Graph Evolution Agent Skill

Graph Evolution

Builds Trailmark code graphs at two source snapshots and computes a

structural diff. Surfaces security-relevant changes that text-level

diffs miss: new attack paths, complexity shifts, blast radius growth,

taint propagation changes, and privilege boundary modifications.

When to Use

• Comparing two git refs to understand what structurally changed
• Auditing a range of commits for security-relevant evolution
• Detecting new attack paths created by code changes
• Finding functions whose blast radius or complexity grew silently
• Identifying taint propagation changes across refactors
• Pre-release structural comparison (tag-to-tag or branch-to-branch)

When NOT to Use

• Line-level code review (use differential-review for text-diff analysis)
• Single-snapshot analysis (use the trailmark skill directly)
• Diagram generation from a single snapshot (use the diagramming-code skill)
• Mutation testing triage (use the genotoxic skill)

Rationalizations to Reject

| Rationalization | Why It's Wrong | Required Action |

|-----------------|----------------|-----------------|

| "We just need the structural diff, skip pre-analysis" | Without pre-analysis, you miss taint changes, blast radius growth, and privilege boundary shifts | Run engine.preanalysis() on both snapshots |

| "Text diff covers what changed" | Text diffs miss new attack paths, transitive complexity shifts, and subgraph membership changes | Use structural diff to complement text diff |

| "Only added nodes matter" | Removed security functions and shifted privilege boundaries are equally dangerous | Review removals and modifications, not just additions |

| "Low-severity structural changes can be ignored" | INFO-level changes (dead code removal) can mask removed security checks | Classify every change, review removals for replaced functionality |

| "One snapshot's graph is enough for comparison" | Single-snapshot analysis can't detect evolution — you need both before and after | Always build and export both graphs |

| "Tool isn't installed, I'll compare manually" | Manual comparison misses what graph analysis catches | Install trailmark first |

Prerequisites

trailmark must be installed. If uv run trailmark fails, run:

uv pip install trailmark

DO NOT fall back to "manual comparison" or reading source files as a

substitute for running trailmark. The tool must be installed and used

programmatically. If installation fails, report the error.

Quick Start

# Compare two git refs (e.g., tags, branches, commits)
# 1. Build graphs at each snapshot
# 2. Run pre-analysis on both
# 3. Compute structural diff
# 4. Generate report

# Step-by-step: see Workflow below

Decision Tree

├─ Need to understand what each metric means?
│  └─ Read: references/evolution-metrics.md
│
├─ Need the report output format?
│  └─ Read: references/report-format.md
│
├─ Already have two graph JSON exports?
│  └─ Jump to Phase 3 (run native diff + graph_diff.py)
│
└─ Starting from two git refs?
   └─ Start at Phase 1

Workflow

Graph Evolution Progress:
- [ ] Phase 1: Create snapshots (git worktrees)
- [ ] Phase 2: Build graphs + pre-analysis on both snapshots
- [ ] Phase 3: Compute structural diff
- [ ] Phase 4: Interpret diff and generate report
- [ ] Phase 5: Clean up worktrees

Phase 1: Create Snapshots

Use git worktrees to get clean copies of each ref without disturbing

the working tree.

# Create temp directories for worktrees
BEFORE_DIR=$(mktemp -d)
AFTER_DIR=$(mktemp -d)

# Create worktrees (run from repo root)
git worktree add "$BEFORE_DIR" {before_ref}
git worktree add "$AFTER_DIR" {after_ref}

If comparing two directories instead of git refs, skip this phase and

use the directory paths directly in Phase 2.

Phase 2: Build Graphs and Run Pre-Analysis

Build Trailmark graphs for both snapshots and run pre-analysis on each.

Pre-analysis computes blast radius, taint propagation, privilege

boundaries, and entrypoint enumeration.

from trailmark.query.api import QueryEngine

def build_and_export(target_dir, output_path, language="auto"):
    """Build graph, run pre-analysis, export JSON."""
    engine = QueryEngine.from_directory(target_dir, language=language)
    engine.preanalysis()
    json_str = engine.to_json()
    with open(output_path, "w") as f:
        f.write(json_str)
    return engine.summary()

import tempfile, os
work_dir = tempfile.mkdtemp(prefix="trailmark_evolution_")
before_json = os.path.join(work_dir, "before_graph.json")
after_json = os.path.join(work_dir, "after_graph.json")

before_summary = build_and_export(
    "{before_dir}", before_json
)
after_summary = build_and_export(
    "{after_dir}", after_json
)

Verify both graphs built successfully by checking the summary output.

If either fails, rerun with an explicit language or comma-separated list

instead of auto.

Phase 3: Compute Structural Diff

Run both:

• Trailmark's native structural diff for nodes, edges, and entrypoints
• The plugin's graph_diff.py helper for subgraph membership changes

Using the same work_dir from Phase 2:

trailmark diff --json "{before_dir}" "{after_dir}" > "{work_dir}/trailmark_diff.json" || \
  uv run trailmark diff --json "{before_dir}" "{after_dir}" > "{work_dir}/trailmark_diff.json"

uv run {baseDir}/scripts/graph_diff.py \
    --before "{before_json}" \
    --after "{after_json}" > "{work_dir}/subgraph_diff.json"

If either diff command fails or writes an empty JSON file, stop and report the

error instead of continuing to Phase 4.

The native Trailmark diff contains:

| Key | Contents |

|-----|----------|

| summary_delta | Changes in node/edge/entrypoint counts |

| nodes.added | New functions, classes, methods |

| nodes.removed | Deleted functions, classes, methods |

| nodes.modified | Functions with changed CC, params, line span |

| edges.added | New call/inheritance/import relationships |

| edges.removed | Deleted relationships |

| entrypoints | Added, removed, and modified entrypoints |

The subgraph diff contains:

| Key | Contents |

|-----|----------|

| subgraphs | Per-subgraph membership changes (tainted, high_blast_radius, etc.) |

Phase 4: Interpret Diff and Generate Report

Read both diff JSON files and generate a security-focused markdown

report.

See references/report-format.md for

the full template.

Interpretation priorities (highest to lowest):

• New tainted paths — nodes entering the tainted subgraph,

especially if they also appear in added edges targeting sensitive

functions

• Privilege boundary changes — new or removed trust transitions

from the native entrypoint/edge diff plus the subgraph diff

• Attack surface growth — new entrypoints, especially

untrusted_external, from trailmark_diff.json

• Blast radius increases — nodes entering high_blast_radius
• Complexity spikes — CC increases > 3 on tainted or

entrypoint-reachable nodes

• Structural additions — new nodes and edges (review needed)
• Structural removals — verify removed security functions were

replaced

Cross-reference structural changes with git diff {before_ref}..{after_ref}

to add source-level context to findings.

Severity classification:

| Severity | Structural Signal |

|----------|------------------|

| CRITICAL | New tainted path to sensitive function, removed auth boundary |

| HIGH | New entrypoint + high blast radius, large CC increase on tainted node |

| MEDIUM | New trust-boundary-crossing edges, moderate CC increase |

| LOW | Added nodes without entrypoint reachability |

| INFO | Dead code removal, complexity reductions |

For detailed metric definitions, see

references/evolution-metrics.md.

Phase 5: Clean Up

Remove git worktrees after the report is written:

git worktree remove "{before_dir}"
git worktree remove "{after_dir}"

Diff Reference

trailmark diff --json BEFORE AFTER
uv run {baseDir}/scripts/graph_diff.py [OPTIONS]

Use trailmark diff for:

• Node/edge changes
• Added/removed/modified entrypoints
• Human-readable structural diff reports

Use graph_diff.py for:

• Subgraph membership changes derived from engine.preanalysis()
• tainted, high_blast_radius, privilege_boundary, and related sets

| Argument | Default | Description |

|----------|---------|-------------|

| --before | required | Path to the "before" graph JSON |

| --after | required | Path to the "after" graph JSON |

| --indent | 2 | JSON output indentation |

graph_diff.py input format: Trailmark JSON exports from engine.to_json().

graph_diff.py output: JSON structural diff for nodes, edges, and subgraphs.

Quality Checklist

Before delivering the report:

• [ ] Both graphs built successfully (check summaries)
• [ ] Pre-analysis ran on both snapshots
• [ ] Native Trailmark diff computed and non-empty (trailmark_diff.json)
• [ ] Subgraph diff computed and non-empty (subgraph_diff.json)
• [ ] All subgraph changes interpreted (tainted, blast radius, etc.)
• [ ] Critical findings include evidence (node IDs, edge diffs)
• [ ] Severity levels assigned to all findings
• [ ] Source-level context added via git diff cross-reference
• [ ] Worktrees cleaned up (or temp dirs removed)
• [ ] Report written to GRAPH_EVOLUTION_*.md

Integration

trailmark skill:

Phase 2 uses the trailmark API for graph building and pre-analysis.

All trailmark query patterns work on either snapshot's engine.

differential-review skill:

Use graph-evolution for structural analysis, differential-review for

line-level code review. The two are complementary — graph-evolution

finds attack paths that text diffs miss, while differential-review

provides git blame context and micro-adversarial analysis.

trailmark-review-gate skill:

Use trailmark-review-gate after graph-evolution when a branch, pull request,

fix commit, or release diff needs a PASS/WARN/FAIL/UNKNOWN structural review

packet. The gate applies deterministic review rules to graph-evolution output;

it does not replace human review.

genotoxic skill:

If graph-evolution reveals new high-CC tainted nodes, feed them to

genotoxic for mutation testing triage.

diagramming-code skill:

Generate before/after diagrams to visualize structural changes.

Use call-graph or data-flow diagrams focused on changed nodes.

Supporting Documentation

• references/evolution-metrics.md —

What each structural metric means and why it matters for security

• references/report-format.md —

Report template, severity classification, and example findings