50 — Call and evidence contract

Status: implemented in 3.0.4 for schema admission and evidence-aware presentation; capability-driven renderer planning landed in 3.0.5; lifecycle and child-run evidence recovery landed in 3.0.8; result-scoped evidence receipts and requirement-linked outcome projection landed in 3.0.10.

vak has one call boundary regardless of whether a turn is temporal, coding, research, data, or ordinary conversation. A provider proposal is not an execution request until the host has validated it against the admitted tool schema, authorized it, and admitted its resource claims.

Call lifecycle

provider proposal
  → schema contract
  → permission / revocation
  → resource claims and budget
  → brokered dispatch
  → typed result and evidence
  → semantic outcome
  → presentation projection

Schema validation is deliberately provider-neutral. It validates the provider-independent subset of the admitted JSON Schema: root and nested types, required properties, enums, arrays, and item types. A malformed call is an error value and never reaches a tool. It is not a network failure and must not enter provider retry or route endurance.

The schema boundary does not pretend to understand domain meaning. Temporal calls still need an explicit time zone and freshness contract; research needs source and claim evidence; coding needs workspace and verification evidence; data comparisons need aligned periods, units, and aggregation basis. Those are result contracts, not tool-name branches.

Evidence and rendering

Plain Markdown is always retained as a lossless fallback document. It is not evidence that a claim is true. A typed semantic result may render as a native card only after its registered payload schema passes. Future domain payloads must carry their comparison period, basis, and evidence references when those fields are material to the claim. If those semantics are absent, the output must remain an ordinary document with an explicit diagnostic rather than being labelled verified.

This keeps the renderer honest: a report can be beautifully formatted while the runtime still distinguishes observed facts, cited facts, model synthesis, and unsupported claims.

Non-goals

• Do not infer a domain from incidental field names.
• Do not repair malformed provider calls by inventing missing arguments.
• Do not calculate a weekly, temporal, or comparative metric from whichever two values happen to be present.
• Do not replace the original tool result with a rendering-oriented rewrite.