diff --git a/packages/utils/test/mermaid/class-arrows.test.ts b/packages/utils/test/mermaid/class-arrows.test.ts
index b979b6e5b..a8d64acac 100644
--- a/packages/utils/test/mermaid/class-arrows.test.ts
+++ b/packages/utils/test/mermaid/class-arrows.test.ts
@@ -8,7 +8,13 @@
  */
 
 import { describe, expect, test } from "bun:test";
-import { renderMermaidAscii } from "../../src/vendor/mermaid-ascii/ascii/index";
+import { type AsciiRenderOptions, renderMermaidAscii as renderRaw } from "../../src/vendor/mermaid-ascii/ascii/index";
+
+// Pin colorMode so assertions stay deterministic even if a concurrent or leaked
+// TTY override makes the renderer auto-detect ANSI color.
+function renderMermaidAscii(text: string, opts: AsciiRenderOptions = {}): string {
+	return renderRaw(text, { colorMode: "none", ...opts });
+}
 
 describe("Class Diagram Arrow Directions", () => {
 	// ============================================================================
diff --git a/packages/utils/test/mermaid/golden.test.ts b/packages/utils/test/mermaid/golden.test.ts
index 5729217a1..a0690caeb 100644
--- a/packages/utils/test/mermaid/golden.test.ts
+++ b/packages/utils/test/mermaid/golden.test.ts
@@ -136,6 +136,7 @@ function runGoldenTests(dir: string, useAscii: boolean): void {
 				useAscii,
 				paddingX: tc.paddingX,
 				paddingY: tc.paddingY,
+				colorMode: "none",
 			});
 
 			const normalizedExpected = normalizeWhitespace(tc.expected);
@@ -172,20 +173,20 @@ describe("Config behavior", () => {
 	const mermaidInput = "graph LR\nA --> B";
 
 	it("ASCII and Unicode outputs should differ", () => {
-		const asciiOutput = renderMermaidAscii(mermaidInput, { useAscii: true });
-		const unicodeOutput = renderMermaidAscii(mermaidInput, { useAscii: false });
+		const asciiOutput = renderMermaidAscii(mermaidInput, { useAscii: true, colorMode: "none" });
+		const unicodeOutput = renderMermaidAscii(mermaidInput, { useAscii: false, colorMode: "none" });
 		expect(asciiOutput).not.toBe(unicodeOutput);
 	});
 
 	it("ASCII output should not contain Unicode box-drawing characters", () => {
-		const output = renderMermaidAscii(mermaidInput, { useAscii: true });
+		const output = renderMermaidAscii(mermaidInput, { useAscii: true, colorMode: "none" });
 		expect(output).not.toContain("┌");
 		expect(output).not.toContain("─");
 		expect(output).not.toContain("│");
 	});
 
 	it("Unicode output should contain Unicode box-drawing characters", () => {
-		const output = renderMermaidAscii(mermaidInput, { useAscii: false });
+		const output = renderMermaidAscii(mermaidInput, { useAscii: false, colorMode: "none" });
 		const hasUnicode = output.includes("┌") || output.includes("─") || output.includes("│");
 		expect(hasUnicode).toBe(true);
 	});
@@ -209,6 +210,7 @@ describe("Diagonal validation", () => {
 				useAscii: true,
 				boxBorderPadding: paddingX,
 				paddingY: paddingY,
+				colorMode: "none",
 			});
 
 			// Check for diagonal characters
@@ -228,6 +230,7 @@ describe("Diagonal validation", () => {
 				useAscii: false,
 				boxBorderPadding: paddingX,
 				paddingY: paddingY,
+				colorMode: "none",
 			});
 
 			// Check for diagonal characters
diff --git a/packages/utils/test/mermaid/multiline.test.ts b/packages/utils/test/mermaid/multiline.test.ts
index 3c26acf1a..1b056f00d 100644
--- a/packages/utils/test/mermaid/multiline.test.ts
+++ b/packages/utils/test/mermaid/multiline.test.ts
@@ -1,5 +1,11 @@
 import { describe, expect, it } from "bun:test";
-import { renderMermaidAscii } from "../../src/vendor/mermaid-ascii/ascii/index";
+import { type AsciiRenderOptions, renderMermaidAscii as renderRaw } from "../../src/vendor/mermaid-ascii/ascii/index";
+
+// Pin colorMode so assertions stay deterministic even if a concurrent or leaked
+// TTY override makes the renderer auto-detect ANSI color.
+function renderMermaidAscii(text: string, opts: AsciiRenderOptions = {}): string {
+	return renderRaw(text, { colorMode: "none", ...opts });
+}
 
 describe("ASCII multi-line labels", () => {
 	describe("flowchart nodes", () => {