Design an Allowlisted Tool Set and Return Contracts
Explore how to design an allowlisted set of tools with strict specifications that enforce runtime validation and predictable return contracts. Understand how enforcing input constraints and shaped outputs enhances workflow reliability by preventing state ambiguity, enabling safe automated calls, and supporting auditability with clear error handling.
Tool use only stays safe and debuggable when the workflow can predict what a tool is allowed to do and what shape it returns. For our support workflow, the requirements are that tools are safe to call automatically, read only by default, return machine-usable fields, and emit results that can be logged end to end without guessing what happened.
This chapter assumes a bounded support loop that moves from triage to lookup to drafting, and each tool writes its output into tool_results in the shared state object. That means the return contract is not documentation, it is the rule that makes downstream routing deterministic. A minimal allowlist that fits these constraints is lookup_customer_status to fetch account signals needed for billing and account decisions, select_reply_template to return a stable template identifier and fill variables, and draft_reply to generate a reply draft without sending anything.
A tool spec that downstream steps can validate
Once the allowlist is short, each tool needs a spec that is strict enough to validate at runtime. A useful template captures what the workflow can enforce before calling the tool and what it can validate after the tool returns, so tool_results.lookup_customer_status can be trusted by select_reply_template and draft_reply.
A practical tool spec template includes:
Tool name as an identifier like
lookup_customer_statusPurpose stated as a single outcome the tool produces for the workflow
Allowed inputs with types, required keys, and constraints like min and max lengths and enums
Output schema with required keys, enums, nullability rules, and any invariants ... ...