AI Client Spend Caps with Webhooks, OpenAI & Data Tables

Put a hard cap on AI spend per client—automatic enforcement with webhooks + Data Tables + OpenAI-compatible limits

This n8n workflow enforces AI Client Spend Caps for multiple clients. Every request is checked against a Data Table usage ledger, and once a client hits (or exceeds) their cap, the workflow immediately returns HTTP 429—without calling your OpenAI-compatible model.

What this workflow does

• Receives client requests via webhook: A POST request includes client_id and prompt, secured with header authentication.
• Resolves usage period + cap: The workflow validates the request, determines the current usage window (month/day/all-time), and calculates the cap using a default value plus optional per-client overrides.
• Reads usage from an n8n Data Table: It looks up the current period record using a row_key formatted as client_id|period.
• Hard-stops at the limit: If the client’s spent amount is at or over the cap, the workflow returns HTTP 429 immediately.
• Calls OpenAI-compatible chat completions only when allowed: If under the cap, it sends the prompt to the configured chat/completions endpoint.
• Records usage + returns results: It extracts token usage from the provider response (or estimates it), updates spent and calls in the Data Table, and returns the answer plus updated usage details.

Use cases

• Agencies running AI workflows for multiple customers and needing predictable usage-based billing controls.
• Freelancers deploying a shared AI endpoint where each client must have a strict spend limit.
• SaaS operators building internal tooling to prevent unexpected model costs per customer and per period.

Technical details

• Webhook with Header Auth for incoming requests.
• n8n Data Table named client_spend with columns: row_key, client_id, period, last_call_at, spent, calls.
• Nodes/logic: if, set, code, webhook, data table (plus sticky note for documentation).
• OpenAI-compatible model call to chat/completions and automatic HTTP 429 rejection when capped.