compare-generations
Compares two tuist generate runs to identify cache hit rate changes and root-cause analysis of cache invalidation. Can be invoked with generation IDs, dashboard URLs, or branch names.
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Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/tuist/agent-skills --skill compare-generations- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads compare-generations automatically when relevant.
- 03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.
SKILL.md
The instructions your agent reads when the skill is triggered.
Compare Generations
Quick Start
You'll typically receive two generation identifiers. Follow these steps:
- Run
tuist generate list --jsonto find generations on each branch. - Run
tuist generate show <id> --jsonfor both base and head generations. - Compare duration, status, and cache hit rates.
- Summarize cache changes with root cause analysis.
Step 1: Resolve Generations
If base/head are generation IDs or dashboard URLs
Fetch each directly:
tuist generate show <base-id> --json
tuist generate show <head-id> --json
If base/head are branch names
List recent generations on each branch:
tuist generate list --git-branch <base-branch> --json --page-size 1
tuist generate list --git-branch <head-branch> --json --page-size 1
Then fetch full details with tuist generate show <id> --json.
Defaults
- If no base is provided, use the project's default branch (usually
main). - If no head is provided, detect the current git branch.
Step 2: Compare Top-Level Metrics
After fetching both generations, compare:
| Metric | What to check |
|---|---|
duration | Flag if head is >10% slower |
status | Flag if base succeeded but head failed |
cacheable_targets | Note if target count changed |
local_cache_target_hits | Compare local hit counts |
remote_cache_target_hits | Compare remote hit counts |
is_ci | Note if one is CI and the other local |
Compute cache hit rates:
- Base:
(local_hits + remote_hits) / cacheable_targets * 100 - Head: same formula
- Delta:
head_rate - base_rate
Step 3: Analyze Cache Invalidation
If cache hit rate dropped, the key question is: which target(s) caused the invalidation cascade?
Cache invalidation typically works like this:
- A "root cause" target has a direct change (source file modified, build setting changed, etc.)
- All targets that depend on the root cause target also get invalidated because their
dependencieshash changes. - This cascade can invalidate many targets from a single root change.
Identifying root cause targets
Without the module cache target detail endpoint (available via MCP), use these heuristics:
- Check
git diffbetween the base and head commits to see which files changed. - Map changed files to their Tuist targets/modules.
- Targets with direct source changes are likely root causes.
- Targets that only changed due to dependency hash cascading are secondary invalidations.
Common root causes of cache invalidation
| Cause | Description |
|---|---|
| Source changes | Files in the target's source directory were modified |
| Resource changes | Assets, XIBs, storyboards, or other resources changed |
| Build settings | Target or project build settings were modified |
| Dependency changes | An external dependency version changed |
| Info.plist changes | The target's Info.plist was modified |
| Entitlements changes | The entitlements file was modified |
| Deployment target | The minimum deployment target changed |
| Headers | Public or project headers changed |
| Project settings | Shared project-level settings changed |
Step 4: Assess Impact
Categorize the cache invalidation:
- Expected: Source files were intentionally changed, causing expected cache misses.
- Unexpected: No source changes but cache was invalidated (build settings, Xcode version, etc.).
- Cascade: A small change invalidated many downstream targets.
Summary Format
Produce a summary with:
- Overall verdict: Cache hit rate improved, regressed, or stable.
- Cache hit rate: Base rate vs head rate with delta.
- Duration: Absolute and percentage change.
- Root cause targets: Which targets had direct changes.
- Cascade impact: How many targets were invalidated due to dependency cascading.
- Recommendations: How to minimize cache invalidation.
Example:
Generation Comparison: base (gen-123 on main) vs head (gen-456 on feature-x)
Duration: 12.5s -> 28.3s (+126%) -- REGRESSION
Cache hit rate: 92% (46/50) -> 64% (32/50) (-28%) -- REGRESSION
Status: success -> success
Root cause: FoundationModule had source changes (3 files modified).
This cascaded to 14 downstream targets that depend on FoundationModule.
Cache invalidation breakdown:
- Direct changes: FoundationModule (sources changed)
- Cascade: NetworkModule, AuthModule, UIModule, + 11 others (dependency hash changed)
Recommendations:
- Consider splitting FoundationModule into smaller modules to reduce cascade impact
- The 14 cascaded targets could benefit from more granular dependency declarations
- If FoundationModule changes frequently, consider interface/implementation module splitting
Done Checklist
- Resolved both base and head generations
- Compared duration and cache hit rates
- Identified root cause targets for cache invalidation
- Analyzed cascade impact
- Provided actionable recommendations for reducing cache misses
FAQ
What does the compare-generations skill do?
Compares two tuist generate runs to identify cache hit rate changes and root-cause analysis of cache invalidation. Can be invoked with generation IDs, dashboard URLs, or branch names.
How do I install compare-generations?
Run npx skills add https://github.com/tuist/agent-skills --skill compare-generations in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/compare-generations/ (personal) or .claude/skills/compare-generations/ (project).
Which AI agents can use compare-generations?
Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.
Is compare-generations safe to install?
No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.
Who maintains compare-generations?
It is published by tuist in tuist/agent-skills. It has 283 recorded installs.
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