feat(coding-agent): added jsonStringify helper and improved frontmatter error messages

- Added jsonStringify Handlebars helper to enable safe JSON serialization of template variables in frontmatter generation.
- Improved frontmatter parsing error messages to include source context, making debugging easier when YAML frontmatter parsing fails.

packages/coding-agent/CHANGELOG.md

@@ -1,7 +1,6 @@
 # Changelog
 
 ## [Unreleased]
-
 ### Added
 
 - Added `task.maxConcurrency` setting to control concurrent limit for subagents (default: 32)
@@ -17,6 +16,10 @@
 
 - Removed `MAX_PARALLEL_TASKS` constant and associated task count validation limit
 
+### Fixed
+
+- Improved frontmatter parsing error messages to include source context for better debugging
+
 ## [10.6.1] - 2026-02-04
 
 ### Added

packages/coding-agent/src/config/prompt-templates.ts

@@ -226,6 +226,8 @@ handlebars.registerHelper("not", (value: unknown): boolean => !value);
 
 handlebars.registerHelper("jtdToTypeScript", (schema: unknown): string => jtdToTypeScript(schema));
 
+handlebars.registerHelper("jsonStringify", (value: unknown): string => JSON.stringify(value));
+
 export function renderPromptTemplate(template: string, context: TemplateContext = {}): string {
 	const compiled = handlebars.compile(template, { noEscape: true, strict: false });
 	const rendered = compiled(context ?? {});

packages/coding-agent/src/prompts/agents/frontmatter.md

@@ -1,8 +1,8 @@
 ---
-name: {{name}}
-description: {{description}}
-{{#if spawns}}spawns: {{spawns}}
-{{/if}}{{#if model}}model: {{model}}
-{{/if}}{{#if thinkingLevel}}thinkingLevel: {{thinkingLevel}}
+name: {{jsonStringify name}}
+description: {{jsonStringify description}}
+{{#if spawns}}spawns: {{jsonStringify spawns}}
+{{/if}}{{#if model}}model: {{jsonStringify model}}
+{{/if}}{{#if thinkingLevel}}thinkingLevel: {{jsonStringify thinkingLevel}}
 {{/if}}---
 {{body}}

packages/coding-agent/src/utils/frontmatter.ts

@@ -18,7 +18,7 @@ export class FrontmatterError extends Error {
 		error: Error,
 		public readonly source?: unknown,
 	) {
-		super(`Failed to parse YAML frontmatter: ${error.message}`, { cause: error });
+		super(`Failed to parse YAML frontmatter (${source}): ${error.message}`, { cause: error });
 		this.name = "FrontmatterError";
 	}