[Changelog] Fundamentals: JSON and RFC 9457 support on 1xxx errors (#28909)

---------

Co-authored-by: Pedro Sousa <680496+pedrosousa@users.noreply.github.com>

Author: smarsh-cf

src/content/changelog/fundamentals/2026-03-11-json-rfc9457-responses-for-1xxx-errors.mdx

@@ -0,0 +1,61 @@
+---
+title: JSON responses and RFC 9457 support for Cloudflare 1xxx errors
+description: "Cloudflare-generated 1xxx errors now return structured JSON when requested with 'Accept: application/json' or 'Accept: application/problem+json', following RFC 9457 (Problem Details for HTTP APIs)."
+products:
+  - fundamentals
+date: 2026-03-11
+---
+
+Cloudflare-generated 1xxx errors now return structured JSON when clients send `Accept: application/json` or `Accept: application/problem+json`. JSON responses follow [RFC 9457 (Problem Details for HTTP APIs)](https://www.rfc-editor.org/rfc/rfc9457), so any HTTP client that understands Problem Details can parse the base members without Cloudflare-specific code.
+
+## Breaking change
+
+The Markdown frontmatter field `http_status` has been renamed to `status`. Agents consuming Markdown frontmatter should update parsers accordingly.
+
+## Changes
+
+**JSON format.** Clients sending `Accept: application/json` or `Accept: application/problem+json` now receive a structured JSON object with the same operational fields as Markdown frontmatter, plus RFC 9457 standard members.
+
+**RFC 9457 standard members (JSON only):**
+
+- `type` — URI pointing to Cloudflare documentation for the specific error code
+- `status` — HTTP status code (matching the response status)
+- `title` — short, human-readable summary
+- `detail` — human-readable explanation specific to this occurrence
+- `instance` — Ray ID identifying this specific error occurrence
+
+**Field renames:**
+
+- `http_status` -> `status` (JSON and Markdown)
+- `what_happened` -> `detail` (JSON only — Markdown prose sections are unchanged)
+
+**Content-Type mirroring.** Clients sending `Accept: application/problem+json` receive `Content-Type: application/problem+json; charset=utf-8` back; `Accept: application/json` receives `application/json; charset=utf-8`. Same body in both cases.
+
+## Negotiation behavior
+
+| Request header sent                             | Response format                                |
+| ----------------------------------------------- | ---------------------------------------------- |
+| `Accept: application/json`                      | JSON (`application/json` content type)         |
+| `Accept: application/problem+json`              | JSON (`application/problem+json` content type) |
+| `Accept: application/json, text/markdown;q=0.9` | JSON                                           |
+| `Accept: text/markdown`                         | Markdown                                       |
+| `Accept: text/markdown, application/json`       | Markdown (equal `q`, first-listed wins)        |
+| `Accept: */*`                                   | HTML (default)                                 |
+
+## Availability
+
+Available now for Cloudflare-generated 1xxx errors.
+
+## Get started
+
+```bash
+curl -s --compressed -H "Accept: application/json" -A "TestAgent/1.0" -H "Accept-Encoding: gzip, deflate" "<YOUR_DOMAIN>/cdn-cgi/error/1015" | jq .
+```
+
+```bash
+curl -s --compressed -H "Accept: application/problem+json" -A "TestAgent/1.0" -H "Accept-Encoding: gzip, deflate" "<YOUR_DOMAIN>/cdn-cgi/error/1015" | jq .
+```
+
+References:
+- [RFC 9457 — Problem Details for HTTP APIs](https://www.rfc-editor.org/rfc/rfc9457)
+- [Cloudflare 1xxx error documentation](/support/troubleshooting/http-status-codes/cloudflare-1xxx-errors/)