diff --git a/src/commands/project.test.ts b/src/commands/project.test.ts
index ae98070..1625773 100644
--- a/src/commands/project.test.ts
+++ b/src/commands/project.test.ts
@@ -16,6 +16,7 @@ import {
   runGet,
   runList,
   runUpdate,
+  MAX_SECRET_FILE_BYTES,
 } from './project.js';
 
 const PROJECT_FIXTURE: CliProject = {
@@ -1423,3 +1424,185 @@ describe('dogfood 2026-06-30 — whitespace-only --name is rejected (parity with
     ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
   });
 });
+
+describe('#282 — secret --*-file flags are guarded (structured error, exit 5, no raw ENOENT)', () => {
+  const noNetwork = () => {
+    throw new Error('network should not be hit');
+  };
+  const deps = (credentialsPath: string) => ({
+    credentialsPath,
+    fetchImpl: makeFetch(noNetwork),
+    stdout: () => {},
+    stderr: () => {},
+  });
+  const missingPath = () => join(mkdtempSync(join(tmpdir(), 'cli-missing-')), 'no-such-secret.txt');
+
+  it('runCredential --credential-file missing → VALIDATION_ERROR (exit 5), no network', async () => {
+    const { credentialsPath } = makeCreds();
+    await expect(
+      runCredential(
+        {
+          profile: 'default',
+          output: 'json',
+          debug: false,
+          projectId: 'p1',
+          authType: 'API key',
+          credentialFile: missingPath(),
+        },
+        deps(credentialsPath),
+      ),
+    ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+  });
+
+  it('runCredential --credential-file pointing at a directory → VALIDATION_ERROR (exit 5)', async () => {
+    const { credentialsPath } = makeCreds();
+    const dir = mkdtempSync(join(tmpdir(), 'cli-cred-dir-'));
+    await expect(
+      runCredential(
+        {
+          profile: 'default',
+          output: 'json',
+          debug: false,
+          projectId: 'p1',
+          authType: 'API key',
+          credentialFile: dir,
+        },
+        deps(credentialsPath),
+      ),
+    ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+  });
+
+  it('runCredential --credential-file over the size cap → PAYLOAD_TOO_LARGE (exit 5)', async () => {
+    const { credentialsPath } = makeCreds();
+    const dir = mkdtempSync(join(tmpdir(), 'cli-cred-big-'));
+    const bigFile = join(dir, 'big.txt');
+    writeFileSync(bigFile, 'a'.repeat(MAX_SECRET_FILE_BYTES + 1));
+    await expect(
+      runCredential(
+        {
+          profile: 'default',
+          output: 'json',
+          debug: false,
+          projectId: 'p1',
+          authType: 'API key',
+          credentialFile: bigFile,
+        },
+        deps(credentialsPath),
+      ),
+    ).rejects.toMatchObject({ code: 'PAYLOAD_TOO_LARGE', exitCode: 5 });
+  });
+
+  it('runCredential reads a valid --credential-file (trimmed) and sends it', async () => {
+    const { credentialsPath } = makeCreds();
+    const dir = mkdtempSync(join(tmpdir(), 'cli-cred-ok-'));
+    const credFile = join(dir, 'cred.txt');
+    writeFileSync(credFile, '  tok-from-file\n');
+    let sentBody: { credential?: string } | undefined;
+    const fetchImpl = makeFetch((_url, init) => {
+      sentBody = init.body ? JSON.parse(init.body as string) : undefined;
+      return { status: 200, body: { projectId: 'p1', authType: 'API key', rewroteCount: 1 } };
+    });
+    await runCredential(
+      {
+        profile: 'default',
+        output: 'json',
+        debug: false,
+        projectId: 'p1',
+        authType: 'API key',
+        credentialFile: credFile,
+      },
+      { credentialsPath, fetchImpl, stdout: () => {}, stderr: () => {} },
+    );
+    // blindfold: manual — the on-disk fixture written above is "  tok-from-file\n"; the guard trims it
+    expect(sentBody?.credential).toBe('tok-from-file');
+  });
+
+  it('runAutoAuth --password-file missing → VALIDATION_ERROR (exit 5), no network', async () => {
+    const { credentialsPath } = makeCreds();
+    await expect(
+      runAutoAuth(
+        {
+          profile: 'default',
+          output: 'json',
+          debug: false,
+          projectId: 'p1',
+          method: 'password',
+          inject: 'bearer',
+          passwordFile: missingPath(),
+        },
+        deps(credentialsPath),
+      ),
+    ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+  });
+
+  it('runAutoAuth --client-secret-file missing → VALIDATION_ERROR (exit 5), no network', async () => {
+    const { credentialsPath } = makeCreds();
+    await expect(
+      runAutoAuth(
+        {
+          profile: 'default',
+          output: 'json',
+          debug: false,
+          projectId: 'p1',
+          method: 'refresh_token',
+          inject: 'bearer',
+          clientSecretFile: missingPath(),
+        },
+        deps(credentialsPath),
+      ),
+    ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+  });
+
+  it('runAutoAuth --refresh-token-file missing → VALIDATION_ERROR (exit 5), no network', async () => {
+    const { credentialsPath } = makeCreds();
+    await expect(
+      runAutoAuth(
+        {
+          profile: 'default',
+          output: 'json',
+          debug: false,
+          projectId: 'p1',
+          method: 'refresh_token',
+          inject: 'bearer',
+          refreshTokenFile: missingPath(),
+        },
+        deps(credentialsPath),
+      ),
+    ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+  });
+
+  it('runCreate --password-file missing → VALIDATION_ERROR (exit 5), no network', async () => {
+    const { credentialsPath } = makeCreds();
+    await expect(
+      runCreate(
+        {
+          profile: 'default',
+          output: 'json',
+          debug: false,
+          type: 'frontend',
+          name: 'FE',
+          targetUrl: 'https://example.com',
+          username: 'u',
+          passwordFile: missingPath(),
+        },
+        deps(credentialsPath),
+      ),
+    ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+  });
+
+  it('runUpdate --password-file missing → VALIDATION_ERROR (exit 5), no network', async () => {
+    const { credentialsPath } = makeCreds();
+    await expect(
+      runUpdate(
+        {
+          profile: 'default',
+          output: 'json',
+          debug: false,
+          projectId: 'p1',
+          passwordFile: missingPath(),
+        },
+        deps(credentialsPath),
+      ),
+    ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+  });
+});
diff --git a/src/commands/project.ts b/src/commands/project.ts
index 6a0b332..4082b90 100644
--- a/src/commands/project.ts
+++ b/src/commands/project.ts
@@ -1,5 +1,6 @@
 import { randomUUID } from 'node:crypto';
-import { readFileSync } from 'node:fs';
+import { readFileSync, statSync } from 'node:fs';
+import { resolve } from 'node:path';
 import { Command } from 'commander';
 import {
   emitDryRunBanner,
@@ -210,7 +211,7 @@ export async function runCreate(
   // Resolve password: flag > file > none
   let password = opts.password;
   if (password === undefined && opts.passwordFile !== undefined) {
-    password = readFileSync(opts.passwordFile, 'utf8').trim();
+    password = readSecretFileGuarded(opts.passwordFile, 'password-file');
   }
 
   const idempotencyKey = opts.idempotencyKey ?? `cli-proj-create-${randomUUID()}`;
@@ -326,7 +327,7 @@ export async function runUpdate(
   // filesystem, even when --password-file is present.
   let password = opts.password;
   if (password === undefined && opts.passwordFile !== undefined) {
-    password = readFileSync(opts.passwordFile, 'utf8').trim();
+    password = readSecretFileGuarded(opts.passwordFile, 'password-file');
   }
 
   const idempotencyKey = opts.idempotencyKey ?? `cli-proj-update-${randomUUID()}`;
@@ -467,7 +468,7 @@ export async function runCredential(
   // except `public` (which clears it).
   let credential = opts.credential;
   if (credential === undefined && opts.credentialFile !== undefined) {
-    credential = readFileSync(opts.credentialFile, 'utf8').trim();
+    credential = readSecretFileGuarded(opts.credentialFile, 'credential-file');
   }
   if (opts.authType !== 'public' && (credential === undefined || credential === '')) {
     throw localValidationError(
@@ -576,16 +577,18 @@ export async function runAutoAuth(
   // Resolve secrets from --*-file variants so they stay out of shell history.
   const password =
     opts.password ??
-    (opts.passwordFile !== undefined ? readFileSync(opts.passwordFile, 'utf8').trim() : undefined);
+    (opts.passwordFile !== undefined
+      ? readSecretFileGuarded(opts.passwordFile, 'password-file')
+      : undefined);
   const clientSecret =
     opts.clientSecret ??
     (opts.clientSecretFile !== undefined
-      ? readFileSync(opts.clientSecretFile, 'utf8').trim()
+      ? readSecretFileGuarded(opts.clientSecretFile, 'client-secret-file')
       : undefined);
   const refreshToken =
     opts.refreshToken ??
     (opts.refreshTokenFile !== undefined
-      ? readFileSync(opts.refreshTokenFile, 'utf8').trim()
+      ? readSecretFileGuarded(opts.refreshTokenFile, 'refresh-token-file')
       : undefined);
 
   const enabled = opts.disable !== true;
@@ -1082,3 +1085,63 @@ function localValidationError(message: string): ApiError {
     },
   });
 }
+
+/** Upper bound for any secret read from a `--*-file` flag. */
+export const MAX_SECRET_FILE_BYTES = 64 * 1024;
+
+/**
+ * Read a secret (password / credential / client-secret / refresh-token) from a
+ * `--*-file` flag, failing with a structured VALIDATION_ERROR (exit 5) instead
+ * of an unwrapped `readFileSync` error. A missing path, a non-regular file, or
+ * an oversized file all surface a typed envelope carrying the flag name — the
+ * same contract the `--code-file` guard in `test.ts` already provides. Without
+ * this, a missing file escaped as a raw `ENOENT` (exit 1) and broke the
+ * `--output json` envelope. `flagName` is the bare flag (e.g. `credential-file`).
+ */
+function readSecretFileGuarded(path: string, flagName: string): string {
+  const absolute = resolve(path);
+  let stat;
+  try {
+    stat = statSync(absolute);
+  } catch (err) {
+    const message = err instanceof Error ? err.message : String(err);
+    throw secretFileError(flagName, 'must point to a readable file', {
+      path: absolute,
+      error: message,
+    });
+  }
+
+  if (!stat.isFile()) {
+    throw secretFileError(flagName, 'must point to a regular file', { path: absolute });
+  }
+
+  if (stat.size > MAX_SECRET_FILE_BYTES) {
+    throw ApiError.fromEnvelope({
+      error: {
+        code: 'PAYLOAD_TOO_LARGE',
+        message: 'Secret file is too large.',
+        nextAction: `Flag \`--${flagName}\` is invalid: file must be at most ${MAX_SECRET_FILE_BYTES} bytes.`,
+        requestId: 'local',
+        details: { field: flagName, sizeBytes: stat.size, maxBytes: MAX_SECRET_FILE_BYTES },
+      },
+    });
+  }
+
+  return readFileSync(absolute, 'utf8').trim();
+}
+
+function secretFileError(
+  flagName: string,
+  reason: string,
+  details: Record<string, unknown>,
+): ApiError {
+  return ApiError.fromEnvelope({
+    error: {
+      code: 'VALIDATION_ERROR',
+      message: 'Invalid request.',
+      nextAction: `Flag \`--${flagName}\` is invalid: ${reason}.`,
+      requestId: 'local',
+      details: { field: flagName, reason, ...details },
+    },
+  });
+}