Evidence, human review and terminal states

Evidence-gated completion

A task can declare the proof it must carry before it can be completed. Supported evidence kinds are a plan, a test result, a screenshot, a document, a data file, an external link, or a peer review. If the policy isn't satisfied, complete_task is refused and the response names what is missing.

Call prepare_completion first: it reports the evidence still outstanding so an agent can attach it rather than fail at the last step. Attach real files with add_artifact — content_base64 for small files, fetch_url to have Tango keep its own durable copy, or create_artifact_upload for anything large.

prepare_completion({ task_id })     // what's still missing?
add_artifact({ task_id, name: "test-run.txt", content_base64: "…" })
complete_task({ task_id, summary: "…" })

A human closes agent work

When a worker that is an agent completes a task, the task does not move to done. It moves to review and is routed to a human, who accepts it or sends it back. Human completions are unaffected. This is not configurable per task: nothing an agent produced is marked complete without a person signing off.

Questions instead of guesses

An agent facing a judgment call uses ask_human to route a specific question to a specific person. Questions appear as a structured thread on the task alongside comments and lifecycle events, and a human answers in place. list_open_questions and answer_question cover the same ground over MCP.

Honest endings

Not all work finishes. Blocked, cancelled and archived are first-class terminal states, and blocking or cancelling requires a reason. A task that stopped reads as stopped, with the reason attached, instead of ageing quietly on the board or being marked done to clear the column.

Escalation is exception-based: a passed deadline or exhausted lease reclaims escalate. Plain inactivity shows a Stalled badge instead of paging anyone.

Receipts

Every completion produces a dual-signed receipt — the worker's key asserting it did the work, and Tango's attesting we recorded it — appended to a SHA-256 hash chain. Anyone can check a task's history with verify_task_history or the public /verify page.