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htmx-endpoint-design

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Design and review htmx endpoint contracts for server-rendered HTML fragments. Use when adding or debugging hx-get, hx-post, hx-put, hx-patch, hx-delete, hx-target, hx-swap, hx-select, out-of-band swaps, HX-Trigger events, partial template responses, redirects, retargeting, or response headers.

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htmx Endpoint Design

Use this skill when the hard part is the server/browser contract: which element makes the request, which endpoint handles it, what HTML comes back, and where that HTML lands.

Contract First

Define the endpoint contract before writing attributes:

  • Request source: the element that sends the request.
  • HTTP method: GET for safe reads; POST, PUT, PATCH, or DELETE for mutations when the server/framework supports them.
  • Input data: normal form fields, included elements, path params, or explicit values.
  • Response shape: full page, partial fragment, empty response, redirect, or event-only response.
  • Swap target: the element to replace or update.
  • Follow-up effects: events, out-of-band fragments, URL changes, focus changes, and messages.

Endpoint Workflow

  1. Run the same auth, permission, and data-loading logic for full-page and htmx requests.
  2. Branch late on whether the request is an htmx request.
  3. Reuse the same partial in the full-page render and the htmx response.
  4. Return a fragment whose root matches the intended hx-target when using outerHTML.
  5. Use response headers or events for cross-component effects instead of coupling unrelated targets.
  6. Test the normal request and the htmx request separately.

Requests

  • Prefer normal form encoding so server validation, CSRF, and file limitations are explicit.
  • Keep mutation requests same-origin unless the project has a deliberate CORS and CSRF design.
  • Use hx-include when one control must submit data from a nearby form or filter panel.
  • Use hx-vals only for small explicit values. Avoid secrets and avoid executable js: values; prefer form fields, hidden inputs, or URL parameters instead.
  • Debounce or throttle chatty triggers such as keyup, input, and polling.
  • Use real links and forms when possible, then enhance them with htmx attributes.

Responses

Choose the smallest response that keeps the UI honest:

SituationResponse
Replace one componentReturn that component fragment
Update several unrelated componentsReturn primary fragment plus hx-swap-oob fragments
Mutation succeeds and another element should refreshReturn 204 plus an event/header when supported
Validation failsReturn the bound form fragment with errors
Browser should navigateReturn a normal redirect for non-htmx, and an htmx redirect/location response for htmx
Server needs a different target/swapUse framework helpers or htmx response headers where available

Targeting And Swapping

  • Use stable IDs for durable targets and keep them unique.
  • Use hx-target="this" for self-contained components.
  • Use outerHTML when replacing the target element itself.
  • Use innerHTML when preserving the target shell and replacing only its contents.
  • Use insert swaps such as beforeend for appending rows, log entries, or feed items.
  • Use hx-select when the server returns a larger document but the client should extract one fragment.
  • Do not swap a parent that contains long-lived local browser state unless that state is intentionally reset.

Out-Of-Band Updates

Use out-of-band swaps for shared chrome and secondary facts:

  • flash messages
  • cart counts
  • notification badges
  • summary totals
  • list counters
  • modal shells outside the main target

Keep out-of-band fragments small and predictable. If many out-of-band updates are required for one action, consider returning the larger owner component instead.

Events

Use events to decouple server results from browser-local behavior:

  • Fire a server-triggered event after save/delete when another component should refresh.
  • Listen for htmx lifecycle events for instrumentation, custom confirmation, and cleanup.
  • Use custom events as boundaries between htmx and Alpine, plain JavaScript, or _hyperscript.
  • Keep event names domain-specific, such as invoice:saved or filters:changed.

Status Codes

  • Use 200 for normal fragment replacement, including many invalid form responses.
  • Use 204 when no visible fragment should be swapped.
  • Use redirects deliberately; do not let htmx silently insert a login page into a small target.
  • For polling, use the framework or htmx convention that stops polling when the server says the job is done.

Testing Checklist

  • Assert status code and key response headers.
  • Assert the partial contains the intended root element.
  • Assert the partial omits full-page chrome when it should.
  • Assert invalid forms render errors into the expected target.
  • Assert out-of-band fragments are present when secondary UI must change.
  • Assert redirects or client events are represented in headers, not only in body text.

Avoid

  • Do not create one endpoint that performs unrelated actions based on arbitrary request parameters.
  • Do not let templates become the authorization layer.
  • Do not return raw JSON for htmx UI updates unless the same endpoint must serve a real API client.
  • Do not use broad selectors that can hit multiple targets by accident.
  • Do not mix server-rendered truth with stale client-side copies of the same state.