feat: add subagents runner config loader

src/config.test.ts

@@ -0,0 +1,67 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { mkdtemp, mkdir, writeFile } from "node:fs/promises";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import { loadSubagentsConfig } from "./config.ts";
+
+async function makeFixture() {
+  const root = await mkdtemp(join(tmpdir(), "pi-subagents-config-"));
+  const homeDir = join(root, "home");
+  const cwd = join(root, "repo");
+  await mkdir(join(homeDir, ".pi", "agent"), { recursive: true });
+  await mkdir(join(cwd, ".pi"), { recursive: true });
+  return { root, homeDir, cwd };
+}
+
+test("loadSubagentsConfig defaults to process when no config files exist", async () => {
+  const { homeDir, cwd } = await makeFixture();
+
+  const config = loadSubagentsConfig(cwd, { homeDir });
+
+  assert.equal(config.runner, "process");
+  assert.equal(config.globalPath, join(homeDir, ".pi", "agent", "subagents.json"));
+  assert.equal(config.projectPath, join(cwd, ".pi", "subagents.json"));
+});
+
+test("loadSubagentsConfig uses global config when project config is absent", async () => {
+  const { homeDir, cwd } = await makeFixture();
+  await writeFile(
+    join(homeDir, ".pi", "agent", "subagents.json"),
+    JSON.stringify({ runner: "tmux" }, null, 2),
+    "utf8",
+  );
+
+  const config = loadSubagentsConfig(cwd, { homeDir });
+
+  assert.equal(config.runner, "tmux");
+});
+
+test("loadSubagentsConfig lets project config override global config", async () => {
+  const { homeDir, cwd } = await makeFixture();
+  await writeFile(
+    join(homeDir, ".pi", "agent", "subagents.json"),
+    JSON.stringify({ runner: "tmux" }, null, 2),
+    "utf8",
+  );
+  await writeFile(
+    join(cwd, ".pi", "subagents.json"),
+    JSON.stringify({ runner: "process" }, null, 2),
+    "utf8",
+  );
+
+  const config = loadSubagentsConfig(cwd, { homeDir });
+
+  assert.equal(config.runner, "process");
+});
+
+test("loadSubagentsConfig throws clear error for invalid runner values", async () => {
+  const { homeDir, cwd } = await makeFixture();
+  const projectPath = join(cwd, ".pi", "subagents.json");
+  await writeFile(projectPath, JSON.stringify({ runner: "fork" }, null, 2), "utf8");
+
+  assert.throws(
+    () => loadSubagentsConfig(cwd, { homeDir }),
+    new RegExp(`Invalid runner .*fork.*${projectPath.replace(/[.*+?^${}()|[\]\\]/g, "\\$&")}`),
+  );
+});

src/config.ts

@@ -0,0 +1,48 @@
+import { existsSync, readFileSync } from "node:fs";
+import { homedir } from "node:os";
+import { join, resolve } from "node:path";
+
+export type RunnerMode = "process" | "tmux";
+
+export interface SubagentsConfig {
+  runner: RunnerMode;
+  globalPath: string;
+  projectPath: string;
+}
+
+export function getSubagentsConfigPaths(cwd: string, homeDir = homedir()) {
+  return {
+    globalPath: join(homeDir, ".pi", "agent", "subagents.json"),
+    projectPath: resolve(cwd, ".pi", "subagents.json"),
+  };
+}
+
+function readConfigFile(path: string): { runner?: RunnerMode } | undefined {
+  if (!existsSync(path)) return undefined;
+
+  let parsed: any;
+  try {
+    parsed = JSON.parse(readFileSync(path, "utf8"));
+  } catch (error) {
+    const message = error instanceof Error ? error.message : String(error);
+    throw new Error(`Failed to parse ${path}: ${message}`);
+  }
+
+  if (parsed.runner !== undefined && parsed.runner !== "process" && parsed.runner !== "tmux") {
+    throw new Error(`Invalid runner ${JSON.stringify(parsed.runner)} in ${path}. Expected "process" or "tmux".`);
+  }
+
+  return parsed;
+}
+
+export function loadSubagentsConfig(cwd: string, options: { homeDir?: string } = {}): SubagentsConfig {
+  const { globalPath, projectPath } = getSubagentsConfigPaths(cwd, options.homeDir);
+  const globalConfig = readConfigFile(globalPath) ?? {};
+  const projectConfig = readConfigFile(projectPath) ?? {};
+
+  return {
+    runner: projectConfig.runner ?? globalConfig.runner ?? "process",
+    globalPath,
+    projectPath,
+  };
+}