diff --git a/tests/relayflows/cases/1602-parentless-worker-inventory/resolve-cargo.test.mjs b/tests/relayflows/cases/1602-parentless-worker-inventory/resolve-cargo.test.mjs
new file mode 100644
index 0000000000..1bdf22906d
--- /dev/null
+++ b/tests/relayflows/cases/1602-parentless-worker-inventory/resolve-cargo.test.mjs
@@ -0,0 +1,216 @@
+// Unit tests for the fail-fast Cargo resolver used by the 1602 PR-proof
+// case runner.
+//
+// Every test drops a real (fake) executable script into a temp directory and
+// exercises the resolver against it. The resolver's timeout is enforced by
+// Node's execFileSync `timeout` + `killSignal: 'SIGKILL'`, so a probe that
+// blocks forever is guaranteed to die within the budget.
+//
+// Run with:
+//   node --test tests/relayflows/cases/1602-parentless-worker-inventory/resolve-cargo.test.mjs
+
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { chmod, mkdir, mkdtemp, realpath, rm, writeFile } from 'node:fs/promises';
+import os from 'node:os';
+import path from 'node:path';
+
+import { CargoNotResolvableError, resolveCargo } from './run.mjs';
+
+async function makeExecutable(filePath, body) {
+  await mkdir(path.dirname(filePath), { recursive: true });
+  await writeFile(filePath, body);
+  await chmod(filePath, 0o755);
+}
+
+async function makeFakeCargo(filePath, { version = '1.75.0' } = {}) {
+  await makeExecutable(
+    filePath,
+    `#!/bin/sh\nif [ "$1" = "--version" ]; then\n  echo "cargo ${version} (fake)"\nfi\n`
+  );
+}
+
+async function makeHangingCargo(filePath) {
+  // Ignore SIGTERM so a hung probe survives a soft signal; only SIGKILL takes
+  // it out. Sleep in a small loop so `trap` can service the signal ignore.
+  await makeExecutable(
+    filePath,
+    '#!/bin/sh\ntrap "" TERM\nwhile true; do sleep 60; done\n'
+  );
+}
+
+async function makeWrongOutputCargo(filePath) {
+  await makeExecutable(filePath, '#!/bin/sh\necho "not cargo at all"\n');
+}
+
+async function makeTmpRoot(prefix) {
+  const raw = await mkdtemp(path.join(os.tmpdir(), prefix));
+  // macOS resolves /var to /private/var — normalize so string comparisons
+  // against paths inside the returned root line up.
+  return realpath(raw);
+}
+
+test('resolveCargo returns a direct hit on PATH when `cargo --version` looks right', async () => {
+  const root = await makeTmpRoot('resolve-cargo-path-');
+  try {
+    const binDir = path.join(root, 'bin');
+    const cargo = path.join(binDir, 'cargo');
+    await makeFakeCargo(cargo);
+
+    const resolved = resolveCargo({
+      env: { PATH: binDir },
+      pathEntries: [binDir],
+      extraSystemPaths: [],
+    });
+    assert.equal(resolved, cargo);
+  } finally {
+    await rm(root, { recursive: true, force: true });
+  }
+});
+
+test('resolveCargo prefers CARGO_HOME/bin/cargo when it works', async () => {
+  const root = await makeTmpRoot('resolve-cargo-cargohome-');
+  try {
+    const cargoHome = path.join(root, '.cargo');
+    const cargo = path.join(cargoHome, 'bin', 'cargo');
+    await makeFakeCargo(cargo);
+
+    const resolved = resolveCargo({
+      env: { PATH: '', CARGO_HOME: cargoHome },
+      pathEntries: [],
+      extraSystemPaths: [],
+    });
+    assert.equal(resolved, cargo);
+  } finally {
+    await rm(root, { recursive: true, force: true });
+  }
+});
+
+test('resolveCargo enumerates HOME/.rustup/toolchains/*/bin/cargo', async () => {
+  const root = await makeTmpRoot('resolve-cargo-rustup-');
+  try {
+    const toolchainCargo = path.join(
+      root,
+      '.rustup',
+      'toolchains',
+      'stable-x86_64-unknown-linux-gnu',
+      'bin',
+      'cargo'
+    );
+    await makeFakeCargo(toolchainCargo);
+
+    const resolved = resolveCargo({
+      env: { PATH: '', HOME: root },
+      pathEntries: [],
+      extraSystemPaths: [],
+    });
+    assert.equal(resolved, toolchainCargo);
+  } finally {
+    await rm(root, { recursive: true, force: true });
+  }
+});
+
+test('resolveCargo falls back to extraSystemPaths when nothing else works', async () => {
+  const root = await makeTmpRoot('resolve-cargo-system-');
+  try {
+    const systemCargo = path.join(root, 'opt', 'cargo', 'bin', 'cargo');
+    await makeFakeCargo(systemCargo);
+
+    const resolved = resolveCargo({
+      env: { PATH: '' },
+      pathEntries: [],
+      extraSystemPaths: [systemCargo],
+    });
+    assert.equal(resolved, systemCargo);
+  } finally {
+    await rm(root, { recursive: true, force: true });
+  }
+});
+
+test('resolveCargo skips a candidate that prints unexpected --version output and picks the next one', async () => {
+  const root = await makeTmpRoot('resolve-cargo-badoutput-');
+  try {
+    // First candidate on PATH is a shim that prints garbage.
+    const badDir = path.join(root, 'bad', 'bin');
+    const badCargo = path.join(badDir, 'cargo');
+    await makeWrongOutputCargo(badCargo);
+
+    // Second candidate on PATH is the real thing.
+    const goodDir = path.join(root, 'good', 'bin');
+    const goodCargo = path.join(goodDir, 'cargo');
+    await makeFakeCargo(goodCargo);
+
+    const resolved = resolveCargo({
+      env: { PATH: `${badDir}${path.delimiter}${goodDir}` },
+      pathEntries: [badDir, goodDir],
+      extraSystemPaths: [],
+    });
+    assert.equal(resolved, goodCargo);
+  } finally {
+    await rm(root, { recursive: true, force: true });
+  }
+});
+
+test(
+  'resolveCargo throws CargoNotResolvableError when every candidate hangs past the timeout',
+  { timeout: 15_000 },
+  async () => {
+    const root = await makeTmpRoot('resolve-cargo-hung-');
+    try {
+      const hungA = path.join(root, 'a', 'cargo');
+      const hungB = path.join(root, 'b', 'cargo');
+      await makeHangingCargo(hungA);
+      await makeHangingCargo(hungB);
+
+      const timeoutMs = 400;
+      const started = Date.now();
+      let threw;
+      try {
+        resolveCargo({
+          env: { PATH: `${path.dirname(hungA)}${path.delimiter}${path.dirname(hungB)}` },
+          pathEntries: [path.dirname(hungA), path.dirname(hungB)],
+          timeoutMs,
+          extraSystemPaths: [],
+        });
+      } catch (error) {
+        threw = error;
+      }
+      const elapsed = Date.now() - started;
+
+      assert.ok(threw instanceof CargoNotResolvableError, `expected CargoNotResolvableError, got ${threw}`);
+      assert.equal(threw.attempts.length, 2, 'both hung candidates must be recorded');
+      for (const attempt of threw.attempts) {
+        assert.match(attempt.reason, /killed by SIGKILL after 400ms/);
+      }
+      assert.match(threw.message, /could not resolve a working cargo executable/);
+      // Two 400ms probes + SIGKILL should be under 5s even on a busy runner.
+      assert.ok(elapsed < 5000, `probes must respect timeout budget, elapsed=${elapsed}ms`);
+    } finally {
+      await rm(root, { recursive: true, force: true });
+    }
+  }
+);
+
+test('resolveCargo dedupes identical candidates across enumeration sources', async () => {
+  const root = await makeTmpRoot('resolve-cargo-dedupe-');
+  try {
+    const cargoHome = path.join(root, '.cargo');
+    const cargo = path.join(cargoHome, 'bin', 'cargo');
+    await makeFakeCargo(cargo);
+
+    // CARGO_HOME/bin/cargo, HOME/.cargo/bin/cargo, and the PATH-based candidate
+    // all resolve to the same string. Dedup must not double-execute it.
+    const resolved = resolveCargo({
+      env: {
+        PATH: path.join(cargoHome, 'bin'),
+        CARGO_HOME: cargoHome,
+        HOME: root,
+      },
+      pathEntries: [path.join(cargoHome, 'bin')],
+      extraSystemPaths: [cargo],
+    });
+    assert.equal(resolved, cargo);
+  } finally {
+    await rm(root, { recursive: true, force: true });
+  }
+});
diff --git a/tests/relayflows/cases/1602-parentless-worker-inventory/run.mjs b/tests/relayflows/cases/1602-parentless-worker-inventory/run.mjs
index 5ede5a470c..cbb78a68ae 100644
--- a/tests/relayflows/cases/1602-parentless-worker-inventory/run.mjs
+++ b/tests/relayflows/cases/1602-parentless-worker-inventory/run.mjs
@@ -1,9 +1,10 @@
 #!/usr/bin/env node
 
-import { spawn } from 'node:child_process';
-import { constants as fsConstants } from 'node:fs';
-import { access, readFile, readdir, realpath, writeFile } from 'node:fs/promises';
+import { execFileSync, spawn } from 'node:child_process';
+import { accessSync, constants as fsConstants, readdirSync } from 'node:fs';
+import { readFile, writeFile } from 'node:fs/promises';
 import path from 'node:path';
+import { pathToFileURL } from 'node:url';
 
 const CASE_ID = '1602-parentless-worker-inventory';
 const MARKER = 'RELAY_PR_PROOF_OBSERVATION=';
@@ -37,48 +38,96 @@ function proofChildEnvironment() {
   return env;
 }
 
-async function isExecutable(filePath) {
-  try {
-    await access(filePath, fsConstants.X_OK);
-    return true;
-  } catch {
-    return false;
+export class CargoNotResolvableError extends Error {
+  constructor(attempts) {
+    const detail = attempts
+      .map((a) => `${a.path} (${a.reason})`)
+      .join('; ');
+    super(
+      `could not resolve a working cargo executable; attempted: ${detail || '(no candidates)'}`
+    );
+    this.name = 'CargoNotResolvableError';
+    this.attempts = attempts;
   }
 }
 
-async function resolveCargo() {
-  const pathEntries = (process.env.PATH ?? '').split(path.delimiter).filter(Boolean);
-  for (const entry of pathEntries) {
-    const candidate = path.join(entry, 'cargo');
-    if (!(await isExecutable(candidate))) continue;
-    const resolved = await realpath(candidate).catch(() => candidate);
-    if (path.basename(resolved) === 'cargo') return resolved;
-  }
+// Ground truth: a candidate is a real cargo iff `cargo --version` prints
+// `cargo <semver>` within a bounded budget. Node's execFileSync `timeout` +
+// `killSignal: 'SIGKILL'` is the OS calling kill(9), so a hung/blocking child
+// can't out-live the probe — no sentinel race, no SIGTERM→SIGKILL escalation,
+// no shim-vs-real heuristics.
+export function resolveCargo(options = {}) {
+  const {
+    env = process.env,
+    pathEntries = (env.PATH ?? '').split(path.delimiter).filter(Boolean),
+    timeoutMs = 3000,
+    extraSystemPaths = [
+      '/usr/local/bin/cargo',
+      '/opt/homebrew/bin/cargo',
+      '/root/.cargo/bin/cargo',
+      '/home/daytona/.cargo/bin/cargo',
+    ],
+  } = options;
+
+  const candidates = [];
+  const seen = new Set();
+  const add = (candidate) => {
+    if (!candidate || seen.has(candidate)) return;
+    seen.add(candidate);
+    candidates.push(candidate);
+  };
 
-  const homes = new Set();
-  for (const entry of pathEntries) {
-    const normalized = entry.replaceAll('\\', '/');
-    for (const suffix of ['/.local/share/mise/shims', '/.cargo/bin']) {
-      if (normalized.endsWith(suffix)) homes.add(normalized.slice(0, -suffix.length));
+  if (env.CARGO_HOME) add(path.join(env.CARGO_HOME, 'bin', 'cargo'));
+  if (env.HOME) add(path.join(env.HOME, '.cargo', 'bin', 'cargo'));
+  if (env.HOME) {
+    const toolchainsDir = path.join(env.HOME, '.rustup', 'toolchains');
+    let entries = [];
+    try {
+      entries = readdirSync(toolchainsDir);
+    } catch {
+      // no toolchains directory
+    }
+    for (const entry of entries.slice().sort().reverse()) {
+      add(path.join(toolchainsDir, entry, 'bin', 'cargo'));
     }
   }
-  if (process.env.RUSTUP_HOME) {
-    homes.add(path.dirname(path.resolve(process.env.RUSTUP_HOME)));
-  }
-
-  for (const home of homes) {
-    const toolchains = path.join(home, '.rustup', 'toolchains');
-    const entries = await readdir(toolchains).catch(() => []);
-    for (const entry of entries.sort().reverse()) {
-      const candidate = path.join(toolchains, entry, 'bin', 'cargo');
-      if (await isExecutable(candidate)) return candidate;
+  for (const entry of pathEntries) add(path.join(entry, 'cargo'));
+  for (const candidate of extraSystemPaths) add(candidate);
+
+  const attempts = [];
+  for (const candidate of candidates) {
+    try {
+      accessSync(candidate, fsConstants.X_OK);
+    } catch {
+      attempts.push({ path: candidate, reason: 'missing or not executable' });
+      continue;
+    }
+    let stdout;
+    try {
+      stdout = execFileSync(candidate, ['--version'], {
+        timeout: timeoutMs,
+        killSignal: 'SIGKILL',
+        stdio: ['ignore', 'pipe', 'pipe'],
+        encoding: 'utf8',
+      });
+    } catch (error) {
+      const signal = error?.signal;
+      const status = error?.status;
+      const reason = signal
+        ? `killed by ${signal} after ${timeoutMs}ms`
+        : `exit ${status ?? 'unknown'}${error?.code ? ` (${error.code})` : ''}`;
+      attempts.push({ path: candidate, reason });
+      continue;
+    }
+    if (!/^cargo \d+\.\d+\.\d+/.test(stdout)) {
+      const preview = (stdout || '').slice(0, 80).replace(/\n/g, '\\n');
+      attempts.push({ path: candidate, reason: `unexpected --version output: ${preview}` });
+      continue;
     }
+    return candidate;
   }
 
-  for (const candidate of ['/usr/local/cargo/bin/cargo', '/opt/rust/bin/cargo']) {
-    if (await isExecutable(candidate)) return candidate;
-  }
-  throw new Error('could not resolve a real Cargo executable outside a version-manager shim');
+  throw new CargoNotResolvableError(attempts);
 }
 
 function run(command, args, options = {}) {
@@ -479,7 +528,9 @@ async function main() {
   );
 }
 
-main().catch((error) => {
-  console.error(error instanceof Error ? error.message : String(error));
-  process.exitCode = 1;
-});
+if (process.argv[1] && import.meta.url === pathToFileURL(process.argv[1]).href) {
+  main().catch((error) => {
+    console.error(error instanceof Error ? error.message : String(error));
+    process.exitCode = 1;
+  });
+}