fix(agent-spawn): fall back to stdin for large prompts

packages/agent-spawn/src/agent-spawn.test.ts

@@ -412,6 +412,19 @@ describe("buildSpawnArgs", () => {
     expect(result.args).not.toContain("test");
   });
 
+  it("builds stdin args for codex when the prompt is too large for argv", () => {
+    const prompt = "a".repeat(64 * 1024 + 1);
+    const result = buildSpawnArgs("codex", { prompt });
+
+    expect(result.args).toEqual([
+      codexSpawnConfig.promptFlag,
+      ...codexSpawnConfig.stdinMode!.extraArgs,
+      ...codexSpawnConfig.defaultArgs,
+      ...codexSpawnConfig.modes.yolo
+    ]);
+    expect(result.args).not.toContain(prompt);
+  });
+
   it("ignores useStdin for agents without stdinMode", () => {
     const result = buildSpawnArgs("opencode", { prompt: "hello", useStdin: true });
 
@@ -743,6 +756,31 @@ describe("spawn", () => {
     expect(child.__capturedStdin()).toBe("hi");
   });
 
+  it("automatically writes large prompts to stdin when supported", async () => {
+    const prompt = "a".repeat(64 * 1024 + 1);
+    const spawnMock = vi
+      .mocked(spawnChildProcess)
+      .mockReturnValue(createMockChildProcess({ stdout: "ok\n", exitCode: 0 }));
+
+    await spawn("codex", { prompt });
+
+    expect(spawnMock).toHaveBeenCalledTimes(1);
+    const [command, args, spawnOptions] = spawnMock.mock.calls[0]!;
+    expect(command).toBe("codex");
+    expect(args).toEqual([
+      codexSpawnConfig.promptFlag,
+      ...(codexSpawnConfig.stdinMode?.extraArgs ?? []),
+      ...codexSpawnConfig.defaultArgs,
+      ...codexSpawnConfig.modes.yolo
+    ]);
+    expect(spawnOptions).toMatchObject({
+      stdio: ["pipe", "pipe", "pipe"]
+    });
+
+    const child = spawnMock.mock.results[0]?.value as any;
+    expect(child.__capturedStdin()).toBe(prompt);
+  });
+
   it("forwards output to tee streams when provided", async () => {
     vi.mocked(spawnChildProcess).mockReturnValue(
       createMockChildProcess({ stdout: "agent output", stderr: "agent progress", exitCode: 0 })

packages/agent-spawn/src/spawn.ts

@@ -17,6 +17,8 @@ import {
   type StdinMode
 } from "./types.js";
 
+const PROMPT_STDIN_FALLBACK_BYTES = 64 * 1024;
+
 function createAbortError(): Error {
   const error = new Error("Agent spawn aborted");
   error.name = "AbortError";
@@ -79,6 +81,21 @@ function getMcpArgsPosition(
   return config.mcpArgsBeforeCommand ? "beforeCommand" : "afterCommand";
 }
 
+function resolveStdinMode(
+  config: CliSpawnConfig,
+  options: Pick<BuildSpawnArgsOptions, "prompt" | "useStdin">
+): StdinMode | undefined {
+  if (!config.stdinMode) {
+    return undefined;
+  }
+  if (options.useStdin) {
+    return config.stdinMode;
+  }
+  return Buffer.byteLength(options.prompt, "utf8") > PROMPT_STDIN_FALLBACK_BYTES
+    ? config.stdinMode
+    : undefined;
+}
+
 function buildCliArgs(
   config: CliSpawnConfig,
   options: BuildSpawnArgsOptions,
@@ -169,7 +186,7 @@ export function buildSpawnArgs(
   options: BuildSpawnArgsOptions
 ): BuildSpawnArgsResult {
   const { binaryName, spawnConfig } = resolveCliConfig(agentId);
-  const stdinMode = options.useStdin && spawnConfig.stdinMode ? spawnConfig.stdinMode : undefined;
+  const stdinMode = resolveStdinMode(spawnConfig, options);
   const result = buildCliArgs(spawnConfig, options, stdinMode);
   return { binaryName, args: result.args, env: result.env };
 }
@@ -185,7 +202,7 @@ export async function spawn(
 
   const { agentId: resolvedId, binaryName, spawnConfig } = resolveCliConfig(agentId);
 
-  const stdinMode = options.useStdin && spawnConfig.stdinMode ? spawnConfig.stdinMode : undefined;
+  const stdinMode = resolveStdinMode(spawnConfig, options);
 
   const { args: spawnArgs, env: modeEnv } = buildCliArgs(spawnConfig, options, stdinMode);