diff --git a/packages/compiler/src/frontend/lowering/lower-exprs.ts b/packages/compiler/src/frontend/lowering/lower-exprs.ts
index edf8fc573..78e0010a2 100644
--- a/packages/compiler/src/frontend/lowering/lower-exprs.ts
+++ b/packages/compiler/src/frontend/lowering/lower-exprs.ts
@@ -2978,6 +2978,13 @@ export function lowerOptionalChain(L: Lowerer, expr: ts.CallExpression | ts.Prop
   export function lowerCondition(L: Lowerer, expr: ts.Expression): IrExpr {
     let e: ts.Expression = expr;
     while (ts.isParenthesizedExpression(e)) e = e.expression;
+    // Node always installs the supported global Buffer constructor. A
+    // captured capability probe (`const b = globalThis.Buffer; if (b)`) is
+    // compile-time true; receiver-position calls through the alias still
+    // resolve via stdlibGlobalNameOf and keep Buffer's per-member fences.
+    if (stdlibGlobalNameOf(L, e) === "Buffer") {
+      return { kind: "boolLit", value: true, type: BOOL, loc: locOf(expr) };
+    }
     if (ts.isBinaryExpression(e)) {
       const op = e.operatorToken.kind;
       if (op === ts.SyntaxKind.AmpersandAmpersandToken || op === ts.SyntaxKind.BarBarToken) {
diff --git a/packages/compiler/src/frontend/lowering/surfaces.ts b/packages/compiler/src/frontend/lowering/surfaces.ts
index 42ff20d9e..fa5f9e28d 100644
--- a/packages/compiler/src/frontend/lowering/surfaces.ts
+++ b/packages/compiler/src/frontend/lowering/surfaces.ts
@@ -1510,10 +1510,19 @@ export const BUILTIN_MODULE_FENCE_HINTS: Record<string, Record<string, string |
     }
     if (ts.isPropertyAccessExpression(expr) && !expr.questionDotToken) {
       if (stdlibGlobalNameOf(L, expr.expression) !== "globalThis") return null;
+      // Capability probes commonly narrow globalThis before reading Buffer:
+      // `(globalThis as { Buffer?: RuntimeBuffer }).Buffer`. The cast's
+      // member symbol belongs to the user-declared probe shape rather than
+      // @types/node, but the receiver is still the unshadowable globalThis
+      // intrinsic and the runtime Buffer is the same supported global.
+      if (expr.name.text === "Buffer") return "Buffer";
       const symbol = L.checker.getSymbolAtLocation(expr.name);
       if (!symbol || !L.isStdlibSymbol(symbol)) return null;
       return symbol.name === "global" ? "globalThis" : symbol.name;
     }
+    if (ts.isAsExpression(expr) || ts.isTypeAssertion(expr)) {
+      return stdlibGlobalNameOf(L, expr.expression);
+    }
     return null;
   }
 
@@ -1564,7 +1573,10 @@ export const BUILTIN_MODULE_FENCE_HINTS: Record<string, Record<string, string |
     // performance — the mockable-clock idiom snapshots it). Function-valued
     // globals (setTimeout) taken as values are a different story — the
     // ordinary value paths (and their fences) apply.
-    if (name !== "process" && name !== "console" && name !== "globalThis" && name !== "performance") return false;
+    if (
+      name !== "process" && name !== "console" && name !== "globalThis" &&
+      name !== "performance" && name !== "Buffer"
+    ) return false;
     const symbol = L.checker.getSymbolAtLocation(nameNode);
     if (!symbol) return false;
     L.stdlibGlobalAliases.set(symbol, name);
diff --git a/tests/harness/global-buffer-alias.test.ts b/tests/harness/global-buffer-alias.test.ts
new file mode 100644
index 000000000..0a20da16d
--- /dev/null
+++ b/tests/harness/global-buffer-alias.test.ts
@@ -0,0 +1,122 @@
+import { execFile } from "node:child_process";
+import { createHash } from "node:crypto";
+import { mkdirSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { promisify } from "node:util";
+import { describe, expect, test } from "vitest";
+import { compile } from "@scriptc/compiler";
+
+const execFileAsync = promisify(execFile);
+const sanitize = process.env["SCRIPTC_SAN"] === "1";
+
+interface RunResult {
+  stdout: Buffer;
+  stderr: Buffer;
+  exitCode: number;
+}
+
+async function run(cmd: string, args: string[]): Promise<RunResult> {
+  try {
+    const { stdout, stderr } = await execFileAsync(cmd, args, { encoding: "buffer" });
+    return { stdout, stderr, exitCode: 0 };
+  } catch (err) {
+    if (
+      typeof err !== "object" || err === null ||
+      !("code" in err) || typeof err.code !== "number" ||
+      !("stdout" in err) || !Buffer.isBuffer(err.stdout) ||
+      !("stderr" in err) || !Buffer.isBuffer(err.stderr)
+    ) {
+      throw err;
+    }
+    return { stdout: err.stdout, stderr: err.stderr, exitCode: err.code };
+  }
+}
+
+async function compileAndCompare(source: string, backend: "c" | "llvm"): Promise<void> {
+  const key = createHash("sha256")
+    .update(source)
+    .update(`${backend}-${sanitize ? "san" : "plain"}`)
+    .digest("hex")
+    .slice(0, 16);
+  const outDir = join(tmpdir(), "scriptc-tests", `global-buffer-alias-${key}`);
+  mkdirSync(outDir, { recursive: true });
+  const file = join(outDir, "main.mts");
+  writeFileSync(file, source);
+  const result = await compile(file, {
+    outPath: join(outDir, "program"),
+    outDir,
+    sanitize,
+    backend,
+  });
+  if (!result.ok) {
+    throw new Error(
+      "guarded global Buffer alias program failed to compile:\n" +
+        result.diagnostics.map((d) => `${d.code}: ${d.message}`).join("\n"),
+    );
+  }
+  const [nodeResult, nativeResult] = await Promise.all([
+    run("node", ["--experimental-transform-types", "--disable-warning=ExperimentalWarning", file]),
+    run(result.binaryPath, []),
+  ]);
+  expect(nativeResult.stdout).toEqual(nodeResult.stdout);
+  expect(nativeResult.stderr).toEqual(nodeResult.stderr);
+  expect(nativeResult.exitCode).toBe(nodeResult.exitCode);
+}
+
+async function compileAndExpectFence(source: string, backend: "c" | "llvm"): Promise<void> {
+  const key = createHash("sha256").update(source).update(backend).digest("hex").slice(0, 16);
+  const outDir = join(tmpdir(), "scriptc-tests", `global-buffer-alias-fence-${key}`);
+  mkdirSync(outDir, { recursive: true });
+  const file = join(outDir, "main.mts");
+  writeFileSync(file, source);
+  const result = await compile(file, {
+    outPath: join(outDir, "program"),
+    outDir,
+    sanitize,
+    backend,
+  });
+  expect(result.ok).toBe(false);
+  if (result.ok) return;
+  expect(result.diagnostics.map((d) => `${d.code}: ${d.message}`).join("\n")).toContain(
+    "SC1090: the reference to 'runtimeBuffer' (a binding form with no lowering) is not supported yet",
+  );
+}
+
+describe.each(["c", "llvm"] as const)(
+  `guarded global Buffer alias, %s backend${sanitize ? " (sanitized)" : ""}`,
+  (backend) => {
+    test("counts UTF-8 bytes through the guarded constructor alias", async () => {
+      await compileAndCompare(`
+interface RuntimeBuffer {
+  byteLength(value: string, encoding?: "utf8"): number;
+}
+
+const runtimeBuffer = (globalThis as { Buffer?: RuntimeBuffer }).Buffer;
+
+function byteLength(content: string): number {
+  return runtimeBuffer
+    ? runtimeBuffer.byteLength(content, "utf8")
+    : content.length;
+}
+
+console.log(byteLength("ascii"));
+console.log(byteLength("é"));
+console.log(byteLength("😀"));
+console.log(runtimeBuffer ? runtimeBuffer.byteLength("Aé😀") : -1);
+`, backend);
+    });
+
+    test("preserves unsupported member fences through the alias", async () => {
+      await compileAndExpectFence(`
+interface RuntimeBuffer {
+  byteLength(value: string): number;
+  poolSize?: number;
+}
+
+const runtimeBuffer = (globalThis as { Buffer?: RuntimeBuffer }).Buffer;
+console.log(runtimeBuffer ? runtimeBuffer.poolSize : -1);
+`, backend);
+    });
+  },
+);