free-recall explain tool: the quizzing-mechanic switch (owner 2026-09-27)

Sycophancy mitigation as a structural commitment: the model writes its
full correctAnswer (plus explanation, in that order) BEFORE the learner
sees the question; the commitment rides the persisted call args, is
revealed on submit, and grades his free text in the model's next reply —
the client never grades free recall.

- tools.explain setting (segmented 'Quizzing — Multiple choice | Free
  recall' in Agent settings' Tools group): free arms explain and drops
  quiz for BOTH agents (prober variant carries no nodeId, same pattern
  as the probe quiz); mechanics keys never pass filterEnabledTools —
  quiz stays unwritable (the 2026-09-19 wedge ruling stands)
- doctrine lives pi-natively in the tool descriptions (owner ruling —
  no conditional system-prompt chunks): explain carries the commitment
  rule; the MC option-building craft moved out of the prompt into the
  quiz description; shared/20-question-tools.md slims to the
  quiz-agnostic core
- display seams: parseExplainInput (LaTeX-debris restoration included),
  dock text-mode with an 'I don't know' row (kbd row 0), AnsweredCard
  reveal (The answer / Why, no verdict chips), [Explain answer] format
- phases/plan seams recognize both quizzing tools' nodeId tags; explain
  answers deliberately feed no ledger evidence (no client grading to
  key on — a future ruling can parse the model's grade)

1049/1049 tests, typecheck/lint/build green.

src/core/phases.ts

@@ -83,7 +83,13 @@ function advance(
 		if (phase === "probe" && part.type === "tool-probe_done" && part.state === "output-available") {
 			phase = "plan";
 		}
-		if (phase === "plan" && part.type === "tool-quiz" && taggedNodeId(part.input)) {
+		// plan → teach via the first nodeId-tagged quizzing CALL — quiz OR
+		// explain, whichever mechanic the settings armed (both carry nodeId).
+		if (
+			phase === "plan" &&
+			(part.type === "tool-quiz" || part.type === "tool-explain") &&
+			taggedNodeId(part.input)
+		) {
 			phase = "teach";
 		}
 	}
@@ -136,7 +142,7 @@ export function phaseAt3Lh(messages: readonly LhMessage[], upToIndex?: number):
 				for (const block of content) {
 					if (
 						block.type === "toolCall" &&
-						block.name === "quiz" &&
+						(block.name === "quiz" || block.name === "explain") &&
 						taggedNodeId(block.arguments)
 					) {
 						phase = "teach";