Direct answer
MCP Tool Call Logging That Others Can Check
Log every MCP call as a receipt with a public URL instead of a private log line. A log line lives on your machine, in a file you control, and proves nothing to anyone else. A receipt is captured when the call runs, carries a commitment over the exact result bytes, and sits at a URL a client, a teammate, or an auditor can open and recheck free, without your help and without trusting you.
Updated October 20, 2026.
What to capture on every call
A useful record of a tool call needs five things, captured at execution time, not reconstructed later:
- The tool and the inputs: what was called, with what arguments, on whose behalf.
- The exact result bytes: what the tool actually returned, preserved as returned.
- The time: when the call ran, so a run reads in order.
- A commitment over the result: a digest fixed at run time, so any later edit to the saved result breaks the check.
- A stable address: one URL per call, so the record can be cited, shared, and searched later.
Miss any one of the five and the record weakens: inputs without results cannot be audited, results without a commitment can be quietly edited, and a record with no public address is just a log line with better formatting.
Receipt vs log: the difference that matters
A private log and a receipt can hold the same facts and still be different objects. The log is written by the system being watched, kept where that system keeps it, and edited by whoever controls the machine. Showing it to a client asks them to trust the file, the machine, and you. A receipt is captured by the layer that ran the call, committed at run time, and published at a URL the reader fetches themselves. The reader recomputes the commitment and compares; trust in you is not part of the procedure.
Keep logs for debugging, by all means: they are fast, local, and detailed. Add receipts for the calls someone else will ever ask about: the ones behind a deliverable, a charge, a decision, or a dispute. The full comparison lives on execution log vs receipt.
Setup: route calls through the Zambo MCP
Point your client at the Zambo MCP and logging is the default, not a project. The endpoint is https://zambo.dev/mcp, carrying 153 tools total with 35 on the default surface:
{
"mcpServers": {
"zambo": { "url": "https://zambo.dev/mcp" }
}
}Every call through it returns the tool result plus its receipt URL. Calls your own servers handle can be receipted the same way the framework pages describe, for example the CrewAI agent receipts pattern. A Day Pass is $0.99 one time and covers 24 hours. The Zambo Pass is $29 a month or $290 a year, and $19 a month for $ZAMBO holders. Humans pay by PayPal or card through PayPal; agents pay USDC on Base via x402. Anyone checking a receipt pays nothing, ever.
Search past calls
A record you cannot find again is a record you do not have. Receipt URLs are stable and citable, so past calls can be gathered the ways real questions arrive: by job (close a multi-step run into one workflow receipt whose root covers every step), by day (review what your agents did today before you trust their summaries), or by link (the receipt URL travels with the deliverable it supports, in the invoice, the handover, or the ticket). When a client asks what actually ran last Tuesday, the answer is a set of links they can check, not a screenshot of a terminal.
Start with the calls behind money and promises, and let the habit spread. Within a week the question changes from whether the work can be checked to why any serious call would run without a receipt.
Honest limits
A receipt proves the saved result was not changed after the fact. It does not prove the tool was the right one to call, the inputs were wise, or the result was correct; it shows exactly what ran and what came back so those judgments can be made on evidence. Receipts also record the calls that run through them: a call made outside the receipted path leaves no receipt, and the honest statement is that it is unrecorded, not that it did not happen.
Frequently asked questions
What is MCP tool call logging?
MCP tool call logging is keeping a record of every tool call an AI makes through the Model Context Protocol: the tool, the inputs, the exact result, and the time. Done as receipts, each record also carries a commitment over the result bytes fixed at run time and a public URL, so anyone can fetch the record and recheck that the saved result was not changed after the fact.
How is a receipt different from a log line?
A log line is written and kept by the system being watched, in a file its operator controls, so showing it to someone asks them to trust the operator. A receipt is captured by the layer that ran the call, committed at run time, and published at a URL the reader fetches and rechecks themselves. Same facts, different trust: the log asks for belief, the receipt allows a check.
How do I start logging MCP calls as receipts?
Point your MCP client at https://zambo.dev/mcp and every call returns its result plus a receipt URL automatically. The server carries 153 tools total, 35 on the default surface. For calls handled by your own servers or frameworks, receipt them at the layer that runs them, the way the CrewAI listener pattern does, so capture happens at execution and outside the agent's control.
Can other people check my MCP call records?
Yes, and that is the point. Each receipt sits at a public URL, and checking it at https://zambo.dev/verify/ is free with no signup. A client, teammate, or auditor opens the link, the verifier recomputes the commitment from the saved bytes, and a match confirms the saved result is the result recorded when the call ran.
What does MCP call logging cost with Zambo?
Light use is covered by the free tier, so you can route real calls and see real receipts before paying. A Day Pass is $0.99 one time for 24 hours of calls. The Zambo Pass is $29 a month or $290 a year, and $19 a month for $ZAMBO holders. Humans pay by PayPal or card through PayPal; agents pay USDC on Base via x402. Checking receipts is always free.