diff --git a/internal/compatibility/generated/node-v24-internal.json b/internal/compatibility/generated/node-v24-internal.json
index 928bea3bb..14a5a4d6a 100644
--- a/internal/compatibility/generated/node-v24-internal.json
+++ b/internal/compatibility/generated/node-v24-internal.json
@@ -79679,6 +79679,7 @@
           "tests/corpus/process-builtin-values.js",
           "tests/corpus/process-hrtime.js",
           "tests/corpus/process-named-signals.cjs",
+          "tests/corpus/process-stream-drain.ts",
           "tests/corpus/process-uncaught-handler-throws.cjs",
           "tests/corpus/process-uncaught-loop.cjs",
           "tests/corpus/process-uncaught-module.mjs",
@@ -79686,6 +79687,7 @@
           "tests/corpus/process-uncaught-rejection.cjs",
           "tests/corpus/process-uncaught-sync.cjs",
           "tests/corpus/process-versions-storage.ts",
+          "tests/corpus/process-write-dynamic-chunks.ts",
           "tests/corpus/stdio-values.mjs",
           "tests/corpus/stdio-write-errors.mjs",
           "tests/corpus/stdio-write-overrides.mjs",
@@ -82049,6 +82051,8 @@
           "tests/corpus/stdio-write-overrides.mjs",
           "tests/corpus/stdio-write-errors.mjs",
           "tests/corpus/stdout-write-patch.ts",
+          "tests/corpus/process-stream-drain.ts",
+          "tests/corpus/process-write-dynamic-chunks.ts",
           "tests/harness/event-loop.test.ts"
         ]
       },
@@ -82092,6 +82096,8 @@
           "tests/corpus/stdio-write-overrides.mjs",
           "tests/corpus/stdio-write-errors.mjs",
           "tests/corpus/stdout-write-patch.ts",
+          "tests/corpus/process-stream-drain.ts",
+          "tests/corpus/process-write-dynamic-chunks.ts",
           "tests/harness/event-loop.test.ts"
         ]
       },
@@ -82135,6 +82141,8 @@
           "tests/corpus/stdio-write-overrides.mjs",
           "tests/corpus/stdio-write-errors.mjs",
           "tests/corpus/stdout-write-patch.ts",
+          "tests/corpus/process-stream-drain.ts",
+          "tests/corpus/process-write-dynamic-chunks.ts",
           "tests/harness/event-loop.test.ts"
         ]
       },
diff --git a/internal/compatibility/static-support.json b/internal/compatibility/static-support.json
index 96e4418db..5b4095f9a 100644
--- a/internal/compatibility/static-support.json
+++ b/internal/compatibility/static-support.json
@@ -314,7 +314,7 @@
     { "chapter": "process", "signatures": ["`process.hrtime([time])`", "`process.hrtime.bigint()`"], "status": "partial", "evidence": ["tests/corpus/process-hrtime.js"] },
     { "chapter": "errors", "signatures": ["`stackTraceLimit` Type: {number}"], "status": "partial", "evidence": ["tests/corpus/error-stack-limit.ts"] },
     { "chapter": "errors", "signatures": ["`stack` Type: {string}"], "status": "partial", "evidence": ["tests/corpus/error-stack-frames.ts", "tests/corpus/error-stack-limit.ts"] },
-    { "chapter": "process", "signatures": ["`stdin` Type: {Stream}", "`stdout` Type: {Stream}", "`stderr` Type: {Stream}"], "status": "partial", "evidence": ["tests/corpus/stdio-values.mjs", "tests/corpus/stdio-write-overrides.mjs", "tests/corpus/stdio-write-errors.mjs", "tests/corpus/stdout-write-patch.ts", "tests/harness/event-loop.test.ts"] },
+    { "chapter": "process", "signatures": ["`stdin` Type: {Stream}", "`stdout` Type: {Stream}", "`stderr` Type: {Stream}"], "status": "partial", "evidence": ["tests/corpus/stdio-values.mjs", "tests/corpus/stdio-write-overrides.mjs", "tests/corpus/stdio-write-errors.mjs", "tests/corpus/stdout-write-patch.ts", "tests/corpus/process-stream-drain.ts", "tests/corpus/process-write-dynamic-chunks.ts", "tests/harness/event-loop.test.ts"] },
     { "chapter": "process", "signatures": ["Signal events"], "status": "partial", "evidence": ["tests/corpus/1443-signal-handlers.ts", "tests/corpus/process-named-signals.cjs"] },
     { "chapter": "process", "signatures": ["Event: `'uncaughtException'`", "Event: `'uncaughtExceptionMonitor'`"], "status": "partial", "evidence": ["tests/corpus/process-uncaught-sync.cjs", "tests/corpus/process-uncaught-loop.cjs", "tests/corpus/process-uncaught-module.mjs", "tests/corpus/process-uncaught-rejection.cjs", "tests/corpus/process-uncaught-monitor.cjs", "tests/corpus/process-uncaught-handler-throws.cjs"] },
     { "chapter": "process", "signatures": ["`process.getBuiltinModule(id)`"], "status": "partial", "evidence": ["tests/corpus/process-builtin-values.js", "packages/compiler/test/process-builtin-values.test.ts"] },
diff --git a/packages/compiler/src/backend/llvm/lib-shared.ts b/packages/compiler/src/backend/llvm/lib-shared.ts
index 754b5eb33..e7f918fdf 100644
--- a/packages/compiler/src/backend/llvm/lib-shared.ts
+++ b/packages/compiler/src/backend/llvm/lib-shared.ts
@@ -379,6 +379,7 @@ export const LIB_FN_SYMS: Record<string, string> = {
   "process.envPairs": "scr_env_pairs",
   "process.stdoutWrite": "scr_process_stdout_write",
   "process.stdio": "scr_process_stdio",
+  "process.stdioWriteDyn": "scr_process_stdio_write_dyn",
   "process.stderrWrite": "scr_process_stderr_write",
   "process.isTTY": "scr_process_is_tty",
   // A promise that never settles (events.once of an event a synchronous
diff --git a/packages/compiler/src/backend/llvm/reference-writes.ts b/packages/compiler/src/backend/llvm/reference-writes.ts
index 56379a443..7ef02a978 100644
--- a/packages/compiler/src/backend/llvm/reference-writes.ts
+++ b/packages/compiler/src/backend/llvm/reference-writes.ts
@@ -102,6 +102,7 @@ const INERT_LIB_CALLS = new Set([
   "json.parse",
   "process.stderrWrite",
   "process.stdoutWrite",
+  "process.stdioWriteDyn",
   "process.envGet",
   "process.envPairs",
   "regex.groupEquals",
diff --git a/packages/compiler/src/frontend/lowering/builtins/process.ts b/packages/compiler/src/frontend/lowering/builtins/process.ts
index 87fd66412..e5c9bd5af 100644
--- a/packages/compiler/src/frontend/lowering/builtins/process.ts
+++ b/packages/compiler/src/frontend/lowering/builtins/process.ts
@@ -1357,6 +1357,24 @@ export function lowerProcessMethodCall(
       if (data.type.kind === "dyn" && isJsSourceFile(ts.getSourceFileOfNode(call))) {
         data = { kind: "dynCheck", value: data, type: STRING, loc };
       }
+      // A chunk known only at run time (`any`, or a string | Uint8Array
+      // union): the runtime dispatches on the value — a string or a
+      // Uint8Array/Buffer writes, null throws ERR_STREAM_NULL_VALUES, any
+      // other value throws ERR_INVALID_ARG_TYPE, as Node's Writable.write
+      // does before writing anything. Only the one-argument form.
+      if (
+        args.length === 1 &&
+        (data.type.kind === "dyn" || data.type.kind === "union") &&
+        (data.type.kind === "dyn" || lowerer.dynConvertible(data.type))
+      ) {
+        return {
+          kind: "libCall",
+          fn: "process.stdioWriteDyn",
+          args: [numLit(stream === "stdout" ? 1 : 2, loc), lowerer.coerceToExpected(data, DYN)],
+          type: BOOL,
+          loc,
+        };
+      }
       const isBytes = data.type.kind === "bytes" && data.type.elem === "u8";
       if (!isBytes && data.type.kind !== "string") {
         lowerer.noLowering(
diff --git a/packages/compiler/src/ir/builtin-effects.ts b/packages/compiler/src/ir/builtin-effects.ts
index 114231730..85005cfc2 100644
--- a/packages/compiler/src/ir/builtin-effects.ts
+++ b/packages/compiler/src/ir/builtin-effects.ts
@@ -107,6 +107,7 @@ export const MAY_THROW_LIB_FNS: ReadonlySet<IrLibFn> = new Set([
   "ffi.argument",
   "ffi.memoryModule",
   "process.stdoutWrite",
+  "process.stdioWriteDyn",
   "process.stderrWrite",
   "process.stdoutWriteBytes",
   "process.stderrWriteBytes",
diff --git a/packages/compiler/src/ir/builtin-signatures.ts b/packages/compiler/src/ir/builtin-signatures.ts
index 55596d087..d8d3ce999 100644
--- a/packages/compiler/src/ir/builtin-signatures.ts
+++ b/packages/compiler/src/ir/builtin-signatures.ts
@@ -3668,6 +3668,9 @@ function buildLibFnSignatures() {
      * never throws. */
     "process.stdoutWrite": { argTypes: [STRING], result: BOOL },
     "process.stdio": { argTypes: [F64], result: DYN },
+    /** process.stdout/stderr.write(chunk) with a run-time chunk: Node's
+     * Writable validation, then a synchronous write. */
+    "process.stdioWriteDyn": { argTypes: [F64, DYN], result: BOOL },
     "process.stderrWrite": { argTypes: [STRING], result: BOOL },
     // error.new's result and the receiver slots are builtin-error classes —
     // program-dependent object types, checked in the libCall case.
@@ -5341,6 +5344,10 @@ function buildLibFnSignatures() {
     /** Validated isatty(3) probe. Stream property lowering maps false to
      * undefined; tty.isatty exposes the boolean result directly. */
     "process.isTTY": { argTypes: [F64], result: BOOL },
+    /** A promise that never settles (events.once of an event that cannot
+     * occur, e.g. 'drain' on a synchronous process stream). The result is
+     * events.once's own Promise<any[]> type. */
+    "events.pendingForever": { argTypes: [], result: { kind: "promise", inner: arrayOf(DYN) } },
     // Like process.envGet: the result is the module's interned
     // `number | undefined` union — checked by arms in the libCall case.
     /** Terminal width over an fd literal (1/2 — process.stdout/stderr
diff --git a/packages/compiler/src/ir/runtime-features.ts b/packages/compiler/src/ir/runtime-features.ts
index 93b78db14..f922204d0 100644
--- a/packages/compiler/src/ir/runtime-features.ts
+++ b/packages/compiler/src/ir/runtime-features.ts
@@ -647,6 +647,7 @@ const PROCESS_EVENT_LIB_FNS: ReadonlySet<IrLibFn> = new Set([
   "process.onExit",
   "process.onSignal",
   "process.stdio",
+  "process.stdioWriteDyn",
   "stdin.nextChunk",
   "stdin.onData",
   "stdin.onEnd",
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/process-write-dynamic-chunks.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/process-write-dynamic-chunks.ts.json
new file mode 100644
index 000000000..0558bbdd6
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/process-write-dynamic-chunks.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/process-write-dynamic-chunks.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/runtime/src/scr_blob.h b/packages/runtime/src/scr_blob.h
index 7c7893322..53d2d2228 100644
--- a/packages/runtime/src/scr_blob.h
+++ b/packages/runtime/src/scr_blob.h
@@ -68,6 +68,10 @@ ScrDyn *scr_console_new(double out_fd, double err_fd, bool ignore_errors);
 void scr_console_bind(ScrDyn *console, double which, ScrStream *stream);
 void scr_console_inst_write(ScrDyn *console, ScrStr *text, bool to_stderr);
 ScrDyn *scr_process_stdio(double fd);
+/* process.stdout/stderr.write(chunk) with a run-time chunk: Node's Writable
+ * validation, then a synchronous write. false with an exception pending when
+ * the chunk is invalid. */
+bool scr_process_stdio_write_dyn(double fd, ScrDyn *chunk);
 bool scr_dyn_process_stdio_is(const ScrDyn *value);
 double scr_dyn_process_stdio_fd(const ScrDyn *value, const ScrDynPath *path);
 
diff --git a/packages/runtime/src/scr_events.c b/packages/runtime/src/scr_events.c
index 9de2c6ce2..8794c270c 100644
--- a/packages/runtime/src/scr_events.c
+++ b/packages/runtime/src/scr_events.c
@@ -1311,6 +1311,52 @@ static void scr_stdio_resize_listener(ScrStdio *stream, ScrDyn *listener, bool r
 }
 
 #endif
+
+/* stream.write(chunk[, encoding][, callback]) on a process output stream:
+ * Node's Writable.write validation, then a synchronous descriptor write.
+ * Returns +1 (true/false) or NULL with an exception pending. */
+static ScrDyn *scr_stdio_write_call(ScrStdio *stream, ScrDyn *const *args, size_t argc) {
+  ScrDyn *arg = argc ? args[0] : scr_dyn_undefined();
+  ScrDyn *result = NULL;
+  ScrDyn *encoding = argc > 1 ? args[1] : scr_dyn_undefined();
+  ScrDyn *callback = argc > 2 ? args[2] : scr_dyn_undefined();
+  if (encoding->kind == SCR_DYN_FUNC) { callback = encoding; encoding = scr_dyn_undefined(); }
+  if (callback->kind != SCR_DYN_UNDEF && callback->kind != SCR_DYN_NULL && callback->kind != SCR_DYN_FUNC) {
+    scr_dyn_arg_type_fail("cb", "of type function", callback);
+    return NULL;
+  }
+  if (arg->kind == SCR_DYN_NULL) {
+    const char *m = "May not write null values to stream";
+    scr_throw_error_msg_code(SCR_ERR_TYPE, m, strlen(m), "ERR_STREAM_NULL_VALUES");
+    return NULL;
+  }
+  if (arg->kind == SCR_DYN_BYTES && arg->v.bytes->elem != SCR_BYTES_U8)
+    return scr_stdio_refusal("write of a typed array other than Uint8Array");
+  ScrBytes *bytes = NULL;
+  if (arg->kind == SCR_DYN_STR) {
+    if (encoding->kind != SCR_DYN_UNDEF && encoding->kind != SCR_DYN_NULL && encoding->kind != SCR_DYN_STR) {
+      return scr_stdio_refusal("write encoding");
+    }
+    ScrStr *enc = encoding->kind == SCR_DYN_STR ? scr_str_retain(encoding->v.str) : scr_str_new("utf8", 4);
+    bytes = scr_bytes_from_str(arg->v.str, enc);
+    scr_str_release(enc);
+  } else if (arg->kind == SCR_DYN_BYTES && arg->v.bytes->elem == SCR_BYTES_U8) {
+    bytes = arg->v.bytes->shared ? scr_bytes_copy(arg->v.bytes) : scr_bytes_retain(arg->v.bytes);
+  } else {
+    scr_dyn_arg_type_fail("chunk", "of type string or an instance of Buffer, TypedArray, or DataView", arg);
+    return NULL;
+  }
+  if (!bytes) return NULL;
+  int error = scr_stdio_write_raw(stream->fd, bytes->data, bytes->len);
+  scr_bytes_release(bytes);
+  if (error) {
+    scr_stdio_queue_write_error(stream, callback, error);
+    return scr_dyn_new_bool(false);
+  }
+  if (callback->kind == SCR_DYN_FUNC) scr_next_tick(scr_stdio_callback(callback, (void *)scr_stdio_write_done));
+  return scr_dyn_new_bool(true);
+}
+
 static ScrDyn *scr_stdio_method(ScrClosure *cb, ScrDyn *const *args, size_t argc) {
   int method = (int)scr_box_get_f64(cb->caps[0]);
   ScrDyn *self = scr_dyn_this_get();
@@ -1328,38 +1374,7 @@ static ScrDyn *scr_stdio_method(ScrClosure *cb, ScrDyn *const *args, size_t argc
     scr_dyn_arr_push(result, columns);
     scr_dyn_arr_push(result, rows);
   } else if (method == STDIO_WRITE && stream->fd != 0) {
-    ScrDyn *encoding = argc > 1 ? args[1] : scr_dyn_undefined();
-    ScrDyn *callback = argc > 2 ? args[2] : scr_dyn_undefined();
-    if (encoding->kind == SCR_DYN_FUNC) { callback = encoding; encoding = scr_dyn_undefined(); }
-    if (callback->kind != SCR_DYN_UNDEF && callback->kind != SCR_DYN_NULL && callback->kind != SCR_DYN_FUNC) {
-      scr_dyn_arg_type_fail("cb", "of type function", callback);
-      goto done;
-    }
-    ScrBytes *bytes = NULL;
-    if (arg->kind == SCR_DYN_STR) {
-      if (encoding->kind != SCR_DYN_UNDEF && encoding->kind != SCR_DYN_NULL && encoding->kind != SCR_DYN_STR) {
-        result = scr_stdio_refusal("write encoding");
-        goto done;
-      }
-      ScrStr *enc = encoding->kind == SCR_DYN_STR ? scr_str_retain(encoding->v.str) : scr_str_new("utf8", 4);
-      bytes = scr_bytes_from_str(arg->v.str, enc);
-      scr_str_release(enc);
-    } else if (arg->kind == SCR_DYN_BYTES && arg->v.bytes->elem == SCR_BYTES_U8) {
-      bytes = arg->v.bytes->shared ? scr_bytes_copy(arg->v.bytes) : scr_bytes_retain(arg->v.bytes);
-    } else {
-      scr_dyn_arg_type_fail("chunk", "of type string or an instance of Buffer, TypedArray, or DataView", arg);
-      goto done;
-    }
-    if (!bytes) goto done;
-    int error = scr_stdio_write_raw(stream->fd, bytes->data, bytes->len);
-    scr_bytes_release(bytes);
-    if (error) {
-      scr_stdio_queue_write_error(stream, callback, error);
-      result = scr_dyn_new_bool(false);
-      goto done;
-    }
-    if (callback->kind == SCR_DYN_FUNC) scr_next_tick(scr_stdio_callback(callback, (void *)scr_stdio_write_done));
-    result = scr_dyn_new_bool(true);
+    result = scr_stdio_write_call(stream, args, argc);
   } else if (method == STDIO_ON || method == STDIO_ONCE || method == STDIO_OFF) {
     ScrDyn *listener = argc > 1 ? args[1] : scr_dyn_undefined();
     if (listener->kind != SCR_DYN_FUNC) {
@@ -1597,6 +1612,19 @@ double scr_dyn_process_stdio_fd(const ScrDyn *value, const ScrDynPath *path) {
   return stream ? stream->fd : 0;
 }
 
+/* process.stdout/stderr.write(chunk) with a run-time chunk: the stream
+ * method's own validation and write, without creating the stream value
+ * or installing the write hook. False with an exception pending when the
+ * chunk is invalid. */
+bool scr_process_stdio_write_dyn(double fd, ScrDyn *chunk) {
+  ScrStdio *stream = &scr_stdio_streams[fd == 2 ? 2 : 1];
+  ScrDyn *result = scr_stdio_write_call(stream, &chunk, 1);
+  if (!result) return false;
+  bool accepted = scr_dyn_truthy(result);
+  scr_dyn_release(result);
+  return accepted;
+}
+
 /* ── the process 'exit' event ─────────────────────────────────────────
  * Listeners run SYNCHRONOUSLY when the process exits: at the end of a
  * normal run (the unit's atexit, registered by scr_events_install after
diff --git a/tests/corpus/process-write-dynamic-chunks.ts b/tests/corpus/process-write-dynamic-chunks.ts
new file mode 100644
index 000000000..4356943b2
--- /dev/null
+++ b/tests/corpus/process-write-dynamic-chunks.ts
@@ -0,0 +1,34 @@
+// process.stdout/stderr.write of chunks whose type is only known at run
+// time: `any` values (a child's 'data' chunk, a JSON field) and
+// string | Uint8Array unions. Strings and Uint8Array/Buffer values write
+// their bytes; null throws ERR_STREAM_NULL_VALUES and any other value
+// throws ERR_INVALID_ARG_TYPE, both TypeErrors raised synchronously before
+// anything is written.
+
+function forward(data: any): boolean {
+  return process.stdout.write(data);
+}
+
+function pick(useBytes: boolean): string | Uint8Array {
+  return useBytes ? new Uint8Array([98, 121, 116, 101, 115, 10]) : "text\n";
+}
+
+console.log("string", forward("any string\n"));
+console.log("buffer", forward(Buffer.from("any buffer\n")));
+console.log("u8", forward(new TextEncoder().encode("any u8\n")));
+process.stdout.write(pick(false));
+process.stdout.write(pick(true));
+process.stderr.write(pick(false));
+const parsed: any = JSON.parse('{"chunk":"from json\\n","n":42}');
+forward(parsed.chunk);
+
+for (const bad of [parsed.n, null, undefined, { a: 1 }]) {
+  try {
+    forward(bad);
+    console.log("no throw");
+  } catch (error) {
+    if (error instanceof TypeError) {
+      console.log((error as NodeJS.ErrnoException).code, error.message);
+    }
+  }
+}
diff --git a/tests/harness/corpus-costs/process-write-dynamic-chunks.ts.json b/tests/harness/corpus-costs/process-write-dynamic-chunks.ts.json
new file mode 100644
index 000000000..b83881cc0
--- /dev/null
+++ b/tests/harness/corpus-costs/process-write-dynamic-chunks.ts.json
@@ -0,0 +1 @@
+{ "plain": 4567, "san": 86527 }