Apply Agent SDK hooks for tool call interception and data normalization

Describe the different timestamp and status formats in the system prompt so the model can reconcile them.

Every call then re-does a mechanical conversion probabilistically, and a mis-parsed date is indistinguishable from a real one downstream.

Normalise deterministically in a PostToolUse hook. The model should receive one shape and spend its reasoning elsewhere.

Add a reviewer subagent that checks whether an action breaches the spend threshold.

Right idea, wrong architectural tier — another probabilistic component reviewing a probabilistic one, and it cannot actually stop the call.

A threshold is arithmetic. Intercept the outgoing call and block it.

Use a PostToolUse hook to stop a policy-violating action.

PostToolUse runs after the tool has executed, so the action — a refund issued, a record written, a deploy started — has already happened by the time the hook sees it.

Prevention intercepts the outgoing call; PostToolUse is for transforming what comes back.

Log the violation and let the call through, so nothing legitimate is ever blocked.

A log is a record that the control did not act. The policy is documented and unenforced.

Block, then redirect into the workflow that can approve it — usually human escalation.

Move every rule into hooks, since deterministic beats probabilistic.

Judgement calls encoded as hooks turn into brittle heuristics that fire on cases nobody anticipated, and the agent loses the discretion it was chosen for.

Hooks carry rules you can state exactly. Everything else stays a matter of prompt and judgement.

Use a SubagentStop hook to clean up or rewrite a subagent's output before the coordinator sees it.

Neither lifecycle hook rewrites subagent output. SubagentStop can block completion with a reason, which sends the subagent back to work — it cannot edit what comes back, so a hook written to reshape the result silently does nothing.

SubagentStart observes spawning; SubagentStop gates completion by blocking with a reason. To change the shape of a result, normalise it in a PostToolUse hook or have the coordinator do it — to reject a result, block it and say why.