60% of my $312 Anthropic bill last month came from a single bug: an MCP router cache key that was missing a tenant ID.
The fix was literally this:
// before
const cacheKey = `mcp:context:${requestId}`;
// after
const cacheKey = `mcp:context:${tenantId}:${requestId}`;
That one missing segment meant warm Cloudflare Worker instances were serving cached Vectorize results from advertiser A into advertiser B's tool responses. In a production ad analytics SaaS. Not a demo.
The counterintuitive part: I assumed V8 isolate boundaries protected me. They don't — not in the way most people think. Isolate-level isolation applies between separate Worker deployments, not between two concurrent requests hitting the same warm Worker instance. Module-scope variables survive across requests. So any context manager or cache object you initialize at module level is shared state, even on Workers.
The failure mode was subtle enough to take 6 weeks to find. Vectorize query volume was 3× expected — that was the first signal. Digging into logs, I found cache hits for tenant a9f2 being served to sessions belonging to tenant b3c1. The corrupted cache contained vector search results, so every bad hit triggered a downstream re-fetch chain. That cascade is what blew up the token spend: wrong cache data → Claude retries with fresh context → Sonnet input tokens accumulate fast.
After fixing the cache key namespace and adding a PostToolUse hook that throws on tenant ID mismatch in tool response metadata, Sonnet input costs dropped from ~$187/month to ~$94. Vectorize queries fell ~40% over the same period.
One thing worth flagging for anyone on a similar stack: this specific fix — scoping everything to Workers' ExecutionContext per request — doesn't translate cleanly to long-running Node processes on something like Fly.io. There, AsyncLocalStorage is the right primitive. Porting the Workers pattern directly will give you a false sense of safety.
I wrote up the full breakdown — including the PostToolUse hook implementation, the KV/D1 cache key enforcement pattern, and the cases where this isolation design is overkill — over on riversealab.com.