@@ -7,3 +7,14 @@ package registries
 //
 // Intended for cargo_test.go's positive-path and transient-error tests only.
 var ValidateCargoREADME = validateCargoREADME
+
+// ValidatePyPIPackage and ValidateNPMPackage expose the package-private
+// validatePyPIPackage / validateNPMPackage to the external _test package so
+// httptest-driven tests can exercise the metadata fetch and status-disambiguation
+// pipeline (missing package vs missing/unpropagated version vs transient upstream)
+// against a mock server, bypassing the exact-baseURL guard that the public
+// ValidatePyPI / ValidateNPM enforce.
+var (
+	ValidatePyPIPackage = validatePyPIPackage
+	ValidateNPMPackage  = validateNPMPackage
+)
@@ -52,6 +52,14 @@ func ValidateNPM(ctx context.Context, pkg model.Package, serverName string) erro
 			pkg.RegistryBaseURL, model.RegistryTypeNPM, model.RegistryURLNPM)
 	}
 
+	return validateNPMPackage(ctx, pkg, serverName)
+}
+
+// validateNPMPackage performs the version-metadata fetch and the mcpName check.
+// It is split out from ValidateNPM so that httptest-based tests can drive the HTTP
+// pipeline against a mock server (exposed via export_test.go), bypassing the
+// exact-baseURL guard that ValidateNPM enforces for callers.
+func validateNPMPackage(ctx context.Context, pkg model.Package, serverName string) error {
 	client := &http.Client{Timeout: 10 * time.Second}
 
 	requestURL := pkg.RegistryBaseURL + "/" + url.PathEscape(pkg.Identifier) + "/" + url.PathEscape(pkg.Version)
@@ -60,7 +68,7 @@ func ValidateNPM(ctx context.Context, pkg model.Package, serverName string) erro
 		return fmt.Errorf("failed to create request: %w", err)
 	}
 
-	req.Header.Set("User-Agent", "MCP-Registry-Validator/1.0")
+	req.Header.Set("User-Agent", userAgent)
 	req.Header.Set("Accept", "application/json")
 
 	resp, err := client.Do(req)
@@ -70,7 +78,7 @@ func ValidateNPM(ctx context.Context, pkg model.Package, serverName string) erro
 	defer resp.Body.Close()
 
 	if resp.StatusCode != http.StatusOK {
-		return fmt.Errorf("NPM package '%s' not found (status: %d)", pkg.Identifier, resp.StatusCode)
+		return npmFetchStatusError(ctx, client, pkg, resp.StatusCode)
 	}
 
 	var npmResp NPMPackageResponse
@@ -88,3 +96,90 @@ func ValidateNPM(ctx context.Context, pkg model.Package, serverName string) erro
 
 	return nil
 }
+
+// npmPackageState is the outcome of probing the package-level NPM metadata
+// endpoint, used to disambiguate a 404 from the version-specific endpoint.
+type npmPackageState int
+
+const (
+	// npmPackageUnknown: the probe returned a status we can't classify.
+	npmPackageUnknown npmPackageState = iota
+	// npmPackageExists: the package exists (200) but the requested version does not.
+	npmPackageExists
+	// npmPackageMissing: the package itself does not exist (404).
+	npmPackageMissing
+	// npmPackageTransient: the probe failed for a retryable reason (network error,
+	// 429, or 5xx). Existence is undetermined and the caller should not report
+	// "not found".
+	npmPackageTransient
+)
+
+// npmFetchStatusError maps a non-200 status from the version-specific metadata
+// endpoint to a caller-actionable error. A 404 is delegated to npmVersion404Error
+// for disambiguation. 429/5xx are transient, not "not found".
+func npmFetchStatusError(ctx context.Context, client *http.Client, pkg model.Package, status int) error {
+	switch {
+	case status == http.StatusNotFound:
+		return npmVersion404Error(ctx, client, pkg)
+	case status == http.StatusTooManyRequests:
+		return fmt.Errorf("NPM rate-limited the metadata request for package '%s' (status: 429). Likely transient, retry later", pkg.Identifier)
+	case status >= 500 && status < 600:
+		return fmt.Errorf("NPM upstream error fetching metadata for package '%s' (status: %d). Likely transient, retry later", pkg.Identifier, status)
+	default:
+		return fmt.Errorf("NPM package '%s' metadata fetch failed (status: %d)", pkg.Identifier, status)
+	}
+}
+
+// npmVersion404Error disambiguates a 404 from the version-specific endpoint: a
+// genuinely-missing package versus a package that exists but whose requested
+// version is absent (commonly because a freshly published release has not yet
+// propagated). It probes the package-level endpoint so the publisher gets an
+// actionable message rather than a blanket "not found".
+func npmVersion404Error(ctx context.Context, client *http.Client, pkg model.Package) error {
+	switch probeNPMPackage(ctx, client, pkg.RegistryBaseURL, pkg.Identifier) {
+	case npmPackageExists:
+		return fmt.Errorf("NPM package '%s' exists, but version '%s' was not found (status: 404). A newly published release can take a moment to appear on the registry. Wait and retry, or publish version '%s' before registering it", pkg.Identifier, pkg.Version, pkg.Version)
+	case npmPackageMissing:
+		return fmt.Errorf("NPM package '%s' not found (status: 404)", pkg.Identifier)
+	case npmPackageTransient:
+		return fmt.Errorf("NPM could not confirm package '%s' version '%s' (version status: 404, package check inconclusive). Likely transient, retry later", pkg.Identifier, pkg.Version)
+	case npmPackageUnknown:
+		// Probe returned an unclassifiable status, so fall through to the
+		// best-effort message below.
+	}
+	return fmt.Errorf("NPM package '%s' version '%s' not found (status: 404)", pkg.Identifier, pkg.Version)
+}
+
+// probeNPMPackage checks whether a package exists on the NPM registry regardless
+// of version, with a HEAD request to the package-level endpoint (/{name}). Only
+// the status code is used, so HEAD avoids downloading the (large) packument.
+func probeNPMPackage(ctx context.Context, client *http.Client, baseURL, identifier string) npmPackageState {
+	// The probe only refines the 404 error message, so it must not extend the
+	// validator's worst case by another full client timeout.
+	ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
+	defer cancel()
+
+	probeURL := baseURL + "/" + url.PathEscape(identifier)
+	req, err := http.NewRequestWithContext(ctx, http.MethodHead, probeURL, nil)
+	if err != nil {
+		return npmPackageUnknown
+	}
+	req.Header.Set("User-Agent", userAgent)
+
+	resp, err := client.Do(req)
+	if err != nil {
+		return npmPackageTransient
+	}
+	defer resp.Body.Close()
+
+	switch {
+	case resp.StatusCode == http.StatusOK:
+		return npmPackageExists
+	case resp.StatusCode == http.StatusNotFound:
+		return npmPackageMissing
+	case resp.StatusCode == http.StatusTooManyRequests, resp.StatusCode >= 500 && resp.StatusCode < 600:
+		return npmPackageTransient
+	default:
+		return npmPackageUnknown
+	}
+}
@@ -2,13 +2,186 @@ package registries_test
 
 import (
 	"context"
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
 	"testing"
+	"time"
 
 	"github.com/modelcontextprotocol/registry/internal/validators/registries"
 	"github.com/modelcontextprotocol/registry/pkg/model"
 	"github.com/stretchr/testify/assert"
 )
 
+// newNPMMock stands in for registry.npmjs.org: it routes the version fetch and
+// package probe by path shape and returns the given statuses (versionBody used on 200).
+func newNPMMock(versionStatus int, versionBody string, packageStatus int) *httptest.Server {
+	return httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// Route on the escaped path: a scoped name like @scope/pkg is sent as the
+		// single segment @scope%2Fpkg, and decoding it would wrongly split it in two.
+		parts := strings.Split(strings.Trim(r.URL.EscapedPath(), "/"), "/")
+		// Pin the method per endpoint (GET fetch, HEAD probe) so a method
+		// regression in the validator surfaces as a 405 instead of passing.
+		switch len(parts) {
+		case 2: // /{name}/{version}
+			if r.Method != http.MethodGet {
+				w.WriteHeader(http.StatusMethodNotAllowed)
+				return
+			}
+			if versionStatus != http.StatusOK {
+				w.WriteHeader(versionStatus)
+				return
+			}
+			w.Header().Set("Content-Type", "application/json")
+			_, _ = io.WriteString(w, versionBody)
+		case 1: // /{name}  (package-existence probe)
+			if r.Method != http.MethodHead {
+				w.WriteHeader(http.StatusMethodNotAllowed)
+				return
+			}
+			w.WriteHeader(packageStatus)
+		default:
+			w.WriteHeader(http.StatusInternalServerError)
+		}
+	}))
+}
+
+// TestValidateNPM_VersionNotYetVisible is the #553 regression for npm: version 404s
+// while the package exists, so the error must report a missing version, not a missing package.
+func TestValidateNPM_VersionNotYetVisible(t *testing.T) {
+	ctx := context.Background()
+	mock := newNPMMock(http.StatusNotFound, "", http.StatusOK)
+	defer mock.Close()
+
+	pkg := model.Package{RegistryType: model.RegistryTypeNPM, RegistryBaseURL: mock.URL, Identifier: "demo-pkg", Version: "9.9.9"}
+	err := registries.ValidateNPMPackage(ctx, pkg, "io.github.test/demo")
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "exists, but version '9.9.9'", "package-exists/version-missing must be distinguished from package-missing")
+}
+
+// TestValidateNPM_PackageMissing: both endpoints 404, so "not found" is correct.
+func TestValidateNPM_PackageMissing(t *testing.T) {
+	ctx := context.Background()
+	mock := newNPMMock(http.StatusNotFound, "", http.StatusNotFound)
+	defer mock.Close()
+
+	pkg := model.Package{RegistryType: model.RegistryTypeNPM, RegistryBaseURL: mock.URL, Identifier: "demo-pkg", Version: "1.0.0"}
+	err := registries.ValidateNPMPackage(ctx, pkg, "io.github.test/demo")
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "not found")
+	assert.NotContains(t, err.Error(), "exists, but version", "a genuinely missing package must not claim the version exists")
+}
+
+// TestValidateNPM_TransientUpstream: a 5xx on the version fetch must be retryable,
+// not "not found".
+func TestValidateNPM_TransientUpstream(t *testing.T) {
+	ctx := context.Background()
+	mock := newNPMMock(http.StatusBadGateway, "", http.StatusOK)
+	defer mock.Close()
+
+	pkg := model.Package{RegistryType: model.RegistryTypeNPM, RegistryBaseURL: mock.URL, Identifier: "demo-pkg", Version: "1.0.0"}
+	err := registries.ValidateNPMPackage(ctx, pkg, "io.github.test/demo")
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "transient")
+	assert.NotContains(t, err.Error(), "not found", "transient upstream errors must not be reported as 'not found'")
+}
+
+// TestValidateNPM_VersionNotFoundProbeInconclusive: version 404 plus a transient
+// probe (503) leaves existence undetermined, so the validator must not say "not found".
+func TestValidateNPM_VersionNotFoundProbeInconclusive(t *testing.T) {
+	ctx := context.Background()
+	mock := newNPMMock(http.StatusNotFound, "", http.StatusServiceUnavailable)
+	defer mock.Close()
+
+	pkg := model.Package{RegistryType: model.RegistryTypeNPM, RegistryBaseURL: mock.URL, Identifier: "demo-pkg", Version: "1.0.0"}
+	err := registries.ValidateNPMPackage(ctx, pkg, "io.github.test/demo")
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "transient")
+	assert.NotContains(t, err.Error(), "not found", "an inconclusive probe must not assert the package is missing")
+}
+
+// TestValidateNPM_ProbeDeadlineBounded: a hung probe must be cut off by the
+// probe's own short deadline instead of riding out the client's full 10s
+// timeout, and the cutoff must read as inconclusive rather than "not found".
+func TestValidateNPM_ProbeDeadlineBounded(t *testing.T) {
+	ctx := context.Background()
+	mock := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		parts := strings.Split(strings.Trim(r.URL.EscapedPath(), "/"), "/")
+		if len(parts) == 2 { // version fetch
+			w.WriteHeader(http.StatusNotFound)
+			return
+		}
+		// Package probe: hang until the client gives up.
+		<-r.Context().Done()
+	}))
+	defer mock.Close()
+
+	pkg := model.Package{RegistryType: model.RegistryTypeNPM, RegistryBaseURL: mock.URL, Identifier: "demo-pkg", Version: "1.0.0"}
+	start := time.Now()
+	err := registries.ValidateNPMPackage(ctx, pkg, "io.github.test/demo")
+	elapsed := time.Since(start)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "transient", "a probe cut off by its deadline is inconclusive, not 'not found'")
+	assert.Less(t, elapsed, 8*time.Second, "a hung probe must be bounded by the probe deadline, not the client timeout")
+}
+
+// TestValidateNPM_ScopedVersionNotYetVisible covers scoped names (@scope/name), the
+// escaped-single-segment case, through the version fetch and the HEAD probe.
+func TestValidateNPM_ScopedVersionNotYetVisible(t *testing.T) {
+	ctx := context.Background()
+	mock := newNPMMock(http.StatusNotFound, "", http.StatusOK)
+	defer mock.Close()
+
+	pkg := model.Package{RegistryType: model.RegistryTypeNPM, RegistryBaseURL: mock.URL, Identifier: "@scope/demo", Version: "9.9.9"}
+	err := registries.ValidateNPMPackage(ctx, pkg, "io.github.test/demo")
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "exists, but version '9.9.9'", "scoped names must route correctly through both endpoints")
+}
+
+// TestValidateNPM_VersionEndpointRateLimited: a 429 on the version fetch is reported
+// as rate-limited/transient.
+func TestValidateNPM_VersionEndpointRateLimited(t *testing.T) {
+	ctx := context.Background()
+	mock := newNPMMock(http.StatusTooManyRequests, "", http.StatusOK)
+	defer mock.Close()
+
+	pkg := model.Package{RegistryType: model.RegistryTypeNPM, RegistryBaseURL: mock.URL, Identifier: "demo-pkg", Version: "1.0.0"}
+	err := registries.ValidateNPMPackage(ctx, pkg, "io.github.test/demo")
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "rate-limited")
+	assert.NotContains(t, err.Error(), "not found")
+}
+
+// TestValidateNPM_VersionNotFoundProbeUnclassified: the version 404s and the probe
+// returns an unclassifiable status, so the validator falls back to a plain
+// version-not-found message.
+func TestValidateNPM_VersionNotFoundProbeUnclassified(t *testing.T) {
+	ctx := context.Background()
+	mock := newNPMMock(http.StatusNotFound, "", http.StatusTeapot)
+	defer mock.Close()
+
+	pkg := model.Package{RegistryType: model.RegistryTypeNPM, RegistryBaseURL: mock.URL, Identifier: "demo-pkg", Version: "1.0.0"}
+	err := registries.ValidateNPMPackage(ctx, pkg, "io.github.test/demo")
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "version '1.0.0' not found")
+	assert.NotContains(t, err.Error(), "exists, but version")
+}
+
+// TestValidateNPM_PositivePathMock: a version response with the matching mcpName validates.
+func TestValidateNPM_PositivePathMock(t *testing.T) {
+	ctx := context.Background()
+	const serverName = "io.github.test/demo"
+	body := fmt.Sprintf(`{"mcpName":%q}`, serverName)
+	mock := newNPMMock(http.StatusOK, body, http.StatusOK)
+	defer mock.Close()
+
+	pkg := model.Package{RegistryType: model.RegistryTypeNPM, RegistryBaseURL: mock.URL, Identifier: "demo-pkg", Version: "1.0.0"}
+	err := registries.ValidateNPMPackage(ctx, pkg, serverName)
+	assert.NoError(t, err, "a version response with the matching mcpName should validate")
+}
+
 func TestValidateNPM_RealPackages(t *testing.T) {
 	ctx := context.Background()
 
@@ -126,6 +299,12 @@ func TestValidateNPM_RealPackages(t *testing.T) {
 
 			err := registries.ValidateNPM(ctx, pkg, tt.serverName)
 
+			// A live 429/5xx from the registry is inconclusive, not a failure
+			// of the case under test.
+			if err != nil && strings.Contains(err.Error(), "retry later") {
+				t.Skipf("transient registry response: %v", err)
+			}
+
 			if tt.expectError {
 				assert.Error(t, err)
 				assert.Contains(t, err.Error(), tt.errorMessage)