diff --git a/internal/testutil/egress_posture_test.go b/internal/testutil/egress_posture_test.go
new file mode 100644
index 000000000..0372f13ea
--- /dev/null
+++ b/internal/testutil/egress_posture_test.go
@@ -0,0 +1,117 @@
+package testutil
+
+import (
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"os"
+	"strings"
+	"testing"
+	"time"
+)
+
+const egressPoisonProxy = "http://127.0.0.1:9"
+
+func TestEgressPosture(t *testing.T) {
+	t.Run("poison_sentinel_denies_HTTP_egress", testPoisonSentinel)
+	t.Run("loopback_bypasses_lane_poison", testLoopbackBypass)
+	t.Run("raw_TCP_remains_open", testRawTCPResidual)
+	t.Run("effects_nil_proxy_remains_open", testEffectsProxyResidual)
+}
+
+func testPoisonSentinel(t *testing.T) {
+	proxyURL, err := url.Parse(egressPoisonProxy)
+	if err != nil {
+		t.Fatalf("parse poison proxy: %v", err)
+	}
+	transport := &http.Transport{Proxy: http.ProxyURL(proxyURL)}
+	t.Cleanup(transport.CloseIdleConnections)
+	client := &http.Client{Transport: transport, Timeout: 5 * time.Second}
+
+	_, err = client.Get("https://example.com")
+	assertPoisonProxyError(t, err)
+}
+
+func testLoopbackBypass(t *testing.T) {
+	if !laneIsPoisoned() {
+		t.Skip("TestEgressPosture/loopback_bypasses_lane_poison requires HTTP_PROXY or HTTPS_PROXY=http://127.0.0.1:9")
+	}
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusNoContent)
+	}))
+	defer server.Close()
+
+	transport := &http.Transport{Proxy: http.ProxyFromEnvironment}
+	t.Cleanup(transport.CloseIdleConnections)
+	response, err := (&http.Client{Transport: transport, Timeout: 5 * time.Second}).Get(server.URL)
+	if err != nil {
+		t.Fatalf("loopback GET under poisoned proxy: %v", err)
+	}
+	defer response.Body.Close()
+	if response.StatusCode != http.StatusNoContent {
+		t.Fatalf("loopback status = %d, want %d", response.StatusCode, http.StatusNoContent)
+	}
+}
+
+func testRawTCPResidual(t *testing.T) {
+	requireLiveEgressPosture(t, "raw_TCP_remains_open")
+	connection, err := net.DialTimeout("tcp", "example.com:443", 10*time.Second)
+	if err != nil {
+		t.Fatalf("raw TCP residual unexpectedly closed: %v", err)
+	}
+	connection.Close()
+}
+
+func testEffectsProxyResidual(t *testing.T) {
+	requireLiveEgressPosture(t, "effects_nil_proxy_remains_open")
+	t.Setenv("HTTP_PROXY", egressPoisonProxy)
+	t.Setenv("HTTPS_PROXY", egressPoisonProxy)
+
+	// This first request intentionally trips red when Option B adds
+	// ProxyFromEnvironment to internal/effects. That red means the residual has
+	// closed; retire this tripwire and its Non-Goals text instead of "fixing" it.
+	effectsTransport := &http.Transport{}
+	t.Cleanup(effectsTransport.CloseIdleConnections)
+	response, err := (&http.Client{Transport: effectsTransport, Timeout: 10 * time.Second}).Get("https://example.com")
+	if err != nil {
+		t.Fatalf("nil-Proxy transport should bypass poison while D5 Option A remains: %v", err)
+	}
+	response.Body.Close()
+
+	controlTransport := &http.Transport{Proxy: http.ProxyFromEnvironment}
+	t.Cleanup(controlTransport.CloseIdleConnections)
+	_, err = (&http.Client{Transport: controlTransport, Timeout: 5 * time.Second}).Get("https://example.com")
+	assertPoisonProxyError(t, err)
+}
+
+func requireLiveEgressPosture(t *testing.T, leg string) {
+	t.Helper()
+	if os.Getenv("AILANG_LIVE_NET") != "1" {
+		t.Skipf("TestEgressPosture/%s requires AILANG_LIVE_NET=1", leg)
+	}
+}
+
+func laneIsPoisoned() bool {
+	return os.Getenv("HTTP_PROXY") == egressPoisonProxy || os.Getenv("HTTPS_PROXY") == egressPoisonProxy
+}
+
+func assertPoisonProxyError(t *testing.T, err error) {
+	t.Helper()
+	if err == nil {
+		t.Fatal("request through poison proxy unexpectedly succeeded")
+	}
+	message := err.Error()
+	// "refused", not "connection refused": the refusal wording is platform-specific.
+	// Unix reports `connect: connection refused`; Windows reports `connectex: No
+	// connection could be made because the target machine actively refused it.`
+	// All three tokens must match, so this stays discriminating — a direct hit or a
+	// DNS failure carries no `proxyconnect`/`127.0.0.1:9`, and a proxy that hangs
+	// rather than refusing carries no `refused`.
+	for _, required := range []string{"proxyconnect", "127.0.0.1:9", "refused"} {
+		if !strings.Contains(strings.ToLower(message), required) {
+			t.Fatalf("poison error %q does not contain %q", message, required)
+		}
+	}
+	t.Logf("observed poison sentinel error: %v", err)
+}
diff --git a/internal/testutil/gatelint/allowlist.go b/internal/testutil/gatelint/allowlist.go
new file mode 100644
index 000000000..d969538e1
--- /dev/null
+++ b/internal/testutil/gatelint/allowlist.go
@@ -0,0 +1,30 @@
+package gatelint
+
+type allowReason struct {
+	text string
+}
+
+func mustReason(text string) allowReason {
+	if text == "" {
+		panic("gatelint allowlist reasons must not be empty")
+	}
+	return allowReason{text: text}
+}
+
+var ruleAllowlist = map[Rule]map[string]allowReason{
+	RuleR2: {
+		"internal/coordinator/provider_script_test.go": mustReason("this is a Unix shell/grandchild signal semantics test, not a live-network test, so the network opt-in helper would misstate its requirement. CI runners skip the known flaky shell behavior while local Unix runs retain coverage."),
+	},
+	RuleR3: {
+		"internal/coordinator/agent_registry_test.go": mustReason("ailang-packages appears only in workspace-path fixtures; this test performs no live network call"),
+		"internal/parser/cli_integration_test.go":     mustReason("httpbin.org appears only in expected CLI diagnostic fixture text; this test performs no live network call"),
+		"internal/parser/suggestion_errors_test.go":   mustReason("httpbin.org appears only in parser suggestion/error fixtures; this test performs no live network call"),
+		"internal/messaging/config_test.go":           mustReason("ailang-packages appears only in registry-mapping configuration fixtures; this test performs no live network call"),
+		"internal/pkg/manifest_test.go":               mustReason("ailang-packages appears only in manifest parsing fixture URLs; this test performs no live network call"),
+	},
+}
+
+func isAllowlisted(rule Rule, path string) bool {
+	_, ok := ruleAllowlist[rule][path]
+	return ok
+}
diff --git a/internal/testutil/gatelint/gatelint_test.go b/internal/testutil/gatelint/gatelint_test.go
new file mode 100644
index 000000000..08584b991
--- /dev/null
+++ b/internal/testutil/gatelint/gatelint_test.go
@@ -0,0 +1,55 @@
+package gatelint
+
+import (
+	"fmt"
+	"path/filepath"
+	"sort"
+	"strings"
+	"testing"
+)
+
+func TestGateLint_SelfTest(t *testing.T) {
+	root := filepath.Join("testdata", "fixtures")
+	violations, scanned := scan(root)
+	if scanned != 4 {
+		t.Fatalf("scanned %d candidate test files, want 4", scanned)
+	}
+
+	got := violationSet(violations)
+	want := []string{
+		"R1:internal/r1_test.go.fixture",
+		"R2:cmd/r2_test.go.fixture",
+		"R3:tests/r3_test.go.fixture",
+	}
+	if strings.Join(got, "\n") != strings.Join(want, "\n") {
+		t.Fatalf("violations mismatch\ngot:\n%s\nwant:\n%s", strings.Join(got, "\n"), strings.Join(want, "\n"))
+	}
+}
+
+func TestGateLint_Repo(t *testing.T) {
+	root, err := filepath.Abs(filepath.Join("..", "..", ".."))
+	if err != nil {
+		t.Fatalf("resolve repository root: %v", err)
+	}
+	violations, scanned := scan(root)
+	if scanned == 0 {
+		t.Fatal("gatelint scanned zero *_test.go files; repository-root or walker scope is broken")
+	}
+	if len(violations) != 0 {
+		lines := make([]string, len(violations))
+		for i, violation := range violations {
+			lines[i] = fmt.Sprintf("%s:%d: %s: %s", violation.Path, violation.Line, violation.Rule, violation.Message)
+		}
+		t.Fatalf("gatelint found %d violation(s) after scanning %d files:\n%s", len(violations), scanned, strings.Join(lines, "\n"))
+	}
+	t.Logf("scanned %d first-party test files", scanned)
+}
+
+func violationSet(violations []Violation) []string {
+	set := make([]string, len(violations))
+	for i, violation := range violations {
+		set[i] = string(violation.Rule) + ":" + violation.Path
+	}
+	sort.Strings(set)
+	return set
+}
diff --git a/internal/testutil/gatelint/scan.go b/internal/testutil/gatelint/scan.go
new file mode 100644
index 000000000..e7628c862
--- /dev/null
+++ b/internal/testutil/gatelint/scan.go
@@ -0,0 +1,137 @@
+// Package gatelint enforces the repository's explicit test-gating conventions.
+package gatelint
+
+import (
+	"bufio"
+	"errors"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+)
+
+// Rule identifies a gatelint rule.
+type Rule string
+
+const (
+	RuleR1 Rule = "R1"
+	RuleR2 Rule = "R2"
+	RuleR3 Rule = "R3"
+)
+
+// Violation describes one rule violation in a first-party test file.
+type Violation struct {
+	Rule    Rule
+	Path    string
+	Line    int
+	Message string
+}
+
+var scanRoots = []string{"internal", "cmd", "runtime", "std", "tests"}
+
+// Scan checks first-party test files below root and returns violations in stable order.
+func Scan(root string) []Violation {
+	violations, _ := scan(root)
+	return violations
+}
+
+func scan(root string) ([]Violation, int) {
+	var violations []Violation
+	scanned := 0
+	for _, scanRoot := range scanRoots {
+		base := filepath.Join(root, scanRoot)
+		err := filepath.WalkDir(base, func(path string, entry fs.DirEntry, walkErr error) error {
+			if walkErr != nil {
+				return walkErr
+			}
+			if entry.IsDir() {
+				if path != base && (strings.HasPrefix(entry.Name(), ".") || entry.Name() == "testdata") {
+					return filepath.SkipDir
+				}
+				return nil
+			}
+
+			logicalName, fixture := logicalFileName(entry.Name())
+			if !strings.HasSuffix(logicalName, "_test.go") {
+				return nil
+			}
+			rel, err := filepath.Rel(root, path)
+			if err != nil {
+				return err
+			}
+			rel = filepath.ToSlash(rel)
+			if strings.HasPrefix(rel, "internal/testutil/gatelint/") && !fixture {
+				return nil
+			}
+
+			contents, err := os.ReadFile(path)
+			if err != nil {
+				return err
+			}
+			scanned++
+			violations = append(violations, inspect(rel, logicalName, string(contents))...)
+			return nil
+		})
+		if err != nil && !errors.Is(err, fs.ErrNotExist) {
+			violations = append(violations, Violation{Rule: "WALK", Path: filepath.ToSlash(scanRoot), Message: err.Error()})
+		}
+	}
+	sort.Slice(violations, func(i, j int) bool {
+		if violations[i].Path != violations[j].Path {
+			return violations[i].Path < violations[j].Path
+		}
+		if violations[i].Line != violations[j].Line {
+			return violations[i].Line < violations[j].Line
+		}
+		return violations[i].Rule < violations[j].Rule
+	})
+	return violations, scanned
+}
+
+func logicalFileName(name string) (string, bool) {
+	const fixtureSuffix = ".fixture"
+	if strings.HasSuffix(name, fixtureSuffix) {
+		return strings.TrimSuffix(name, fixtureSuffix), true
+	}
+	return name, false
+}
+
+func inspect(path, name, contents string) []Violation {
+	var violations []Violation
+	if line := tokenLine(contents, "testing.Short("); line != 0 && !isAllowlisted(RuleR1, path) {
+		violations = append(violations, Violation{Rule: RuleR1, Path: path, Line: line, Message: "testing.Short is inert in CI; delete the gate or use an explicit testutil opt-in helper"})
+	}
+	if line := firstTokenLine(contents, `Getenv("CI")`, `Getenv("GITHUB_ACTIONS")`); line != 0 && !isAllowlisted(RuleR2, path) {
+		violations = append(violations, Violation{Rule: RuleR2, Path: path, Line: line, Message: "CI environment opt-out gate; use the explicit testutil gating convention"})
+	}
+	if !strings.HasSuffix(name, "_live_test.go") &&
+		!strings.Contains(contents, "testutil.RequiresLiveNetwork(") &&
+		!isAllowlisted(RuleR3, path) {
+		if line := firstTokenLine(contents, "httpbin.org", "ailang-packages"); line != 0 {
+			violations = append(violations, Violation{Rule: RuleR3, Path: path, Line: line, Message: "known third-party token outside a live-network gate or documented allowlist"})
+		}
+	}
+	return violations
+}
+
+func firstTokenLine(contents string, tokens ...string) int {
+	first := 0
+	for _, token := range tokens {
+		line := tokenLine(contents, token)
+		if line != 0 && (first == 0 || line < first) {
+			first = line
+		}
+	}
+	return first
+}
+
+func tokenLine(contents, token string) int {
+	scanner := bufio.NewScanner(strings.NewReader(contents))
+	for line := 1; scanner.Scan(); line++ {
+		if strings.Contains(scanner.Text(), token) {
+			return line
+		}
+	}
+	return 0
+}
diff --git a/internal/testutil/gatelint/testdata/fixtures/cmd/r2_test.go.fixture b/internal/testutil/gatelint/testdata/fixtures/cmd/r2_test.go.fixture
new file mode 100644
index 000000000..061528361
--- /dev/null
+++ b/internal/testutil/gatelint/testdata/fixtures/cmd/r2_test.go.fixture
@@ -0,0 +1,3 @@
+package fixture
+
+func testGate() string { return os.Getenv("CI") }
diff --git a/internal/testutil/gatelint/testdata/fixtures/internal/.hidden/hidden_test.go.fixture b/internal/testutil/gatelint/testdata/fixtures/internal/.hidden/hidden_test.go.fixture
new file mode 100644
index 000000000..431ccae64
--- /dev/null
+++ b/internal/testutil/gatelint/testdata/fixtures/internal/.hidden/hidden_test.go.fixture
@@ -0,0 +1,3 @@
+package fixture
+
+func hiddenGate() { if testing.Short() {} }
diff --git a/internal/testutil/gatelint/testdata/fixtures/internal/r1_test.go.fixture b/internal/testutil/gatelint/testdata/fixtures/internal/r1_test.go.fixture
new file mode 100644
index 000000000..45fb3ad1b
--- /dev/null
+++ b/internal/testutil/gatelint/testdata/fixtures/internal/r1_test.go.fixture
@@ -0,0 +1,5 @@
+package fixture
+
+func testGate() {
+	if testing.Short() {}
+}
diff --git a/internal/testutil/gatelint/testdata/fixtures/runtime/production.go.fixture b/internal/testutil/gatelint/testdata/fixtures/runtime/production.go.fixture
new file mode 100644
index 000000000..02d39540a
--- /dev/null
+++ b/internal/testutil/gatelint/testdata/fixtures/runtime/production.go.fixture
@@ -0,0 +1,3 @@
+package fixture
+
+const inertRegistryFixture = "ailang-packages"
diff --git a/internal/testutil/gatelint/testdata/fixtures/std/clean_test.go.fixture b/internal/testutil/gatelint/testdata/fixtures/std/clean_test.go.fixture
new file mode 100644
index 000000000..4aeb30a79
--- /dev/null
+++ b/internal/testutil/gatelint/testdata/fixtures/std/clean_test.go.fixture
@@ -0,0 +1,3 @@
+package fixture
+
+func TestClean(t *testing.T) {}
diff --git a/internal/testutil/gatelint/testdata/fixtures/tests/r3_test.go.fixture b/internal/testutil/gatelint/testdata/fixtures/tests/r3_test.go.fixture
new file mode 100644
index 000000000..335100fc4
--- /dev/null
+++ b/internal/testutil/gatelint/testdata/fixtures/tests/r3_test.go.fixture
@@ -0,0 +1,3 @@
+package fixture
+
+const endpoint = "https://httpbin.org/post"