diff --git a/tools/subagent-prompt-prefix.test.mjs b/tools/subagent-prompt-prefix.test.mjs
new file mode 100644
index 00000000..e3f2469a
--- /dev/null
+++ b/tools/subagent-prompt-prefix.test.mjs
@@ -0,0 +1,92 @@
+#!/usr/bin/env node
+/**
+ * Tests for tools/subagent-prompt-prefix.mjs — PreToolUse Task git-safety header inject.
+ *
+ * Per Pravila §15.1 — hook injects cwd/branch/HEAD/worktree-root into каждый Task-prompt.
+ * FAIL-OPEN: any error → return {continue: true} без модификации.
+ *
+ * Spec: docs/superpowers/specs/2026-05-18-parallel-sessions-coordination-design.md §4
+ */
+import { test } from 'node:test';
+import { strict as assert } from 'node:assert';
+import { spawnSync } from 'node:child_process';
+import { fileURLToPath } from 'node:url';
+import { dirname, join } from 'node:path';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const HOOK_PATH = join(__dirname, 'subagent-prompt-prefix.mjs');
+
+function runHook(stdinJson) {
+  return spawnSync('node', [HOOK_PATH], {
+    input: JSON.stringify(stdinJson),
+    encoding: 'utf8',
+    timeout: 5000,
+  });
+}
+
+test('hook injects SUBAGENT GIT-SAFETY HEADER into Task prompt', () => {
+  const result = runHook({
+    tool_name: 'Task',
+    tool_input: { prompt: 'do something' },
+  });
+  assert.equal(result.status, 0, `stderr: ${result.stderr}`);
+  const out = JSON.parse(result.stdout);
+  assert.ok(out.hookSpecificOutput, 'has hookSpecificOutput');
+  assert.equal(out.hookSpecificOutput.hookEventName, 'PreToolUse');
+  assert.equal(out.hookSpecificOutput.permissionDecision, 'allow');
+  const newPrompt = out.hookSpecificOutput.updatedInput.prompt;
+  assert.match(newPrompt, /=== SUBAGENT GIT-SAFETY HEADER/);
+  assert.match(newPrompt, /=== END SUBAGENT GIT-SAFETY HEADER ===/);
+  assert.match(newPrompt, /do something/, 'preserves original prompt');
+});
+
+test('hook injects real cwd, branch, HEAD values', () => {
+  const result = runHook({
+    tool_name: 'Task',
+    tool_input: { prompt: 'noop' },
+  });
+  assert.equal(result.status, 0);
+  const out = JSON.parse(result.stdout);
+  const newPrompt = out.hookSpecificOutput.updatedInput.prompt;
+  // cwd — absolute path
+  assert.match(newPrompt, /Working directory \(cwd\): [A-Za-z]:[\\/]|Working directory \(cwd\): \//);
+  // branch — non-empty
+  assert.match(newPrompt, /Branch \(git branch --show-current\): \S+/);
+  // HEAD — 40-char SHA
+  assert.match(newPrompt, /Parent commit \(git rev-parse HEAD\): [0-9a-f]{40}/);
+});
+
+test('hook passes through non-Task tools without modification', () => {
+  const result = runHook({
+    tool_name: 'Edit',
+    tool_input: { file_path: '/foo', old_string: 'a', new_string: 'b' },
+  });
+  assert.equal(result.status, 0);
+  const out = JSON.parse(result.stdout);
+  // Non-Task → either {continue: true} OR no updatedInput
+  if (out.hookSpecificOutput) {
+    assert.equal(out.hookSpecificOutput.updatedInput, undefined);
+  } else {
+    assert.equal(out.continue, true);
+  }
+});
+
+test('hook fail-open on malformed stdin', () => {
+  const result = spawnSync('node', [HOOK_PATH], {
+    input: 'not-json',
+    encoding: 'utf8',
+    timeout: 5000,
+  });
+  assert.equal(result.status, 0, `should not crash; stderr: ${result.stderr}`);
+  // Either {continue: true} or empty output — both acceptable fail-open
+});
+
+test('hook fail-open when git not available', () => {
+  const result = spawnSync('node', [HOOK_PATH], {
+    input: JSON.stringify({ tool_name: 'Task', tool_input: { prompt: 'x' } }),
+    encoding: 'utf8',
+    timeout: 5000,
+    env: { ...process.env, PATH: '' }, // strip PATH → git not found
+  });
+  assert.equal(result.status, 0, `should not crash when git missing; stderr: ${result.stderr}`);
+});