diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index a029fa70d..1baee1786 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -26,6 +26,7 @@
 - Fixed a symlinked `AGENTS.md` in a git worktree nested inside its main repo causing no project context file to load ([#10681](https://github.com/earendil-works/pi/issues/10681))
 - Fixed the code mode tool description claiming every failed tool call rejects; a tool that declares an output schema resolves with its value on failure too (e.g. `bash` with a nonzero exit)
 - Fixed runs started by `pi.sendMessage(..., { triggerTurn: true })` removing system prompt sections added in `before_agent_start` after their first tool call, which changed the prompt mid-run and broke the provider prompt cache twice per run ([#10267](https://github.com/earendil-works/pi/issues/10267))
+- Fixed codemode blocking the process for seconds to minutes after scripts with many `text()` calls; joining the output items was quadratic ([#10678](https://github.com/earendil-works/pi/issues/10678))
 
 ## [1.1.0] - 2026-10-07
 
diff --git a/packages/coding-agent/src/extensions/codemode/execute.ts b/packages/coding-agent/src/extensions/codemode/execute.ts
index 468b4638f..68f896a3b 100644
--- a/packages/coding-agent/src/extensions/codemode/execute.ts
+++ b/packages/coding-agent/src/extensions/codemode/execute.ts
@@ -285,18 +285,34 @@ function formatOutput(output: readonly CodemodeOutputItem[]): (TextContent | Ima
 	return items;
 }
 
-/** Join adjacent text items into one, each part starting on its own line. */
+/**
+ * Join adjacent text items into one, each part starting on its own line. The separator check looks
+ * at the last non-empty part, not the joined text: reading the end of a growing concatenation makes
+ * V8 copy it on every item, which is quadratic (#10678).
+ */
 function joinAdjacentText(items: (TextContent | ImageContent)[]): (TextContent | ImageContent)[] {
 	const joined: (TextContent | ImageContent)[] = [];
+	let parts: string[] = [];
+	let lastPart = "";
+	const flush = () => {
+		if (parts.length > 0) joined.push({ type: "text", text: parts.join("") });
+		parts = [];
+		lastPart = "";
+	};
 	for (const item of items) {
-		const last = joined.at(-1);
-		if (item.type === "text" && last?.type === "text") {
-			const separator = last.text === "" || last.text.endsWith("\n") ? "" : "\n";
-			joined[joined.length - 1] = { type: "text", text: `${last.text}${separator}${item.text}` };
-		} else {
+		if (item.type !== "text") {
+			flush();
 			joined.push(item);
+			continue;
 		}
+		if (lastPart !== "" && !lastPart.endsWith("\n")) {
+			parts.push("\n");
+			lastPart = "\n";
+		}
+		parts.push(item.text);
+		if (item.text !== "") lastPart = item.text;
 	}
+	flush();
 	return joined;
 }
 
diff --git a/packages/coding-agent/test/suite/agent-session-codemode.test.ts b/packages/coding-agent/test/suite/agent-session-codemode.test.ts
index 4869645a3..4b6b53c99 100644
--- a/packages/coding-agent/test/suite/agent-session-codemode.test.ts
+++ b/packages/coding-agent/test/suite/agent-session-codemode.test.ts
@@ -541,6 +541,18 @@ describe("codemode options and store", () => {
 		expect((small.details as unknown as CodemodeToolDetails).fullOutputPath).toBeUndefined();
 	});
 
+	// #10678: joining the text items was quadratic, about 10 s here for the item limit.
+	it("formats output at the item limit in linear time", async () => {
+		const harness = await setup();
+		const startedAt = performance.now();
+		const result = await run(harness, 'for (let i = 0; i < 100000; i++) text("");');
+		const elapsed = performance.now() - startedAt;
+		rmSync((result.details as unknown as CodemodeToolDetails).fullOutputPath ?? "", { force: true });
+		expect(result.isError).toBe(false);
+		expect(resultText(result)).toContain("==> text 100000/100000 <==");
+		expect(elapsed).toBeLessThan(5000);
+	});
+
 	it("resolves bash calls to structured results, also for non-zero exit codes", async () => {
 		const harness = await createHarness({
 			initialActiveToolNames: ["codemode", "bash"],
diff --git a/packages/durable/src/codemode/output.ts b/packages/durable/src/codemode/output.ts
index 5b364378f..98a086288 100644
--- a/packages/durable/src/codemode/output.ts
+++ b/packages/durable/src/codemode/output.ts
@@ -60,18 +60,34 @@ function formatItems(output: readonly CodemodeOutputItem[]): Item[] {
 	return items;
 }
 
-/** Join adjacent text items into one, each part starting on its own line. */
+/**
+ * Join adjacent text items into one, each part starting on its own line. The separator check looks at the last
+ * non-empty part, not the joined text: reading the end of a growing concatenation makes V8 copy it on every item, which
+ * is quadratic.
+ */
 function joinAdjacentText(items: readonly Item[]): Item[] {
 	const joined: Item[] = [];
+	let parts: string[] = [];
+	let lastPart = "";
+	const flush = () => {
+		if (parts.length > 0) joined.push({ type: "text", text: parts.join("") });
+		parts = [];
+		lastPart = "";
+	};
 	for (const item of items) {
-		const last = joined.at(-1);
-		if (item.type === "text" && last?.type === "text") {
-			const separator = last.text === "" || last.text.endsWith("\n") ? "" : "\n";
-			joined[joined.length - 1] = { type: "text", text: `${last.text}${separator}${item.text}` };
-		} else {
+		if (item.type !== "text") {
+			flush();
 			joined.push(item);
+			continue;
 		}
+		if (lastPart !== "" && !lastPart.endsWith("\n")) {
+			parts.push("\n");
+			lastPart = "\n";
+		}
+		parts.push(item.text);
+		if (item.text !== "") lastPart = item.text;
 	}
+	flush();
 	return joined;
 }