add dependencies cooldown

config/config.go

@@ -76,6 +76,16 @@ type Config struct {
 	// Sandbox enables sandboxing of package manager processes with controlled filesystem,
 	// network, and process execution access. Provides defense-in-depth against supply chain attacks.
 	Sandbox SandboxConfig `mapstructure:"sandbox"`
+
+	// DependencyCooldown blocks installation of recently-published package versions.
+	DependencyCooldown DependencyCooldownConfig `mapstructure:"dependency_cooldown"`
+}
+
+// DependencyCooldownConfig blocks installation of package versions published within a
+// configurable time window, reducing exposure to supply chain attacks.
+type DependencyCooldownConfig struct {
+	Enabled bool `mapstructure:"enabled"`
+	Days    int  `mapstructure:"days"`
 }
 
 // SandboxConfig configures the sandbox system for isolating package manager processes.
@@ -225,6 +235,10 @@ func DefaultConfig() RuntimeConfig {
 				Enabled:       false,
 				EnforceAlways: false,
 			},
+			DependencyCooldown: DependencyCooldownConfig{
+				Enabled: true,
+				Days:    5,
+			},
 		},
 		DryRun:               false,
 		InsecureInstallation: insecureInstallation,

config/config.template.yml

@@ -137,3 +137,19 @@ sandbox:
     uv:
       enabled: true
       profile: pypi-restrictive
+
+# Dependency cooldown blocks installation of package versions published within a configurable
+# time window. Malicious packages are often caught within the first few days of publication,
+# so enforcing a cooldown period reduces exposure to supply chain attacks.
+#
+# When a package version is published within the cooldown window, PMG will block the
+# installation with a message indicating when the cooldown expires.
+#
+# Applies to all packages universally (new and existing dependencies alike).
+# Currently supported: NPM in proxy mode only.
+dependency_cooldown:
+  # Enable dependency cooldown. Default is true.
+  enabled: true
+
+  # Cooldown period in days. Default is 5.
+  days: 5

internal/flows/proxy_flow.go

@@ -180,6 +180,17 @@ func (f *proxyFlow) Run(ctx context.Context, args []string, parsedCmd *packagema
 	reportData.BlockedPackages = statsCollector.GetBlockedPackages()
 	reportData.ConfirmedPackages = statsCollector.GetConfirmedPackages()
 
+	for _, cb := range statsCollector.GetCooldownBlocks() {
+		reportData.CooldownBlockedPackages = append(reportData.CooldownBlockedPackages, ui.CooldownBlockedPackage{
+			Name:         cb.Name,
+			Version:      cb.Version,
+			PublishDate:  cb.PublishDate,
+			DaysAgo:      cb.DaysAgo,
+			DaysLeft:     cb.DaysLeft,
+			CooldownDays: cb.CooldownDays,
+		})
+	}
+
 	// Set outcome based on execution result using shared inference logic
 	reportData.Outcome = inferOutcome(cfg.InsecureInstallation, cfg.DryRun, reportData.BlockedCount, stats.UserCancelledCount, executionError)
 

internal/ui/report.go

@@ -57,6 +57,16 @@ func (o ExecutionOutcome) String() string {
 	}
 }
 
+// CooldownBlockedPackage is a package blocked by the dependency cooldown policy.
+type CooldownBlockedPackage struct {
+	Name         string
+	Version      string
+	PublishDate  time.Time
+	DaysAgo      int
+	DaysLeft     int
+	CooldownDays int
+}
+
 // ReportData captures execution statistics for the post-execution report.
 // This is a pure data model with no rendering logic.
 type ReportData struct {
@@ -78,6 +88,9 @@ type ReportData struct {
 	BlockedPackages   []*analyzer.PackageVersionAnalysisResult
 	ConfirmedPackages []*analyzer.PackageVersionAnalysisResult
 
+	// Packages blocked by the dependency cooldown policy (proxy mode only)
+	CooldownBlockedPackages []CooldownBlockedPackage
+
 	// Configuration context
 	FlowType          FlowType
 	DryRun            bool
@@ -106,7 +119,7 @@ func (r *ReportData) Finalize() {
 
 // HasIssues returns true if any packages were blocked or required confirmation
 func (r *ReportData) HasIssues() bool {
-	return r.BlockedCount > 0 || r.ConfirmedCount > 0
+	return r.BlockedCount > 0 || r.ConfirmedCount > 0 || len(r.CooldownBlockedPackages) > 0
 }
 
 // WasSuccessful returns true if execution completed without blocks or errors
@@ -172,28 +185,46 @@ func reportNormal(data *ReportData) {
 
 	switch data.Outcome {
 	case OutcomeBlocked:
-		fmt.Println()
-		fmt.Printf("%s %s\n", Colors.Red("✗"), Colors.Red("Malicious package blocked"))
+		if len(data.BlockedPackages) > 0 {
+			fmt.Println()
+			fmt.Printf("%s %s\n", Colors.Red("✗"), Colors.Red("Malicious package blocked"))
+			printMaliciousPackagesList(data.BlockedPackages)
+			fmt.Println()
+		}
 
-		printMaliciousPackagesList(data.BlockedPackages)
-		fmt.Println()
+		if len(data.CooldownBlockedPackages) > 0 {
+			fmt.Println()
+			n := len(data.CooldownBlockedPackages)
+			fmt.Printf("%s %s\n",
+				Colors.Yellow("⊘"),
+				Colors.Yellow(fmt.Sprintf("Dependency cooldown — %s blocked", pluralizePackages(n))))
+			printCooldownPackagesList(data.CooldownBlockedPackages)
+			fmt.Println()
+		}
 
-		icon = Colors.Red("✗")
-		message = fmt.Sprintf("PMG: %d packages analyzed, %d blocked",
-			data.TotalAnalyzed, data.BlockedCount)
+		onlyCooldown := len(data.BlockedPackages) == 0 && len(data.CooldownBlockedPackages) > 0
+		if onlyCooldown {
+			icon = Colors.Yellow("⊘")
+			message = fmt.Sprintf("PMG: %s analyzed, %s blocked by cooldown",
+				pluralizePackages(data.TotalAnalyzed), pluralizePackages(data.BlockedCount))
+		} else {
+			icon = Colors.Red("✗")
+			message = fmt.Sprintf("PMG: %s analyzed, %s blocked",
+				pluralizePackages(data.TotalAnalyzed), pluralizePackages(data.BlockedCount))
+		}
 	case OutcomeUserCancelled:
 		icon = Colors.Yellow("✗")
-		message = fmt.Sprintf("PMG: %d packages analyzed, installation cancelled",
-			data.TotalAnalyzed)
+		message = fmt.Sprintf("PMG: %s analyzed, installation cancelled",
+			pluralizePackages(data.TotalAnalyzed))
 	default:
 		// Success case
 		if data.HasIssues() {
 			icon = Colors.Yellow("!")
-			message = fmt.Sprintf("PMG: %d packages analyzed (%d confirmed)",
-				data.TotalAnalyzed, data.ConfirmedCount)
+			message = fmt.Sprintf("PMG: %s analyzed (%s confirmed)",
+				pluralizePackages(data.TotalAnalyzed), pluralizePackages(data.ConfirmedCount))
 		} else {
 			icon = Colors.Green("✓")
-			message = fmt.Sprintf("PMG: %d packages analyzed", data.TotalAnalyzed)
+			message = fmt.Sprintf("PMG: %s analyzed", pluralizePackages(data.TotalAnalyzed))
 		}
 	}
 
@@ -217,9 +248,9 @@ func reportVerbose(data *ReportData) {
 			data.TotalAnalyzed,
 			data.TrustedSkipped)
 	} else {
-		fmt.Printf("  %s %d analyzed\n",
+		fmt.Printf("  %s %s analyzed\n",
 			Colors.Bold("Packages:"),
-			data.TotalAnalyzed)
+			pluralizePackages(data.TotalAnalyzed))
 	}
 
 	fmt.Printf("  %s %s (allowed: %d, confirmed: %d, blocked: %d)\n",
@@ -251,12 +282,29 @@ func reportVerbose(data *ReportData) {
 	// Show blocked/confirmed package details in verbose mode
 	if len(data.BlockedPackages) > 0 {
 		fmt.Println()
-		fmt.Println(Colors.Red("  Blocked packages:"))
+		fmt.Println(Colors.Red("  Blocked packages (malware):"))
 		for _, pkg := range data.BlockedPackages {
 			printPackageDetail(pkg)
 		}
 	}
 
+	if len(data.CooldownBlockedPackages) > 0 {
+		fmt.Println()
+		fmt.Println(Colors.Yellow("  Blocked by dependency cooldown:"))
+		for _, pkg := range data.CooldownBlockedPackages {
+			dateStr := ""
+			if !pkg.PublishDate.IsZero() {
+				dateStr = fmt.Sprintf(" (%s)", pkg.PublishDate.Format("2006-01-02"))
+			}
+			fmt.Printf("    %s %s\n", Colors.Yellow("⊘"), Colors.Yellow(fmt.Sprintf("%s@%s", pkg.Name, pkg.Version)))
+			fmt.Printf("      %s\n", Colors.Dim(fmt.Sprintf(
+				"Published %s ago%s — cooldown: %d days, available in %s",
+				pluralizeDays(pkg.DaysAgo), dateStr, pkg.CooldownDays, pluralizeDays(pkg.DaysLeft),
+			)))
+			fmt.Printf("      %s\n", Colors.Dim("Tip: add to trusted_packages in your PMG config to bypass"))
+		}
+	}
+
 	if len(data.ConfirmedPackages) > 0 {
 		fmt.Println()
 		fmt.Println(Colors.Yellow("  User-confirmed packages:"))
@@ -273,7 +321,16 @@ func printOutcomeLine(data *ReportData) {
 	case OutcomeSuccess:
 		fmt.Printf("  %s %s\n", Colors.Green("✓"), Colors.Green("Installation completed successfully"))
 	case OutcomeBlocked:
-		fmt.Printf("  %s %s\n", Colors.Red("✗"), Colors.Red("Installation blocked - malicious package detected"))
+		hasMalware := len(data.BlockedPackages) > 0
+		hasCooldown := len(data.CooldownBlockedPackages) > 0
+		switch {
+		case hasMalware && hasCooldown:
+			fmt.Printf("  %s %s\n", Colors.Red("✗"), Colors.Red("Installation blocked — malicious package detected + cooldown policy"))
+		case hasCooldown:
+			fmt.Printf("  %s %s\n", Colors.Yellow("⊘"), Colors.Yellow("Installation blocked — dependency cooldown policy"))
+		default:
+			fmt.Printf("  %s %s\n", Colors.Red("✗"), Colors.Red("Installation blocked — malicious package detected"))
+		}
 	case OutcomeUserCancelled:
 		fmt.Printf("  %s %s\n", Colors.Yellow("✗"), Colors.Yellow("Installation cancelled by user"))
 	case OutcomeDryRun:
@@ -312,3 +369,19 @@ func boolToOnOff(b bool) string {
 	}
 	return "off"
 }
+
+// pluralizePackages returns "1 package" or "N packages" with correct grammar.
+func pluralizePackages(n int) string {
+	if n == 1 {
+		return "1 package"
+	}
+	return fmt.Sprintf("%d packages", n)
+}
+
+// pluralizeDays returns "1 day" or "N days" with correct grammar.
+// func pluralizeDays(n int) string {
+// 	if n == 1 {
+// 		return "1 day"
+// 	}
+// 	return fmt.Sprintf("%d days", n)
+// }

internal/ui/ui.go

@@ -141,6 +141,33 @@ func Fatalf(msg string, args ...interface{}) {
 	os.Exit(1)
 }
 
+func printCooldownPackagesList(packages []CooldownBlockedPackage) {
+	for _, pkg := range packages {
+		fmt.Println()
+		fmt.Printf("  %s %s\n",
+			Colors.Yellow("⊘"),
+			Colors.Yellow(fmt.Sprintf("%s@%s", pkg.Name, pkg.Version)))
+
+		dateStr := ""
+		if !pkg.PublishDate.IsZero() {
+			dateStr = fmt.Sprintf(" (%s)", pkg.PublishDate.Format("2006-01-02"))
+		}
+
+		fmt.Printf("    %s\n", Colors.Dim(fmt.Sprintf(
+			"Published %s ago%s — available in %s",
+			pluralizeDays(pkg.DaysAgo), dateStr, pluralizeDays(pkg.DaysLeft),
+		)))
+		fmt.Printf("    %s\n", Colors.Dim("Tip: add to trusted_packages in your PMG config to bypass"))
+	}
+}
+
+func pluralizeDays(n int) string {
+	if n == 1 {
+		return "1 day"
+	}
+	return fmt.Sprintf("%d days", n)
+}
+
 func printMaliciousPackagesList(malwarePackages []*analyzer.PackageVersionAnalysisResult) {
 	for _, mp := range malwarePackages {
 		fmt.Println()

proxy/interceptors/npm_cooldown_test.go

@@ -0,0 +1,400 @@
+package interceptors
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/url"
+	"testing"
+	"time"
+
+	"github.com/safedep/dry/log"
+	"github.com/safedep/pmg/config"
+	"github.com/safedep/pmg/proxy"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// setCooldownConfig sets the global cooldown config for the duration of a test
+// and restores the original value via t.Cleanup.
+func setCooldownConfig(t *testing.T, cfg config.DependencyCooldownConfig) {
+	t.Helper()
+	orig := config.Get().Config.DependencyCooldown
+	t.Cleanup(func() { config.Get().Config.DependencyCooldown = orig })
+	config.Get().Config.DependencyCooldown = cfg
+}
+
+// TestParseNpmMetadataTime verifies that publish dates are correctly extracted from
+// the "time" field of NPM package metadata, with skip keys omitted.
+func TestParseNpmMetadataTime(t *testing.T) {
+	tests := []struct {
+		name        string
+		body        string
+		expectCount int
+		expectErr   bool
+	}{
+		{
+			name: "valid metadata with multiple versions",
+			body: `{
+				"name": "lodash",
+				"time": {
+					"created":  "2012-04-23T16:52:34.248Z",
+					"modified": "2024-01-15T10:30:00.000Z",
+					"1.0.0":    "2012-04-23T16:52:34.248Z",
+					"2.0.0":    "2013-05-10T12:00:00.000Z",
+					"4.17.21":  "2021-02-20T15:42:16.000Z"
+				}
+			}`,
+			expectCount: 3,
+			expectErr:   false,
+		},
+		{
+			name:        "metadata without time field",
+			body:        `{"name": "lodash"}`,
+			expectCount: 0,
+			expectErr:   false,
+		},
+		{
+			name: "only skip keys present in time",
+			body: `{
+				"time": {
+					"created":  "2012-04-23T16:52:34.248Z",
+					"modified": "2024-01-15T10:30:00.000Z"
+				}
+			}`,
+			expectCount: 0,
+			expectErr:   false,
+		},
+		{
+			name:        "invalid JSON body",
+			body:        `not valid json`,
+			expectCount: 0,
+			expectErr:   true,
+		},
+		{
+			name: "unparseable date values are skipped",
+			body: `{
+				"time": {
+					"1.0.0": "2024-01-15T10:30:00.000Z",
+					"2.0.0": "not-a-date-at-all"
+				}
+			}`,
+			expectCount: 1,
+			expectErr:   false,
+		},
+		{
+			name: "RFC3339 format without milliseconds is accepted",
+			body: `{
+				"time": {
+					"1.0.0": "2024-01-15T10:30:00Z"
+				}
+			}`,
+			expectCount: 1,
+			expectErr:   false,
+		},
+		{
+			name: "millisecond precision format is accepted",
+			body: `{
+				"time": {
+					"1.0.0": "2024-01-15T10:30:00.000Z"
+				}
+			}`,
+			expectCount: 1,
+			expectErr:   false,
+		},
+		{
+			name:        "empty body",
+			body:        ``,
+			expectCount: 0,
+			expectErr:   true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			dates, err := parseNpmMetadataTime([]byte(tt.body))
+			if tt.expectErr {
+				assert.Error(t, err)
+			} else {
+				require.NoError(t, err)
+				assert.Len(t, dates, tt.expectCount)
+			}
+		})
+	}
+}
+
+// TestParseNpmMetadataTime_CorrectDates verifies that the parsed dates match the
+// exact timestamps from the metadata JSON.
+func TestParseNpmMetadataTime_CorrectDates(t *testing.T) {
+	body := `{
+		"name": "lodash",
+		"time": {
+			"created":  "2012-04-23T16:52:34.248Z",
+			"modified": "2024-01-15T10:30:00.000Z",
+			"4.17.20":  "2021-01-07T14:24:45.000Z",
+			"4.17.21":  "2021-02-20T15:42:16.000Z"
+		}
+	}`
+
+	dates, err := parseNpmMetadataTime([]byte(body))
+	require.NoError(t, err)
+	require.Len(t, dates, 2)
+
+	expected4_17_20, err := time.Parse("2006-01-02T15:04:05.000Z", "2021-01-07T14:24:45.000Z")
+	require.NoError(t, err)
+	assert.Equal(t, expected4_17_20, dates["4.17.20"])
+
+	expected4_17_21, err := time.Parse("2006-01-02T15:04:05.000Z", "2021-02-20T15:42:16.000Z")
+	require.NoError(t, err)
+	assert.Equal(t, expected4_17_21, dates["4.17.21"])
+
+	_, hasCreated := dates["created"]
+	assert.False(t, hasCreated, "created key must be skipped")
+
+	_, hasModified := dates["modified"]
+	assert.False(t, hasModified, "modified key must be skipped")
+}
+
+// TestNpmCooldown_BlocksRecentPackage verifies that a metadata response for a package
+// with a recently published version strips that version from the response.
+func TestNpmCooldown_BlocksRecentPackage(t *testing.T) {
+	setCooldownConfig(t, config.DependencyCooldownConfig{Enabled: true, Days: 5})
+
+	interceptor := NewNpmRegistryInterceptor(nil, NewInMemoryAnalysisCache(), nil, nil)
+
+	ctx := &proxy.RequestContext{
+		Hostname: "registry.npmjs.org",
+		URL:      mustParseURL("https://registry.npmjs.org/lodash"),
+	}
+
+	resp, err := interceptor.HandleRequest(ctx)
+	require.NoError(t, err)
+	require.Equal(t, proxy.ActionModifyResponse, resp.Action)
+
+	recentDate := time.Now().Add(-2 * 24 * time.Hour).UTC().Format(time.RFC3339)
+	body, _ := json.Marshal(map[string]any{
+		"name":      "lodash",
+		"dist-tags": map[string]string{"latest": "4.18.0"},
+		"versions":  map[string]any{"4.17.21": map[string]string{"version": "4.17.21"}, "4.18.0": map[string]string{"version": "4.18.0"}},
+		"time":      map[string]string{"4.17.21": "2021-02-20T15:42:16.000Z", "4.18.0": recentDate},
+	})
+
+	_, _, outBody, err := resp.ResponseModifier(http.StatusOK, http.Header{}, body)
+	require.NoError(t, err)
+
+	var result map[string]json.RawMessage
+	require.NoError(t, json.Unmarshal(outBody, &result))
+
+	var versions map[string]json.RawMessage
+	require.NoError(t, json.Unmarshal(result["versions"], &versions))
+	assert.NotContains(t, versions, "4.18.0", "recent version should be stripped")
+	assert.Contains(t, versions, "4.17.21")
+
+	var distTags map[string]string
+	require.NoError(t, json.Unmarshal(result["dist-tags"], &distTags))
+	assert.Equal(t, "4.17.21", distTags["latest"], "dist-tags.latest should point to oldest eligible version")
+}
+
+// TestNpmCooldown_AllowsOldPackage verifies that a metadata response with no versions
+// in the cooldown window is returned unchanged.
+func TestNpmCooldown_AllowsOldPackage(t *testing.T) {
+	setCooldownConfig(t, config.DependencyCooldownConfig{Enabled: true, Days: 5})
+
+	interceptor := NewNpmRegistryInterceptor(nil, NewInMemoryAnalysisCache(), nil, nil)
+
+	ctx := &proxy.RequestContext{
+		Hostname: "registry.npmjs.org",
+		URL:      mustParseURL("https://registry.npmjs.org/lodash"),
+	}
+
+	resp, err := interceptor.HandleRequest(ctx)
+	require.NoError(t, err)
+	require.Equal(t, proxy.ActionModifyResponse, resp.Action)
+
+	body, _ := json.Marshal(map[string]any{
+		"name":      "lodash",
+		"dist-tags": map[string]string{"latest": "4.17.21"},
+		"versions":  map[string]any{"4.17.21": map[string]string{"version": "4.17.21"}},
+		"time":      map[string]string{"4.17.21": "2021-02-20T15:42:16.000Z"},
+	})
+
+	_, _, outBody, err := resp.ResponseModifier(http.StatusOK, http.Header{}, body)
+	require.NoError(t, err)
+	assert.Equal(t, body, outBody, "body should be unchanged when no versions are in cooldown")
+}
+
+// TestNpmCooldown_DisabledByConfig verifies that when cooldown is disabled, metadata
+// requests are allowed through without modification.
+func TestNpmCooldown_DisabledByConfig(t *testing.T) {
+	setCooldownConfig(t, config.DependencyCooldownConfig{Enabled: false, Days: 5})
+
+	interceptor := NewNpmRegistryInterceptor(nil, NewInMemoryAnalysisCache(), nil, nil)
+
+	ctx := &proxy.RequestContext{
+		Hostname: "registry.npmjs.org",
+		URL:      mustParseURL("https://registry.npmjs.org/lodash"),
+	}
+
+	resp, err := interceptor.HandleRequest(ctx)
+	require.NoError(t, err)
+	// Cooldown disabled: metadata requests must be allowed through without a modifier
+	assert.Equal(t, proxy.ActionAllow, resp.Action)
+	assert.Nil(t, resp.ResponseModifier)
+}
+
+// TestNpmCooldown_MetadataResponseRegistersModifier verifies that a metadata request
+// when cooldown is enabled returns ActionModifyResponse with a modifier set.
+func TestNpmCooldown_MetadataResponseRegistersModifier(t *testing.T) {
+	setCooldownConfig(t, config.DependencyCooldownConfig{Enabled: true, Days: 5})
+
+	interceptor := NewNpmRegistryInterceptor(nil, NewInMemoryAnalysisCache(), nil, nil)
+
+	ctx := &proxy.RequestContext{
+		Hostname: "registry.npmjs.org",
+		URL:      mustParseURL("https://registry.npmjs.org/lodash"),
+	}
+
+	resp, err := interceptor.HandleRequest(ctx)
+	require.NoError(t, err)
+	assert.Equal(t, proxy.ActionModifyResponse, resp.Action)
+	assert.NotNil(t, resp.ResponseModifier, "response modifier must be set for metadata requests when cooldown is enabled")
+}
+
+// TestNpmCooldown_MetadataStripsRecentVersions verifies that the metadata modifier
+// removes versions within the cooldown window from "versions", "time", and fixes "dist-tags".
+// This is the key behavior for npm update: npm's resolver never sees the too-new versions
+// and naturally falls back to the latest eligible version.
+func TestNpmCooldown_MetadataStripsRecentVersions(t *testing.T) {
+	setCooldownConfig(t, config.DependencyCooldownConfig{Enabled: true, Days: 5})
+
+	interceptor := NewNpmRegistryInterceptor(nil, NewInMemoryAnalysisCache(), NewAnalysisStatsCollector(), nil)
+
+	ctx := &proxy.RequestContext{
+		Hostname: "registry.npmjs.org",
+		URL:      mustParseURL("https://registry.npmjs.org/some-pkg"),
+	}
+
+	resp, err := interceptor.HandleRequest(ctx)
+	require.NoError(t, err)
+	require.NotNil(t, resp.ResponseModifier)
+
+	// Build metadata where 1.3.0 was published 1 day ago (within cooldown)
+	recentDate := time.Now().Add(-1 * 24 * time.Hour).UTC().Format("2006-01-02T15:04:05.000Z")
+	oldDate := "2024-01-15T10:30:00.000Z"
+
+	metadata := map[string]interface{}{
+		"name": "some-pkg",
+		"dist-tags": map[string]string{
+			"latest": "1.3.0",
+		},
+		"time": map[string]string{
+			"created":  "2023-01-01T00:00:00.000Z",
+			"modified": recentDate,
+			"1.0.0":    oldDate,
+			"1.2.0":    "2024-06-01T00:00:00.000Z",
+			"1.3.0":    recentDate,
+		},
+		"versions": map[string]interface{}{
+			"1.0.0": map[string]string{"version": "1.0.0"},
+			"1.2.0": map[string]string{"version": "1.2.0"},
+			"1.3.0": map[string]string{"version": "1.3.0"},
+		},
+	}
+	body, err := json.Marshal(metadata)
+	require.NoError(t, err)
+
+	_, _, outBody, modErr := resp.ResponseModifier(http.StatusOK, http.Header{}, body)
+	require.NoError(t, modErr)
+
+	// Parse the modified body
+	var result map[string]json.RawMessage
+	require.NoError(t, json.Unmarshal(outBody, &result))
+
+	// 1.3.0 should be stripped from "versions"
+	var versions map[string]json.RawMessage
+	require.NoError(t, json.Unmarshal(result["versions"], &versions))
+	assert.Contains(t, versions, "1.0.0")
+	assert.Contains(t, versions, "1.2.0")
+	assert.NotContains(t, versions, "1.3.0", "1.3.0 should be stripped from versions")
+
+	// 1.3.0 should be stripped from "time"
+	var timeMap map[string]string
+	require.NoError(t, json.Unmarshal(result["time"], &timeMap))
+	assert.Contains(t, timeMap, "1.0.0")
+	assert.Contains(t, timeMap, "1.2.0")
+	assert.NotContains(t, timeMap, "1.3.0", "1.3.0 should be stripped from time")
+	assert.Contains(t, timeMap, "created", "created should be preserved")
+
+	// dist-tags.latest should be updated to 1.2.0 (latest non-cooldown version)
+	var distTags map[string]string
+	require.NoError(t, json.Unmarshal(result["dist-tags"], &distTags))
+	assert.Equal(t, "1.2.0", distTags["latest"], "dist-tags.latest should point to latest non-cooldown version")
+
+}
+
+// TestStripCooldownVersions_AllVersionsTooNew verifies behavior when every version
+// is within the cooldown window — all versions are stripped.
+func TestStripCooldownVersions_AllVersionsTooNew(t *testing.T) {
+	now := time.Now()
+	dates := map[string]time.Time{
+		"1.0.0": now.Add(-1 * 24 * time.Hour),
+		"1.1.0": now.Add(-2 * 24 * time.Hour),
+	}
+
+	body, _ := json.Marshal(map[string]interface{}{
+		"name": "new-pkg",
+		"dist-tags": map[string]string{
+			"latest": "1.1.0",
+		},
+		"versions": map[string]interface{}{
+			"1.0.0": map[string]string{"version": "1.0.0"},
+			"1.1.0": map[string]string{"version": "1.1.0"},
+		},
+		"time": map[string]string{
+			"1.0.0": now.Add(-1 * 24 * time.Hour).Format(time.RFC3339),
+			"1.1.0": now.Add(-2 * 24 * time.Hour).Format(time.RFC3339),
+		},
+	})
+
+	result, stripped, remaining := stripCooldownVersions(body, dates, 5)
+	assert.Equal(t, 2, stripped)
+	assert.Equal(t, 0, remaining)
+
+	var parsed map[string]json.RawMessage
+	require.NoError(t, json.Unmarshal(result, &parsed))
+
+	var versions map[string]json.RawMessage
+	require.NoError(t, json.Unmarshal(parsed["versions"], &versions))
+	assert.Empty(t, versions, "all versions should be stripped")
+
+	// dist-tags should be empty since no eligible version exists
+	var distTags map[string]string
+	require.NoError(t, json.Unmarshal(parsed["dist-tags"], &distTags))
+	assert.Empty(t, distTags)
+}
+
+// TestStripCooldownVersions_NoVersionsTooNew verifies that when no versions are
+// within the cooldown window, the body is returned unchanged.
+func TestStripCooldownVersions_NoVersionsTooNew(t *testing.T) {
+	dates := map[string]time.Time{
+		"1.0.0": time.Now().Add(-30 * 24 * time.Hour),
+		"1.1.0": time.Now().Add(-20 * 24 * time.Hour),
+	}
+
+	body := []byte(`{"name":"pkg","versions":{"1.0.0":{},"1.1.0":{}},"time":{"1.0.0":"2024-01-01T00:00:00Z","1.1.0":"2024-02-01T00:00:00Z"}}`)
+
+	result, stripped, remaining := stripCooldownVersions(body, dates, 5)
+	assert.Equal(t, 0, stripped)
+	assert.Equal(t, 2, remaining)
+	assert.Equal(t, body, result, "body should be unchanged when no versions are in cooldown")
+}
+
+
+func mustParseURL(rawURL string) *url.URL {
+	u, err := url.Parse(rawURL)
+	if err != nil {
+		log.Errorf("mustParseURL: %s" + err.Error())
+		return nil
+	}
+
+	return u
+}

proxy/interceptors/npm_registry.go

@@ -1,9 +1,15 @@
 package interceptors
 
 import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"time"
+
 	packagev1 "buf.build/gen/go/safedep/api/protocolbuffers/go/safedep/messages/package/v1"
 	"github.com/safedep/dry/log"
 	"github.com/safedep/pmg/analyzer"
+	"github.com/safedep/pmg/config"
 	"github.com/safedep/pmg/proxy"
 )
 
@@ -30,8 +36,14 @@ var npmRegistryDomains = registryConfigMap{
 	},
 }
 
-// NpmRegistryInterceptor intercepts NPM registry requests and analyzes packages for malware
-// It embeds baseRegistryInterceptor to reuse ecosystem agnostic functionality
+// npmMetadataTimeSkipKeys are non-version keys present in the NPM metadata "time" object.
+var npmMetadataTimeSkipKeys = map[string]bool{
+	"created":  true,
+	"modified": true,
+}
+
+// NpmRegistryInterceptor intercepts NPM registry requests and analyzes packages for malware.
+// It embeds baseRegistryInterceptor to reuse ecosystem agnostic functionality.
 type NpmRegistryInterceptor struct {
 	baseRegistryInterceptor
 }
@@ -75,37 +87,39 @@ func (i *NpmRegistryInterceptor) ShouldIntercept(ctx *proxy.RequestContext) bool
 	return npmRegistryDomains.ContainsHostname(ctx.Hostname)
 }
 
-// HandleRequest processes the request and returns response action
-// We take a fail-open approach here, allowing requests that we can't parse the package information from the URL.
+// HandleRequest processes the request and returns response action.
+// We take a fail-open approach, allowing requests we can't parse.
 func (i *NpmRegistryInterceptor) HandleRequest(ctx *proxy.RequestContext) (*proxy.InterceptorResponse, error) {
 	log.Debugf("[%s] Handling NPM registry request: %s", ctx.RequestID, ctx.URL.Path)
 
-	// Get registry configuration
-	config := npmRegistryDomains.GetConfigForHostname(ctx.Hostname)
-	if config == nil {
-		// Shouldn't happen if ShouldIntercept is working correctly
+	registryConfig := npmRegistryDomains.GetConfigForHostname(ctx.Hostname)
+	if registryConfig == nil {
 		log.Warnf("[%s] No registry config found for hostname: %s", ctx.RequestID, ctx.Hostname)
 		return &proxy.InterceptorResponse{Action: proxy.ActionAllow}, nil
 	}
 
-	// Skip analysis for registries that are not supported for analysis
-	if !config.SupportedForAnalysis {
+	if !registryConfig.SupportedForAnalysis {
 		log.Debugf("[%s] Skipping analysis for %s registry (not supported for analysis): %s",
-			ctx.RequestID, config.Host, ctx.URL.String())
+			ctx.RequestID, registryConfig.Host, ctx.URL.String())
 		return &proxy.InterceptorResponse{Action: proxy.ActionAllow}, nil
 	}
 
-	// Parse URL using registry-specific strategy
-	pkgInfo, err := config.Parser.ParseURL(ctx.URL.Path)
+	pkgInfo, err := registryConfig.Parser.ParseURL(ctx.URL.Path)
 	if err != nil {
 		log.Warnf("[%s] Failed to parse NPM registry URL %s for %s: %v",
-			ctx.RequestID, ctx.URL.Path, config.Host, err)
+			ctx.RequestID, ctx.URL.Path, registryConfig.Host, err)
 		return &proxy.InterceptorResponse{Action: proxy.ActionAllow}, nil
 	}
 
-	// Only analyze tarball downloads (these have a specific version)
-	// Metadata requests (without version) are allowed through
+	depCooldownConfig := config.Get().Config.DependencyCooldown
+
+	// Metadata requests: strip versions within the cooldown window so npm's resolver
+	// naturally falls back to the latest eligible version.
 	if !pkgInfo.IsFileDownload() {
+		if depCooldownConfig.Enabled {
+			return i.handleMetadataRequest(ctx, pkgInfo.GetName())
+		}
+
 		log.Debugf("[%s] Skipping analysis for metadata request: %s", ctx.RequestID, pkgInfo.GetName())
 		return &proxy.InterceptorResponse{Action: proxy.ActionAllow}, nil
 	}
@@ -123,3 +137,214 @@ func (i *NpmRegistryInterceptor) HandleRequest(ctx *proxy.RequestContext) (*prox
 
 	return i.handleAnalysisResult(ctx, packagev1.Ecosystem_ECOSYSTEM_NPM, pkgInfo.GetName(), pkgInfo.GetVersion(), result)
 }
+
+// handleMetadataRequest registers a response modifier that extracts publish dates from the
+// NPM metadata JSON body, caches them, and strips versions within the cooldown window.
+// By removing recent versions from "versions", "time", and "dist-tags", npm's resolver
+// naturally falls back to the latest eligible version — matching the behavior of npm's
+// own --min-release-age flag.
+func (i *NpmRegistryInterceptor) handleMetadataRequest(ctx *proxy.RequestContext, packageName string) (*proxy.InterceptorResponse, error) {
+	log.Debugf("[%s] Registering metadata response modifier for %s", ctx.RequestID, packageName)
+
+	modifier := func(statusCode int, headers http.Header, body []byte) (int, http.Header, []byte, error) {
+		dates, err := parseNpmMetadataTime(body)
+		if err != nil {
+			log.Warnf("[%s] Failed to parse NPM metadata time for %s: %v", ctx.RequestID, packageName, err)
+			return statusCode, headers, body, nil
+		}
+
+		log.Debugf("[%s] Parsed %d publish dates for %s", ctx.RequestID, len(dates), packageName)
+
+		cooldownDays := config.Get().Config.DependencyCooldown.Days
+		strippedBody, stripped, remaining := stripCooldownVersions(body, dates, cooldownDays)
+		if stripped > 0 {
+			log.Infof("[%s] Stripped %d version(s) from %s metadata (cooldown: %d days, %d eligible remain)",
+				ctx.RequestID, stripped, packageName, cooldownDays, remaining)
+
+			// Only report a cooldown block when npm has no eligible version to fall back to.
+			// If older versions remain, npm resolves to them silently — nothing to report.
+			if remaining == 0 && i.statsCollector != nil {
+				// Report the most recently published (would-have-been-latest) stripped version
+				latestStripped, latestDate := mostRecentVersion(dates)
+				if latestStripped != "" {
+					cooldownDuration := time.Duration(cooldownDays) * 24 * time.Hour
+					age := time.Since(latestDate)
+					daysAgo := int(age.Hours() / 24)
+					daysLeft := int((cooldownDuration-age).Hours()/24) + 1
+					i.statsCollector.RecordCooldownBlocked(packageName, latestStripped, latestDate, daysAgo, daysLeft, cooldownDays)
+				}
+			}
+
+			return statusCode, headers, strippedBody, nil
+		}
+
+		return statusCode, headers, body, nil
+	}
+
+	return &proxy.InterceptorResponse{
+		Action:           proxy.ActionModifyResponse,
+		ResponseModifier: modifier,
+	}, nil
+}
+
+
+
+// parseNpmMetadataTime extracts version publish dates from an NPM package metadata body.
+// The NPM registry embeds a "time" object mapping version strings to RFC3339 timestamps.
+// Non-version keys (created, modified) are skipped.
+func parseNpmMetadataTime(body []byte) (map[string]time.Time, error) {
+	var metadata struct {
+		Time map[string]string `json:"time"`
+	}
+
+	if err := json.Unmarshal(body, &metadata); err != nil {
+		return nil, fmt.Errorf("failed to unmarshal npm metadata: %w", err)
+	}
+
+	if metadata.Time == nil {
+		return map[string]time.Time{}, nil
+	}
+
+	dates := make(map[string]time.Time, len(metadata.Time))
+	for version, dateStr := range metadata.Time {
+		if npmMetadataTimeSkipKeys[version] {
+			continue
+		}
+
+		t, err := time.Parse(time.RFC3339, dateStr)
+		if err != nil {
+			// NPM sometimes uses millisecond precision: 2024-01-15T10:30:00.000Z
+			t, err = time.Parse("2006-01-02T15:04:05.000Z", dateStr)
+			if err != nil {
+				log.Debugf("Skipping unparseable publish date for version %s: %q", version, dateStr)
+				continue
+			}
+		}
+
+		dates[version] = t
+	}
+
+	return dates, nil
+}
+
+// stripCooldownVersions removes versions published within the cooldown window from the
+// NPM metadata response. It strips entries from "versions", "time", and updates "dist-tags"
+// so npm's resolver naturally picks the latest eligible version.
+// Returns the modified body, count of stripped versions, and count of remaining eligible versions.
+// If no versions need stripping, returns the original body unchanged with counts (0, total).
+func stripCooldownVersions(body []byte, dates map[string]time.Time, cooldownDays int) ([]byte, int, int) {
+	cooldownDuration := time.Duration(cooldownDays) * 24 * time.Hour
+	now := time.Now()
+
+	// Identify which versions to strip
+	tooNew := make(map[string]bool)
+	for version, publishDate := range dates {
+		if now.Sub(publishDate) < cooldownDuration {
+			tooNew[version] = true
+		}
+	}
+
+	remaining := len(dates) - len(tooNew)
+
+	if len(tooNew) == 0 {
+		return body, 0, remaining
+	}
+
+	// Parse into a generic map to preserve all fields we don't care about
+	var metadata map[string]json.RawMessage
+	if err := json.Unmarshal(body, &metadata); err != nil {
+		return body, 0, remaining
+	}
+
+	// Strip from "versions"
+	if raw, ok := metadata["versions"]; ok {
+		var versions map[string]json.RawMessage
+		if err := json.Unmarshal(raw, &versions); err == nil {
+			for v := range tooNew {
+				delete(versions, v)
+			}
+			if updated, err := json.Marshal(versions); err == nil {
+				metadata["versions"] = updated
+			}
+		}
+	}
+
+	// Strip from "time"
+	if raw, ok := metadata["time"]; ok {
+		var timeMap map[string]string
+		if err := json.Unmarshal(raw, &timeMap); err == nil {
+			for v := range tooNew {
+				delete(timeMap, v)
+			}
+			if updated, err := json.Marshal(timeMap); err == nil {
+				metadata["time"] = updated
+			}
+		}
+	}
+
+	// Fix "dist-tags" — if any tag points to a stripped version, update it to the
+	// latest remaining version
+	if raw, ok := metadata["dist-tags"]; ok {
+		var distTags map[string]string
+		if err := json.Unmarshal(raw, &distTags); err == nil {
+			changed := false
+			for tag, version := range distTags {
+				if tooNew[version] {
+					latest := latestNonCooldownVersion(dates, tooNew)
+					if latest != "" {
+						distTags[tag] = latest
+					} else {
+						delete(distTags, tag)
+					}
+					changed = true
+				}
+			}
+			if changed {
+				if updated, err := json.Marshal(distTags); err == nil {
+					metadata["dist-tags"] = updated
+				}
+			}
+		}
+	}
+
+	result, err := json.Marshal(metadata)
+	if err != nil {
+		return body, 0, remaining
+	}
+
+	return result, len(tooNew), remaining
+}
+
+// mostRecentVersion returns the version with the most recent publish date from dates.
+func mostRecentVersion(dates map[string]time.Time) (string, time.Time) {
+	var latest string
+	var latestTime time.Time
+
+	for version, publishDate := range dates {
+		if publishDate.After(latestTime) {
+			latest = version
+			latestTime = publishDate
+		}
+	}
+
+	return latest, latestTime
+}
+
+// latestNonCooldownVersion finds the most recently published version that is NOT in the
+// tooNew set.
+func latestNonCooldownVersion(dates map[string]time.Time, tooNew map[string]bool) string {
+	var latest string
+	var latestTime time.Time
+
+	for version, publishDate := range dates {
+		if tooNew[version] {
+			continue
+		}
+		if publishDate.After(latestTime) {
+			latest = version
+			latestTime = publishDate
+		}
+	}
+
+	return latest
+}

proxy/interceptors/stats.go

@@ -2,17 +2,29 @@ package interceptors
 
 import (
 	"sync"
+	"time"
 
 	"github.com/safedep/pmg/analyzer"
 )
 
+// CooldownBlock records a package blocked by the dependency cooldown policy.
+type CooldownBlock struct {
+	Name         string
+	Version      string
+	PublishDate  time.Time
+	DaysAgo      int
+	DaysLeft     int
+	CooldownDays int
+}
+
 // AnalysisStats contains aggregated statistics from analysis results
 type AnalysisStats struct {
-	TotalAnalyzed      int
-	AllowedCount       int
-	ConfirmedCount     int
-	BlockedCount       int
-	UserCancelledCount int
+	TotalAnalyzed        int
+	AllowedCount         int
+	ConfirmedCount       int
+	BlockedCount         int
+	UserCancelledCount   int
+	CooldownBlockedCount int
 }
 
 // AnalysisStatsCollector tracks analysis statistics during proxy execution.
@@ -23,6 +35,7 @@ type AnalysisStatsCollector struct {
 	stats             AnalysisStats
 	blockedPackages   []*analyzer.PackageVersionAnalysisResult
 	confirmedPackages []*analyzer.PackageVersionAnalysisResult
+	cooldownBlocks    []CooldownBlock
 }
 
 // NewAnalysisStatsCollector creates a new stats collector
@@ -118,3 +131,33 @@ func (c *AnalysisStatsCollector) GetConfirmedPackages() []*analyzer.PackageVersi
 	copy(result, c.confirmedPackages)
 	return result
 }
+
+// RecordCooldownBlocked records a package blocked by the dependency cooldown policy.
+// It increments both the cooldown-specific counter and the overall BlockedCount so that
+// inferOutcome correctly treats cooldown-only sessions as OutcomeBlocked.
+func (c *AnalysisStatsCollector) RecordCooldownBlocked(name, version string, publishDate time.Time, daysAgo, daysLeft, cooldownDays int) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.stats.TotalAnalyzed++
+	c.stats.BlockedCount++
+	c.stats.CooldownBlockedCount++
+	c.cooldownBlocks = append(c.cooldownBlocks, CooldownBlock{
+		Name:         name,
+		Version:      version,
+		PublishDate:  publishDate,
+		DaysAgo:      daysAgo,
+		DaysLeft:     daysLeft,
+		CooldownDays: cooldownDays,
+	})
+}
+
+// GetCooldownBlocks returns all packages blocked by the cooldown policy.
+func (c *AnalysisStatsCollector) GetCooldownBlocks() []CooldownBlock {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	result := make([]CooldownBlock, len(c.cooldownBlocks))
+	copy(result, c.cooldownBlocks)
+	return result
+}

proxy/proxy.go

@@ -1,12 +1,15 @@
 package proxy
 
 import (
+	"bytes"
 	"context"
 	"crypto/tls"
 	"fmt"
+	"io"
 	"net"
 	"net/http"
 	"net/url"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -494,10 +497,37 @@ func (ps *proxyServer) registerHandlers() {
 			return resp
 		}
 
-		// TODO: Implement response body modification
-		// This requires buffering the response body, modifying it, and creating a new response
-		// For now, lets skip it
+		modifiedResp, err := applyResponseModifier(resp, modifier)
+		if err != nil {
+			log.Errorf("[%s] Response modifier failed for %s: %v", reqCtx.RequestID, ctx.Req.URL.String(), err)
+			return resp
+		}
 
-		return resp
+		return modifiedResp
 	})
 }
+
+// applyResponseModifier reads the full response body, passes it to the modifier function,
+// and returns a new response with the modifier's output. Content-Length is updated to match.
+func applyResponseModifier(resp *http.Response, modifier ResponseModifierFunc) (*http.Response, error) {
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return resp, fmt.Errorf("failed to read response body: %w", err)
+	}
+	_ = resp.Body.Close()
+
+	newStatus, newHeaders, newBody, err := modifier(resp.StatusCode, resp.Header, body)
+	if err != nil {
+		// Restore original body so the response remains usable on error
+		resp.Body = io.NopCloser(bytes.NewReader(body))
+		return resp, fmt.Errorf("modifier returned error: %w", err)
+	}
+
+	resp.StatusCode = newStatus
+	resp.Header = newHeaders
+	resp.Body = io.NopCloser(bytes.NewReader(newBody))
+	resp.ContentLength = int64(len(newBody))
+	resp.Header.Set("Content-Length", strconv.Itoa(len(newBody)))
+
+	return resp, nil
+}

proxy/proxy_test.go

@@ -2,9 +2,13 @@ package proxy
 
 import (
 	"crypto/tls"
+	"fmt"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"strconv"
+	"strings"
 	"testing"
 	"time"
 
@@ -128,9 +132,9 @@ func TestNormalizeRequestURLNilSafety(t *testing.T) {
 
 func TestProxyWithLoopbackBypass(t *testing.T) {
 	tests := []struct {
-		name          string
-		url           string
-		shouldBypass  bool
+		name         string
+		url          string
+		shouldBypass bool
 	}{
 		{"localhost bypassed", "http://localhost:9876/", true},
 		{"127.0.0.1 bypassed", "http://127.0.0.1:9876/", true},
@@ -225,3 +229,98 @@ func TestResponseProtoNormalisedToHTTP11(t *testing.T) {
 	assert.Equal(t, 1, resp.ProtoMajor, "response ProtoMajor should be 1 (HTTP/1.1)")
 	assert.Equal(t, 1, resp.ProtoMinor, "response ProtoMinor should be 1 (HTTP/1.1)")
 }
+
+func TestApplyResponseModifier(t *testing.T) {
+	tests := []struct {
+		name           string
+		body           string
+		modifier       ResponseModifierFunc
+		expectedBody   string
+		expectedStatus int
+	}{
+		{
+			name: "modifier returns body unchanged",
+			body: `{"name":"lodash"}`,
+			modifier: func(statusCode int, headers http.Header, body []byte) (int, http.Header, []byte, error) {
+				return statusCode, headers, body, nil
+			},
+			expectedBody:   `{"name":"lodash"}`,
+			expectedStatus: http.StatusOK,
+		},
+		{
+			name: "modifier rewrites body",
+			body: `original`,
+			modifier: func(statusCode int, headers http.Header, body []byte) (int, http.Header, []byte, error) {
+				return statusCode, headers, []byte(`rewritten`), nil
+			},
+			expectedBody:   `rewritten`,
+			expectedStatus: http.StatusOK,
+		},
+		{
+			name: "modifier changes status code",
+			body: `body`,
+			modifier: func(statusCode int, headers http.Header, body []byte) (int, http.Header, []byte, error) {
+				return http.StatusTeapot, headers, body, nil
+			},
+			expectedBody:   `body`,
+			expectedStatus: http.StatusTeapot,
+		},
+		{
+			name: "modifier error restores original body",
+			body: `original`,
+			modifier: func(statusCode int, headers http.Header, body []byte) (int, http.Header, []byte, error) {
+				return 0, nil, nil, fmt.Errorf("modifier failed")
+			},
+			expectedBody:   `original`,
+			expectedStatus: http.StatusOK,
+		},
+		{
+			name: "empty body handled",
+			body: ``,
+			modifier: func(statusCode int, headers http.Header, body []byte) (int, http.Header, []byte, error) {
+				return statusCode, headers, body, nil
+			},
+			expectedBody:   ``,
+			expectedStatus: http.StatusOK,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			resp := &http.Response{
+				StatusCode: http.StatusOK,
+				Header:     http.Header{"Content-Type": []string{"application/json"}},
+				Body:       io.NopCloser(strings.NewReader(tt.body)),
+			}
+
+			got, _ := applyResponseModifier(resp, tt.modifier)
+
+			assert.Equal(t, tt.expectedStatus, got.StatusCode)
+
+			gotBody, err := io.ReadAll(got.Body)
+			require.NoError(t, err)
+			assert.Equal(t, tt.expectedBody, string(gotBody))
+		})
+	}
+}
+
+func TestApplyResponseModifier_ContentLengthUpdated(t *testing.T) {
+	original := `short`
+	replacement := `this is a longer body`
+
+	resp := &http.Response{
+		StatusCode:    http.StatusOK,
+		Header:        http.Header{},
+		Body:          io.NopCloser(strings.NewReader(original)),
+		ContentLength: int64(len(original)),
+	}
+
+	modifier := func(statusCode int, headers http.Header, body []byte) (int, http.Header, []byte, error) {
+		return statusCode, headers, []byte(replacement), nil
+	}
+
+	got, err := applyResponseModifier(resp, modifier)
+	require.NoError(t, err)
+	assert.Equal(t, int64(len(replacement)), got.ContentLength)
+	assert.Equal(t, strconv.Itoa(len(replacement)), got.Header.Get("Content-Length"))
+}