beacon-diff

name	beacon-diff
description	Use when you need to analyze git diffs or pull requests to understand what changed, affected components, and risks

/beacon-diff

Analyze the current code changes against the knowledge graph at .beacon/knowledge-graph.json.

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

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

Instructions

1. Check that .beacon/knowledge-graph.json exists. If not, tell the user to run /beacon first.

2. 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

3. Read project metadata only — use Grep or Read with a line limit to extract just the "project" section for context.

4. 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

5. 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

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

7. 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

8. Write diff overlay for dashboard — after producing the analysis, write the diff data to .beacon/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 /beacon:beacon-dashboard to see the diff overlay visually.