Understand Diff Agent Skill

/understand-diff

Analyze the current code changes against the knowledge graph in the project's data directory (.ua/knowledge-graph.json, or the legacy .understand-anything/knowledge-graph.json when that directory is present).

Graph Structure Reference

The knowledge graph JSON has this structure:

• project — {name, description, languages, frameworks, analyzedAt, gitCommitHash}
• nodes[] — each has {id, type, name, filePath?, summary, tags[], complexity, languageNotes?}
• Code node types: file, function, class, module, concept
• Non-code node types: config, document, service, table, endpoint, pipeline, schema, resource
• Domain/knowledge node types: domain, flow, step, article, entity, topic, claim, source
• IDs use the node type as prefix, e.g. file:path, function:path:name, config:path, article:path
• edges[] — each has {source, target, type, direction, weight}
• Key types: imports, contains, calls, depends_on, configures, documents, deploys, triggers, contains_flow, flow_step, related, cites
• layers[] — each has {id, name, description, nodeIds[]}
• tour[] — each has {order, title, description, nodeIds[]}

How to Read Efficiently

• Use Grep to search within the JSON for relevant entries BEFORE reading the full file
• Only read sections you need — don't dump the entire graph into context
• Node names and summaries are the most useful fields for understanding
• Edges tell you how components connect — follow imports and calls for dependency chains

Instructions

• Resolve the data directory $UA_DIR. Run UA_DIR=$([ -d .understand-anything ] && echo .understand-anything || echo .ua) — this is the legacy .understand-anything/ when it already exists, otherwise the new .ua/. Check that $UA_DIR/knowledge-graph.json exists. If not, tell the user to run /understand first.

• Get the changed files list (do NOT read the graph yet):
• If on a branch with uncommitted changes: git diff --name-only
• If on a feature branch: git diff main...HEAD --name-only (or the base branch)
• If the user specifies a PR number: get the diff from that PR

• Read project metadata and check graph freshness — use Grep or Read with a line limit to extract the "project" section, including gitCommitHash as GRAPH_COMMIT_RAW, then:
• Resolve it as a commit before using it in any Git diff. From the project root, compare the resolved commit with git rev-parse HEAD and inspect project-scoped committed and working-tree changes:

     GRAPH_COMMIT=$(git rev-parse --verify --end-of-options "${GRAPH_COMMIT_RAW}^{commit}" 2>/dev/null)
     git rev-parse HEAD
     git diff --name-only "$GRAPH_COMMIT" HEAD -- .
     git diff --cached --name-only -- .
     git diff --name-only -- .
     git ls-files --others --exclude-standard -- .

• The -- . pathspec is required: commits that only touch a sibling monorepo project must not make this graph stale. A hash mismatch alone is not stale when the project diff is empty.
• Ignore the selected data directory (.ua/ or legacy .understand-anything/) in every command's output because it contains generated graph artifacts, not project source drift.
• If the committed diff or any working-tree command reports project files, warn before impact analysis that the graph may omit those changes. Suggest: Run /understand to refresh the graph.
• Run the commit diff only when GRAPH_COMMIT_RAW resolves successfully. If the graph commit or Git metadata is missing, invalid, or unavailable, give a brief best-effort warning and continue instead of blocking.

• Find nodes for changed files — for each changed file path, use Grep to search the knowledge graph for:
• Nodes with matching "filePath" values (e.g., grep "changed/file/path")
• This finds file-level nodes (including non-code types) AND function/class nodes defined in those files
• Note the id values of all matched nodes

• Find connected edges (1-hop) — for each matched node ID, Grep for that ID in the edges to find:
• What imports or depends on the changed nodes (upstream callers)
• What the changed nodes import or call (downstream dependencies)
• These are the "affected components" — things that might break or need updating

• Identify affected layers — Grep for the matched node IDs in the "layers" section to determine which architectural layers are touched.

• Provide structured analysis:
• Changed Components: What was directly modified (with summaries from matched nodes)
• Affected Components: What might be impacted (from 1-hop edges)
• Affected Layers: Which architectural layers are touched and cross-layer concerns
• Risk Assessment: Based on node complexity values, number of cross-layer edges, and blast radius (number of affected components)
• Suggest what to review carefully and any potential issues

• Write diff overlay for dashboard — after producing the analysis, write the diff data to $UA_DIR/diff-overlay.json so the dashboard can visualize changed and affected components. The file contains:

   {
     "version": "1.0.0",
     "baseBranch": "<the base branch used>",
     "generatedAt": "<ISO timestamp>",
     "changedFiles": ["<list of changed file paths>"],
     "changedNodeIds": ["<node IDs from step 4>"],
     "affectedNodeIds": ["<node IDs from step 5, excluding changedNodeIds>"]
   }

After writing, tell the user they can run /understand-anything:understand-dashboard to see the diff overlay visually.