wottpal sem-diff
Purpose
Use sem when you need to understand code changes semantically: functions, classes, methods, types, and structured sections instead of raw line hunks.
sem is an entity-level layer on top of Git. It uses tree-sitter parsing and AST-normalized structural hashes to distinguish cosmetic edits from structural changes.
When to use
- Reviewing a diff and needing a concise "what changed" summary.
- Explaining staged, working tree, commit, or range changes to a user.
- Finding who last touched an entity.
- Estimating dependency impact from a changed function/class/method.
- Building AI context around an entity and its related code.
Do not replace tests, typechecks, or domain reasoning with sem; use it to focus investigation.
Install and setup
Check whether it is already available:
command -v sem && sem --help
Install with Homebrew:
brew install sem-cli
Optional Git integration:
sem setup
sem setup configures git diff to use sem. Revert with:
sem unsetup
Build from source when Homebrew is not appropriate:
git clone https://github.com/Ataraxy-Labs/sem
cd sem/crates
cargo install --path sem-cli
Core workflow
1) Start with entity diff
sem diff
sem diff --staged
sem diff --commit abc1234
sem diff --from HEAD~5 --to HEAD
sem diff file1.ts file2.ts
Prefer sem diff before raw git diff when the user asks what changed. It groups changes by entity and reports added, modified, deleted, moved, renamed, reordered, and cosmetic-vs-structural edits.
Use verbose mode when the entity list is too coarse:
sem diff -v
2) Use machine-readable output for agents and scripts
sem diff --format json
sem diff --format plain
sem diff --format markdown
sem diff --stdin --format json
When consuming results programmatically, prefer JSON and summarize the summary counts plus the highest-risk changes entries.
Some commands also accept --json; if --format json is rejected, retry with --json.
3) Trace ownership and impact
sem blame src/auth.ts --json
sem graph --entity validateToken --format json
sem impact validateToken --json
Use blame for per-entity ownership, graph for calls and callers, and impact for transitive dependents.
4) Pull focused context
When available in the installed sem version, use:
sem entities src/auth.ts
sem log authenticateUser --json
sem context authenticateUser --json
Use entities to identify exact names, log to inspect how one entity evolved, and context to provide a token-budgeted prompt with the target entity plus dependencies and dependents.
Practical command choices
- Current worktree:
sem diff - Staged review:
sem diff --staged --format json - One commit:
sem diff --commit <sha> --format json - Release/range review:
sem diff --from <base> --to <head> --format json - Language-limited scan:
sem diff --file-exts .ts .tsx --format json - Dependency check:
sem impact <entity> --json - Caller/callee graph:
sem graph --entity <entity> --format json - Entity ownership:
sem blame <path> --json
Interpreting output
- Treat
modified (logic)or structural changes as review priorities. - Treat cosmetic-only edits as lower risk unless they touch generated, formatted, or whitespace-sensitive files.
- Treat deleted, renamed, moved, or reordered public API entities as compatibility risks.
- For impact analysis, inspect directly affected tests and transitive callers before claiming safety.
- Use raw
git diffonly aftersemhas identified the entities that deserve line-level inspection.
Language coverage
sem supports common code and structured data formats including TypeScript, JavaScript, Python, Go, Rust, Java, C/C++, C#, Ruby, PHP, Swift, Kotlin, Elixir, Bash, HCL/Terraform, Vue, Svelte, Dart, Scala, Nix, Zig, JSON, YAML, TOML, CSV, Markdown, and more. Unsupported files fall back to chunk-based diffing.