diff --git a/docs/gateguard/design-notes.md b/docs/gateguard/design-notes.md
index 3e0e14124..5809dc9aa 100644
--- a/docs/gateguard/design-notes.md
+++ b/docs/gateguard/design-notes.md
@@ -58,6 +58,8 @@ and trivial edits) is in [change-profile.md](change-profile.md).
   detection and sibling-collapse eligibility. Stateless; filesystem access is
   limited to worktree `.git` checks, realpath/lstat of the target's parent
   chain, and lstat/stat of the target for its link count.
+- `scripts/lib/file-tail.js`: the bounded tail read shared with the
+  strategic-compact hook's `transcript-context.js`.
 - `scripts/lib/gateguard-turn-scan.js`: one bounded tail read of the Claude Code
   JSONL transcript, walked back to the start of the current turn. Yields the
   turn id, the turn's completed non-error search calls (newest first), their
diff --git a/scripts/lib/file-tail.js b/scripts/lib/file-tail.js
new file mode 100644
index 000000000..807a1a937
--- /dev/null
+++ b/scripts/lib/file-tail.js
@@ -0,0 +1,41 @@
+'use strict';
+
+const fs = require('fs');
+
+const DEFAULT_TRANSCRIPT_TAIL_BYTES = 256 * 1024;
+
+/** Last `tailBytes` of a file as UTF-8 text, with whether earlier bytes were skipped; null when unreadable. */
+function readFileTail(filePath, tailBytes) {
+  let fd;
+  try {
+    fd = fs.openSync(filePath, 'r');
+  } catch {
+    return null;
+  }
+
+  try {
+    const size = fs.fstatSync(fd).size;
+    const start = Math.max(0, size - tailBytes);
+    const length = size - start;
+    if (length <= 0) {
+      return { text: '', truncated: false };
+    }
+
+    const buffer = Buffer.alloc(length);
+    const bytesRead = fs.readSync(fd, buffer, 0, length, start);
+    return {
+      text: buffer.toString('utf8', 0, bytesRead),
+      truncated: start > 0
+    };
+  } catch {
+    return null;
+  } finally {
+    try {
+      fs.closeSync(fd);
+    } catch {
+      /* ignore */
+    }
+  }
+}
+
+module.exports = { DEFAULT_TRANSCRIPT_TAIL_BYTES, readFileTail };
diff --git a/scripts/lib/gateguard-turn-scan.js b/scripts/lib/gateguard-turn-scan.js
index 687623763..26cf20041 100644
--- a/scripts/lib/gateguard-turn-scan.js
+++ b/scripts/lib/gateguard-turn-scan.js
@@ -3,7 +3,7 @@
 const crypto = require('crypto');
 const fs = require('fs');
 const path = require('path');
-const { readFileTail, DEFAULT_TRANSCRIPT_TAIL_BYTES } = require('./transcript-context');
+const { readFileTail, DEFAULT_TRANSCRIPT_TAIL_BYTES } = require('./file-tail');
 
 const TRANSCRIPT_SCAN_MAX_LINES = 2000;
 // see docs/gateguard/design-notes.md#read-is-not-evidence
diff --git a/scripts/lib/transcript-context.js b/scripts/lib/transcript-context.js
index b6665a986..87c6bb2d4 100644
--- a/scripts/lib/transcript-context.js
+++ b/scripts/lib/transcript-context.js
@@ -17,14 +17,13 @@
  * keeping the PreToolUse hook fast even for very large sessions.
  */
 
-const fs = require('fs');
+const { readFileTail, DEFAULT_TRANSCRIPT_TAIL_BYTES } = require('./file-tail');
 
 const STANDARD_CONTEXT_WINDOW_TOKENS = 200000;
 const LARGE_CONTEXT_WINDOW_TOKENS = 1000000;
 const DEFAULT_CONTEXT_THRESHOLD_STANDARD = 160000;
 const DEFAULT_CONTEXT_THRESHOLD_LARGE = 250000;
 const DEFAULT_CONTEXT_INTERVAL_TOKENS = 60000;
-const DEFAULT_TRANSCRIPT_TAIL_BYTES = 256 * 1024;
 const MAX_TOKEN_SETTING = 10000000;
 const LARGE_WINDOW_MODEL_MARKER = '[1m]';
 
@@ -59,38 +58,6 @@ function isKnownModelFamilyMatch(model, familyId) {
  * Read the trailing `tailBytes` of a file as UTF-8.
  * Returns null when the file is missing or unreadable.
  */
-function readFileTail(filePath, tailBytes) {
-  let fd;
-  try {
-    fd = fs.openSync(filePath, 'r');
-  } catch {
-    return null;
-  }
-
-  try {
-    const size = fs.fstatSync(fd).size;
-    const start = Math.max(0, size - tailBytes);
-    const length = size - start;
-    if (length <= 0) {
-      return { text: '', truncated: false };
-    }
-
-    const buffer = Buffer.alloc(length);
-    const bytesRead = fs.readSync(fd, buffer, 0, length, start);
-    return {
-      text: buffer.toString('utf8', 0, bytesRead),
-      truncated: start > 0
-    };
-  } catch {
-    return null;
-  } finally {
-    try {
-      fs.closeSync(fd);
-    } catch {
-      /* ignore */
-    }
-  }
-}
 
 /**
  * Extract the context token total from a transcript record's usage block.
diff --git a/tests/hooks/gateguard-fact-force.test.js b/tests/hooks/gateguard-fact-force.test.js
index b77ca8dcf..e0b794300 100644
--- a/tests/hooks/gateguard-fact-force.test.js
+++ b/tests/hooks/gateguard-fact-force.test.js
@@ -7394,7 +7394,7 @@ function runTests() {
       'const hook = require(process.argv[1]);',
       'hook.run(process.argv[2]);',
       "const names = Object.keys(require.cache).map(f => require('path').basename(f, '.js'));",
-      "const libs = names.filter(n => /^(gateguard-|transcript-context$)/.test(n) && !/^gateguard-(fact-force|heredoc)$/.test(n));",
+      "const libs = names.filter(n => /^(gateguard-|transcript-context$|file-tail$)/.test(n) && !/^gateguard-(fact-force|heredoc)$/.test(n));",
       'process.stdout.write(JSON.stringify(libs.sort()));'
     ].join('\n');
     try {
@@ -7411,7 +7411,7 @@ function runTests() {
     ['a first shell command loads only the read-only check', lazyBash('npm test'), [], ['gateguard-readonly-shell', 'gateguard-state']],
     ['a shell command after the routine gate loads only the state helpers', lazyBash('npm run build'), [lazyBash('npm test')], ['gateguard-state']],
     ['an edit of a checked file loads only the target classification', lazyEdit, [lazyEdit], ['gateguard-state', 'gateguard-target-class']],
-    ['a first touch without a transcript loads no search matching', lazyEdit, [], ['gateguard-change-profile', 'gateguard-code-lexer', 'gateguard-file-context', 'gateguard-state', 'gateguard-target-class', 'gateguard-turn-scan', 'transcript-context']]
+    ['a first touch without a transcript loads no search matching', lazyEdit, [], ['file-tail', 'gateguard-change-profile', 'gateguard-code-lexer', 'gateguard-file-context', 'gateguard-state', 'gateguard-target-class', 'gateguard-turn-scan']]
   ];
   for (const [label, payload, setup, expected] of lazyCases) {
     if (test(`lazy loading: ${label}`, () => assert.deepStrictEqual(loadedLibs(payload, setup), expected))) passed++;
diff --git a/tests/lib/file-tail.test.js b/tests/lib/file-tail.test.js
new file mode 100644
index 000000000..c5aa68a7c
--- /dev/null
+++ b/tests/lib/file-tail.test.js
@@ -0,0 +1,65 @@
+'use strict';
+/**
+ * Tests for scripts/lib/file-tail.js.
+ *
+ * Run with: node tests/lib/file-tail.test.js
+ */
+
+const assert = require('assert');
+const fs = require('fs');
+const os = require('os');
+const path = require('path');
+
+const { readFileTail, DEFAULT_TRANSCRIPT_TAIL_BYTES } = require(path.join(__dirname, '..', '..', 'scripts', 'lib', 'file-tail.js'));
+
+console.log('=== Testing file-tail.js ===\n');
+
+let passed = 0;
+let failed = 0;
+
+function test(desc, fn) {
+  try {
+    fn();
+    console.log(`  ✓ ${desc}`);
+    passed++;
+  } catch (e) {
+    console.log(`  ✗ ${desc}: ${e.message}`);
+    failed++;
+  }
+}
+
+const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'file-tail-'));
+const file = path.join(dir, 'f.txt');
+fs.writeFileSync(file, 'abcdef');
+
+test('the default tail is 256 KiB', () => {
+  assert.strictEqual(DEFAULT_TRANSCRIPT_TAIL_BYTES, 256 * 1024);
+});
+
+test('a file smaller than the tail is read whole', () => {
+  assert.deepStrictEqual(readFileTail(file, 1024), { text: 'abcdef', truncated: false });
+});
+
+test('a larger file is cut to its last bytes and marked truncated', () => {
+  assert.deepStrictEqual(readFileTail(file, 2), { text: 'ef', truncated: true });
+});
+
+test('an empty file gives empty text', () => {
+  const empty = path.join(dir, 'empty.txt');
+  fs.writeFileSync(empty, '');
+  assert.deepStrictEqual(readFileTail(empty, 10), { text: '', truncated: false });
+});
+
+test('a missing file gives null', () => {
+  assert.strictEqual(readFileTail(path.join(dir, 'missing.txt'), 10), null);
+});
+
+test('transcript-context still exports the same reader', () => {
+  const context = require(path.join(__dirname, '..', '..', 'scripts', 'lib', 'transcript-context.js'));
+  assert.strictEqual(context.readFileTail, readFileTail);
+  assert.strictEqual(context.DEFAULT_TRANSCRIPT_TAIL_BYTES, DEFAULT_TRANSCRIPT_TAIL_BYTES);
+});
+
+fs.rmSync(dir, { recursive: true, force: true });
+console.log(`\nResults: Passed: ${passed}, Failed: ${failed}`);
+process.exit(failed > 0 ? 1 : 0);