diff --git a/packages/bun-uws/src/App.h b/packages/bun-uws/src/App.h
index e326e63a6368..8b3181eb1d99 100644
--- a/packages/bun-uws/src/App.h
+++ b/packages/bun-uws/src/App.h
@@ -418,19 +418,16 @@ struct TemplatedApp {
 
         /* Terminate on misleading idleTimeout values */
         if (behavior.idleTimeout && behavior.idleTimeout < 8) {
-            std::cerr << "Error: idleTimeout must be either 0 or greater than 8!" << std::endl;
             std::terminate();
         }
 
         /* Maximum idleTimeout is 16 minutes */
         if (behavior.idleTimeout > 240 * 4) {
-            std::cerr << "Error: idleTimeout must not be greater than 960 seconds!" << std::endl;
             std::terminate();
         }
 
         /* Maximum maxLifetime is 4 hours */
         if (behavior.maxLifetime > 240) {
-            std::cerr << "Error: maxLifetime must not be greater than 240 minutes!" << std::endl;
             std::terminate();
         }
 
diff --git a/packages/bun-uws/src/AsyncSocket.h b/packages/bun-uws/src/AsyncSocket.h
index 5e2fafae7eb6..e608eedf98d0 100644
--- a/packages/bun-uws/src/AsyncSocket.h
+++ b/packages/bun-uws/src/AsyncSocket.h
@@ -27,7 +27,6 @@
  * to signal error with -1 (which is how the entire UNIX syscalling is built). */
 
 #include <cstring>
-#include <iostream>
 
 #include "libusockets.h"
 #include "bun-usockets/src/internal/internal.h"
diff --git a/packages/bun-uws/src/HttpContext.h b/packages/bun-uws/src/HttpContext.h
index 67d209e5e54f..6e3bd96fe15d 100644
--- a/packages/bun-uws/src/HttpContext.h
+++ b/packages/bun-uws/src/HttpContext.h
@@ -30,7 +30,6 @@
 #include <string>
 #include <map>
 #include <string_view>
-#include <iostream>
 #include "MoveOnlyFunction.h"
 #include "HttpParser.h"
 #include <span>
@@ -465,8 +464,6 @@ struct HttpContext {
 
             /* Returning from a request handler without responding or attaching an onAborted handler is ill-use */
             if (!((HttpResponse<SSL> *) s)->hasResponded() && !httpResponseData->onAborted && !httpResponseData->socketData) {
-                /* Throw exception here? */
-                std::cerr << "Error: Returning from a request handler without responding or attaching an abort handler is forbidden!" << std::endl;
                 std::terminate();
             }
 
diff --git a/packages/bun-uws/src/HttpRouter.h b/packages/bun-uws/src/HttpRouter.h
index 2783237a40f1..8a48cd6b8ea7 100644
--- a/packages/bun-uws/src/HttpRouter.h
+++ b/packages/bun-uws/src/HttpRouter.h
@@ -27,7 +27,6 @@
 #include <memory>
 #include <utility>
 #include <span>
-#include <iostream>
 
 #include "MoveOnlyFunction.h"
 
diff --git a/packages/bun-uws/src/Loop.h b/packages/bun-uws/src/Loop.h
index c33f271f626b..d8d8003c74b5 100644
--- a/packages/bun-uws/src/Loop.h
+++ b/packages/bun-uws/src/Loop.h
@@ -23,7 +23,6 @@
 
 #include "LoopData.h"
 #include <libusockets.h>
-#include <iostream>
 #include "AsyncSocket.h"
 
 extern "C" int bun_is_exiting();
diff --git a/packages/bun-uws/src/TopicTree.h b/packages/bun-uws/src/TopicTree.h
index 0df624b04d18..ec3a61d1671a 100644
--- a/packages/bun-uws/src/TopicTree.h
+++ b/packages/bun-uws/src/TopicTree.h
@@ -18,7 +18,6 @@
 #pragma once
 #include <map>
 #include <list>
-#include <iostream>
 #include <unordered_set>
 #include <utility>
 #include <memory>
@@ -106,9 +105,7 @@ struct TopicTree {
     std::vector<T> outgoingMessages;
 
     void checkIteratingSubscriber(Subscriber *s) {
-        /* Notify user that they are doing something wrong here */
         if (iteratingSubscriber == s) {
-            std::cerr << "Error: WebSocket must not subscribe or unsubscribe to topics while iterating its topics!" << std::endl;
             std::terminate();
         }
     }
@@ -163,7 +160,6 @@ struct TopicTree {
 
     /* Subscribe fails if we already are subscribed */
     Topic *subscribe(Subscriber *s, std::string_view topic) {
-        /* Notify user that they are doing something wrong here */
         checkIteratingSubscriber(s);
 
         /* Lookup or create new topic */
@@ -187,7 +183,6 @@ struct TopicTree {
 
     /* Returns ok, last, newCount */
     std::tuple<bool, bool, int> unsubscribe(Subscriber *s, std::string_view topic) {
-        /* Notify user that they are doing something wrong here */
         checkIteratingSubscriber(s);
 
         /* Lookup topic */
diff --git a/scripts/build/flags.ts b/scripts/build/flags.ts
index 2ea21c66e91d..c31aaa2de3c8 100644
--- a/scripts/build/flags.ts
+++ b/scripts/build/flags.ts
@@ -1512,6 +1512,12 @@ export const stripFlags: Flag[] = [
 export function bunIncludes(cfg: Config): string[] {
   const { cwd, codegenDir, vendorDir } = cfg;
   const includes: string[] = [
+    // Release builds shadow <iostream> with a #error shim. A single <iostream>
+    // include anywhere (our headers, bun-uws, or a WebKit header pulled into a
+    // Bun TU) drags libstdc++'s globals_io.o into the link and runs
+    // std::ios_base::Init + the full locale facet set before main. Debug builds
+    // keep the real header available for ad-hoc printf-debugging.
+    ...(cfg.release ? [join(cwd, "src/banned-includes")] : []),
     join(cwd, "packages"),
     join(cwd, "packages/bun-usockets"),
     join(cwd, "packages/bun-usockets/src"),
diff --git a/src/banned-includes/iostream b/src/banned-includes/iostream
new file mode 100644
index 000000000000..dfd898b0077f
--- /dev/null
+++ b/src/banned-includes/iostream
@@ -0,0 +1,27 @@
+// This directory is placed first on the -I search path for Bun's C++ compile
+// (release profile), so `#include <iostream>` resolves here instead of the
+// toolchain header.
+//
+// <iostream> is unlike <ostream>/<istream>/<sstream>/<fstream>: on libstdc++
+// it emits a reference to std::ios_base_library_init (or, on configurations
+// without the init-priority attribute, a static `std::ios_base::Init __ioinit`
+// object) in every translation unit that includes it. One such reference
+// anywhere in the link pulls libstdc++'s globals_io.o in, whose
+// _GLOBAL__sub_I.00090_globals_io.cc static initializer constructs
+// cin/cout/cerr/clog and their wchar_t siblings before main. That in turn
+// references the full std::locale facet set (ctype / numpunct / moneypunct /
+// timepunct / messages, for both char and wchar_t), so roughly fifty libstdc++
+// functions run on every Bun process start.
+//
+// Bun never touches C++ iostreams at runtime. Use fputs/fprintf for error
+// output, or WTF's dataLog()/PrintStream in JSC-adjacent code.
+//
+// Because Bun's own headers include wtf/SIMDUTF.h (and others) from the
+// WebKit prebuilt, this shim also catches a WebKit header that starts
+// including <iostream>: it will fail Bun's release build rather than silently
+// regressing startup.
+#ifndef BUN_ALLOW_IOSTREAM
+#error "<iostream> is banned in Bun release builds: it drags std::ios_base::Init and the full std::locale facet set into pre-main startup. Use <cstdio> fputs/fprintf for stderr output. See src/banned-includes/iostream."
+#else
+#include_next <iostream>
+#endif
diff --git a/test/internal/source-lints/no-iostream-include.test.ts b/test/internal/source-lints/no-iostream-include.test.ts
new file mode 100644
index 000000000000..2ef0fa212c33
--- /dev/null
+++ b/test/internal/source-lints/no-iostream-include.test.ts
@@ -0,0 +1,51 @@
+import { Glob } from "bun";
+import { expect, test } from "bun:test";
+import { readFileSync } from "node:fs";
+import path from "node:path";
+
+// <iostream> is unique among the C++ stream headers: on libstdc++ it emits a
+// reference to std::ios_base_library_init in every TU that includes it, which
+// forces libstdc++'s globals_io.o into the link. That object's
+// _GLOBAL__sub_I.00090_globals_io.cc initializer constructs cin/cout/cerr/clog
+// (and the wchar_t variants) before main, dragging the full std::locale facet
+// set (ctype/numpunct/moneypunct/timepunct/messages for char and wchar_t) into
+// every Bun process startup. Bun never touches C++ iostreams at runtime.
+//
+// <ostream>, <istream>, <sstream> and <fstream> are fine: they declare the
+// stream types but do not emit the static Init object. If you need to print to
+// stderr from C++, use fputs/fprintf.
+//
+// The upstream source of the original leak was the vendored simdutf header
+// inside WebKit (Source/WTF/wtf/simdutf/simdutf_impl.h); that is handled by
+// the WebKit pin. This test guards Bun's own compiled C++ so the initializer
+// cannot creep back in through packages/ or src/.
+test("C++ sources compiled into Bun do not include <iostream>", async () => {
+  const repoRoot = path.resolve(import.meta.dir, "..", "..", "..");
+
+  const roots = ["src", "packages/bun-uws", "packages/bun-usockets"];
+  // sizegen.cpp is a build-time code generator, not linked into the bun binary.
+  const allowlist = new Set(["src/jsc/headergen/sizegen.cpp"]);
+
+  const iostreamInclude = /^\s*#\s*include\s*<iostream>/m;
+  const violations: string[] = [];
+
+  for (const root of roots) {
+    let scanned = 0;
+    const glob = new Glob("**/*.{h,hpp,hxx,cpp,cc,cxx}");
+    for await (const rel of glob.scan({ cwd: path.join(repoRoot, root) })) {
+      scanned++;
+      const relFromRepo = path.join(root, rel).replaceAll("\\", "/");
+      if (allowlist.has(relFromRepo)) continue;
+      const source = readFileSync(path.join(repoRoot, root, rel), "utf8");
+      if (iostreamInclude.test(source)) {
+        violations.push(relFromRepo);
+      }
+    }
+    // Guard against repoRoot resolving wrong (test file moved) or a scanned
+    // root going away, which would make the ban below pass vacuously.
+    expect(scanned).toBeGreaterThan(0);
+  }
+
+  violations.sort();
+  expect(violations).toEqual([]);
+});