Secure File Downloads
Serve private files without turning storage URLs into permanent public links.
What it adds
Authorization checked at download time, followed by a short-lived signed URL that the storage layer serves directly.
What your agent is told to do
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What your agent is told to do
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Route every download through an app endpoint that checks the current user's authorization for that specific file, then redirects to a signed URL with a short expiry.
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Keep the expiry measured in minutes. A signed link is a bearer credential and will end up in browser history, chat messages, and referrer headers.
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Set Content-Disposition to attachment and a conservative Content-Type for user-uploaded files so the browser downloads rather than renders them.
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Let the storage layer handle range requests and large transfers; the app issues the URL and gets out of the way.
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Do NOT rely on an unguessable storage path as the access control. Unguessable is not private — it is a permanent public link nobody has found yet.
Edge cases it handles
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Edge cases it handles
6- Authorization must be re-checked at every download, not cached from when the page listing the file was rendered.
- Removing a user from a workspace must break their outstanding signed URLs faster than those URLs expire, or the expiry is too long.
- Filenames from users can contain newlines, quotes, and non-ASCII characters — sanitise and encode them or the header is an injection point.
- SVG and HTML uploads execute script when served inline. Force attachment disposition, or serve user content from a separate origin.
- A resumed or partially failed download must not require a fresh authorization round-trip mid-transfer.
- Log who downloaded what and when; for private files that record is often the point.
Definition of done
8
Definition of done
8- Every download re-checks authorization for the specific file at request time.
- Signed URLs are short-lived and never stored or shared as permanent links.
- User-uploaded files are served with attachment disposition and a safe content type.
- Filenames are sanitised and encoded in headers.
- Large files and range requests are served by storage, not proxied through the app.
- Access changes invalidate outstanding links promptly.
- The feature matches the existing design system.
- No existing functionality is broken.
Related features
Password Generator
Password Generator
Offer a strong password in one click wherever someone sets or changes credentials.
What it does
A generate control on password fields that produces a strong value, shows it, and hands it to the user safely.
How it works
- 1 Add the control to every place a password is set: sign-up, password change, password reset completion, and any admin screen that provisions credentials for someone else.
- 2 Generate the value in the browser using the platform's cryptographic random source. The value must never be produced on the server or sent anywhere.
- 3 Reveal the generated password by default so the user can record it, and offer a regenerate control beside it. A hidden generated password that the user cannot see is a password they will immediately reset.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/password-generator
Idempotent Form Submission
Idempotent Form Submission
Make one submit mean one result, even when the network retries.
What it does
Idempotency keys on high-impact writes so a double-click, a retry, or a refresh cannot create the same record twice.
How it works
- 1 Find the writes where a duplicate is expensive — payments, orders, invitations, provisioning, anything that sends money or email.
- 2 Generate an idempotency key when the form is rendered, send it with the submission, and persist it server-side alongside the result of the first successful attempt.
- 3 When the same key arrives again, return the stored result instead of doing the work a second time. When the same key arrives with a different payload, reject it — that is a client bug, not a retry.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/idempotent-form-submission
Client Version Compatibility
Client Version Compatibility
Handle the tab that has been open since two deploys ago.
What it does
The server tells the client which build it expects, and the client responds — quietly for an optional update, insistently when the old code can no longer talk to the API.
How it works
- 1 Stamp every build with a version identifier and return it on API responses, either in a header or a small metadata endpoint. The client compares it against the version it was built as.
- 2 Separate two cases and treat them differently. A newer build being available is an optional update — offer it and let the user finish what they are doing. An API contract the running client cannot satisfy is a hard incompatibility and must block further writes.
- 3 For an optional update, show an unobtrusive, dismissible prompt and apply the new version at the next natural navigation. Do not interrupt.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/client-version-compatibility
How it works
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Copy the link
Grab the Markdown instruction URL for this feature.
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Give it to your AI
Paste it into Claude Code, Cursor, v0, Lovable — whatever you build with.
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It inspects, then implements
Your agent reads your existing app first, then adds the feature to fit it.
Works with your stack
These instructions are written to adapt. They tell the agent to detect your framework, match your existing design system, and reuse what you already have — rather than assuming a particular stack.
Need it tighter than that? Customize the feature and tell it exactly what you're running.