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.agents/skills/turborepo/references/caching/RULE.md

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+# How Turborepo Caching Works
+
+Turborepo's core principle: **never do the same work twice**.
+
+## The Cache Equation
+
+```
+fingerprint(inputs) → stored outputs
+```
+
+If inputs haven't changed, restore outputs from cache instead of re-running the task.
+
+## What Determines the Cache Key
+
+### Global Hash Inputs
+
+These affect ALL tasks in the repo:
+
+- `package-lock.json` / `yarn.lock` / `pnpm-lock.yaml`
+- Files listed in `globalDependencies`
+- Environment variables in `globalEnv`
+- `turbo.json` configuration
+
+```json
+{
+  "globalDependencies": [".env", "tsconfig.base.json"],
+  "globalEnv": ["CI", "NODE_ENV"]
+}
+```
+
+### Task Hash Inputs
+
+These affect specific tasks:
+
+- All files in the package (unless filtered by `inputs`)
+- `package.json` contents
+- Environment variables in task's `env` key
+- Task configuration (command, outputs, dependencies)
+- Hashes of dependent tasks (`dependsOn`)
+
+```json
+{
+  "tasks": {
+    "build": {
+      "dependsOn": ["^build"],
+      "inputs": ["src/**", "package.json", "tsconfig.json"],
+      "env": ["API_URL"]
+    }
+  }
+}
+```
+
+## What Gets Cached
+
+1. **File outputs** - files/directories specified in `outputs`
+2. **Task logs** - stdout/stderr for replay on cache hit
+
+```json
+{
+  "tasks": {
+    "build": {
+      "outputs": ["dist/**", ".next/**"]
+    }
+  }
+}
+```
+
+## Local Cache Location
+
+```
+.turbo/cache/
+├── <hash1>.tar.zst    # compressed outputs
+├── <hash2>.tar.zst
+└── ...
+```
+
+Add `.turbo` to `.gitignore`.
+
+## Cache Restoration
+
+On cache hit, Turborepo:
+
+1. Extracts archived outputs to their original locations
+2. Replays the logged stdout/stderr
+3. Reports the task as cached (shows `FULL TURBO` in output)
+
+## Example Flow
+
+```bash
+# First run - executes build, caches result
+turbo build
+# → packages/ui: cache miss, executing...
+# → packages/web: cache miss, executing...
+
+# Second run - same inputs, restores from cache
+turbo build
+# → packages/ui: cache hit, replaying output
+# → packages/web: cache hit, replaying output
+# → FULL TURBO
+```
+
+## Key Points
+
+- Cache is content-addressed (based on input hash, not timestamps)
+- Empty `outputs` array means task runs but nothing is cached
+- Tasks without `outputs` key cache nothing (use `"outputs": []` to be explicit)
+- Cache is invalidated when ANY input changes