Hey there,

I just mass spent hours debugging one of the most frustrating issues I've encountered in a while, and I wanted to share it because I'm sure others will hit the same wall.

The Setup

I was building JSON-RPC endpoints for my blog's A2A (Agent-to-Agent) protocol - APIs that let AI agents query blog content. Everything worked perfectly in local development.

The Problem

In production on Vercel, different POST requests started returning the same response:

• Request with {"type": "site"} → Returns site metadata ✓
• Request with {"type": "stats"} → Returns site metadata ✗ (should be stats!)

Same URL, different POST body, identical cached response.

What Didn't Work

I tried everything that should prevent caching:

• Cache-Control: no-store, no-cache headers
• x-vercel-no-cache: 1 header
• Query parameter cache busting
• CDN-specific headers
• vercel.json configuration

None of it worked. ISR ignored all of it.

The Root Cause

Vercel's ISR caches POST requests by URL path only. It completely ignores: - The POST body content - Query string parameters - All cache-control headers

For JSON-RPC, GraphQL, or any API where the request body determines the response, this silently breaks everything.

The Fix

Explicitly exclude your API routes from ISR:

// astro.config.mjs
adapter: vercel({
  isr: {
    exclude: ['/api/your-endpoint', '/api/another-endpoint'],
  },
})

That's it. Once deployed, each request is processed independently.

Full Details

I wrote up the complete debugging journey with all the code examples, what to look for in response headers, and solutions for both Astro and Next.js:

→ https://colinmcnamara.com/blog/vercel-isr-caching-post-requests-json-rpc/

If you're building APIs on Vercel, bookmark this one. It'll save you hours when you inevitably hit this issue.

Until next time, Colin

P.S. - If you've encountered similar Vercel gotchas, hit reply and let me know. Always looking for more topics to cover.