diff --git a/packages/coding-agent/src/core/bash-executor.ts b/packages/coding-agent/src/core/bash-executor.ts
index 679543ec4..9745ff217 100644
--- a/packages/coding-agent/src/core/bash-executor.ts
+++ b/packages/coding-agent/src/core/bash-executor.ts
@@ -53,6 +53,7 @@ export async function executeBashWithOperations(
 ): Promise<BashResult> {
 	const outputChunks: string[] = [];
 	let outputBytes = 0;
+	let discardedOutput = false;
 	const maxOutputBytes = DEFAULT_MAX_BYTES * 2;
 
 	let tempFilePath: string | undefined;
@@ -95,6 +96,7 @@ export async function executeBashWithOperations(
 		while (outputBytes > maxOutputBytes && outputChunks.length > 1) {
 			const removed = outputChunks.shift()!;
 			outputBytes -= removed.length;
+			discardedOutput = true;
 		}
 
 		// Stream to callback
@@ -140,7 +142,7 @@ export async function executeBashWithOperations(
 		output: truncationResult.truncated ? truncationResult.content : fullOutput,
 		exitCode: cancelled ? undefined : (exitCode ?? undefined),
 		cancelled,
-		truncated: truncationResult.truncated,
+		truncated: discardedOutput || truncationResult.truncated,
 		fullOutputPath: tempFilePath,
 	};
 }
diff --git a/packages/coding-agent/test/bash-executor-truncation.test.ts b/packages/coding-agent/test/bash-executor-truncation.test.ts
new file mode 100644
index 000000000..7c480fe46
--- /dev/null
+++ b/packages/coding-agent/test/bash-executor-truncation.test.ts
@@ -0,0 +1,59 @@
+import { readFile, rm } from "node:fs/promises";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { executeBashWithOperations } from "../src/core/bash-executor.ts";
+import { bashExecutionToText } from "../src/core/messages.ts";
+
+describe("user bash output truncation (#10164)", () => {
+	const tempFiles: string[] = [];
+
+	afterEach(async () => {
+		await Promise.all(tempFiles.splice(0).map((path) => rm(path, { force: true })));
+	});
+
+	// #10164: the retained tail can fit even though earlier output was discarded.
+	it.each([false, true])("reports discarded chunks when cancelled=%s", async (cancelled) => {
+		const first = "error: missing build dependency\n".padEnd(64 * 1024, "x");
+		const last = "remaining build output\n".padEnd(44 * 1024, "y");
+		const controller = new AbortController();
+		const result = await executeBashWithOperations(
+			"cat build.log",
+			process.cwd(),
+			{
+				exec: async (_command, _cwd, { onData }) => {
+					onData(Buffer.from(first));
+					onData(Buffer.from(last));
+					if (cancelled) {
+						controller.abort();
+						throw new Error("aborted");
+					}
+					return { exitCode: 0 };
+				},
+			},
+			{ signal: controller.signal },
+		);
+		if (result.fullOutputPath) tempFiles.push(result.fullOutputPath);
+		// The executor closes its file stream without awaiting its finish event.
+		await vi.waitFor(async () => {
+			expect(await readFile(result.fullOutputPath!, "utf-8")).toBe(first + last);
+		});
+
+		expect(result.cancelled).toBe(cancelled);
+		expect(result.output).toBe(last);
+		expect(result.truncated).toBe(true);
+		expect(
+			bashExecutionToText({ role: "bashExecution", command: "cat build.log", timestamp: 0, ...result }),
+		).toContain(`[Output truncated. Full output: ${result.fullOutputPath}]`);
+	});
+
+	it("does not mark short output as truncated", async () => {
+		const result = await executeBashWithOperations("echo done", process.cwd(), {
+			exec: async (_command, _cwd, { onData }) => {
+				onData(Buffer.from("done\n"));
+				return { exitCode: 0 };
+			},
+		});
+		expect(result.output).toBe("done\n");
+		expect(result.truncated).toBe(false);
+		expect(result.fullOutputPath).toBeUndefined();
+	});
+});