Content Security Policy
Cut script-injection risk with a policy the existing app can actually run under.
What it adds
A CSP built from an inventory of what the app really loads, rolled out in report-only mode before it is enforced.
What your agent is told to do
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What your agent is told to do
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Inventory first: every inline script and style, every third-party origin, every worker, frame, font, and image host the app loads. Report that list before writing a single directive.
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Add a per-response nonce to the inline scripts you must keep, and hashes for the ones you cannot make dynamic.
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Deploy in report-only mode, collect violations from real traffic for long enough to cover the slow paths, and only then enforce.
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Set a violation report endpoint that deduplicates by directive and blocked URI, so one broken third-party script does not produce a million rows.
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Do NOT reach for unsafe-inline or unsafe-eval to make the errors stop. A policy with unsafe-inline on script-src provides essentially none of the protection you added it for.
Edge cases it handles
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Edge cases it handles
6- Asset hosts, API origins, and analytics domains differ between development, staging, and production — generate the policy per environment.
- Browser extensions inject scripts and will generate violation reports you cannot fix. Filter them out or they will drown the real signal.
- Some third-party scripts load further scripts at runtime; the origin you allowlist is rarely the only one they need.
- Violation reports can include URL fragments and query strings — strip anything sensitive before storing them.
- The nonce must be unique per response and never cached; a cached page with a stale nonce blocks its own scripts.
- Blocked content usually fails silently for the user. Watch error rates as well as reports when you flip to enforcing.
Definition of done
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Definition of done
8- The policy is derived from an inventory of actual app dependencies.
- Inline scripts use per-response nonces or hashes; unsafe-inline is not present on script-src.
- A report-only rollout preceded enforcement, with violations reviewed.
- A deduplicating report endpoint exists and filters extension noise.
- Policies differ correctly by environment.
- No sensitive data is retained in stored violation reports.
- 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.