fix: deliver Claude teaching prose through an output tool

src/agent/tools/send-to-user.ts

@@ -0,0 +1,36 @@
+import { Type } from "typebox";
+import type { AgentTool } from "@earendil-works/pi-agent-core";
+import { SEND_TO_USER_TOOL } from "../../core/native-messages.ts";
+import { serverExecute } from "../tool-execute.ts";
+
+const sendToUserInput = Type.Object({
+	message: Type.String({
+		minLength: 1,
+		pattern: "\\S",
+		description: "The complete content to display to the learner. Markdown and LaTeX are supported.",
+	}),
+});
+
+/** Claude's documented verbatim mid-turn output channel. Availability is
+ * model-gated at the shared conversation-agent construction boundary;
+ * the native call remains in history, and the view renders its message. */
+export const sendToUserTool: AgentTool<typeof sendToUserInput> = {
+	name: SEND_TO_USER_TOOL,
+	label: SEND_TO_USER_TOOL,
+	description:
+		"## REQUIRED OUTPUT CHANNEL — ALWAYS USE `send_to_user`\n\n" +
+		"**ALL learner-facing messages MUST be sent using `send_to_user`.** This includes explanations, teaching, " +
+		"feedback, corrections, hints, introductions, progress messages, and final responses. " +
+		"There is no exception for short messages or the end of a turn.\n\n" +
+		"Ordinary response text may be converted into summarized thinking instead of reaching the learner. " +
+		"**Never rely on ordinary response text to communicate with the learner. Thinking is not delivery.**\n\n" +
+		"Put the complete message in `message`. Markdown and LaTeX are supported. " +
+		"Do not duplicate it in ordinary response text.\n\n" +
+		"Question content still belongs in the appropriate question tool; send any accompanying teaching or " +
+		"commentary through `send_to_user` before calling that tool.\n\n" +
+		"This tool does not pause or end your turn. Continue with other tools when needed. " +
+		"After delivering your final message, end the turn without additional ordinary response text.",
+	parameters: sendToUserInput,
+	executionMode: "sequential",
+	execute: serverExecute(SEND_TO_USER_TOOL, sendToUserInput, () => "Message received for display."),
+};