diff --git a/.github/workflows/source-lints.yml b/.github/workflows/source-lints.yml
index ffbca8fcfc30..c4d54fc01ad3 100644
--- a/.github/workflows/source-lints.yml
+++ b/.github/workflows/source-lints.yml
@@ -16,6 +16,7 @@ on:
       - "src/**/*.classes.ts"
       - "src/codegen/class-definitions.ts"
       - "src/jsc/bindings/**"
+      - "packages/bun-types/bun.d.ts"
       - "packages/bun-types/redis.d.ts"
       - "scripts/build/**"
       - "scripts/glob-sources.ts"
@@ -34,6 +35,7 @@ on:
       - "src/**/*.classes.ts"
       - "src/codegen/class-definitions.ts"
       - "src/jsc/bindings/**"
+      - "packages/bun-types/bun.d.ts"
       - "packages/bun-types/redis.d.ts"
       - "scripts/build/**"
       - "scripts/glob-sources.ts"
diff --git a/docs/runtime/child-process.mdx b/docs/runtime/child-process.mdx
index 60bf1a2e733b..bf9624d207f5 100644
--- a/docs/runtime/child-process.mdx
+++ b/docs/runtime/child-process.mdx
@@ -575,7 +575,8 @@ interface Subprocess extends AsyncDisposable {
   readonly stdin: FileSink | number | undefined | null;
   readonly stdout: ReadableStream<Uint8Array<ArrayBuffer>> | number | undefined | null;
   readonly stderr: ReadableStream<Uint8Array<ArrayBuffer>> | number | undefined | null;
-  readonly readable: ReadableStream<Uint8Array<ArrayBuffer>> | number | undefined | null;
+  readonly writable: Subprocess["stdin"]; // the same value as stdin
+  readonly readable: Subprocess["stdout"]; // the same value as stdout
   readonly terminal: Terminal | undefined;
   readonly pid: number;
   readonly exited: Promise<number>;
diff --git a/packages/bun-types/bun.d.ts b/packages/bun-types/bun.d.ts
index d8c9402e2871..0b7d988bf0f9 100644
--- a/packages/bun-types/bun.d.ts
+++ b/packages/bun-types/bun.d.ts
@@ -7578,6 +7578,14 @@ declare module "bun" {
      */
     readonly stdio: [null, null, null, ...(number | null)[]];
 
+    /**
+     * The same value as {@link Subprocess.stdin}
+     *
+     * The counterpart of {@link Subprocess.readable}. With `stdin: "pipe"` this is
+     * the {@link FileSink} for the process's stdin, not a `WritableStream`.
+     */
+    readonly writable: SpawnOptions.WritableToIO<In>;
+
     /**
      * The same value as {@link Subprocess.stdout}
      *
diff --git a/test/integration/bun-types/fixture/spawn.ts b/test/integration/bun-types/fixture/spawn.ts
index 2036413158a0..91441364e5eb 100644
--- a/test/integration/bun-types/fixture/spawn.ts
+++ b/test/integration/bun-types/fixture/spawn.ts
@@ -188,6 +188,32 @@ tsd.expectAssignable<NullSubprocess>(Bun.spawn([], { stdio: [null, null, null] }
 
 tsd.expectAssignable<SyncSubprocess<Bun.SpawnOptions.Readable, Bun.SpawnOptions.Readable>>(Bun.spawnSync([], {}));
 
+// `writable` and `readable` return the same values as `stdin` and `stdout`, so they have the
+// same types as `stdin` and `stdout` whatever the process was spawned with.
+{
+  function aliases<
+    In extends Bun.SpawnOptions.Writable,
+    Out extends Bun.SpawnOptions.Readable,
+    Err extends Bun.SpawnOptions.Readable,
+  >(proc: Bun.Subprocess<In, Out, Err>) {
+    tsd.expectType(proc.writable).is<typeof proc.stdin>();
+    tsd.expectType(proc.readable).is<typeof proc.stdout>();
+  }
+  aliases(Bun.spawn(["cat"]));
+}
+{
+  const proc = Bun.spawn(["cat"], { stdin: "pipe" });
+  tsd.expectType(proc.writable).is<FileSink>();
+  proc.writable.write("hello");
+
+  // @ts-expect-error writable is a read-only getter, like stdin
+  proc.writable = proc.stdin;
+}
+tsd.expectType<PipedSubprocess["writable"]>().is<FileSink>();
+tsd.expectType<WritableSubprocess["writable"]>().is<FileSink>();
+tsd.expectType<NullSubprocess["writable"]>().is<undefined>();
+tsd.expectType<ReadableSubprocess["readable"]>().is<ReadableStream<Uint8Array<ArrayBuffer>>>();
+
 // Lazy option types (async only)
 {
   // valid: lazy usable with async spawn
diff --git a/test/internal/source-lints/subprocess-types.test.ts b/test/internal/source-lints/subprocess-types.test.ts
new file mode 100644
index 000000000000..c157d2f13e4c
--- /dev/null
+++ b/test/internal/source-lints/subprocess-types.test.ts
@@ -0,0 +1,117 @@
+import { expect, test } from "bun:test";
+import { readFileSync } from "node:fs";
+import path from "node:path";
+import bunClasses from "../../../src/runtime/api/BunObject.classes.ts";
+
+// `interface Subprocess` in packages/bun-types/bun.d.ts is a hand-written mirror
+// of the Subprocess `proto` table in src/runtime/api/BunObject.classes.ts, which
+// is what the class codegen installs on Subprocess.prototype. Nothing else
+// compares the two: `writable` was registered next to `readable` in 2022 and
+// stayed undeclared until 2026, so `proc.writable` worked at runtime and failed
+// to type-check. Same check as redis-client-types.test.ts, reading an interface
+// body instead of a class body.
+
+// Registered members that are not declared yet, each with the open PR that
+// declares it. Delete the entry when that PR lands: the lint fails while a name
+// listed here is declared (or is no longer registered).
+const pendingDeclarations: Record<string, string> = {
+  connected: "#38677",
+};
+
+// Registered members the interface declares through its `extends` clause rather
+// than in its body, keyed to the interface that declares them.
+const declaredByExtends: Record<string, string> = {
+  "@@asyncDispose": "AsyncDisposable",
+};
+
+const classesFile = "src/runtime/api/BunObject.classes.ts";
+const dtsFile = "packages/bun-types/bun.d.ts";
+const root = path.resolve(import.meta.dir, "..", "..", "..");
+
+const definition = bunClasses.find(c => c.name === "Subprocess");
+if (definition === undefined) throw new Error(`${classesFile} no longer defines Subprocess`);
+// An interface body can only mirror prototype members. A constructor or statics
+// would need a value declaration this lint does not read.
+if (!definition.noConstructor || Object.keys(definition.klass).length !== 0) {
+  throw new Error(
+    `Subprocess in ${classesFile} has a constructor or statics, which \`interface Subprocess\` cannot declare`,
+  );
+}
+
+// Every member the codegen installs on the prototype. A well-known symbol stays
+// in the table's `@@x` spelling; parseInterfaceBody maps `[Symbol.x]` onto it.
+const registered = new Set<string>();
+for (const [name, field] of Object.entries(definition.proto)) {
+  // Installed under a private name or a Symbol.for() symbol, or (internal) not
+  // installed at all; none of these has a declaration to mirror.
+  if ("internal" in field || "privateSymbol" in field || "publicSymbol" in field) continue;
+  registered.add(name);
+}
+
+const { extended, declared, unrecognized } = parseInterfaceBody(readFileSync(path.join(root, dtsFile), "utf8"));
+for (const [name, base] of Object.entries(declaredByExtends)) {
+  if (extended.includes(base)) declared.add(name);
+}
+
+function parseInterfaceBody(dts: string): { extended: string[]; declared: Set<string>; unrecognized: string[] } {
+  // The header spans several lines because of the type parameter list.
+  const open = /^  interface Subprocess<\n(?: {4}.*\n)*  >(?: extends (.+))? \{$/m.exec(dts);
+  if (open === null) throw new Error(`${dtsFile} no longer declares \`interface Subprocess<...> {\``);
+  const extended = open[1] === undefined ? [] : open[1].split(",").map(base => base.trim());
+  const bodyStart = open.index + open[0].length;
+  const bodyEnd = dts.indexOf("\n  }\n", bodyStart);
+  if (bodyEnd === -1) throw new Error(`${dtsFile}: unterminated interface Subprocess body`);
+
+  const body = dts
+    .slice(bodyStart, bodyEnd)
+    .replace(/\/\*[\s\S]*?\*\//g, "")
+    .replace(/^[ \t]*\/\/.*$/gm, "");
+
+  const declared = new Set<string>();
+  const unrecognized: string[] = [];
+  // Members start at the body's four-space indent. The only other lines at that
+  // indent are the `): ...;` closers of multi-line signatures, which start with
+  // punctuation, so everything else at that indent is a member and has to
+  // parse: a declaration shape this does not know is reported rather than
+  // skipped.
+  const member = /^(?:readonly )?(?:\[Symbol\.(\w+)\]|([A-Za-z_$][\w$]*))\s*\??\s*[(:<]/;
+  for (const [line] of body.matchAll(/^    [^\s)\]}>].*$/gm)) {
+    const m = member.exec(line.slice(4));
+    if (m === null) {
+      unrecognized.push(line.trim());
+      continue;
+    }
+    declared.add(m[1] !== undefined ? `@@${m[1]}` : m[2]!);
+  }
+  return { extended, declared, unrecognized };
+}
+
+test(`every member of interface Subprocess in ${dtsFile} has a shape this lint can read`, () => {
+  expect(unrecognized).toEqual([]);
+});
+
+test(`${dtsFile} declares every Subprocess member ${classesFile} installs`, () => {
+  const undeclared = [...registered]
+    .filter(name => !declared.has(name) && !Object.hasOwn(pendingDeclarations, name))
+    .sort();
+  expect(undeclared).toEqual([]);
+});
+
+test(`${dtsFile} declares no Subprocess member ${classesFile} does not install`, () => {
+  const phantom = [...declared].filter(name => !registered.has(name)).sort();
+  expect(phantom).toEqual([]);
+});
+
+test("pendingDeclarations and declaredByExtends describe the current files", () => {
+  const stale = [
+    ...Object.entries(pendingDeclarations).flatMap(([name, pr]) => {
+      if (!registered.has(name)) return [`${name} (${pr}) is no longer registered in ${classesFile}`];
+      if (declared.has(name)) return [`${name} (${pr}) is declared in ${dtsFile} now; delete its entry`];
+      return [];
+    }),
+    ...Object.entries(declaredByExtends).flatMap(([name, base]) =>
+      extended.includes(base) ? [] : [`interface Subprocess no longer extends ${base}, which declared ${name}`],
+    ),
+  ].sort();
+  expect(stale).toEqual([]);
+});