diff --git a/packages/bun-types/bun.d.ts b/packages/bun-types/bun.d.ts
index d8c9402e2871..ab0df74c4be5 100644
--- a/packages/bun-types/bun.d.ts
+++ b/packages/bun-types/bun.d.ts
@@ -7119,11 +7119,12 @@ declare module "bun" {
        *
        * For stdout and stderr you may pass:
        *
-       * - `"pipe"`, `undefined`: The process has a {@link ReadableStream} for standard output/error
+       * - `"pipe"`: The process has a {@link ReadableStream} for standard output/error
        * - `"ignore"`, `null`: The process has no standard output/error
        * - `"inherit"`: The process inherits the standard output/error of the current process
        * - `ArrayBufferView`: The process writes to the preallocated buffer. Not implemented.
        * - `number`: The process writes to the file descriptor
+       * - `undefined`: The default for that position, see below
        *
        * At indices >= 3, `"socket-fd"` (POSIX only) is also accepted:
        * creates a socketpair like `"pipe"`, but the parent-end fd exposed
@@ -7152,11 +7153,12 @@ declare module "bun" {
       /**
        * The file descriptor for the standard output. It may be:
        *
-       * - `"pipe"`, `undefined`: The process has a {@link ReadableStream} for standard output/error
+       * - `"pipe"`: The process has a {@link ReadableStream} for standard output/error
        * - `"ignore"`, `null`: The process has no standard output/error
        * - `"inherit"`: The process inherits the standard output/error of the current process
        * - `ArrayBufferView`: The process writes to the preallocated buffer. Not implemented.
        * - `number`: The process writes to the file descriptor
+       * - `undefined`: The default below
        *
        * @default "pipe"
        */
@@ -7164,11 +7166,13 @@ declare module "bun" {
       /**
        * The file descriptor for the standard error. It may be:
        *
-       * - `"pipe"`, `undefined`: The process has a {@link ReadableStream} for standard output/error
+       * - `"pipe"`: The process has a {@link ReadableStream} for standard output/error
        * - `"ignore"`, `null`: The process has no standard output/error
        * - `"inherit"`: The process inherits the standard output/error of the current process
        * - `ArrayBufferView`: The process writes to the preallocated buffer. Not implemented.
        * - `number`: The process writes to the file descriptor
+       * - `undefined`: The default below, so with {@link spawn} the process inherits standard error
+       *   and {@link Subprocess.stderr} is `undefined`
        *
        * @default "inherit" for `spawn`
        * "pipe" for `spawnSync`
@@ -7436,11 +7440,22 @@ declare module "bun" {
       terminal?: TerminalOptions | Terminal;
     }
 
-    type ReadableToIO<X extends Readable> = X extends "pipe" | undefined
-      ? ReadableStream<Uint8Array<ArrayBuffer>>
-      : X extends BunFile | ArrayBufferView | number
-        ? number
-        : undefined;
+    /**
+     * The type of {@link Subprocess.stdout} / {@link Subprocess.stderr} for a `stdout` / `stderr`
+     * option of type `X`.
+     *
+     * An `undefined` option means the slot's default, and {@link spawn} has a different default
+     * for each slot: `"pipe"` for stdout, `"inherit"` for stderr. `Default` is that slot default,
+     * so `ReadableToIO<undefined, "pipe">` (stdout) is a {@link ReadableStream} and
+     * `ReadableToIO<undefined, "inherit">` (stderr) is `undefined`.
+     */
+    type ReadableToIO<X extends Readable, Default extends Exclude<Readable, undefined> = "pipe"> = X extends undefined
+      ? ReadableToIO<Default>
+      : X extends "pipe"
+        ? ReadableStream<Uint8Array<ArrayBuffer>>
+        : X extends BunFile | ArrayBufferView | number
+          ? number
+          : undefined;
 
     type ReadableToSyncIO<X extends Readable> = X extends "pipe" | undefined ? Buffer : undefined;
 
@@ -7545,8 +7560,8 @@ declare module "bun" {
     Err extends SpawnOptions.Readable = SpawnOptions.Readable,
   > extends AsyncDisposable {
     readonly stdin: SpawnOptions.WritableToIO<In>;
-    readonly stdout: SpawnOptions.ReadableToIO<Out>;
-    readonly stderr: SpawnOptions.ReadableToIO<Err>;
+    readonly stdout: SpawnOptions.ReadableToIO<Out, "pipe">;
+    readonly stderr: SpawnOptions.ReadableToIO<Err, "inherit">;
 
     /**
      * The terminal attached to this subprocess, if spawned with the `terminal` option.
@@ -7583,7 +7598,7 @@ declare module "bun" {
      *
      * Exists for compatibility with {@link ReadableStream.pipeThrough}
      */
-    readonly readable: SpawnOptions.ReadableToIO<Out>;
+    readonly readable: SpawnOptions.ReadableToIO<Out, "pipe">;
 
     /**
      * The process ID of the child process
diff --git a/test/integration/bun-types/bun-types.test.ts b/test/integration/bun-types/bun-types.test.ts
index 05fa801f45e5..e1fca224640e 100644
--- a/test/integration/bun-types/bun-types.test.ts
+++ b/test/integration/bun-types/bun-types.test.ts
@@ -400,6 +400,35 @@ describe("@types/bun integration test", () => {
     });
   });
 
+  // Also runs on debug builds, where the in-process typeTest cases (which cover the whole
+  // fixture directory) are skipped: checks fixture/spawn.ts alone against the packed bun-types.
+  describe("Bun.spawn", () => {
+    test("fixture/spawn.ts type-checks", async () => {
+      const checkDir = join(TEMP_DIR, "spawn-fixture-check");
+      const tsconfig = structuredClone(sourceTsconfig);
+      tsconfig.files = [join(BASE_FIXTURE_DIR, "spawn.ts")];
+      tsconfig.compilerOptions.typeRoots = [join(BASE_FIXTURE_DIR, "node_modules", "@types")];
+      await mkdir(checkDir, { recursive: true });
+      await makeTree(checkDir, {
+        "tsconfig.json": JSON.stringify(tsconfig, null, 2),
+      });
+
+      await using proc = Bun.spawn({
+        cmd: [bunExe(), join(BASE_FIXTURE_DIR, "node_modules", "typescript", "bin", "tsc"), "-p", "."],
+        env: bunEnv,
+        cwd: checkDir,
+        stdout: "pipe",
+        stderr: "pipe",
+      });
+
+      const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+      expect(stderr.trim()).toBe("");
+      expect(stdout.trim()).toBe("");
+      expect(exitCode).toBe(0);
+    });
+  });
+
   describe("Test Globals", () => {
     const code = `
       const test_shouldBeAFunction: Function = test;
diff --git a/test/integration/bun-types/fixture/spawn.ts b/test/integration/bun-types/fixture/spawn.ts
index 2036413158a0..4363551fb4f9 100644
--- a/test/integration/bun-types/fixture/spawn.ts
+++ b/test/integration/bun-types/fixture/spawn.ts
@@ -226,3 +226,78 @@ tsd.expectAssignable<SyncSubprocess<Bun.SpawnOptions.Readable, Bun.SpawnOptions.
   Bun.spawnSync({ cmd: ["echo", "hello"], stdout: "pipe", stderr: "pipe", lazy: true,
   });
 }
+
+// An undefined stdout/stderr option means that slot's default, which differs between the two slots
+// of Bun.spawn: stdout defaults to "pipe", stderr to "inherit" (so proc.stderr is undefined).
+// Bun.spawnSync defaults both to "pipe".
+//
+// Without exactOptionalPropertyTypes, TypeScript drops undefined from a union passed to the optional
+// `stderr` property (`stderr: cond ? "pipe" : undefined` infers Err = "pipe"), so the union case is
+// asserted through the stdio tuple and through explicit type arguments below.
+declare const pipeWhenCapturing: boolean;
+{
+  const proc = Bun.spawn(["cat"], { stderr: undefined });
+  tsd.expectType(proc).is<Bun.Subprocess<"ignore", "pipe", undefined>>();
+  tsd.expectType(proc.stderr).is<undefined>();
+}
+{
+  const proc = Bun.spawn({ cmd: ["cat"], stderr: undefined });
+  tsd.expectType(proc.stderr).is<undefined>();
+}
+{
+  const proc = Bun.spawn(["cat"], { stdout: undefined });
+  tsd.expectType(proc).is<Bun.Subprocess<"ignore", undefined, "inherit">>();
+  tsd.expectType(proc.stdout).is<ReadableStream<Uint8Array<ArrayBuffer>>>();
+  tsd.expectType(proc.readable).is<ReadableStream<Uint8Array<ArrayBuffer>>>();
+}
+{
+  const proc = Bun.spawn(["cat"], { stdio: [undefined, undefined, undefined] });
+  tsd.expectType(proc.stdin).is<undefined>();
+  tsd.expectType(proc.stdout).is<ReadableStream<Uint8Array<ArrayBuffer>>>();
+  tsd.expectType(proc.stderr).is<undefined>();
+}
+{
+  const proc = Bun.spawn(["cat"], {
+    stdio: ["ignore", pipeWhenCapturing ? "pipe" : undefined, pipeWhenCapturing ? "pipe" : undefined],
+  });
+  tsd.expectType(proc).is<Bun.Subprocess<"ignore", "pipe" | undefined, "pipe" | undefined>>();
+  tsd.expectType(proc.stdout).is<ReadableStream<Uint8Array<ArrayBuffer>>>();
+  tsd.expectType(proc.stderr).is<ReadableStream<Uint8Array<ArrayBuffer>> | undefined>();
+}
+{
+  Bun.spawn(["cat"], {
+    stderr: undefined,
+    onExit(proc) {
+      tsd.expectType(proc.stderr).is<undefined>();
+    },
+    ipc(message, proc) {
+      tsd.expectType(proc.stderr).is<undefined>();
+    },
+  });
+}
+{
+  const proc = Bun.spawnSync(["cat"], { stdout: undefined, stderr: undefined });
+  tsd.expectType(proc.stdout).is<Buffer>();
+  tsd.expectType(proc.stderr).is<Buffer>();
+}
+{
+  const proc = Bun.spawnSync(["cat"], { stdio: ["ignore", undefined, undefined] });
+  tsd.expectType(proc.stdout).is<Buffer>();
+  tsd.expectType(proc.stderr).is<Buffer>();
+}
+tsd
+  .expectType<Bun.Subprocess<"ignore", "pipe", "pipe" | undefined>["stderr"]>()
+  .is<ReadableStream<Uint8Array<ArrayBuffer>> | undefined>();
+tsd
+  .expectType<Bun.Subprocess<"ignore", "pipe" | undefined, "inherit">["stdout"]>()
+  .is<ReadableStream<Uint8Array<ArrayBuffer>>>();
+tsd.expectType<NullSubprocess["stderr"]>().is<undefined>();
+// Configurations that are not known statically still see every possible value.
+tsd.expectType<Bun.Subprocess["stderr"]>().is<ReadableStream<Uint8Array<ArrayBuffer>> | number | undefined>();
+tsd.expectType<WritableSubprocess["stderr"]>().is<ReadableStream<Uint8Array<ArrayBuffer>> | number | undefined>();
+// The one-parameter form of the alias keeps stdout's default.
+tsd.expectType<Bun.Spawn.ReadableToIO<undefined>>().is<ReadableStream<Uint8Array<ArrayBuffer>>>();
+tsd.expectType<Bun.Spawn.ReadableToIO<undefined, "inherit">>().is<undefined>();
+tsd
+  .expectType<Bun.Spawn.ReadableToIO<"pipe" | undefined, "inherit">>()
+  .is<ReadableStream<Uint8Array<ArrayBuffer>> | undefined>();