> ## Documentation Index
> Fetch the complete documentation index at: https://docs.fallow.tools/llms.txt
> Use this file to discover all available pages before exploring further.
# fallow dupes
> CLI reference for fallow dupes. Detect duplicated code blocks across your codebase using suffix-array analysis with configurable thresholds.
Detect copy-pasted code blocks across your entire codebase using suffix-array analysis.
Start with the default `mild` mode. Use `semantic` when you want to catch clones with renamed variables.
```bash theme={null}
fallow dupes
```
## Options
### Detection
| Flag | Description |
| :---------------------- | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `--mode ` | Detection mode: `strict`, `mild` (default), `weak`, `semantic` |
| `--min-tokens ` | Minimum tokens per clone (default: 50) |
| `--min-lines ` | Minimum lines per clone (default: 5) |
| `--min-occurrences ` | Minimum number of occurrences before a clone group is reported (default: 2, must be ≥ 2). Raise to skip pair-only clones and focus on widespread copy-paste worth refactoring. |
| `--skip-local` | Only report cross-directory duplicates |
| `--cross-language` | Strip TS types for TS to JS matching |
| `--ignore-imports` | Exclude import declarations from clone detection |
### Output
| Flag | Description |
| :---------------------- | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `-f, --format ` | Output format: `human` (default), `json`, `sarif`, `compact`, `markdown`, `codeclimate`, `gitlab-codequality`, `pr-comment-github`, `pr-comment-gitlab`, `review-github`, `review-gitlab` |
| `--top ` | Show only the N most-duplicated clone groups. Groups are sorted by instance count descending (tiebreak: line count descending, then deterministic path/line). Summary stats reflect the full project, not just the shown groups. |
| `--quiet` | Suppress progress output |
| `--threshold ` | Fail if duplication exceeds N% |
| `--group-by ` | Partition the report into per-group sections. `MODE`: `owner` (CODEOWNERS), `directory` (first path component), `package` (workspace), or `section` (GitLab `[Section]` headers). Each clone group is attributed to its largest owner (most instances; alphabetical tiebreak), so a group split 2 src / 1 lib appears under `src`. JSON adds `grouped_by` plus a `groups` array of per-bucket dedup-aware `stats`, attributed `clone_groups` (each carrying `primary_owner` and per-instance `owner`), and `clone_families`. SARIF results carry `properties.group` and CodeClimate issues carry a top-level `group` field. Compact and markdown fall back to ungrouped output with a stderr note. |
| `--explain` | Add metric explanations. In human format, explanations are always shown. In JSON format, adds a `_meta` object with metric descriptions and docs links. |
| `--explain-skipped` | In human/markdown output, show the per-pattern breakdown for files skipped by default duplicates ignores. Machine formats suppress this note. |
| `--summary` | Print a one-line summary of duplication counts at the end of the run. In JSON format, adds a `summary` counts object. |
### Incremental
| Flag | Description |
| :----------------------- | :------------------------------------------------------- |
| `--changed-since [` | Only report duplication in files changed since a git ref |
| `--baseline ` | Compare against a previously saved baseline file |
| `--save-baseline ` | Save current duplication as a baseline file |
#### Token cache
For projects with at least `duplicates.minCorpusSizeForTokenCache` source files (default `5000`), `fallow dupes` keeps a per-project tokenized-source cache under `/.fallow/cache/dupes-tokens-vN/` so warm runs skip re-tokenizing unchanged files. The cache writes a sibling `.gitignore` containing `*` on first save, so its contents stay out of `git status`. Pass `--no-cache` to disable the cache for one run, or set a higher `minCorpusSizeForTokenCache` to keep it disabled even on larger projects. Below the threshold the cache stays off because the load/save overhead exceeds the tokenize savings on small corpora.
When `--changed-since` is set on a project at or above `duplicates.minCorpusSizeForShingleFilter` files (default `1024`), the detector additionally drops unchanged files whose k-token shingles do not overlap any changed file before building the suffix array. Both knobs are scoped to `[duplicates]` in `.fallowrc.json` / `.fallowrc.jsonc` / `fallow.toml` / `.fallow.toml`.
#### Default generated-output ignores
`fallow dupes` skips common framework output by default: `.next`, `.nuxt`, `.svelte-kit`, `.turbo`, `.parcel-cache`, `.vite`, `.cache`, `out`, and `storybook-static`. These defaults merge with `duplicates.ignore`; they do not replace your configured ignores.
If your duplication percentage drops after upgrading, fallow is now excluding generated framework output from duplicate detection by default. Use `--explain-skipped` in human or markdown output to see the per-pattern skipped-file counts.
```jsonc theme={null}
{
"duplicates": {
"ignoreDefaults": true,
"ignore": ["**/__generated__/**"]
}
}
```
Set `ignoreDefaults` to `false` to opt out and use only your `duplicates.ignore` list.
### Monorepo scoping
Both flags retain only clone groups where at least one instance is under a selected workspace root. The full cross-workspace index is still built; reported groups and the `duplication_percentage` stat are recomputed from the scoped slice.
| Flag | Description |
| :--------------------------- | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `-w, --workspace ` | Scope output to one or more workspaces. Supports exact package names, globs (`apps/*`, `@scope/*`) matched against both the package name and the workspace path, and `!`-prefixed negation. Comma-separated values or repeated flag. |
| `--changed-workspaces ][` | Git-derived monorepo CI scoping: scope to workspaces containing any file changed since `REF` (e.g. `origin/main`). Mutually exclusive with `--workspace`. Missing ref is a hard error (exit 2), not silent full-scope fallback. |
### Debugging
| Flag | Description |
| :-------------------- | :--------------------------------------------- |
| `--trace ` | Show all clones of code at a specific location |
| `--performance` | Show pipeline timing breakdown |
## Detection modes
Each mode progressively normalizes more syntax before comparing, trading precision for recall.
| Mode | What it normalizes | Best for |
| :----------- | :----------------------------------------------------------------------------- | :--------------------------------------- |
| **strict** | Nothing: exact token match | Finding verbatim copies |
| **mild** | Currently equivalent to strict (AST tokenization inherently strips whitespace) | General-purpose detection (default) |
| **weak** | + string literal values | Catching copies with different messages |
| **semantic** | + variable names and numeric values | Finding structural clones after renaming |
As you move from `strict` to `semantic`, you'll find more clones but also more potential false positives. Start with `mild` and increase sensitivity as needed.
## Examples
```bash Default analysis theme={null}
# Mild mode (default)
fallow dupes
```
```bash Filtering theme={null}
# Show only the 10 largest clone groups
fallow dupes --top 10
# Top 5 semantic clones, cross-directory only
fallow dupes --top 5 --mode semantic --skip-local
```
```bash Advanced detection theme={null}
# Catch renamed-variable clones
fallow dupes --mode semantic
# Only cross-directory duplicates
fallow dupes --skip-local
# TS to JS comparison
fallow dupes --cross-language
# Ignore sorted import blocks
fallow dupes --ignore-imports
```
```bash CI integration theme={null}
# Only check duplication in changed files
fallow dupes --changed-since main
# Fail CI if duplication exceeds 5%
fallow dupes --threshold 5
# SARIF output for GitHub
fallow dupes --format sarif > dupes.sarif
```
```bash Debugging theme={null}
# Trace clones at a specific line
fallow dupes --trace src/utils.ts:42
# Performance breakdown
fallow dupes --performance
```
## Example output
```bash title="$ fallow dupes --top 3" theme={null}
● Duplicates (3 clone groups)
5,433 lines 2 instances
deno/lib/types.ts:45-5477
src/types.ts:45-5477
914 lines 2 instances
deno/lib/__tests__/string.test.ts:8-921
src/__tests__/string.test.ts:7-920
42 lines 3 instances
src/features/forecasting/server/procedures/analytics.ts:141-181
src/features/forecasting/server/procedures/cashflow.ts:153-194
src/features/forecasting/server/procedures/income.ts:590-631
Identical code blocks detected via suffix-array analysis — https://docs.fallow.tools/explanations/duplication#clone-groups
✓ 27,255 lines (19.4%) duplicated across 398 files (0.23s)
```
```bash title="$ fallow dupes --threshold 15" theme={null}
● Duplicates (3 clone groups)
5,433 lines 2 instances
deno/lib/types.ts:45-5477
src/types.ts:45-5477
914 lines 2 instances
deno/lib/__tests__/string.test.ts:8-921
src/__tests__/string.test.ts:7-920
42 lines 3 instances
src/features/forecasting/server/procedures/analytics.ts:141-181
src/features/forecasting/server/procedures/cashflow.ts:153-194
src/features/forecasting/server/procedures/income.ts:590-631
Identical code blocks detected via suffix-array analysis — https://docs.fallow.tools/explanations/duplication#clone-groups
✗ 27,255 lines (19.4%) duplicated across 398 files (0.23s)
Duplication (19.4%) exceeds threshold (15%)
```
## JSON output
### `mirrored_directories`
When fallow detects directory pairs that are largely duplicated (e.g., a `deno/` mirror of `src/`), the JSON output includes a `mirrored_directories` array:
```json theme={null}
{
"mirrored_directories": [
{
"dir_a": "deno/lib",
"dir_b": "src",
"shared_files": ["client.ts", "handlers.ts", "types.ts", "utils.ts"],
"total_lines": 6347
}
]
}
```
Each entry identifies two directories with significant overlap, the shared filenames, and the total duplicated lines across those files.
## See also
How fallow's detection engine works.
Set default modes and thresholds in your config.
Coming from jscpd? See the migration guide.
]