fix(interactive): treat missing thinking/text payloads as empty instead of crashing

packages/coding-agent/src/modes/interactive/changes.md

@@ -1,5 +1,26 @@
 # changes
 
+## Malformed assistant blocks render as empty instead of crashing (2026-08-05)
+
+### What changed
+
+- `components/assistant-render-descriptors.ts` and `streaming-reveal-content.ts` now treat a missing `text`/`thinking`
+  payload on a content block as an empty string instead of reading `.length`/`.trim()` off `undefined`.
+- Previously a provider extension emitting a block in the wrong shape (e.g. `{ type: "thinking", text: ... }` instead
+  of `{ type: "thinking", thinking: ... }`, or a bare block with no payload) crashed the interactive TUI with
+  `TypeError: Cannot read properties of undefined (reading 'length')` during smooth streaming reveal, and could also
+  crash transcript rendering for already-stored messages (`content.thinking.trim()`).
+- This was changed in core UI because assistant content rendering and streaming reveal are private built-in TUI
+  behavior; an extension cannot harden the renderer from outside, and the crash otherwise terminates the whole TUI
+  process (uncaught exception) instead of degrading gracefully.
+- `test/streaming-reveal-content.test.ts` covers malformed thinking/text blocks end-to-end; the regression fails
+  with the exact production `TypeError` when the fix is reverted.
+
+### Expected merge conflict zones
+
+- LOW: `streaming-reveal-content.ts` `countVisibleUnits`/`buildDisplayMessage` and
+  `components/assistant-render-descriptors.ts` payload reads.
+
 ## Server fallback abort uses one TUI notice (2026-08-05)
 
 ### What changed

packages/coding-agent/src/modes/interactive/components/assistant-render-descriptors.ts

@@ -28,9 +28,9 @@ function assertNever(value: never): never {
 function isVisibleContent(content: AssistantMessage["content"][number], providerNativeVisible: boolean): boolean {
 	switch (content.type) {
 		case "text":
-			return Boolean(content.text.trim());
+			return Boolean((content.text ?? "").trim());
 		case "thinking":
-			return Boolean(content.thinking.trim());
+			return Boolean((content.thinking ?? "").trim());
 		case "providerNative":
 			return providerNativeVisible;
 		case "toolCall":
@@ -50,7 +50,7 @@ export function createAssistantRenderDescriptors(
 		const content = message.content[i];
 		switch (content.type) {
 			case "text": {
-				const text = content.text.trim();
+				const text = (content.text ?? "").trim();
 				if (text) descriptors.push({ kind: "text-md", text });
 				break;
 			}
@@ -74,7 +74,7 @@ export function createAssistantRenderDescriptors(
 							maxEnd = Math.max(maxEnd, endedAt);
 						}
 					}
-					const thinking = thinkingContent.thinking.trim();
+					const thinking = (thinkingContent.thinking ?? "").trim();
 					if (thinking) thinkingBlocks.push(thinking);
 				}
 				i--;

packages/coding-agent/src/modes/interactive/streaming-reveal-content.ts

@@ -96,9 +96,9 @@ export function countVisibleUnits(message: AssistantMessage, hideThinking: boole
 	for (let index = 0; index < message.content.length; index++) {
 		const block = message.content[index];
 		if (block?.type === "text") {
-			total += countOf(index, block.text);
+			total += countOf(index, block.text ?? "");
 		} else if (block?.type === "thinking" && !hideThinking) {
-			total += countOf(index, block.thinking);
+			total += countOf(index, block.thinking ?? "");
 		}
 	}
 	return total;
@@ -121,27 +121,29 @@ export function buildDisplayMessage(
 		const block = target.content[index];
 		if (!block) continue;
 		if (block.type === "text") {
-			const units = countOf(index, block.text);
+			const text = block.text ?? "";
+			const units = countOf(index, text);
 			content.push(
 				remaining <= 0
-					? block.text.length === 0
+					? text.length === 0
 						? block
 						: { ...block, text: "" }
 					: remaining >= units
 						? block
-						: { ...block, text: sliceOf(index, block.text, remaining) },
+						: { ...block, text: sliceOf(index, text, remaining) },
 			);
 			remaining = Math.max(0, remaining - units);
 		} else if (block.type === "thinking" && !hideThinking) {
-			const units = countOf(index, block.thinking);
+			const thinking = block.thinking ?? "";
+			const units = countOf(index, thinking);
 			content.push(
 				remaining <= 0
-					? block.thinking.length === 0
+					? thinking.length === 0
 						? block
 						: { ...block, thinking: "" }
 					: remaining >= units
 						? block
-						: { ...block, thinking: sliceOf(index, block.thinking, remaining) },
+						: { ...block, thinking: sliceOf(index, thinking, remaining) },
 			);
 			remaining = Math.max(0, remaining - units);
 		} else {

packages/coding-agent/test/streaming-reveal-content.test.ts

@@ -1,6 +1,12 @@
+import type { AssistantMessage } from "@earendil-works/pi-ai";
 import { getGraphemeSegmenter } from "@earendil-works/pi-tui";
 import { describe, expect, test } from "vitest";
-import { BlockUnitCounter, buildDisplayMessage, visibleUnits } from "../src/modes/interactive/streaming-reveal.ts";
+import {
+	BlockUnitCounter,
+	buildDisplayMessage,
+	countVisibleUnits,
+	visibleUnits,
+} from "../src/modes/interactive/streaming-reveal.ts";
 import { makeMessage, textAt, thinkingAt } from "./helpers/streaming-reveal.ts";
 
 function fullSlice(text: string, units: number): string {
@@ -83,4 +89,20 @@ describe("streaming reveal content helpers", () => {
 		expect(block.startedAt).toBe(1_000);
 		expect(block.endedAt).toBe(4_200);
 	});
+
+	test("#given a block missing its payload key #when revealing #then treats it as empty instead of crashing", () => {
+		const malformedContent = [
+			// Real-world corrupt shape: a provider extension emitting `text` instead of `thinking`.
+			{ type: "thinking", text: "reasoning" },
+			// Bare block with no payload at all.
+			{ type: "thinking" },
+			{ type: "text" },
+		] as unknown as AssistantMessage["content"];
+		const target = makeMessage(malformedContent);
+
+		expect(() => countVisibleUnits(target, false, (_index, text) => text.length)).not.toThrow();
+		expect(() => buildDisplayMessage(target, 0, false)).not.toThrow();
+		expect(() => buildDisplayMessage(target, 5, false)).not.toThrow();
+		expect(visibleUnits(target, false)).toBe(0);
+	});
 });