diff --git a/CHANGELOG.md b/CHANGELOG.md
index 518ce9d..767c097 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -36,6 +36,11 @@ Cutting that release is tracked in
 - Recording and transport regression coverage for every ffmpeg format/quality
   mapping, executable discovery, capture-failure and stop bounds, capabilities
   accuracy, and oversized Unix-socket requests.
+- Cross-uid Unix-socket rejection and host E2E classification checks for
+  blocked top-frame navigation and denied page-initiated downloads.
+- macOS explicitly converts download-intent links, attachment responses, and
+  unsupported response types into cancellable `WKDownload` objects so every
+  denied download is classified without writing page-controlled bytes.
 - Progressive context pruning: `inspect --context summary|outline|text|actions|full`
   with `--task` ranking, `--within @rN` scoping, and `--limit` / `--budget` /
   `--depth` bounds. Every focused response reports `contextStats` and `omitted`.
diff --git a/apps/headless/Tests/Fixtures/dashboard.html b/apps/headless/Tests/Fixtures/dashboard.html
index 8e3cb0e..a6628ec 100644
--- a/apps/headless/Tests/Fixtures/dashboard.html
+++ b/apps/headless/Tests/Fixtures/dashboard.html
@@ -30,7 +30,9 @@ <h2>Continue setup</h2>
       <p><a href="about:blank">Non-web browser URL</a></p>
       <p><a href="http://qa:secret@127.0.0.1/private">Credential-bearing URL</a></p>
       <p><a href="/suspicious-installer.dmg">Suspicious installer</a></p>
+      <p><a href="/download.txt" download>Download fixture</a></p>
       <button onclick="location.href='data:text/html,blocked'">Scripted non-web navigation</button>
+      <button onclick="location.href='about:blank'">Scripted host-blocked navigation</button>
       <button onclick="location.href='/next'">Continue</button>
     </section>
   </main>
diff --git a/apps/headless/Tests/HeadlessProtocolTests/ProtocolTests.swift b/apps/headless/Tests/HeadlessProtocolTests/ProtocolTests.swift
index 48fb01f..ff5ee8a 100644
--- a/apps/headless/Tests/HeadlessProtocolTests/ProtocolTests.swift
+++ b/apps/headless/Tests/HeadlessProtocolTests/ProtocolTests.swift
@@ -1095,6 +1095,48 @@ struct ProtocolTests {
         try expect(response.result == nil, "oversized request must not reach the command handler")
     }
 
+    static func differentPeerUserIsRejected() throws {
+        #if os(Linux)
+        // The Linux CI container runs this suite as root, which lets the test
+        // launch one deliberately unprivileged peer. Normal developer runs
+        // still exercise every other transport boundary without requiring
+        // privilege escalation.
+        guard getuid() == 0 else { return }
+        let setpriv = "/usr/bin/setpriv"
+        try expect(FileManager.default.isExecutableFile(atPath: setpriv), "Linux CI should provide setpriv")
+        try LocalRuntime.preparePrivateDirectory()
+        let socketPath = LocalRuntime.directoryURL
+            .appendingPathComponent("peer-uid-\(UUID().uuidString).sock").path
+        let server = LocalSocketServer(socketPath: socketPath)
+        try server.start { request in CommandResponse.success(id: request.id) }
+        defer {
+            server.stop()
+            _ = Glibc.chmod(LocalRuntime.directoryURL.path, 0o700)
+        }
+        // The production modes are 0700/0600. Open them only inside this
+        // disposable test so a different uid can reach accept(), where the
+        // credential check must still fail closed.
+        try expect(Glibc.chmod(LocalRuntime.directoryURL.path, 0o777) == 0, "test runtime directory chmod failed")
+        try expect(Glibc.chmod(socketPath, 0o666) == 0, "test socket chmod failed")
+
+        let process = Process()
+        process.executableURL = URL(fileURLWithPath: setpriv)
+        process.arguments = [
+            "--reuid=65534", "--regid=65534", "--clear-groups",
+            CommandLine.arguments[0], "--peer-denied-client", socketPath,
+        ]
+        process.standardOutput = FileHandle.nullDevice
+        let errors = Pipe()
+        process.standardError = errors
+        try process.run()
+        process.waitUntilExit()
+        let errorText = String(
+            decoding: errors.fileHandleForReading.readDataToEndOfFile(), as: UTF8.self
+        )
+        try expect(process.terminationStatus == 0, "different-uid peer was not rejected: \(errorText)")
+        #endif
+    }
+
     static func screenshotSeriesHelpers() throws {
         let rawPlan = JSONValue.object([
             "initialY": .number(240),
@@ -1386,6 +1428,24 @@ struct ProtocolTests {
     }
 
     static func main() {
+        if CommandLine.arguments.count == 3,
+           CommandLine.arguments[1] == "--peer-denied-client" {
+            do {
+                let descriptor = try connectRawUnixSocket(path: CommandLine.arguments[2])
+                defer { closeRawSocket(descriptor) }
+                let response = try ProtocolCodec.decodeLine(
+                    CommandResponse.self, from: readRawSocketLine(descriptor: descriptor)
+                )
+                guard response.error?.code == "PEER_DENIED" else {
+                    fputs("expected PEER_DENIED\n", stderr)
+                    exit(1)
+                }
+                exit(0)
+            } catch {
+                fputs("peer client failed: \(error)\n", stderr)
+                exit(1)
+            }
+        }
         let tests: [TestCase] = [
             ("request round-trip", requestRoundTrip),
             ("unsafe navigation schemes", rejectsUnsafeNavigationSchemes),
@@ -1426,6 +1486,7 @@ struct ProtocolTests {
             ("private socket directory", serverRejectsSocketOutsidePrivateDirectory),
             ("shutdown bypasses busy request", shutdownBypassesBusyRequest),
             ("oversized socket request", oversizedSocketRequestIsRejected),
+            ("different peer uid", differentPeerUserIsRejected),
         ]
 
         var failures = 0
diff --git a/apps/headless/Tests/fixture-server.mjs b/apps/headless/Tests/fixture-server.mjs
index 176fe2f..1847275 100644
--- a/apps/headless/Tests/fixture-server.mjs
+++ b/apps/headless/Tests/fixture-server.mjs
@@ -25,6 +25,17 @@ const server = createServer(async (request, response) => {
     response.end(body);
     return;
   }
+  if (pathname === '/download.txt') {
+    const body = Buffer.from('download must remain blocked');
+    response.writeHead(200, {
+      'content-type': 'text/plain; charset=utf-8',
+      'content-length': body.length,
+      'content-disposition': 'attachment; filename="download.txt"',
+      'cache-control': 'no-store',
+    });
+    response.end(body);
+    return;
+  }
   const fixture = routes.get(pathname);
   if (!fixture) {
     response.writeHead(404, {'content-type': 'text/plain; charset=utf-8'});
diff --git a/apps/headless/Tests/linux-e2e.sh b/apps/headless/Tests/linux-e2e.sh
index dce6326..dd1599d 100755
--- a/apps/headless/Tests/linux-e2e.sh
+++ b/apps/headless/Tests/linux-e2e.sh
@@ -128,8 +128,12 @@ if SUSPICIOUS_RESULT="$(headless --session qa click --role link --name 'Suspicio
   exit 1
 fi
 echo "$SUSPICIOUS_RESULT" | grep -q 'UNSAFE_RESOURCE_TYPE'
-headless --session qa click --role button --name 'Scripted non-web navigation' | grep -q '"clicked"'
+headless --session qa click --role button --name 'Scripted host-blocked navigation' | grep -q '"clicked"'
 headless --session qa wait --url /designers/dashboard --settled --timeout 10000 | grep -q 'Designers Dashboard'
+BLOCKED_NAVIGATION_REPORT="$(headless --session qa qa report)"
+echo "$BLOCKED_NAVIGATION_REPORT" | grep -q '"kind":"navigation-blocked"'
+echo "$BLOCKED_NAVIGATION_REPORT" | grep -q 'about:blank'
+headless --session qa qa clear | grep -q '"cleared"'
 headless --session qa screenshot --output viewport.png | grep -q '"name":"viewport.png"'
 headless --session qa screenshot --full-page --output full-page.png | grep -q '"name":"full-page.png"'
 headless --session qa screenshot --role button --name Continue --output continue.png | grep -q '"name":"continue.png"'
diff --git a/apps/headless/Tests/macos-e2e.sh b/apps/headless/Tests/macos-e2e.sh
index c26ad9f..499d419 100755
--- a/apps/headless/Tests/macos-e2e.sh
+++ b/apps/headless/Tests/macos-e2e.sh
@@ -185,28 +185,41 @@ echo "$QA_REPORT" | grep -q '"kind":"local-not-found"'
 STEP="safe-input-navigation"
 "$CLI" --session qa fill @e1 'Ada Lovelace' | grep -q '"valueLength":12'
 "$CLI" --session qa press Escape | grep -q '"pressed":"Escape"'
+STEP="safe-input-external-link"
 if EXTERNAL_RESULT="$("$CLI" --session qa click --role link --name 'External application')"; then
   echo "external application link was not blocked" >&2
   fail
 fi
 echo "$EXTERNAL_RESULT" | grep -q 'UNSAFE_NAVIGATION'
+STEP="safe-input-non-web-link"
 if NON_WEB_RESULT="$("$CLI" --session qa click --role link --name 'Non-web browser URL')"; then
   echo "non-HTTP browser URL was not blocked" >&2
   fail
 fi
 echo "$NON_WEB_RESULT" | grep -q 'UNSAFE_NAVIGATION'
+STEP="safe-input-credential-link"
 if CREDENTIAL_RESULT="$("$CLI" --session qa click --role link --name 'Credential-bearing URL')"; then
   echo "credential-bearing browser URL was not blocked" >&2
   fail
 fi
 echo "$CREDENTIAL_RESULT" | grep -q 'UNSAFE_NAVIGATION'
+STEP="safe-input-suspicious-link"
 if SUSPICIOUS_RESULT="$("$CLI" --session qa click --role link --name 'Suspicious installer')"; then
   echo "suspicious installer link was not blocked" >&2
   fail
 fi
 echo "$SUSPICIOUS_RESULT" | grep -q 'UNSAFE_RESOURCE_TYPE'
+STEP="safe-input-scripted-navigation"
 "$CLI" --session qa click --role button --name 'Scripted non-web navigation' | grep -q '"clicked"'
 "$CLI" --session qa wait --url /designers/dashboard --settled --timeout 10000 | grep -q 'Designers Dashboard'
+STEP="safe-input-download"
+"$CLI" --session qa click --role link --name 'Download fixture' | grep -q '"clicked"'
+sleep 1
+BLOCKED_DOWNLOAD_REPORT="$("$CLI" --session qa qa report)"
+STEP="safe-input-download-report"
+echo "$BLOCKED_DOWNLOAD_REPORT" | grep -q '"kind":"download-blocked"'
+echo "$BLOCKED_DOWNLOAD_REPORT" | grep -q '/download.txt'
+"$CLI" --session qa qa clear | grep -q '"cleared"'
 STEP="artifacts-visual"
 "$CLI" --session qa screenshot --output viewport.png | grep -q '"name":"viewport.png"'
 "$CLI" --session qa screenshot --full-page --output full-page.png | grep -q '"name":"full-page.png"'
diff --git a/apps/headless/main.swift b/apps/headless/main.swift
index 50a98d4..57962b4 100644
--- a/apps/headless/main.swift
+++ b/apps/headless/main.swift
@@ -706,6 +706,12 @@ final class BrowserWindowController: NSWindowController, NSWindowDelegate,
             decisionHandler(.cancel)
             return
         }
+        if navigationAction.shouldPerformDownload {
+            // Convert explicit download intent into WKDownload so the
+            // delegate below can classify and cancel it deterministically.
+            decisionHandler(.download)
+            return
+        }
         // Hand non-web schemes (mailto:, facetime:, app links…) to the system.
         if let url = navigationAction.request.url, let scheme = url.scheme?.lowercased(),
            !["http", "https", "file", "about", "data", "blob", "javascript"].contains(scheme) {
@@ -721,10 +727,17 @@ final class BrowserWindowController: NSWindowController, NSWindowDelegate,
         if let response = navigationResponse.response as? HTTPURLResponse {
             qaBridge.store.append(kind: "response", url: response.url?.absoluteString,
                                   method: "GET", status: Double(response.statusCode))
+            if response.value(forHTTPHeaderField: "Content-Disposition")?
+                .lowercased().contains("attachment") == true {
+                decisionHandler(.download)
+                return
+            }
         }
         if !navigationResponse.canShowMIMEType {
-            showToast("Can’t display this file type")
-            decisionHandler(.cancel)
+            // Unsupported navigation responses would otherwise become
+            // implicit downloads. Route them through the same fail-closed
+            // cancellation and diagnostic path.
+            decisionHandler(.download)
             return
         }
         decisionHandler(.allow)
diff --git a/docs/roadmap/improvements-backlog.md b/docs/roadmap/improvements-backlog.md
index f5e1697..7dfb897 100644
--- a/docs/roadmap/improvements-backlog.md
+++ b/docs/roadmap/improvements-backlog.md
@@ -287,7 +287,10 @@ screenshot-plan caps and 96 px deduplication, budget text fallback, and stale
 element-reference errors. Recording coverage now locks every format/quality
 argument mapping, strict executable discovery, consecutive-failure aborts, and
 bounded stop timeouts; capabilities and oversized socket requests are covered.
-Peer-credential and host navigation/download classification gaps remain.
+Linux CI also connects through a deliberately different uid and requires
+`PEER_DENIED`; Linux and macOS E2E assert `navigation-blocked` and
+`download-blocked` diagnostics respectively. **Done:** every gap named in D2
+now has direct regression coverage.
 
 **D3. Web CI:** ~~`next build` + eslint on PR (site can break invisibly today).~~
 **Done** — the `web` job in `ci.yml` runs `pnpm --filter @headless/web lint`