diff --git a/packages/coding-agent/test/transcript-streaming-commit-repro.test.ts b/packages/coding-agent/test/transcript-streaming-commit-repro.test.ts
index 2fd6f8b46..d4896913b 100644
--- a/packages/coding-agent/test/transcript-streaming-commit-repro.test.ts
+++ b/packages/coding-agent/test/transcript-streaming-commit-repro.test.ts
@@ -219,4 +219,19 @@ describe("transcript streaming commit (assistant text)", () => {
 		expect(codeRow).toBeGreaterThanOrEqual(0);
 		expect(rows.slice(codeRow, codeRow + 3)).toEqual(["  +a", "", "  +streaming"]);
 	});
+
+	it("does not duplicate a leading blank row when the streamed diff grows", () => {
+		const block = new StreamingMarkdownBlock();
+
+		block.setStreamingText("```diff\n\n+partial");
+		block.render(40);
+
+		block.setStreamingText("```diff\n\n+done\n-streaming");
+
+		const rows = block.render(40).map(row => Bun.stripANSI(row).trimEnd());
+		const fenceRow = rows.indexOf("```diff");
+
+		expect(fenceRow).toBeGreaterThanOrEqual(0);
+		expect(rows.slice(fenceRow, fenceRow + 5)).toEqual(["```diff", "", "  +done", "  -streaming", "```"]);
+	});
 });
diff --git a/packages/tui/src/components/markdown.ts b/packages/tui/src/components/markdown.ts
index 9d87eac21..67789a496 100644
--- a/packages/tui/src/components/markdown.ts
+++ b/packages/tui/src/components/markdown.ts
@@ -1499,7 +1499,7 @@ export class Markdown implements Component {
 		) {
 			if (completedText.length === cache.text.length) return cache.lines;
 			const lines = cache.lines.slice();
-			const addedText = completedText.slice(cache.text.length === 0 ? 0 : cache.text.length + 1);
+			const addedText = completedText.slice(cache.text.length + 1);
 			for (const codeLine of addedText.split("\n")) {
 				lines.push(...highlightCode(codeLine, lang));
 			}