fix(coding-agent): correct bash truncation line count

closes #4818

packages/coding-agent/CHANGELOG.md

@@ -9,6 +9,7 @@
 ### Fixed
 
 - Fixed final bash tool cards to avoid rendering duplicate full-output truncation paths ([#4819](https://github.com/earendil-works/pi/issues/4819)).
+- Fixed bash tool truncation line counts to ignore the trailing newline as an extra output line ([#4818](https://github.com/earendil-works/pi/issues/4818)).
 
 ## [0.75.4] - 2026-05-20
 

packages/coding-agent/src/core/tools/output-accumulator.ts

@@ -45,8 +45,10 @@ export class OutputAccumulator {
 	private tailStartsAtLineBoundary = true;
 	private totalRawBytes = 0;
 	private totalDecodedBytes = 0;
-	private totalLines = 1;
+	private completedLines = 0;
+	private totalLines = 0;
 	private currentLineBytes = 0;
+	private hasOpenLine = false;
 	private finished = false;
 
 	private tempFilePath: string | undefined;
@@ -164,10 +166,14 @@ export class OutputAccumulator {
 		}
 		if (newlines === 0) {
 			this.currentLineBytes += bytes;
+			this.hasOpenLine = true;
 		} else {
-			this.totalLines += newlines;
-			this.currentLineBytes = byteLength(text.slice(lastNewline + 1));
+			this.completedLines += newlines;
+			const tail = text.slice(lastNewline + 1);
+			this.currentLineBytes = byteLength(tail);
+			this.hasOpenLine = tail.length > 0;
 		}
+		this.totalLines = this.completedLines + (this.hasOpenLine ? 1 : 0);
 	}
 
 	private trimTail(): void {

packages/coding-agent/src/core/tools/truncate.ts

@@ -44,6 +44,17 @@ export interface TruncationOptions {
 	maxBytes?: number;
 }
 
+function splitLinesForCounting(content: string): string[] {
+	if (content.length === 0) {
+		return [];
+	}
+	const lines = content.split("\n");
+	if (content.endsWith("\n")) {
+		lines.pop();
+	}
+	return lines;
+}
+
 /**
  * Format bytes as human-readable size.
  */
@@ -69,7 +80,7 @@ export function truncateHead(content: string, options: TruncationOptions = {}):
 	const maxBytes = options.maxBytes ?? DEFAULT_MAX_BYTES;
 
 	const totalBytes = Buffer.byteLength(content, "utf-8");
-	const lines = content.split("\n");
+	const lines = splitLinesForCounting(content);
 	const totalLines = lines.length;
 
 	// Check if no truncation needed
@@ -159,7 +170,7 @@ export function truncateTail(content: string, options: TruncationOptions = {}):
 	const maxBytes = options.maxBytes ?? DEFAULT_MAX_BYTES;
 
 	const totalBytes = Buffer.byteLength(content, "utf-8");
-	const lines = content.split("\n");
+	const lines = splitLinesForCounting(content);
 	const totalLines = lines.length;
 
 	// Check if no truncation needed

packages/coding-agent/test/tools.test.ts

@@ -580,6 +580,28 @@ describe("Coding Agent Tools", () => {
 			expect(getTextOutput(result)).toContain("line 4999");
 		});
 
+		it("should not count a trailing newline as an extra truncated bash output line", async () => {
+			const operations: BashOperations = {
+				exec: async (_command, _cwd, { onData }) => {
+					for (let i = 1; i <= 4000; i++) {
+						onData(Buffer.from(`line-${String(i).padStart(4, "0")}\n`, "utf-8"));
+					}
+					return { exitCode: 0 };
+				},
+			};
+			const bash = createBashTool(testDir, { operations });
+
+			const result = await bash.execute("test-call-trailing-newline-line-count", { command: "many-lines" });
+			const output = getTextOutput(result);
+
+			expect(result.details?.truncation?.totalLines).toBe(4000);
+			expect(result.details?.truncation?.outputLines).toBe(2000);
+			expect(output).toContain("line-2001");
+			expect(output).toContain("line-4000");
+			expect(output).toMatch(/\[Showing lines 2001-4000 of 4000\. Full output: /);
+			expect(output).not.toContain("4001");
+		});
+
 		it("should decode UTF-8 characters split across output chunks", async () => {
 			const euro = Buffer.from("€\n", "utf-8");
 			const operations: BashOperations = {