Why: acceptance clause 1 requires the anthropic preset to complete a
tool-call round against recorded HTTP fixtures; audit N4's root cause
was an offline body builder the real client never used, with no
tool_use/thinking support on the wire.
Evidence: GOWORK=off go vet + go test -count=1 ./... green;
TestAnthropicPresetToolRound drives the preset through tools request →
tool_use → tool_result → final text with thinking-signature round-trip
against byte-recorded SSE fixtures; TestAnthropicBodyShape pins block
forms and cache_control; legacy OpenAI path untouched (Client.Protocol
zero value).
Impact: the anthropic preset now means the native Messages API (the
compat shim remains reachable via models.json custom providers); audit
N4 closes. BuildBody shares anthropicBody with the live client.
Refs: Agent-System-Documents#42 (N2 plan WP2), audit N4
weichen