diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
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+++ b/.github/copilot-instructions.md
@@ -0,0 +1,18 @@
+# AI Instructions
+
+Read and follow the CONTRIBUTING.md file in this repository for all code style conventions, commit message format, and development guidelines.
+
+## Quick Reference
+
+- Commit format: Conventional Commits — `type(scope): lowercase description`
+- Version releases are the only exception: just the version number (e.g. `0.21.1`)
+- Import order: internal (`crate::`/`super::`) → external crates → `std::`, prefer merged imports
+- Use descriptive generic names (`Size`, `Report`), not single letters
+- Prefer `where` clauses for multiple trait bounds
+- Derive order: std traits → comparison traits → `Hash` → derive_more → feature-gated
+- Custom errors: `#[derive(Debug, Display, Error)]` + `#[non_exhaustive]`
+- Minimize `unwrap()` in non-test code — use proper error handling
+- `#![deny(warnings)]` is active — code must be warning-free
+- Install toolchain before running tests: `rustup toolchain install "$(< rust-toolchain)" && rustup component add --toolchain "$(< rust-toolchain)" rustfmt clippy`
+- If the AI agent is Claude Code, `gh` (GitHub CLI) is not installed — do not attempt to use it
+- Run `FMT=true LINT=true BUILD=true TEST=true DOC=true ./test.sh` to validate changes
diff --git a/AGENTS.md b/AGENTS.md
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+++ b/AGENTS.md
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+# AI Instructions
+
+Read and follow the CONTRIBUTING.md file in this repository for all code style conventions, commit message format, and development guidelines.
+
+## Quick Reference
+
+- Commit format: Conventional Commits — `type(scope): lowercase description`
+- Version releases are the only exception: just the version number (e.g. `0.21.1`)
+- Import order: internal (`crate::`/`super::`) → external crates → `std::`, prefer merged imports
+- Use descriptive generic names (`Size`, `Report`), not single letters
+- Prefer `where` clauses for multiple trait bounds
+- Derive order: std traits → comparison traits → `Hash` → derive_more → feature-gated
+- Custom errors: `#[derive(Debug, Display, Error)]` + `#[non_exhaustive]`
+- Minimize `unwrap()` in non-test code — use proper error handling
+- `#![deny(warnings)]` is active — code must be warning-free
+- Install toolchain before running tests: `rustup toolchain install "$(< rust-toolchain)" && rustup component add --toolchain "$(< rust-toolchain)" rustfmt clippy`
+- If the AI agent is Claude Code, `gh` (GitHub CLI) is not installed — do not attempt to use it
+- Run `FMT=true LINT=true BUILD=true TEST=true DOC=true ./test.sh` to validate changes
diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
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--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,18 @@
+# AI Instructions
+
+Read and follow the CONTRIBUTING.md file in this repository for all code style conventions, commit message format, and development guidelines.
+
+## Quick Reference
+
+- Commit format: Conventional Commits — `type(scope): lowercase description`
+- Version releases are the only exception: just the version number (e.g. `0.21.1`)
+- Import order: internal (`crate::`/`super::`) → external crates → `std::`, prefer merged imports
+- Use descriptive generic names (`Size`, `Report`), not single letters
+- Prefer `where` clauses for multiple trait bounds
+- Derive order: std traits → comparison traits → `Hash` → derive_more → feature-gated
+- Custom errors: `#[derive(Debug, Display, Error)]` + `#[non_exhaustive]`
+- Minimize `unwrap()` in non-test code — use proper error handling
+- `#![deny(warnings)]` is active — code must be warning-free
+- Install toolchain before running tests: `rustup toolchain install "$(< rust-toolchain)" && rustup component add --toolchain "$(< rust-toolchain)" rustfmt clippy`
+- If the AI agent is Claude Code, `gh` (GitHub CLI) is not installed — do not attempt to use it
+- Run `FMT=true LINT=true BUILD=true TEST=true DOC=true ./test.sh` to validate changes
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 00000000..c1c0d086
--- /dev/null
+++ b/CONTRIBUTING.md
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+# Contributing to parallel-disk-usage
+
+## Commit Message Convention
+
+This project uses [Conventional Commits](https://www.conventionalcommits.org/).
+
+### Format
+
+```
+type(scope): lowercase description
+```
+
+### Rules
+
+- **Types:** `feat`, `fix`, `refactor`, `perf`, `docs`, `style`, `chore`, `ci`, `test`, `lint`
+- **Scopes** (optional): `cli`, `api`, `deps`, `readme`, `benchmark`, `toolchain`, `test`, or other relevant area
+- **Description:** always lowercase after the colon, no trailing period, brief (3-7 words preferred)
+- **Breaking changes:** append `!` before the colon (e.g. `feat(cli)!: remove deprecated flag`)
+- **Code identifiers** in descriptions should be wrapped in backticks (e.g. `` chore(deps): update `rand` ``)
+
+### Exception: Version Releases
+
+Version release commits use **only** the version number as the message — no type prefix:
+
+```
+0.21.1
+```
+
+## Code Style
+
+Automated tools enforce formatting (`cargo fmt`) and linting (`cargo clippy`). The following conventions are **not** enforced by those tools and must be followed manually.
+
+### Import Organization
+
+Prefer **merged imports** — combine multiple items from the same crate or module into a single `use` statement with braces rather than separate `use` lines.
+
+Imports are grouped in this order, separated by blank lines:
+
+1. `use super::...` or `use crate::...` (internal)
+2. External crate imports (alphabetical)
+3. `use std::...` (standard library)
+
+Within each group, items are ordered alphabetically. Platform-specific imports (`#[cfg(unix)]`) go in a separate block after the main imports.
+
+```rust
+use crate::{
+    args::{Args, Quantity, Threads},
+    bytes_format::BytesFormat,
+    size,
+};
+use clap::Parser;
+use pipe_trait::Pipe;
+use std::{io::stdin, time::Duration};
+
+#[cfg(unix)]
+use crate::get_size::{GetBlockCount, GetBlockSize};
+```
+
+### Module Organization
+
+- Use the flat file pattern (`module.rs`) rather than `module/mod.rs` for submodules.
+- List `pub mod` declarations first, then `pub use` re-exports, then private imports and items.
+- Use `pub use` to re-export key types at the module level for convenience.
+
+```rust
+pub mod error_only_reporter;
+pub mod error_report;
+pub mod event;
+
+pub use error_only_reporter::ErrorOnlyReporter;
+pub use error_report::ErrorReport;
+pub use event::Event;
+```
+
+- Type aliases using `pub use ... as ...` are used to provide semantic alternative names:
+
+```rust
+pub use Reflection as DataTreeReflection;
+```
+
+### Derive Macro Ordering
+
+When deriving multiple traits, use this order and split across multiple `#[derive(...)]` lines for readability:
+
+1. **Standard traits:** `Debug`, `Default`, `Clone`, `Copy`
+2. **Comparison traits:** `PartialEq`, `Eq`, `PartialOrd`, `Ord`
+3. **Hash**
+4. **`derive_more` traits:** `Display`, `From`, `Into`, `Add`, `AddAssign`, etc.
+5. **Feature-gated derives** on a separate `#[cfg_attr(...)]` line
+
+```rust
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(From, Into, Add, AddAssign, Sub, SubAssign, Sum)]
+#[cfg_attr(feature = "json", derive(Deserialize, Serialize))]
+pub struct Bytes(u64);
+```
+
+### Generic Parameter Naming
+
+Use **descriptive names** for type parameters, not single letters:
+
+- `Size`, `Name`, `SizeGetter`, `HardlinksRecorder`, `Report`
+
+Single-letter generics are acceptable only in very short, self-contained trait impls.
+
+### Trait Bounds
+
+Prefer `where` clauses over inline bounds when there are multiple constraints:
+
+```rust
+impl<Size, SizeGetter, HardlinksRecorder, Report>
+    From<FsTreeBuilder<'a, Size, SizeGetter, HardlinksRecorder, Report>>
+    for DataTree<OsStringDisplay, Size>
+where
+    Report: Reporter<Size> + Sync + ?Sized,
+    Size: size::Size + Send + Sync,
+    SizeGetter: GetSize<Size = Size> + Sync,
+    HardlinksRecorder: RecordHardlinks<Size, Report> + Sync + ?Sized,
+```
+
+### Visibility
+
+- Use `pub` for the public API surface.
+- Use `pub(crate)` for items shared within the crate but not exposed externally.
+- Default to private for everything else.
+
+### Error Handling
+
+- Define custom error enums with `#[derive(Debug, Display, Error)]` from `derive_more`.
+- Mark error enums as `#[non_exhaustive]`.
+- Minimize `unwrap()` in non-test code — use proper error propagation. `unwrap()` is acceptable in tests and for provably infallible operations (with a comment explaining why). When deliberately ignoring an error, use `.ok()` with a comment explaining why.
+
+```rust
+#[derive(Debug, Display, Error)]
+#[non_exhaustive]
+pub enum RuntimeError {
+    #[display("SerializationFailure: {_0}")]
+    SerializationFailure(serde_json::Error),
+}
+```
+
+### Documentation Comments
+
+- Use `///` doc comments for all public types, traits, functions, and fields.
+- Use `//!` module-level doc comments at the top of `lib.rs` and significant modules.
+- Include usage examples with `/// ```no_run` blocks for key public APIs.
+- Reference related types with `[`backtick links`](crate::path)` syntax.
+
+### Feature Gating
+
+- Use `#[cfg(feature = "...")]` for optional functionality (e.g., `json`, `cli`).
+- Use `#[cfg(unix)]` for POSIX-specific code.
+- Use `#[cfg_attr(feature = "json", derive(Deserialize, Serialize))]` for conditional derives.
+
+### Pattern Matching
+
+Use exhaustive matching. When mapping enum variants to values, prefer the concise wrapping style:
+
+```rust
+ExitCode::from(match self {
+    RuntimeError::SerializationFailure(_) => 2,
+    RuntimeError::DeserializationFailure(_) => 3,
+})
+```
+
+### Struct Field Ordering
+
+Order fields logically by purpose, not alphabetically. Group related fields together. Document every public field with `///` comments.
+
+### Macros
+
+Use macros to reduce boilerplate for repetitive patterns (e.g. newtype wrappers, trait impls for multiple numeric types). Keep macros well-scoped and documented.
+
+### Warnings Policy
+
+The crate uses `#![deny(warnings)]` — all warnings are treated as errors. Code must compile warning-free.
+
+## Setup
+
+Install the required Rust toolchain and components before running any checks:
+
+```sh
+rustup toolchain install "$(< rust-toolchain)"
+rustup component add --toolchain "$(< rust-toolchain)" rustfmt clippy
+```
+
+## Automated Checks
+
+Before submitting, ensure:
+
+- `cargo fmt -- --check` passes
+- `cargo clippy` passes (on all feature combinations)
+- `cargo test` passes
+- The project builds with no default features, default features, and all features
+
+The CI script `test.sh` runs all of these across 5 feature combinations. You can run it locally with:
+
+```sh
+FMT=true LINT=true BUILD=true TEST=true DOC=true ./test.sh
+```
diff --git a/tests/sync_ai_instructions.rs b/tests/sync_ai_instructions.rs
new file mode 100644
index 00000000..f863fb10
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+++ b/tests/sync_ai_instructions.rs
@@ -0,0 +1,25 @@
+//! The following tests check whether the AI instruction files are in sync.
+//!
+//! All three files (CLAUDE.md, AGENTS.md, .github/copilot-instructions.md) should be identical.
+
+macro_rules! check {
+    ($name:ident: $a_path:literal == $b_path:literal) => {
+        #[test]
+        fn $name() {
+            let a = include_str!($a_path);
+            let b = include_str!($b_path);
+            assert!(
+                a == b,
+                concat!(
+                    "AI instruction files are out of sync: ",
+                    $a_path,
+                    " != ",
+                    $b_path,
+                ),
+            );
+        }
+    };
+}
+
+check!(claude_md_vs_agents_md: "../CLAUDE.md" == "../AGENTS.md");
+check!(claude_md_vs_copilot_instructions: "../CLAUDE.md" == "../.github/copilot-instructions.md");