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/tool-parts.ts

@@ -29,7 +29,7 @@
 
 import { Value } from "typebox/value";
 import type { TSchema } from "typebox";
-import { quizInputBase, askInputSchema, askOptionProblems, quizInputProblems, type AskToolInput, type QuizToolInput } from "./question-core.ts";
+import { quizInputBase, askInputSchema, askOptionProblems, quizInputProblems, explainInputBase, explainInputProblems, type AskToolInput, type QuizToolInput, type ExplainToolInput } from "./question-core.ts";
 import { restoreMathEscapes } from "./math-delims.ts";
 import { probeDoneInputSchema, type ProbeDoneInput } from "./contracts/probe.ts";
 
@@ -103,6 +103,17 @@ export function parseAskInput(input: unknown): AskToolInput | undefined {
 	return cleaned;
 }
 
+/** Explain parse for DISPLAY: the shared authoring base (the teacher's
+ *  nodeId is an unknown key Value.Clean strips, same as the quiz seam) —
+ *  both explain variants share the same fields, and the commitment
+ *  refinement (explainInputProblems) mirrors the boundary chain. */
+export function parseExplainInput(input: unknown): ExplainToolInput | undefined {
+	const cleaned = boundaryParse<ExplainToolInput>(explainInputBase, restoreMathEscapesDeep(input));
+	if (!cleaned) return undefined;
+	if (explainInputProblems(cleaned).length > 0) return undefined;
+	return cleaned;
+}
+
 /** probe_done parse — the core-owned schema the tool itself declares. */
 export function parseProbeDoneInput(input: unknown): ProbeDoneInput | undefined {
 	return boundaryParse<ProbeDoneInput>(probeDoneInputSchema, input);