diff --git a/packages/chord/src/output.ts b/packages/chord/src/output.ts
index a31cdfd2a..e0e36a183 100644
--- a/packages/chord/src/output.ts
+++ b/packages/chord/src/output.ts
@@ -32,13 +32,16 @@ export interface OutputSkip {
 
 const NEWLINE = 0x0a;
 const INVALID_OUTPUT = /[\x00-\x08\x0b-\x1f\ufff9-\ufffb]/g;
+/** Whole 7-bit escape sequences: strings (OSC, DCS, APC, PM, SOS) up to BEL or ST, CSI, and short `ESC x` forms. */
+const ESCAPE_SEQUENCE =
+	/\x1b[\]PX^_][^\x07\x1b]*(?:\x07|\x1b\\)|\x1b\[[\x30-\x3f]*[\x20-\x2f]*[\x40-\x7e]|\x1b[\x20-\x2f]*[\x30-\x7e]/g;
 const encoder = new TextEncoder();
 /** Slices decode exactly: a U+FEFF at a slice's start is text, not a byte-order mark. */
 const decoder = new TextDecoder("utf-8", { ignoreBOM: true });
 
-/** Remove control characters that break display and transcripts; tabs and newlines stay. */
+/** Remove escape sequences and control characters that break display and transcripts; tabs and newlines stay. */
 export function sanitizeOutput(text: string): string {
-	return text.replace(INVALID_OUTPUT, "");
+	return text.replace(ESCAPE_SEQUENCE, "").replace(INVALID_OUTPUT, "");
 }
 
 /**
diff --git a/packages/chord/test/output.test.ts b/packages/chord/test/output.test.ts
index 00e7d647e..b35a62bf6 100644
--- a/packages/chord/test/output.test.ts
+++ b/packages/chord/test/output.test.ts
@@ -57,6 +57,18 @@ describe("tool output bounds", () => {
 		expect(sanitizeOutput("a\0b\tc\nd\re\u0007f\ufff9g\ufffbh😀")).toBe("ab\tc\ndefgh😀");
 	});
 
+	// #11604
+	it("removes whole ANSI escape sequences, not only their ESC bytes", () => {
+		expect(sanitizeOutput("\x1b[31mERROR\x1b[0m\n")).toBe("ERROR\n");
+		expect(sanitizeOutput("\x1b[1;38;5;208mx\x1b[?25l\x1b[2K\x1b(By\x1b7z")).toBe("xyz");
+		expect(sanitizeOutput("\x1b]8;;https://pi.dev\x07link\x1b]8;;\x1b\\ \x1b]0;title\x07ok")).toBe("link ok");
+		const output = new OutputBuffer({ retain: "tail", maxBytes: 1024, maxLines: 100 });
+		output.push("\x1b[3");
+		output.push("1mERROR\x1b[0");
+		output.push("m\n");
+		expect(output.snapshot().text).toBe("ERROR\n");
+	});
+
 	it("keeps output within the limits unchanged", () => {
 		expect(bound("a\nb\n", head(2))).toEqual({ kept: "a\nb\n", droppedBytes: 0, droppedLines: 0 });
 		expect(bound("a\nb", tail(2))).toEqual({ kept: "a\nb", droppedBytes: 0, droppedLines: 0 });