fix(coding-agent): preserved markerless thinking titles

- Limited markerless fallback cleanup to leading leaked-thinking envelopes so literal reasoning syntax in plain titles survives.

- Added markerless regression coverage for think tags and thinking-fence titles.

Fixes #5122

packages/coding-agent/src/utils/title-generator.ts

@@ -35,6 +35,8 @@ const TITLE_MAX_TOKENS = 1024;
 /** Matches the title the model wraps in `<title>...</title>`. */
 const TITLE_MARKER_GLOBAL_RE = /<title>([\s\S]*?)<\/title>/gi;
 const TITLE_VISIBILITY_SENTINEL = "\uE000omp-title-visible\uE000";
+const LEADING_THINKING_TAG_RE = /^\s*<(think|thinking|reasoning)>\s*[\s\S]*?<\/\1>\s*/i;
+const LEADING_THINKING_FENCE_RE = /^\s*```(?:thinking|reasoning)\b[\s\S]*?```\s*/i;
 
 function getTitleModel(registry: ModelRegistry, settings: Settings, currentModel?: Model<Api>): Model<Api> | undefined {
 	const availableModels = registry.getAvailable();
@@ -244,12 +246,12 @@ function extractGeneratedTitle(contentBlocks: AssistantMessage["content"]): stri
 		}
 	}
 	// Stay lenient: prefer the first closed title marker in visible text, then
-	// fall back to a plain sentence after stripping leaked thinking plus any
-	// stray/unclosed title tag fragment (e.g. output truncated before closing).
+	// fall back to a plain sentence after stripping only known leading leaked
+	// thinking envelopes plus any stray/unclosed title tag fragment.
 	const markedTitle = extractVisibleMarkedTitle(textTitle);
 	const cleanedTextTitle =
 		markedTitle ??
-		stripLeakedThinkingMarkup(textTitle)
+		stripLeadingLeakedThinkingMarkup(textTitle)
 			.replace(/<\/?title>/gi, "")
 			.trim();
 	return unwrapJsonTitle(cleanedTextTitle);
@@ -272,6 +274,16 @@ function isVisibleTitleMarker(text: string, markerIndex: number): boolean {
 	);
 }
 
+function stripLeadingLeakedThinkingMarkup(text: string): string {
+	let current = text;
+	while (true) {
+		const withoutTag = current.replace(LEADING_THINKING_TAG_RE, "");
+		const withoutFence = withoutTag.replace(LEADING_THINKING_FENCE_RE, "");
+		if (withoutFence === current) return current;
+		current = withoutFence;
+	}
+}
+
 function stripLeakedThinkingMarkup(text: string): string {
 	const healer = new StreamMarkupHealing({ pattern: "thinking" });
 	return healer.feed(text) + healer.flushPending();

packages/coding-agent/test/title-generator.test.ts

@@ -377,6 +377,39 @@ describe("title generator", () => {
 		expect(title).toBe("Fix login button on mobile");
 	});
 
+	it("preserves a markerless title that mentions a <think> tag", async () => {
+		const model = getModelFor("deepseek", "deepseek-v4-pro");
+		vi.spyOn(ai, "completeSimple").mockResolvedValue({
+			stopReason: "stop",
+			content: [{ type: "text", text: "Fix <think> tag parsing" }],
+		} as never);
+
+		const title = await generateSessionTitle(
+			"fix title generation for <think> tag parsing",
+			createRegistry(model),
+			createSettings(model),
+		);
+
+		expect(title).toBe("Fix <think> tag parsing");
+	});
+
+	it("preserves a markerless title that mentions a ```thinking fence", async () => {
+		const model = getModelFor("deepseek", "deepseek-v4-pro");
+		vi.spyOn(ai, "completeSimple").mockResolvedValue({
+			stopReason: "stop",
+			content: [{ type: "text", text: "Fix ```thinking fence parsing" }],
+		} as never);
+
+		const title = await generateSessionTitle(
+			"fix title generation for a ```thinking fence",
+			createRegistry(model),
+			createSettings(model),
+		);
+
+		expect(title).toContain("```thinking");
+		expect(title).toContain("fence");
+	});
+
 	it("strips an unclosed <title> tag from a truncated response", async () => {
 		const model = getModelFor("deepseek", "deepseek-v4-pro");
 		vi.spyOn(ai, "completeSimple").mockResolvedValue({