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To keep a tool-using AI agent from guessing, return an evidence envelope—not just a bare value—and instruct it to preserve any requested field that the evidence does not support. For example, instead of replying only "active" to “Is this French supplier still active, and what is its legal name?”, return the claim alongside its source, retrieval time, support status and unresolved fields. The envelope is an application design choice carried over MCP, not a feature MCP itself guarantees.
Why a bare tool response can lead to a guess
A response such as "active" says nothing about where that value came from, when it was checked, or whether the tool also found the supplier’s legal name. The agent may produce a fluent answer that fills in those gaps, but a plausible model completion is not evidence.
An evidence-linked response makes the distinction visible: which claims have support, what evidence supports them, and what remains unknown. Give the agent an explicit instruction to report unresolved fields rather than infer them. Baron Sigma’s FACTRAIL tutorial puts the rule succinctly: “The design rule behind it: unknown is better than invented.”
What an evidence envelope contains
The FACTRAIL tutorial describes schema 1.2. Its envelope organizes the result into these parts:
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status: the overall result. Depending on the evidence, it can indicate a supported, contradicted, insufficient-evidence, stale or conflicting result.facts[]: individual values, their support levels and the IDs of evidence associated with them.evidence[]: source or publisher, URL, retrieval time and source status.coverage: which requested fields were resolved, which were not, and why.conflicts[]: competing claims retained for the agent to report rather than silently reconcile.freshness: information about retrieval and staleness.receipt_id: an identifier for retrieving the same envelope again and checking whether it has changed.
The value of the structure is not that it makes every answer complete. It makes incompleteness legible, so the agent can say “I don’t know” about a specific field while still reporting supported facts.
How partial evidence should shape the answer
In the tutorial’s EDF example, five fields are supported, while legal_form is unsupported. The overall status is insufficient_evidence. That status does not erase the five supported fields; it tells the agent that the requested result is only partly established.
A well-behaved agent should report the supported fields with their provenance, then state that the legal form was not established by the returned evidence. It should not turn a missing value into a confident-sounding answer. Likewise, if the envelope preserves conflicting claims, the agent should surface the conflict instead of picking one without justification.
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MCP defines the client/server protocol and transports used to exchange messages. The fields and behavior described above belong to FACTRAIL’s application-level response design; they are not built-in MCP evidence semantics. Using MCP to carry a response does not, by itself, establish that the response is complete, current or true.
The official MCP transport documentation describes Streamable HTTP message delivery. In that documented revision, messages are sent as POST requests to one MCP endpoint, with replies returned as JSON or request-scoped SSE. The documentation also requires servers to validate the Origin header to help prevent DNS rebinding. Transport details are revision-specific; identify the MCP revision your implementation targets rather than treating one revision’s behavior as timeless.
What FACTRAIL’s example does—and does not—cover
Baron Sigma’s tutorial describes FACTRAIL as a read-only service with two example capabilities:
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company_frchecks French companies using INSEE Sirene and BODACC. It is limited to French companies, not a global company-record service.- An EU import assessment is described as beta. The tutorial reports that official EU TARIC data was not installed at the time described.
The tutorial says a company check alone is not a complete KYC/KYB compliance check. Do not treat it as a sanctions screening, beneficial-ownership check or general-purpose fact-checking service.
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The operational details below are claims in the tutorial, not durable MCP guarantees or independently established service facts. The tutorial’s examples and endpoint observations are dated 2026-09-29; service limits, listings and prices may change.
| Reported detail | Qualification |
|---|---|
| 7 tools | The endpoint’s tool listing as reported in the tutorial at the time checked; four tools were described in detail. |
| 60 MCP requests per minute per client IP | Service-specific limit reported by the tutorial for its 2026-09-29 snapshot. |
| 600 requests per minute through known MCP gateways | A separate service-specific allowance reported by the tutorial for its 2026-09-29 snapshot. |
| $0.005 per successful verification | Third-party Apify Actor price reported as checked on 2026-09-29; not a price for the direct endpoint. |
| $0.03 per successful import assessment | Third-party Apify Actor price reported as checked on 2026-09-29; not a price for the direct endpoint. |
| Version 2.4.1 | Server response reported by the tutorial with a sample envelope captured 2026-09-29; the tutorial also recounts an earlier 2.4.0 capture. |
A receipt helps check integrity, not truth
According to the tutorial, retrieving an envelope by its receipt ID lets a client check that the returned envelope has the same identity or content under the service’s content-addressed mechanism. That is an integrity and retrieval aid: it can help detect whether the envelope changed.
It does not prove that the source facts were accurate, digitally sign the claims or certify them as true. Keep those questions separate: the receipt concerns the returned envelope; the quality and correctness of its underlying sources still need to be assessed on their own.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Put the design into practice
- Define the requested fields. For a supplier check, make “active status” and “legal name” separate fields so the tool can show whether each was resolved.
- Return claims with support. Include per-fact support levels and evidence IDs, plus the source metadata needed to trace the evidence.
- Represent missing and conflicting evidence explicitly. Populate coverage reasons for unresolved fields and preserve competing claims in
conflicts[]. - Instruct the agent not to fill gaps. Require it to report supported facts with their sources and to say “I don’t know” for fields not supported by the envelope.
- Treat receipts and transport as separate concerns. Use a receipt to retrieve or check envelope integrity where the service supports it; implement MCP transport requirements for the protocol revision you use.
This pattern gives an agent a structured basis for distinguishing “the tool found support for this” from “the tool did not establish this.” It does not guarantee correctness, but it avoids asking the model to invent provenance or silently complete missing fields.
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