Prove OpenAI Retry Cache HITs in n8n with Metrecept JWKS

Prove OpenAI Retry Cache HITs in n8n (with Metrecept JWKS verification)

This n8n workflow sends the exact same OpenAI-style chat completion request to Metrecept twice—first to demonstrate a cache MISS, then (optionally) to produce and prove a cache HIT—verifying the returned receipt signature using Metrecept’s public JWKS.

What this workflow does

• Accepts input via manual run or webhook: a POST request containing prompt or messages (optionally model and prove_retry).
• Normalizes and freezes the payload: converts your input into an OpenAI Chat Completions request body, freezes it as a JSON string, and returns HTTP 400 if no messages are provided.
• Sends the request to Metrecept: posts the frozen payload to /v1/chat/completions and captures cache/billing headers. Returns HTTP 402 on budget errors and HTTP 502 for other upstream failures.
• Generates a retry HIT (optional): when prove_retry is enabled and the first response is a cache MISS, the workflow sends the identical payload again to trigger a cache HIT.
• Verifies the receipt signature (JWKS): if a receipt JWS header is present, it fetches Metrecept’s public key directory (JWKS) and verifies the Ed25519 signature. If missing, it reports verification as unavailable.
• Returns a consolidated proof object: includes mode, cache status, billed amounts, receipt verification result, and current usage from Metrecept /v1/usage.

Use cases

• Audit that “retry” behavior in your OpenAI-style integrations actually yields cache HITs.
• Validate Metrecept receipt integrity for compliance or internal reliability testing.
• Estimate billed impact by comparing MISS vs HIT scenarios with returned usage data.

Technical details

• n8n nodes: webhook, if, set, code, switch, plus sticky notes for clarity.
• Metrecept integration: HTTP calls to /v1/chat/completions and /v1/usage, plus public JWKS fetch for receipt verification.
• Auth setup: an n8n HTTP Header Auth credential using Authorization: Bearer <your_intermediate_key>.