Evolve an Existing Agent

Improve, extend, or create an advanced variant of an agent that was originally authored with create-agent. This procedure covers the maintenance side of the agent lifecycle: assessing gaps against best practices, applying targeted improvements to the persona definition, bumping versions, and keeping the registry and cross-references in sync.

When to Use

• An agent's skills list is outdated after new skills were added to the library
• User feedback reveals missing capabilities, unclear purpose, or weak examples
• Tool requirements have changed (new MCP server, tool removed, privilege reduction needed)
• An agent's scope needs sharpening — it overlaps with another agent or is too broad
• An advanced variant is needed alongside the original (e.g., r-developer and r-developer-advanced)
• Related agents or teams were added and cross-references in See Also are stale

Inputs

• Required: Path to the existing agent file to evolve (e.g., agents/r-developer.md)
• Required: Evolution trigger (feedback, new skills, tool change, scope overlap, team integration, discovered limitations)
• Optional: Target version bump magnitude (patch, minor, major)
• Optional: Whether to create an advanced variant instead of refining in-place (default: refine in-place)

Procedure

Step 1: Assess the Current Agent

Read the existing agent file and evaluate each section against the quality checklist from guides/agent-best-practices.md:

Section	What to Check	Common Issues
Frontmatter	All required fields present (name, description, tools, model, version, author)	Missing tags, stale version, wrong priority
Purpose	Specific problem statement, not generic "helps with X"	Vague or overlapping with another agent
Capabilities	Concrete, verifiable capabilities with bold lead-ins	Generic ("handles development"), no grouping
Available Skills	Matches frontmatter skills list, all IDs exist in registry	Stale IDs, missing new skills, lists default skills unnecessarily
Usage Scenarios	2-3 realistic scenarios with invocation patterns	Placeholder text, unrealistic examples
Examples	Shows user request and agent behavior	Missing or trivial examples
Limitations	3-5 honest constraints	Too few, too vague, or missing entirely
See Also	Valid cross-references to agents, guides, teams	Stale links to renamed or removed files

# Read the agent file
cat agents/<agent-name>.md

# Check frontmatter parses
head -20 agents/<agent-name>.md

# Verify skills in frontmatter exist in registry
grep "skills:" -A 20 agents/<agent-name>.md

# Check if agent is referenced by any team
grep -r "<agent-name>" teams/*.md

Expected: A list of specific gaps, weaknesses, or improvement opportunities organized by section.

On failure: If the agent file does not exist or has no frontmatter, this skill does not apply — use create-agent instead to author it from scratch.

Step 2: Gather Evolution Requirements

Identify and categorize what triggered the evolution:

Trigger	Example	Typical Scope
User feedback	"Agent missed XSS in review"	Add skill or capability
New skills available	Library gained analyze-api-security	Update skills list
Tool change	New MCP server available	Add to tools/mcp_servers
Scope overlap	Two agents both claim "code review"	Sharpen purpose and limitations
Team integration	Agent added to a new team	Update See Also, verify capabilities
Model upgrade	Task requires deeper reasoning	Change model field
Privilege reduction	Agent has Bash but only reads files	Remove unnecessary tools

Document the specific changes needed before editing. List each change with its target section:

- Frontmatter: add `new-skill-id` to skills list
- Capabilities: add "API Security Analysis" capability
- Available Skills: add `new-skill-id` with description
- Limitations: remove outdated limitation about missing skill
- See Also: add link to new team that includes this agent

Expected: A concrete list of changes, each mapped to a specific section of the agent file.

On failure: If the changes are unclear, consult the user for clarification before proceeding. Vague evolution goals produce vague improvements.

Step 3: Choose Evolution Scope

Use this decision matrix to determine whether to refine in-place or create a variant:

Criteria	Refinement (in-place)	Advanced Variant (new agent)
Agent ID	Unchanged	New ID: <agent>-advanced or <agent>-<specialty>
File path	Same .md file	New file in agents/
Version bump	Patch or minor	Starts at 1.0.0
Model	May change	Often higher (e.g., sonnet → opus)
Registry	Update existing entry	New entry added
Original agent	Modified directly	Left intact, gains See Also cross-reference

Refinement: Choose when updating skills, fixing documentation, sharpening scope, or adjusting tools. The agent keeps its identity.

Variant: Choose when the evolved version would serve a substantially different audience, require a different model, or add capabilities that would make the original too broad. The original stays as-is for simpler use cases.

Expected: A clear decision — refinement or variant — with rationale.

On failure: If unsure, default to refinement. You can always extract a variant later; it is harder to merge one back.

Step 4: Apply Changes to the Agent File

For Refinements

Edit the existing agent file directly:

• Frontmatter: Update skills, tools, tags, model, priority, mcp_servers as needed
• Purpose/Capabilities: Revise to reflect new scope or added functionality
• Available Skills: Add new skills with descriptions, remove deprecated ones
• Usage Scenarios: Add or revise scenarios to demonstrate new capabilities
• Limitations: Remove constraints that no longer apply, add new honest ones
• See Also: Update cross-references to reflect current agent/team/guide landscape

Follow these editing rules:

• Preserve all existing sections — add content, do not remove sections
• Keep the Available Skills section in sync with the frontmatter skills list
• Do not add default skills (meditate, heal) to frontmatter unless they are core to the agent's methodology
• Verify each skill ID exists: grep "id: skill-name" skills/_registry.yml

For Variants

# Copy the original as a starting point
cp agents/<agent-name>.md agents/<agent-name>-advanced.md

# Edit the variant:
# - Change `name` to `<agent-name>-advanced`
# - Update `description` to reflect the advanced scope
# - Raise `model` if needed (e.g., sonnet → opus)
# - Reset `version` to "1.0.0"
# - Expand skills, capabilities, and examples for the advanced use case
# - Reference the original in See Also as a simpler alternative

Expected: The agent file (refined or new variant) passes the assessment checklist from Step 1.

On failure: If an edit breaks the document structure, use git diff to review changes and revert partial edits with git checkout -- <file>.

Step 5: Update Version and Metadata

Bump the version field in frontmatter following semantic versioning:

Change Type	Version Bump	Example
Typo fix, wording clarification	Patch: 1.0.0 → 1.0.1	Fixed unclear limitation
New skills added, capability expanded	Minor: 1.0.0 → 1.1.0	Added 3 new skills from library
Restructured purpose, changed model	Major: 1.0.0 → 2.0.0	Narrowed scope, upgraded to opus

Also update:

• updated date to the current date
• tags if the agent's domain coverage changed
• description if the purpose is materially different
• priority if the agent's importance relative to others changed

Expected: Frontmatter version and updated reflect the magnitude and date of changes. New variants start at "1.0.0".

On failure: If you forget to bump the version, the next evolution will have no way to distinguish the current state from the previous one. Always bump before committing.

Step 6: Update Registry and Cross-References

For Refinements

Update the existing entry in agents/_registry.yml to match the revised frontmatter:

# Find the agent's registry entry
grep -A 10 "id: <agent-name>" agents/_registry.yml

Update description, tags, tools, and skills fields to match the agent file. No count change is needed.

Update cross-references in other files if the agent's capabilities or name changed:

# Check if any team references this agent
grep -r "<agent-name>" teams/*.md

# Check if any guide references this agent
grep -r "<agent-name>" guides/*.md

For Variants

Add the new agent to agents/_registry.yml in alphabetical position:

  - id: <agent-name>-advanced
    path: agents/<agent-name>-advanced.md
    description: One-line description of the advanced variant
    tags: [domain, specialty, advanced]
    priority: normal
    tools: [Read, Write, Edit, Bash, Grep, Glob]
    skills:
      - skill-id-one
      - skill-id-two

Then:

1. Increment total_agents at the top of the registry
2. Add See Also cross-reference in the original agent pointing to the variant
3. Add See Also cross-reference in the variant pointing to the original
4. The .claude/agents/ symlink to agents/ means the variant is automatically discoverable

Expected: Registry entry matches the agent file frontmatter. For variants, total_agents equals the actual number of agent entries.

On failure: Count entries with grep -c "^ - id:" agents/_registry.yml and verify it matches total_agents.

Step 7: Validate the Evolved Agent

Run the full validation checklist:

• Agent file exists at the expected path
• YAML frontmatter parses without errors
• version was bumped (refinement) or set to "1.0.0" (variant)
• updated date reflects today
• All required sections present: Purpose, Capabilities, Available Skills, Usage Scenarios, Examples, Limitations, See Also
• Skills in frontmatter match the Available Skills section
• All skill IDs exist in skills/_registry.yml
• Default skills (meditate, heal) are not listed unless core to methodology
• Tools list follows least-privilege principle
• Registry entry exists and matches frontmatter
• For variants: total_agents count matches actual count on disk
• Cross-references are bidirectional (original ↔ variant)
• git diff shows no accidental deletions from the original content

# Verify frontmatter
head -20 agents/<agent-name>.md

# Check skills exist
for skill in skill-a skill-b; do
  grep "id: $skill" skills/_registry.yml
done

# Count agents on disk vs registry
ls agents/*.md | grep -v template | wc -l
grep total_agents agents/_registry.yml

# Review all changes
git diff

Expected: All checklist items pass. The evolved agent is ready to commit.

On failure: Address each failing item individually. The most common post-evolution issues are stale skill IDs in the Available Skills section and a forgotten updated date.

Validation

• Agent file exists and has valid YAML frontmatter
• version field reflects the changes made
• updated date is current
• All sections present and internally consistent
• Frontmatter skills array matches the Available Skills section
• All skill IDs exist in skills/_registry.yml
• Default skills not listed unnecessarily
• Registry entry matches the agent file
• For variants: new entry in agents/_registry.yml with correct path
• For variants: total_agents count updated
• Cross-references are valid (no broken links in See Also)
• git diff confirms no accidental content removal

Common Pitfalls

• Forgetting to bump version: Without version bumps, there is no way to track what changed or when. Always update version and updated in frontmatter before committing.
• Skills list drift: The frontmatter skills array and the ## Available Skills section must stay in sync. Updating one without the other creates confusion for both humans and tooling.
• Listing default skills unnecessarily: Adding meditate or heal to the frontmatter when they are already inherited from the registry. Only list them if they are core to the agent's methodology (e.g., mystic, alchemist).
• Tool over-provisioning during evolution: Adding Bash or WebFetch during an evolution "just in case." Every tool addition should be justified by a specific new capability.
• Stale See Also after variant creation: When creating a variant, both the original and the variant need to reference each other. One-directional references leave the graph incomplete.
• Registry entry not updated: After changing an agent's skills, tools, or description, the agents/_registry.yml entry must be updated to match. Stale registry entries cause discovery and tooling failures.

Related Skills

• create-agent — foundation for authoring new agents; evolve-agent assumes this was followed originally
• evolve-skill — the parallel procedure for evolving SKILL.md files
• commit-changes — commit the evolved agent with a descriptive message