Agent definitions with colons in description frontmatter fail to register

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Environment:

• Claude Code version: 2.1.1
• OS: macOS (Darwin 25.2.0)

Description:

Agent definition files in .claude/agents/ fail to be recognized by Claude Code when the description field in the YAML frontmatter contains unquoted colons (:).

Expected Behavior:

The agent should be recognized and available for use. YAML string values containing colons should be parsed correctly, as colons within a value (not at key position) are valid YAML.

Actual Behavior:

The agent is not recognized by Claude Code. The YAML parser appears to misinterpret colons within the description value.

Workaround:

Enclosing the description value in quotes resolves the issue:

---
name: test-agent
description: "Use this agent when you need help. This includes: task A, task B, and task C."
model: sonnet
---

What Should Happen?

Agent should register as it did prior to v. 2.1.1.

Error Messages/Logs

Attempts to invoke the agent are met with "Agent type '[agent-name]' not found"

Agent does not appear in `/agents` list.

Steps to Reproduce

1. Create an agent file at .claude/agents/test-agent.md with the following content:

---
name: test-agent
description: Use this agent when you need help. This includes: task A, task B, and task C.
model: sonnet
---

Agent instructions here.

1. Run /agents or attempt to invoke the agent
2. The agent does not appear in the list / cannot be invoked

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.0.76

Claude Code Version

2.1.1 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

Additional Context:

• This appears to be a regression - agent definitions with colons in descriptions worked in previous versions
• Multiple existing agents that previously functioned correctly stopped being recognized after upgrading to 2.1.1
• The issue specifically affects colons (:); other special characters may or may not be affected

Suggested Fix:

The YAML frontmatter parser should either:

1. Handle unquoted string values containing colons correctly (per YAML spec, colons are only special when followed by a space at key position)
2. Or provide a clear error message indicating the description needs to be quoted