+	})
+	suite.mux.HandleFunc("/v3/products/nat_gateways/"+productUID+"/diagnostics/routes/operation", func(w http.ResponseWriter, r *http.Request) {
+		// Deliver one in-progress 400, then cancel the caller's context so the
+		// next poll iteration surfaces the caller's cancellation.
+		w.WriteHeader(http.StatusBadRequest)
+		fmt.Fprint(w, diagnosticsInProgressBody)
+		cancel()
+	})
+
+	_, err := natSvc.ListNATGatewayIPRoutes(ctx, productUID, "")
+	suite.ErrorIs(err, context.Canceled)
 }
 
 // --- Prefix list round-trip ----------------------------------------------