diff --git a/docs/fleet/FLEET-LAUNCH.md b/docs/fleet/FLEET-LAUNCH.md
index 3979fd4..c6e2167 100644
--- a/docs/fleet/FLEET-LAUNCH.md
+++ b/docs/fleet/FLEET-LAUNCH.md
@@ -68,6 +68,10 @@ the shell sidecar only provides fallback state when the native marker is stale o
 
 `mosaic agent comms-block <exact-member>` can inspect that exact roster member's resolved Fleet-Comms
 block. It is a read-only inspection tool and fails loudly for an unknown exact member or missing roster.
+On Linux, the installed roster, TOOLS contract, and executable helper are opened through a held
+descriptor chain rooted at `/`; every managed path component uses no-follow traversal, and content plus
+execute validation stay bound to the same opened file. Systems without Linux `/proc/self/fd` support
+fail closed rather than falling back to pathname revalidation.
 
 ## Current M2 boundary
 
diff --git a/docs/scratchpads/issue-766-exact-fleet-comms.md b/docs/scratchpads/issue-766-exact-fleet-comms.md
index 3bad5dc..cdad958 100644
--- a/docs/scratchpads/issue-766-exact-fleet-comms.md
+++ b/docs/scratchpads/issue-766-exact-fleet-comms.md
@@ -71,3 +71,10 @@ Replace inference-prone fleet onboarding guidance with one roster-resolved contr
 - Final remediated gates passed: four-runtime regression 39/39; six changed-suite matrix 345/345; connector schema regression PASS; Mosaic package 910/910; installer migration 21/21; `agent-send.test.sh` 11/11; named-socket isolation PASS; Matrix/tmux transport 12/12; repository format PASS; typecheck 42/42 tasks; lint 23/23 tasks; tests 42/42 tasks; build 23/23 tasks.
 - No live tmux/session/fleet mutation, commit, push, PR mutation, issue mutation, context mutation, or reviewer launch performed.
 - Exact synthetic-tree reconstruction and frozen evidence are included in the coordinator handoff.
+- Sole-remediation preflight reverified the clean committed checkout at head `0dc47cac92c93a3ffd39ba9dd6685ac4165f6361`, tree `538de6ccce1f8c44ba288a7493286e63a3413e75`, branch `fix/766-exact-fleet-comms`; issue and PR state were read only through Mosaic wrappers.
+- Deterministic red-first ancestor substitution swapped validated `root/tools` for an external symlink immediately after `lstat`; current head returned `external marker` (`1 failed, 4 passed`) before implementation.
+- Secure reads now hold `/` and every root/descendant directory descriptor, traverse appended components through Linux `/proc/self/fd` with `O_DIRECTORY|O_NOFOLLOW`, and read plus effective-identity execute-check the same final descriptor. Non-Linux or unavailable proc-fd capability fails closed; stable errors redact managed paths while retaining Node `code` compatibility for missing/non-executable repair behavior.
+- Added deterministic root-selection, descendant-ancestor, and final-target substitution coverage. All return trusted descriptor-bound bytes after rename/symlink replacement; the race suite passed 50/50 repeated runs.
+- Isolated CLI verification drove `mosaic agent --mosaic-home <fixture> comms-block self` while repeatedly swapping `fleet/` with an external symlink: `trusted=2 fail_closed=10 external_marker=0`; a persistent symlink ancestor exited 1 with a redacted unsafe-ancestor error. No live fleet state was used or mutated.
+- Remediation gates: focused secure-file/comms/launch/tmux/Matrix `110/110`; full `@mosaicstack/mosaic` `914/914`; package and repository typecheck pass (`42/42` repository tasks); package and repository lint pass (`23/23` repository tasks); repository format check and `git diff --check` pass.
+- Independent review found one production hardening blocker (nonblocking final open), one redacted-error blocker, and a deterministic ancestor-test gap. Remediation added `O_NONBLOCK`, normalized execute errors while preserving errno, proved the ancestor hook fires, and added final-target substitution coverage; post-remediation review evidence is clean on the production invariant.
diff --git a/packages/mosaic/src/fleet/secure-file.spec.ts b/packages/mosaic/src/fleet/secure-file.spec.ts
index 95a7049..7d97384 100644
--- a/packages/mosaic/src/fleet/secure-file.spec.ts
+++ b/packages/mosaic/src/fleet/secure-file.spec.ts
@@ -1,7 +1,42 @@
-import { afterEach, beforeEach, describe, expect, it } from 'vitest';
-import { chmodSync, mkdirSync, mkdtempSync, rmSync, symlinkSync, writeFileSync } from 'node:fs';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import {
+  chmodSync,
+  mkdirSync,
+  mkdtempSync,
+  renameSync,
+  rmSync,
+  symlinkSync,
+  writeFileSync,
+  type PathLike,
+} from 'node:fs';
+import type * as NodeFs from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
+
+interface FilesystemRaceState {
+  afterLstat?: (path: string) => void;
+  afterOpen?: (path: string) => void;
+}
+
+const filesystemRaceState = vi.hoisted<FilesystemRaceState>(() => ({}));
+
+vi.mock('node:fs', async (importOriginal) => {
+  const actual = await importOriginal<typeof NodeFs>();
+  return {
+    ...actual,
+    lstatSync: (path: PathLike) => {
+      const result = actual.lstatSync(path);
+      filesystemRaceState.afterLstat?.(String(path));
+      return result;
+    },
+    openSync: (path: PathLike, flags: string | number, mode?: number) => {
+      const fd = actual.openSync(path, flags, mode);
+      filesystemRaceState.afterOpen?.(String(path));
+      return fd;
+    },
+  };
+});
+
 import { assertCanonicalContainment, readRegularFileSecure } from './secure-file.js';
 
 describe('secure file reads', () => {
@@ -9,9 +44,15 @@ describe('secure file reads', () => {
 
   beforeEach(() => {
     root = mkdtempSync(join(tmpdir(), 'mosaic-secure-file-'));
+    filesystemRaceState.afterLstat = undefined;
+    filesystemRaceState.afterOpen = undefined;
   });
 
-  afterEach(() => rmSync(root, { recursive: true, force: true }));
+  afterEach(() => {
+    filesystemRaceState.afterLstat = undefined;
+    filesystemRaceState.afterOpen = undefined;
+    rmSync(root, { recursive: true, force: true });
+  });
 
   it('rejects canonical path escape', () => {
     expect(() => assertCanonicalContainment(root, join(root, '..', 'outside'))).toThrow(
@@ -40,6 +81,91 @@ describe('secure file reads', () => {
     );
   });
 
+  it('keeps ancestor traversal bound when an opened directory is substituted', () => {
+    const tools = join(root, 'tools');
+    const displacedTools = join(root, 'tools.displaced');
+    const external = join(root, 'external');
+    const helper = join(tools, 'helper.sh');
+    mkdirSync(tools);
+    mkdirSync(external);
+    writeFileSync(helper, 'trusted\n', { mode: 0o755 });
+    writeFileSync(join(external, 'helper.sh'), 'external marker\n', { mode: 0o755 });
+
+    let substituted = false;
+    filesystemRaceState.afterOpen = (openedPath: string): void => {
+      if (substituted || !openedPath.startsWith('/proc/self/fd/')) return;
+      if (openedPath.split('/').at(-1) !== 'tools') return;
+      substituted = true;
+      renameSync(tools, displacedTools);
+      symlinkSync(external, tools);
+    };
+
+    const result = readRegularFileSecure(helper, { root, executable: true });
+    expect(substituted).toBe(true);
+    expect(result.content.toString('utf8')).toBe('trusted\n');
+  });
+
+  it('keeps root selection bound when the opened root is substituted', () => {
+    const displacedRoot = `${root}.displaced`;
+    const externalRoot = `${root}.external`;
+    const helper = join(root, 'helper.sh');
+    mkdirSync(externalRoot);
+    writeFileSync(helper, 'trusted root\n', { mode: 0o755 });
+    writeFileSync(join(externalRoot, 'helper.sh'), 'external root marker\n', { mode: 0o755 });
+
+    let substituted = false;
+    filesystemRaceState.afterOpen = (openedPath: string): void => {
+      if (substituted || !openedPath.startsWith('/proc/self/fd/')) return;
+      const match = openedPath.match(/\/([^/]+)$/);
+      if (match?.[1] !== root.split('/').filter(Boolean).at(-1)) return;
+      substituted = true;
+      renameSync(root, displacedRoot);
+      symlinkSync(externalRoot, root);
+    };
+
+    const result = readRegularFileSecure(helper, { root, executable: true });
+    expect(substituted).toBe(true);
+    expect(result.content.toString('utf8')).toBe('trusted root\n');
+    filesystemRaceState.afterOpen = undefined;
+    rmSync(root);
+    renameSync(displacedRoot, root);
+  });
+
+  it('keeps target read and execute validation bound to the opened file', () => {
+    const file = join(root, 'helper.sh');
+    const displaced = join(root, 'helper.displaced.sh');
+    const external = join(root, 'external-helper.sh');
+    writeFileSync(file, 'trusted target\n', { mode: 0o755 });
+    writeFileSync(external, 'external target marker\n', { mode: 0o755 });
+
+    let substituted = false;
+    filesystemRaceState.afterOpen = (openedPath: string): void => {
+      if (substituted || !openedPath.startsWith('/proc/self/fd/')) return;
+      if (openedPath.split('/').at(-1) !== 'helper.sh') return;
+      substituted = true;
+      renameSync(file, displaced);
+      symlinkSync(external, file);
+    };
+
+    const result = readRegularFileSecure(file, { root, executable: true });
+    expect(substituted).toBe(true);
+    expect(result.content.toString('utf8')).toBe('trusted target\n');
+  });
+
+  it('uses a stable redacted executable error while retaining the error code', () => {
+    const file = join(root, 'helper.sh');
+    writeFileSync(file, '#!/bin/sh\n', { mode: 0o644 });
+
+    try {
+      readRegularFileSecure(file, { root, executable: true });
+      throw new Error('expected executable validation to fail');
+    } catch (error) {
+      expect(error).toMatchObject({ message: 'managed file is not executable', code: 'EACCES' });
+      expect(String(error)).not.toContain('/proc/self/fd/');
+      expect(String(error)).not.toContain(root);
+    }
+  });
+
   it('uses effective-identity execute access after regular-file validation', () => {
     const file = join(root, 'helper.sh');
     writeFileSync(file, '#!/bin/sh\n', { mode: 0o644 });
diff --git a/packages/mosaic/src/fleet/secure-file.ts b/packages/mosaic/src/fleet/secure-file.ts
index b07fbb0..82f2720 100644
--- a/packages/mosaic/src/fleet/secure-file.ts
+++ b/packages/mosaic/src/fleet/secure-file.ts
@@ -8,6 +8,7 @@ import {
   openSync,
   readFileSync,
 } from 'node:fs';
+import { platform } from 'node:os';
 import { dirname, isAbsolute, relative, resolve, sep } from 'node:path';
 
 export interface SecureFileReadOptions {
@@ -30,6 +31,109 @@ function sameIdentity(
   return left.dev === right.dev && left.ino === right.ino;
 }
 
+function secureFilesystemError(message: string, cause: unknown): Error {
+  const error = new Error(message);
+  if (cause instanceof Error && 'code' in cause && typeof cause.code === 'string') {
+    Object.defineProperty(error, 'code', { value: cause.code, enumerable: true });
+  }
+  return error;
+}
+
+function closeDescriptors(descriptors: number[]): void {
+  for (const fd of descriptors.reverse()) {
+    try {
+      closeSync(fd);
+    } catch {
+      // Best-effort cleanup must not replace the security decision already made.
+    }
+  }
+}
+
+function procDescriptorPath(fd: number, component?: string): string {
+  const descriptor = `/proc/self/fd/${fd}`;
+  return component === undefined ? descriptor : `${descriptor}/${component}`;
+}
+
+/**
+ * Hold each directory while opening its child through Linux proc-fd. The only
+ * symlink followed is the kernel-owned descriptor link; O_NOFOLLOW protects
+ * every appended filesystem component from substitution.
+ */
+function openDirectoryChain(absoluteDirectory: string): { fd: number; descriptors: number[] } {
+  if (platform() !== 'linux') {
+    throw new Error('secure descriptor traversal is unsupported on this platform');
+  }
+
+  const descriptors: number[] = [];
+  try {
+    let fd = openSync(sep, constants.O_RDONLY | constants.O_DIRECTORY | constants.O_NOFOLLOW);
+    descriptors.push(fd);
+    for (const component of absoluteDirectory.split(sep).filter(Boolean)) {
+      fd = openSync(
+        procDescriptorPath(fd, component),
+        constants.O_RDONLY | constants.O_DIRECTORY | constants.O_NOFOLLOW,
+      );
+      descriptors.push(fd);
+      if (!fstatSync(fd).isDirectory()) {
+        throw new Error('secure descriptor traversal encountered a non-directory component');
+      }
+    }
+    return { fd, descriptors };
+  } catch (error) {
+    closeDescriptors(descriptors);
+    throw secureFilesystemError(
+      'secure descriptor traversal failed: symbolic link, unavailable, or not a directory',
+      error,
+    );
+  }
+}
+
+function openFileBeneathRoot(root: string, target: string): { fd: number; descriptors: number[] } {
+  const canonicalRoot = resolve(root);
+  const canonicalTarget = resolve(target);
+  assertCanonicalContainment(canonicalRoot, canonicalTarget);
+  const components = relative(canonicalRoot, canonicalTarget).split(sep).filter(Boolean);
+  const fileName = components.pop();
+  if (fileName === undefined) throw new Error('managed file path names the managed root');
+
+  const rootChain = openDirectoryChain(canonicalRoot);
+  try {
+    let parentFd = rootChain.fd;
+    for (const component of components) {
+      try {
+        parentFd = openSync(
+          procDescriptorPath(parentFd, component),
+          constants.O_RDONLY | constants.O_DIRECTORY | constants.O_NOFOLLOW,
+        );
+      } catch (error) {
+        throw secureFilesystemError(
+          'path ancestor is a symbolic link, unavailable, or not a directory',
+          error,
+        );
+      }
+      rootChain.descriptors.push(parentFd);
+      if (!fstatSync(parentFd).isDirectory()) {
+        throw new Error('path ancestor is a symbolic link or not a directory');
+      }
+    }
+    let fd: number;
+    try {
+      fd = openSync(
+        procDescriptorPath(parentFd, fileName),
+        constants.O_RDONLY | constants.O_NONBLOCK | constants.O_NOFOLLOW,
+      );
+    } catch (error) {
+      throw secureFilesystemError('file is a symbolic link or unavailable', error);
+    }
+    rootChain.descriptors.push(fd);
+    return { fd, descriptors: rootChain.descriptors };
+  } catch (error) {
+    closeDescriptors(rootChain.descriptors);
+    if (error instanceof Error) throw error;
+    throw new Error('secure managed file open failed');
+  }
+}
+
 export function assertCanonicalContainment(root: string, target: string): void {
   const canonicalRoot = resolve(root);
   const canonicalTarget = resolve(target);
@@ -99,43 +203,32 @@ export function readRegularFileSecure(
   path: string,
   options: SecureFileReadOptions,
 ): SecureFileSnapshot {
-  assertCanonicalContainment(options.root, path);
-  assertNoSymlinkAncestors(path);
-  const before = lstatSync(path);
-  if (before.isSymbolicLink()) throw new Error(`file is a symbolic link: ${path}`);
-  if (!before.isFile()) throw new Error(`file is not a regular file: ${path}`);
-  if (options.maxBytes !== undefined && before.size > options.maxBytes) {
-    throw new Error(`file exceeds ${options.maxBytes} bytes: ${path}`);
-  }
-
-  const fd = openSync(path, constants.O_RDONLY | constants.O_NOFOLLOW);
+  const openedFile = openFileBeneathRoot(options.root, path);
   try {
-    const opened = fstatSync(fd);
-    if (!opened.isFile() || !sameIdentity(before, opened)) {
-      throw new Error(`file changed during secure open: ${path}`);
+    const opened = fstatSync(openedFile.fd);
+    if (!opened.isFile()) throw new Error('managed file is not a regular file');
+    if (options.maxBytes !== undefined && opened.size > options.maxBytes) {
+      throw new Error(`managed file exceeds ${options.maxBytes} bytes`);
     }
     if (options.executable) {
-      accessSync(path, constants.X_OK);
-      const afterAccess = lstatSync(path);
-      if (
-        afterAccess.isSymbolicLink() ||
-        !afterAccess.isFile() ||
-        !sameIdentity(opened, afterAccess)
-      ) {
-        throw new Error(`file changed during executable access check: ${path}`);
+      try {
+        accessSync(procDescriptorPath(openedFile.fd), constants.X_OK);
+      } catch (error) {
+        throw secureFilesystemError('managed file is not executable', error);
+      }
+      const afterAccess = fstatSync(openedFile.fd);
+      if (!afterAccess.isFile() || !sameIdentity(opened, afterAccess)) {
+        throw new Error('managed file changed during executable access check');
       }
     }
-    const content = readFileSync(fd);
-    const after = fstatSync(fd);
-    const current = lstatSync(path);
-    if (
-      !after.isFile() ||
-      !sameIdentity(opened, after) ||
-      current.isSymbolicLink() ||
-      !current.isFile() ||
-      !sameIdentity(opened, current)
-    ) {
-      throw new Error(`file changed during secure read: ${path}`);
+
+    const content = readFileSync(openedFile.fd);
+    const after = fstatSync(openedFile.fd);
+    if (!after.isFile() || !sameIdentity(opened, after)) {
+      throw new Error('managed file changed during secure read');
+    }
+    if (options.maxBytes !== undefined && content.byteLength > options.maxBytes) {
+      throw new Error(`managed file exceeds ${options.maxBytes} bytes`);
     }
     return {
       content,
@@ -144,6 +237,6 @@ export function readRegularFileSecure(
       ino: opened.ino,
     };
   } finally {
-    closeSync(fd);
+    closeDescriptors(openedFile.descriptors);
   }
 }