fix: resolve agent-browser via module resolution (fixes #15)

src/runner.ts

@@ -1,5 +1,6 @@
+import { existsSync } from "node:fs";
+import { createRequire } from "node:module";
 import { dirname, join } from "node:path";
-import { fileURLToPath } from "node:url";
 import type {
   FlowError,
   FlowResult,
@@ -83,26 +84,45 @@ function generateSessionName(): string {
 }
 
 /**
- * Get the path to the agent-browser binary
+ * Get the path to the agent-browser binary.
+ *
+ * Resolves through Node module resolution rather than a hardcoded
+ * `<flowspec>/node_modules/.bin` path, because that layout only exists
+ * under nested installs (pnpm, or flowspec's own repo). Hoisting package
+ * managers (bun, npm) place agent-browser at the consumer root, where the
+ * old path pointed at nothing and every flow died with "Browser command
+ * failed" (#15).
+ *
+ * Preference order: the `.bin` shim beside the resolved package (same
+ * spawn semantics as before), then the package's own bin script, then a
+ * bare `agent-browser` for PATH lookup.
+ *
+ * @param fromUrl module URL to resolve from — overridable for tests
  */
-function getAgentBrowserPath(): string {
-  // Try to find agent-browser in node_modules/.bin
-  // This works whether we're running from src/ or from dist/
-  const currentFile = import.meta.url;
-  const currentDir = dirname(fileURLToPath(currentFile));
+export function getAgentBrowserPath(fromUrl: string = import.meta.url): string {
+  try {
+    const require = createRequire(fromUrl);
+    const pkgJsonPath = require.resolve("agent-browser/package.json");
+    const pkgDir = dirname(pkgJsonPath);
 
-  // From src/ it's ../node_modules/.bin/agent-browser
-  // From dist/ it's ../node_modules/.bin/agent-browser
-  const binPath = join(
-    currentDir,
-    "..",
-    "node_modules",
-    ".bin",
-    "agent-browser",
-  );
+    // The .bin shim lives in the node_modules that contains the package.
+    const shimPath = join(dirname(pkgDir), ".bin", "agent-browser");
+    if (existsSync(shimPath)) return shimPath;
+
+    const pkg = require(pkgJsonPath) as {
+      bin?: string | Record<string, string>;
+    };
+    const bin =
+      typeof pkg.bin === "string" ? pkg.bin : pkg.bin?.["agent-browser"];
+    if (bin) {
+      const binPath = join(pkgDir, bin);
+      if (existsSync(binPath)) return binPath;
+    }
+  } catch {
+    // Not resolvable as a module — fall through to PATH lookup.
+  }
 
-  // If that doesn't work, fall back to npx
-  return binPath;
+  return "agent-browser";
 }
 
 /**

test/agent-browser-resolution.test.ts

@@ -0,0 +1,94 @@
+import {
+  mkdirSync,
+  mkdtempSync,
+  realpathSync,
+  rmSync,
+  symlinkSync,
+  writeFileSync,
+} from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { pathToFileURL } from "node:url";
+import { afterEach, describe, expect, it } from "vitest";
+import { getAgentBrowserPath } from "../src/runner.js";
+
+/**
+ * Regression tests for #15: flowspec hardcoded its own nested
+ * node_modules/.bin path, which doesn't exist under hoisting package
+ * managers (bun, npm) — so every flow failed at step 0 when flowspec was
+ * installed as a dependency.
+ */
+describe("getAgentBrowserPath", () => {
+  const tmpDirs: string[] = [];
+
+  function makeTmpDir(): string {
+    // realpath so expectations match module resolution, which returns
+    // resolved paths (macOS tmpdir lives behind a /var → /private/var symlink)
+    const dir = realpathSync(mkdtempSync(join(tmpdir(), "flowspec-abr-")));
+    tmpDirs.push(dir);
+    return dir;
+  }
+
+  afterEach(() => {
+    for (const dir of tmpDirs.splice(0)) {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+
+  /** Lay out <root>/node_modules/agent-browser with a bin script. */
+  function scaffoldPackage(root: string, { withShim }: { withShim: boolean }) {
+    const pkgDir = join(root, "node_modules", "agent-browser");
+    mkdirSync(join(pkgDir, "bin"), { recursive: true });
+    writeFileSync(
+      join(pkgDir, "package.json"),
+      JSON.stringify({
+        name: "agent-browser",
+        version: "0.0.0-test",
+        bin: { "agent-browser": "./bin/agent-browser.js" },
+      }),
+    );
+    writeFileSync(
+      join(pkgDir, "bin", "agent-browser.js"),
+      "#!/usr/bin/env node\n",
+    );
+    if (withShim) {
+      const binDir = join(root, "node_modules", ".bin");
+      mkdirSync(binDir, { recursive: true });
+      symlinkSync(
+        join("..", "agent-browser", "bin", "agent-browser.js"),
+        join(binDir, "agent-browser"),
+      );
+    }
+    // The module URL resolution starts from — a file at the consumer root,
+    // as if flowspec code were running from <root>/node_modules/flowspec/.
+    return pathToFileURL(join(root, "entry.js")).href;
+  }
+
+  it("resolves via the real install in this repo", () => {
+    const result = getAgentBrowserPath();
+    expect(result).not.toBe("agent-browser");
+    expect(result).toContain("agent-browser");
+  });
+
+  it("prefers the .bin shim beside the resolved package (hoisted layout)", () => {
+    const root = makeTmpDir();
+    const fromUrl = scaffoldPackage(root, { withShim: true });
+    expect(getAgentBrowserPath(fromUrl)).toBe(
+      join(root, "node_modules", ".bin", "agent-browser"),
+    );
+  });
+
+  it("falls back to the package's own bin script when no shim exists", () => {
+    const root = makeTmpDir();
+    const fromUrl = scaffoldPackage(root, { withShim: false });
+    expect(getAgentBrowserPath(fromUrl)).toBe(
+      join(root, "node_modules", "agent-browser", "bin", "agent-browser.js"),
+    );
+  });
+
+  it("falls back to PATH lookup when agent-browser is not resolvable", () => {
+    const root = makeTmpDir();
+    const fromUrl = pathToFileURL(join(root, "entry.js")).href;
+    expect(getAgentBrowserPath(fromUrl)).toBe("agent-browser");
+  });
+});