# Oxygen (`oxy`) — Version Control System Specification

| | |

|---|---|

| **Status** | Draft 0.6p |

| **Format version** | 1 |

| **Date** | 2026-09-26 |

| **Audience** | Implementers, security reviewers, and contributors |

The key words MUST, MUST NOT, SHOULD, SHOULD NOT, and MAY are to be interpreted as described in RFC 2119.

---

## Table of contents

1. [Overview](#1-overview)

2. [Goals, non-goals, and lineage](#2-goals-non-goals-and-lineage)

3. [Threat model](#3-threat-model)

4. [Terminology](#4-terminology)

5. [Object model and hashing](#5-object-model-and-hashing)

6. [The change graph](#6-the-change-graph)

7. [Files, trees, and atoms](#7-files-trees-and-atoms)

8. [Dependencies and commutation](#8-dependencies-and-commutation)

9. [Conflicts](#9-conflicts)

10. [States, bookmarks, and tags](#10-states-bookmarks-and-tags)

11. [Checkpoints](#11-checkpoints)

12. [Operation log, workspaces, and the working copy](#12-operation-log-workspaces-and-the-working-copy)

13. [Storage engine](#13-storage-engine)

14. [Identity, signatures, and trust policy](#14-identity-signatures-and-trust-policy)

15. [Transport](#15-transport)

16. [Wire protocol](#16-wire-protocol)

17. [Partial, sparse, and shallow repositories](#17-partial-sparse-and-shallow-repositories)

18. [Locks for unmergeable files](#18-locks-for-unmergeable-files)

19. [Repository configuration files](#19-repository-configuration-files)

20. [Hooks and server policy](#20-hooks-and-server-policy)

21. [Garbage collection](#21-garbage-collection)

22. [Client profiles and retro hardware](#22-client-profiles-and-retro-hardware)

23. [Git interoperability](#23-git-interoperability)

24. [Command-line interface](#24-command-line-interface)

25. [Performance targets](#25-performance-targets)

26. [Conformance and test vectors](#26-conformance-and-test-vectors)

27. [Open questions](#27-open-questions)

---

## 1. Overview

Oxygen (`oxy`) is an open-source, distributed version control system written in Rust. It is built for multi-gigabyte repositories, monorepos, and high-frequency development loops, and it defines a reduced client profile so that older and low-specification machines, which current version control systems exclude, can still take part.

Its distinguishing design choices are:

- **History is a set of changes, not a sequence of snapshots.** A change records what it did to identified lines and tree nodes, and which earlier changes it depends on. Changes that do not depend on each other commute: applying them in any order yields the same result.

- **Every change has two identifiers.** A random, stable *change ID* that survives rewriting, and a content-addressed *change hash* that makes history tamper-evident. Neither is optional.

- **Conflicts are data, not errors.** A conflicted state can be committed, pushed, fetched, and built on. Resolution is itself a change.

- **Every mutation of repository state is logged and undoable** through an operation log.

- **Content is stored in a content-defined-chunked, BLAKE3-addressed store**, so large files and large histories deduplicate naturally and can be fetched lazily.

- **Every change and every reference update is authenticated** with composite Ed25519 + ML-DSA-65 signatures, either on the change itself or on a push-time seal covering it, and verification is driven by an in-repository trust policy.

- **Transport is pluggable.** HTTPS is mandatory for every server, Noise is available for peer-to-peer use, and a plaintext transport exists for constrained clients on trusted networks, with integrity provided end to end by the object model rather than by the channel.

## 2. Goals, non-goals, and lineage

### 2.1 Goals

- Correct, deterministic merges of concurrent work without merge commits for independent changes.

- Stable identifiers for human and tooling use across rebase, amend, and reorder.

- Tamper-evident history and verifiable authorship, robust against future large quantum computers.

- Repositories of tens of gigabytes with millions of files, with `status` and `commit` latency independent of repository size.

- Partial clone by path, depth, and blob size as a first-class mode, not an afterthought.

- Interoperability with Git repositories in both directions.

- A documented Thin client profile for machines with a 68000-class CPU and 1 MiB of RAM.

### 2.2 Non-goals

- Running on machines with less than 512 KiB of RAM.

- Replacing code review, CI, or issue tracking.

- Semantic (language-aware) merging. Merging is line-based for text and replace-based for binary.

- Confidentiality of repository contents at rest in version 1 (see §27).

### 2.3 Lineage

The change model is the patch theory of Darcs and Pijul. The stable-ID-plus-hash split, the first-class working-copy revision, and the operation log come from Jujutsu. The chunked content store follows bup, restic, and Xet. The signed reference model follows The Update Framework (TUF) and gittuf. Oxygen's contribution is combining these with a partial-fetch-first protocol, composite post-quantum signatures, and a constrained-client profile in a single coherent design.

## 3. Threat model

### 3.1 Adversaries

| Adversary | Capabilities |

|---|---|

| **Network attacker** | Observe, modify, drop, delay, and replay traffic between client and server. |

| **Malicious or compromised server** | Serve arbitrary objects and references, withhold objects, and serve stale references. |

| **Malicious collaborator** | Hold a valid signing key authorized for some references and attempt to exceed that authorization, forge others' authorship, or introduce colliding identifiers. |

| **Compromised client host** | Full control of one contributor's machine, including their private keys. |

### 3.2 Security goals

- **G1 Integrity.** A client that has verified a state's heads can detect any modification to any object reachable from them.

- **G2 Authenticity.** Every change and every reference update carries a signature verifiable against an identity, and policy decides whether that identity is authorized.

- **G3 Freshness.** A client can detect when a server presents a reference older than one it has already seen.

- **G4 Non-downgrade.** An attacker cannot strip the post-quantum half of a composite signature and have it verify.

- **G5 Availability under adversarial input.** Pathological file contents cannot cause super-linear chunking, hashing, or graph work.

### 3.3 Explicitly out of scope

- Confidentiality against a malicious server (repository contents are visible to whoever hosts them).

- Recovery from a compromised signing key beyond revocation and policy update.

- Protection against a compromised client host altering its own working copy.

Every mechanism in §§5, 14, 15, and 16 is annotated with the goal it serves.

## 4. Terminology

| Term | Meaning |

|---|---|

| **Object** | An immutable, content-addressed byte string with a type. |

| **ObjectRef** | The 32-byte BLAKE3 hash of an object, including its domain-separation header. |

| **Change** | The unit of history: tree operations, edits, dependencies, and metadata. |

| **ChangeId** | A 128-bit random identifier that is stable across rewrites of a change. |

| **ChangeHash** | The ObjectRef of a change. Changes with different content have different hashes. |

| **Atom** | An identified unit of file content: one line of a text file. |

| **AtomId** | `(ChangeHash of introducing change, u32 index)`. |

| **Node** | A file, directory, or symlink, identified by a 128-bit random **NodeId** stable across renames. |

| **Placement** | The `(parent NodeId, name)` pair that positions a node in the tree. |

| **State** | A dependency-closed set of changes. Fully determined by its maximal elements (**heads**), and named by a `state` object that lists them. |

| **Bookmark** | A named, mutable pointer to a state. Equivalent to a Git branch. |

| **Tag** | A named, immutable, signed pointer to a state. |

| **Checkpoint** | A materialized snapshot of a state, used as a cache. |

| **Operation** | A recorded mutation of local repository state (bookmarks, workspaces). |

| **Workspace** | A checkout directory associated with a repository. |

| **Attestation** | A signed statement about a reference, tag, batch of changes, or key. |

| **Identity** | An object holding a contributor's public keys and metadata. |

## 5. Object model and hashing

### 5.1 Encoding

All objects are encoded as deterministic CBOR (RFC 8949 §4.2.1): definite lengths, shortest integer encodings, map keys sorted bytewise, no floating-point values, no indefinite-length items. Map keys are small unsigned integers; the tables in this document give the key number in brackets, e.g. `deps [2]`.

Every object begins with the field `version [0]`, an unsigned integer. This document defines version 1. Readers MUST reject objects whose version they do not understand.

### 5.2 Hashing (G1)

```

ObjectRef = BLAKE3-256( header || body )

header    = "oxy1 " || type-name || " " || decimal(len(body)) || 0x00

```

`type-name` is one of `chunk`, `blob`, `change`, `checkpoint`, `op`, `attest`, `identity`, `policy`, `state`. The header prevents an object of one type from being presented as another. BLAKE3 with a 256-bit output retains 128-bit collision resistance against quantum attackers.

The all-zero ObjectRef is reserved and means "self" inside a change (§7.4).

### 5.3 Object types

| Type | Purpose | Section |

|---|---|---|

| `chunk` | Raw bytes, at most 1 MiB. | §13.2 |

| `blob` | Ordered list of chunk refs plus total length; represents file content. | §13.2 |

| `change` | A history entry. | §6 |

| `checkpoint` | Materialized state. | §11 |

| `op` | Operation log entry. | §12 |

| `attest` | A signed statement. | §14.4 |

| `identity` | A contributor's keys. | §14.2 |

| `policy` | The repository's trust policy. | §14.5 |

| `state` | A sorted list of heads naming a state. | §10.1 |

### 5.4 Signatures are outside the hash

Signatures never appear inside a hashed object. A signed object is transported and stored as:

```

Signed = { object [0]: bytes, sigs [1]: [Signature] }

```

Adding a signature, re-signing, or attaching a seal (§14.4) therefore never changes an ObjectRef.

### 5.5 Encoding conventions

The field tables in this document give each map key in brackets. Where a structure is a choice between variants, or a compact reference, the following encodings apply:

| Item | Encoding |

|---|---|

| `TreeOp` | Map with key 0 = variant tag: 0 `AddNode`, 1 `RemoveNode`, 2 `Move`, 3 `SetMode`; key 1 = node id; `AddNode` adds 2 kind, 3 mode, 4 parent, 5 name; `RemoveNode` adds 2 placement ref; `Move` adds 2 old placement ref, 4 parent, 5 name; `SetMode` adds 2 old mode ref, 3 mode. |

| `Edit` | Map with key 0 = variant tag: 0 `Insert`, 1 `Delete`, 2 `Replace`; key 1 = node id; `Insert` adds 2 after, 3 before, 4 lines (array of byte strings) or 5 `lines_blob`; `Delete` adds 2 atom ranges; `Replace` adds 2 old (bytes or null), 3 new (bytes or null). |

| `NodeKind` | Unsigned integer: 0 file, 1 dir, 2 symlink. |

| `Mode` | Unsigned integer bit field: bit 0 executable, bit 1 opaque content mode. Directories carry 0. |

| `OpRef` (placement or mode reference) | `[bytes(32) change hash, uint index]`; the all-zero hash means this change. |

| `AtomRef` | `0` for `START`, `1` for `END`, otherwise `[bytes(32) change hash, uint index]` with the all-zero hash meaning this change. |

| `AtomRange` | `[bytes(32) change hash, uint from, uint to]`, half-open, non-empty. |

| `View` (in `Op`) | Key 0 bookmarks: array of `[text name, bytes(32) state ref]` sorted by name; key 1 workspaces: array of `[bytes(16) id, bytes(32) change]` sorted by id; key 2 remotes: array of `[text remote, [[text bookmark, bytes(32) state ref]]]`; key 3 policy ref or null; key 4 tags: array of `[text name, bytes(32) state ref]`. Repository format 1 stored `[name, [heads]]`; format 2 stores the state ref. |

| `Signature` | Map: 0 key id, 1 alg (1 = Ed25519 + ML-DSA-65), 2 Ed25519 bytes(64), 3 ML-DSA-65 bytes(3309). A signature list is a CBOR array of these. |

| Secret key file | Map: 0 version, 1 key id, 2 Ed25519 seed bytes(32), 3 ML-DSA-65 seed bytes(32). Kept only in the user's key directory. |

Atom indices count the atoms one change introduces for one node, in edit order, across all of that change's `Insert` edits on the node.

## 6. The change graph

### 6.1 Change object

```

Change {

  version   [0]: 1

  change_id [1]: bytes(16)            // random, CSPRNG; see §6.3

  deps      [2]: [ChangeHash]         // sorted, unique; see §8

  author    [3]: KeyId                // ObjectRef of an identity object

  hlc       [4]: { wall_ms: u64, logical: u32 }

  message   [5]: text

  tree_ops  [6]: [TreeOp]             // §7.2

  edits     [7]: [Edit]               // §7.4

  kind      [8]: 0 = normal, 1 = resolution, 2 = working-copy

  extra     [9]: map                  // reserved; MUST be empty in v1

}

```

### 6.2 Two identifiers

- **ChangeHash** is the integrity handle (G1). All references between objects use it. Dependencies, bookmark heads, tags, and checkpoints reference changes by hash only.

- **ChangeId** is the stable handle. It is what users type, what tooling tracks, and what survives amend, rebase, and reorder. It carries no integrity meaning and is never used alone as a reference in any object.

A change may be *rewritten*: a new change object is created with the same ChangeId and a different hash. Changes that depended on the old hash are *orphaned* and must be rewritten in turn (§8.4). A repository may hold several changes with the same ChangeId; when more than one is reachable from the same state the ChangeId is *divergent*, and commands MUST report divergence rather than pick silently.

### 6.3 ChangeId generation

ChangeIds MUST be produced by a cryptographically secure random generator with 128 bits of entropy on Full and Lite profiles. On the Thin profile they are generated by the proxy (§22). Two changes with equal ChangeId and unequal hash are not an error; they are divergence (§6.2). An implementation MUST NOT infer anything about content from a ChangeId.

### 6.4 Timestamps

`hlc` is a hybrid logical clock: `wall_ms` is the author's wall clock in milliseconds since the Unix epoch, and `logical` is a counter. On observing a change with a later HLC, a client advances its own clock per the Kulkarni et al. HLC rules. `wall_ms` is untrusted metadata and MUST NOT be used for ordering, security, or freshness decisions. Sorting for display uses `(hlc, ChangeHash)`.

**Drift bound.** A change with a far-future `wall_ms` must not be able to drag every peer's clock forward. On observing a change, a client MUST NOT advance its own HLC more than `core.hlc-max-drift` (default 5 minutes) ahead of its local wall clock; the change itself is still accepted and stored, so whether a state is well-formed never depends on the observer's clock. A server MAY reject a push containing a change whose `wall_ms` is more than `core.hlc-max-drift` ahead of the server clock, since the author is present and can correct their clock. `logical` is a `u32`; a client whose counter would overflow MUST refuse to create the change and report clock skew.

## 7. Files, trees, and atoms

### 7.1 Tree model

The repository tree is a set of nodes. Each node has a **NodeId**, a **kind** (`file`, `dir`, `symlink`), a **mode**, and zero or more **placements**. The mode of a file is its executable bit and its **content mode**, `text` or `opaque` (§7.5); a symlink's content is its target path and its content mode is always `opaque`; a directory has no mode. The root directory has the fixed NodeId of 16 zero bytes and no placement.

A placement is `(parent NodeId, name)`. Names are byte strings that MUST be valid UTF-8, MUST NOT be empty, `.`, or `..`, and MUST NOT contain `/` or NUL. Placements that a particular file system cannot represent distinctly, such as names differing only by case, are workspace conflicts (§9.7), not errors in the state.

Renames and moves are placement changes, so rename tracking is exact once recorded. A working copy has no way to report a move, so when a snapshot finds a removed file and an added file of the same kind whose contents are at least 50% alike (the share of the larger file's bytes found in lines both have; opaque files must be identical), it records a `Move` of the existing node rather than a removal and an addition. Beyond 10,000 candidate pairs only identical contents are paired. The same detection applies to Git import (§23).

### 7.2 Tree operations

Placements are atoms in the sense of §7.3: each is introduced by exactly one change and may be deleted by later changes. Because a placement is defined relative to the parent's NodeId rather than a path, moving a directory moves its whole subtree with a single placement change; no operations are generated for descendants. The cost of this is that two concurrent moves can create a cycle, handled in §9.2.

```

TreeOp = AddNode  { node: NodeId, kind, mode, place: (parent, name) }   // introduces node + placement

       | RemoveNode { node: NodeId, placement: PlacementRef }            // deletes the placement; node dead when no live placements

       | Move     { node: NodeId, old: PlacementRef, place: (parent, name) } // deletes old placement, introduces new one; implicitly moves all descendants

       | SetMode  { node: NodeId, old: ModeRef, mode }                  // mode is an atom too

PlacementRef = (ChangeHash, u32 index into that change's tree_ops)

```

A node is **live** in a state when it has at least one live placement and its parent is live. A node with more than one live placement, or two live nodes with the same placement, is a conflict (§9.2).

### 7.3 Atoms and the file graph

The content of a text file is a directed graph whose vertices are atoms. Each atom is one line, including its terminator (bytes up to and including `\n`, or to end of file). Edges express "immediately after". An atom is introduced by exactly one change and has an `AtomId = (ChangeHash, index)`, where `index` counts atoms introduced by that change for that node, starting at 0.

Every file graph contains two virtual atoms, `START` and `END`. An atom is **live** unless a change in the state deletes it. Deleted atoms remain in the graph as tombstones; they may still be referenced as anchors.

The graph of a file in a state is the union of the atoms and edges introduced by every change in the state, minus nothing: deletion sets a flag, it does not remove vertices. **The graph is a function of the set of changes alone and not of the order they were applied.** Implementations MUST preserve this invariant; the conformance suite (§26) tests it by applying random permutations.

### 7.4 Edits

```

Edit = Insert  { node: NodeId, after: AtomRef, before: AtomRef, lines: [bytes] | lines_blob: ObjectRef }

     | Delete  { node: NodeId, atoms: [AtomRange] }

     | Replace { node: NodeId, old: ObjectRef | null, new: ObjectRef }     // opaque nodes only

AtomRef   = START | END | AtomId

AtomRange = { change: ChangeHash, from: u32, to: u32 }   // half-open

```

- `Insert` introduces `len(lines)` new atoms, indexed consecutively, and edges `after → first`, `last → before`. `after` and `before` MUST be adjacent in the author's view at authoring time; the pair is the insert's **anchor**.

- Atoms introduced by the same change are referenced with the all-zero "self" hash. This is how a change can insert a line and then insert another after it without knowing its own hash.

- `lines_blob` MAY replace inline `lines` when the inserted content exceeds 64 KiB; the blob's content is split into atoms by the same line rule.

- `Replace` is the only edit for `opaque` nodes. `old` is the blob currently expected; two concurrent `Replace` operations with the same `old` and different `new` conflict (§9.3).

### 7.5 Opaque nodes

A node's content mode is `opaque` if its content is not line-mergeable. A node MUST be opaque if it matches a `binary` pattern in `.oxyattributes`, if it contains a NUL byte in its first 8 KiB, if it exceeds `core.max-text-size` (default 4 MiB), or if it is a symlink. Symlink targets are stored as small blobs and edited only by `Replace`, bypassing the line graph. Opaque content is stored as a blob and versioned by `Replace`.

Content mode is part of the node's mode atom, so a switch is a `SetMode`. A change switching a node to `opaque` MUST also `Delete` every live atom of the node and `Replace` with `old = null`; a change switching to `text` MUST also `Replace` with `new = null` and `Insert` the content between `START` and `END`. Concurrent edits made under the other content mode then conflict through the mode atom (§9.2) rather than silently combining.

### 7.6 Line endings and encoding

Oxygen stores bytes verbatim and never rewrites line endings unless `.oxyattributes` requests `eol=lf` or `eol=crlf`, in which case normalization happens on snapshot and denormalization on checkout, exactly as Git does with `text eol=`. Implementations MUST NOT guess.

### 7.7 Checkout safety

Checkout writes only inside the workspace directory, whatever the tree contains.

- Checkout MUST create a symlink without following it and MUST NOT write any file through a path that contains a symlink, whether that symlink came from the repository or already existed in the workspace. Implementations SHOULD open parent directories with `O_NOFOLLOW` or the platform equivalent and write relative to that handle.

- A placement named `.oxy` under the root, or any placement that resolves to a path inside `.oxy/`, MUST be refused at snapshot and at checkout.

- `.oxyignore` and `.oxyattributes` files that are symlinks MUST be treated as absent, with a warning from `oxy status`.

- On file systems without symlink support, a symlink node is written as a plain file containing the target path, and `oxy status` reports it; snapshot MUST NOT convert such a file back into a content change.

## 8. Dependencies and commutation

### 8.1 Definition

The **scope** of a change `C` is the set of atoms and placements it references: every atom it deletes, every atom used as an anchor, every placement it deletes or moves, and every node it operates on.

`deps(C)` MUST contain the ChangeHash of the introducing change of every element of `scope(C)`, excluding `C` itself, START, END, and the root node. 

**Explicit (semantic) dependencies.** Changes that share no atoms commute even when they are semantically coupled, for example an API change in one file and the update of its caller in another. To prevent such changes from being separated, `deps(C)` MAY contain additional hashes that the author or tooling chose to require. `oxy commit --depends-on <change>` adds one; the argument is resolved to a ChangeHash as in §24, and a divergent ChangeId is an error. Implementations SHOULD support the `semantic-deps` hook (§20) so that build tools or language servers can inject cross-file dependencies at commit time.

An explicit dependency is a hard dependency with every consequence of §8.2 and §8.4: the change no longer commutes with its target, and rewriting the target orphans it. Tooling that injects dependencies MUST inject the minimal set and MUST be enabled per repository by the user, because injecting dependencies liberally turns history back into a linear sequence and forfeits the model's benefits. `deps(C)` MUST NOT be transitively reduced by the writer; readers MAY reduce it.

A change with `kind = resolution` MUST additionally depend on every change that contributed an atom or placement to the conflict it resolves.

### 8.2 Commutation

Two changes `A` and `B` **commute** if neither is in the transitive dependency closure of the other. By §7.3 applying `{A, B}` in either order produces the same graph, so no rewriting, re-hashing, or merge change is needed to reorder them. Commutation is a consequence of the model, not a separate rule to implement; the implementer's obligation is the invariant in §7.3.

### 8.3 Well-formedness

A state `S` is well-formed iff for every `C ∈ S`, `deps(C) ⊆ S`. Every reference to a state (bookmark, tag, checkpoint) names it by its heads, and the state is the dependency closure of the heads. Implementations MUST refuse to construct a state that is not well-formed and MUST refuse to store a change whose dependencies are unavailable unless the repository is shallow (§17).

### 8.4 Rewriting

Rewriting a change `C` to `C'` (amend, rebase, split, squash) produces a new object with the same ChangeId. Amending an older change with the working copy's edits (`oxy amend --into C`) is allowed when every atom, placement, and node those edits reference exists in `C`'s closure; the edits are appended to `C` with references to `C` turned into self references, so `C`'s own atoms keep their indices and dependents are rewritten with an identity correspondence. Splitting a head (`oxy split C PATH..`) rebuilds it as a first change holding its effect on the named paths and a second holding the rest under the original ChangeId. Any change `D` with `hash(C) ∈ deps(D)` is orphaned relative to `C'`. `oxy` MUST automatically rewrite orphaned descendants by translating their AtomRefs and PlacementRefs from `C` to `C'` where a correspondence exists. Where none exists the reference is **reified by re-anchoring**: an insert anchored on a vanished atom is anchored instead on the nearest surviving atom of the same insert chain of `C` (the preceding one for `after`, the following one for `before`), or on that chain's own anchors when none survives, so that an order conflict (§9.1) arises exactly when `C'` placed other content there; a deletion of a vanished atom is dropped; an operation or edit on a node that vanished with `C` is dropped. Each adjustment MUST be reported to the user. Removing `C` altogether rewires each dependent to depend on `deps(C)` and reifies references into `C` the same way. Changes that do not depend on `C` are untouched, and a rewritten change by another author is stored without that author's signature, which only they can renew. Rewriting is recorded in the operation log (§12) and is undoable.

## 9. Conflicts

### 9.1 Text conflicts

The live atoms of a file graph form a partial order. The file is **conflicted** if that order is not total. Two shapes arise:

- **Order conflict.** Two or more chains of live atoms are incomparable (concurrent inserts at the same anchor, or concurrent edits of overlapping regions). The conflict region is the minimal interval of the graph containing all incomparable chains.

- **Delete/insert conflict.** An atom `x` is deleted by change `D`, and a live atom `y` is anchored after `x` by a change `I`, where `D` and `I` commute. The inserted content has no live neighbour to attach to.

### 9.2 Tree conflicts

- **Name conflict.** Two live nodes share a placement.

- **Multi-placement.** One node has two live placements (concurrent renames).

- **Orphan.** A live node's parent directory is not live (concurrent delete of a directory and modification within it).

- **Cycle.** A directory is placed inside its own descendant, which arises when two commuting changes each move a directory under the other. Both placements are live and the tree is conflicted. For materialization the placement whose introducing change has the greater `(hlc, ChangeHash)` is treated as absent; resolution deletes one of the two placements.

- **Kind conflict.** Concurrent `AddNode` of the same placement with different kinds is a name conflict; concurrent `SetMode` with different modes is an order conflict on the mode atom.

### 9.3 Opaque conflicts

Two live `Replace` edits with the same `old` and different `new` are a **replace conflict**. The state exposes both candidate blobs.

### 9.4 Properties

- A conflicted state is well-formed and valid. It MAY be committed, pushed, fetched, tagged, and built upon.

- Conflicts nest and compose automatically because they are graph shapes, not special objects. A resolution that is itself concurrent with another resolution yields a new order conflict between the two resolutions.

- Servers MAY refuse conflicted heads on protected bookmarks through policy (§20). Clients MUST be able to push conflicted heads to unprotected bookmarks.

### 9.5 Materialization in the working copy

Conflicted text files are written with markers that identify the conflict and each side so that tools can parse them unambiguously:

```

<<<<<<< oxy conflict 1 of 2, side 1: change k7m2p9 "Add retry loop"

... lines ...

======= oxy conflict 1 of 2, side 2: change q3x8w1 "Refactor client"

... lines ...

>>>>>>> oxy conflict 1 of 2 end

```

The short identifiers are ChangeId prefixes. Delete/insert conflicts render the deleted context as a `-------` section. Tree conflicts are materialized by writing each candidate with a suffix `.oxy-conflict-<n>` and are listed by `oxy status`. Replace conflicts materialize as `<name>.oxy-conflict-<n>` files.

`oxy status` MUST list every conflict, MUST exit with status 2 when any conflict exists in the working-copy state, MUST exit with status 3 when only workspace conflicts (§9.7) exist, and MUST support `--json` output enumerating conflicts by node, kind, and contributing ChangeIds.

### 9.6 Resolution

A **resolution** is a change with `kind = resolution` whose deletions and insertions leave the affected graphs totally ordered and the tree conflict-free. `oxy resolve` computes it from the edited working copy by parsing the markers; a resolution MAY also be authored by any tool that can produce a change. Because a resolution depends on all contributing changes (§8.1), any peer that fetches it necessarily has the conflict it resolves.

### 9.7 Workspace conflicts

A state can be conflict-free and still not be representable on a particular file system. These are **workspace conflicts**: the state is valid, peers on other file systems see nothing, and no resolution change is involved.

- Two live placements in one directory whose names are equal under the file system's case folding (`Foo.c` and `foo.c` on default Windows and macOS volumes).

- Two live placements whose names are equal after the file system's Unicode normalization (NFC and NFD forms of the same name on macOS).

- A name the file system reserves or alters: Windows device names such as `CON` and `NUL`, names ending in a dot or a space, and names containing `:` or other characters the platform forbids.

- A path longer than the platform limit.

Each colliding node MUST be materialized with a `.oxy-conflict-<n>` suffix so that no content is lost, and `oxy status` MUST list the conflict with its kind. A workspace conflict is cleared by any ordinary change that renames one of the nodes; it is never counted by `allow_conflicts` (§14.5) or by server rules that forbid conflicted heads (§20).

## 10. States, bookmarks, and tags

### 10.1 References

References live in a namespace:

```

bookmarks/<name>                 local bookmarks

remotes/<remote>/bookmarks/<name>  last-seen remote bookmarks

tags/<name>                      tags (immutable once created)

policy                           the trust policy head (§14.5)

```

`<name>` is UTF-8, 1–255 bytes, without control characters, leading `-`, or the sequences `..`, `//`, `@{`.

A bookmark or tag points at a **state object**:

```

State { version [0]: 1, heads [1]: [ChangeHash] }   // sorted, unique; may be empty

```

Every reference, attestation, view, and checkpoint therefore names a state by one ObjectRef however many heads it has. Multiple heads are normal in this model: every independent change is a head until something depends on it, so a busy bookmark may have dozens. That is not a conflict in the §9 sense; `oxy log` shows the heads as parallel, and a state object keeps the cost of naming them constant for everything that refers to the bookmark. The empty state, `State { heads: [] }`, is what a new bookmark points at.

### 10.2 Reference attestations (G2, G3)

Every published bookmark update is accompanied by an attestation (§14.4) of kind `ref` that binds the repository ID, the reference name, the previous state, the new state, and a per-reference monotonically increasing `serial`. Servers MUST store the latest attestation per reference and serve it with the reference. Clients MUST verify the attestation against policy before updating a `remotes/` reference and MUST reject an attestation whose `serial` is lower than one they have already recorded for that reference (rollback protection). Clients SHOULD warn when the newest attestation's `hlc.wall_ms` is older than `remote.max-age` (default 30 days) and the remote claims no update.

Tags are attestations of kind `tag` and are never updated; a server MUST reject a second attestation for an existing tag name, and any receiver MUST refuse a tag whose name it already holds with a different state. `oxy tag create NAME [CHANGE..]` tags the named state (default the bookmark's heads); tags travel with `refs` and `push` (§16.3), and a policy rule for `tags/<name>` governs who may create them.

## 11. Checkpoints

Materializing a state by replaying every change is linear in history length. Checkpoints make checkout proportional to state size instead.

```

Checkpoint {

  version [0]: 1

  heads   [1]: [ChangeHash]        // sorted

  count   [2]: u64                 // changes in the state

  payload [3]: ObjectRef           // blob holding the serialized state

}

```

The payload blob is a deterministic CBOR map `{0: version, 1: changes, 2: tree, 3: files, 4: replaces}`:

- `changes`: for every change in the state, `[hash, change id, [deps], message, hlc wall, hlc logical, kind, author]`, sorted by hash. Operations are not repeated: once applied they live in the graphs, and the store keeps the change objects.

- `tree`: the tree graph, `{0: nodes [[node, kind, introducer]], 1: placements [[change, index, node, parent, name]], 2: retired placements [[change, index, [deleters]]], 3: modes [[change, index, node, mode]], 4: retired modes [[change, index, [deleters]]]}`.

- `files`: `[[node, file graph]]` where a file graph is `{0: atoms [[change, index, content or null, [deleters]]], 1: edges [[from, to]]}` with vertices encoded as in §5.5 (`0` start, `1` end, `[hash, index]`). Tombstones keep their content so conflicts can show deleted context.

- `replaces`: `[[node, [[change, old blob or null, new blob or null]]]]`.

A checkpoint is valid only if it is bit-for-bit reproducible from its heads. Implementations MUST create one at least every `core.checkpoint-interval` changes (default 256) along each bookmark and MAY discard any checkpoint at any time; keeping only the newest per bookmark is enough. To load a state from heads `H`, a client walks from `H` and stops at any change the checkpoint contains; the checkpoint applies iff every one of its heads was reached, since a state is closed downward, and the walk yields exactly the changes to replay on top. Clients fetching a state SHOULD request the nearest checkpoint and replay only the difference.

## 12. Operation log, workspaces, and the working copy

### 12.1 Operations

Every command that mutates local repository state (bookmarks, workspaces, the working-copy change, fetched remote references) records an operation:

```

Op {

  version [0]: 1

  parents [1]: [ObjectRef]            // usually one; two when merging concurrent operations

  hlc     [2]: HLC

  command [3]: text                   // e.g. "oxy rebase -s k7m2p9 -d main"

  view    [4]: { bookmarks: map<name, state ObjectRef>, remotes: map, tags: map, workspaces: map<WorkspaceId, ChangeHash> }

}

```

`view` is the complete state of all references and workspaces after the operation. The operation log is a DAG; concurrent processes each append to their own head, and the next command merges heads by three-way merge of views, with both sides kept when they disagree on a bookmark (the bookmark then has multiple heads).

`oxy undo` moves the current operation to its parent's view. `oxy redo` and `oxy op log` expose the DAG. Operations are retained for `core.op-retention` (default 90 days) and then eligible for garbage collection.

Because all mutation goes through the operation log and object writes are idempotent, no global repository lock is needed. A short-lived lock is held only while snapshotting a single workspace's working copy.

### 12.2 Workspaces

A repository has one or more workspaces, each with a `WorkspaceId` (128-bit random) and a checkout directory. Each workspace has exactly one **working-copy change** at any time. The primary workspace is the directory holding `.oxy/`; a secondary workspace holds a `.oxy` file instead, `oxydir: <path>` and `workspace: <id>`, pointing at the shared repository, and the repository records each workspace's directory under `.oxy/workspaces/<id>/`. `oxy workspace add PATH [CHANGE..]` creates one, `list` shows them, and `forget` drops one from the view without touching its files. Workspaces do not nest.

### 12.3 The working-copy change

The working copy is a real change with `kind = working-copy`. Before running any command that reads repository state, `oxy` snapshots the working copy: it diffs the file system against the working-copy change's materialized state and rewrites the working-copy change to include the difference. The change therefore always describes the working directory. To prevent operation log bloat, a command that snapshots the working copy MUST NOT record a new Operation (§12.1) if the filesystem diff was empty (i.e., the working-copy change's hash did not change).

- The working-copy change is unsigned and never pushed while `kind = working-copy`.

- `oxy commit` (alias `oxy describe`) sets the message, sets `kind = normal`, and starts a fresh working-copy change on top of it. The change is signed immediately if `core.sign = "change"`; with the default `core.sign = "seal"` it is left unsigned locally and covered by a seal at push time (§14.4).

- `oxy new`, `oxy edit <change>`, and `oxy abandon` move which change the workspace edits.

- There is no staging area and no stash. Partial commits are made with `oxy split`.

### 12.4 Snapshot performance

Implementations MUST support a file-system monitor (Watchman or a native inotify/FSEvents/ReadDirectoryChangesW watcher) and MUST fall back to a stat-based crawl with a persistent cache keyed by `(path, size, mtime, inode, ctime)`. Snapshot latency targets are in §25.

## 13. Storage engine

### 13.1 Layout

```

.oxy/

  format              "oxy repository format 2\n"

  config              repository-local configuration (TOML)

  objects/

    pack-<id>.pack    append-only pack file

    pack-<id>.idx     sorted (ObjectRef → offset, length, type) index

    loose/            objects written since the last pack, as header || body

    sigs/             signature lists keyed by the ObjectRef they cover (§5.4)

  ops/                operation objects and op heads

  refs/               current view cache (derived from ops; rebuildable)

  workspaces/<id>/    snapshot cache, fsmonitor cookie

  policy/             cached policy and identities

```

Object writes go to a temporary file, are `fsync`ed, and are renamed into place. Pack files are `fsync`ed before their index is written. An interrupted write can never corrupt an existing object. The refs cache is derivable from the operation log and is rebuilt if missing.

### 13.2 Chunking and blobs (G5)

File content is split with FastCDC (Xia et al., 2016) using a 32 KiB minimum, 64 KiB target, and 256 KiB maximum chunk size, with normalized chunking level 2 and cut-point skipping from the minimum size. Files of 32 KiB or less are a single chunk, and the final chunk of a file may be shorter than the minimum. The minimum and maximum bounds guarantee that adversarial content produces at most `len / 32 KiB` chunks and at least `len / 256 KiB` chunks, bounding both index growth and dedup loss.

So that every implementation produces identical boundaries, the parameters are fixed:

- **Gear table.** `GEAR[i]` for `i` in `0..256` is the little-endian `u64` at bytes `8i..8i+8` of the BLAKE3 extendable output of the ASCII string `oxy1 fastcdc gear`.

- **Rolling hash.** `h = (h << 1) + GEAR[byte]` in 64-bit wrapping arithmetic, starting from zero at the minimum size.

- **Masks.** A mask with `k` one-bits has them at bit positions `63 - floor(i * 64 / k)` for `i` in `0..k`. The small mask has 18 one-bits (used until the target size) and the large mask has 14 (used until the maximum). A cut occurs at the first position where `h & mask == 0`.

```

Blob { version [0]: 1, length [1]: u64, chunks [2]: [ObjectRef] }

```

Chunks and blobs are the only object types that can be large. Every other object type MUST be under 1 MiB after encoding; a change exceeding that MUST use `lines_blob`.

### 13.3 Compression

Each object in a pack is individually compressed with zstd (level 3 by default). Objects whose compressed size is not smaller are stored raw, with a flag. Chunks matching a `nocompress` attribute pattern (already compressed media) are stored raw.

### 13.4 Packing

`oxy gc` (and an automatic trigger when loose object count exceeds 4096) writes loose objects into a new pack and merges packs when there are more than 16. Pack indexes MUST support lookup by ObjectRef prefix and MUST be memory-mappable. A repository with two million objects MUST be openable with under 64 MiB of resident index memory on the Full profile.

The pack and index formats are fixed so that tools can read them without the implementation:

```

pack:  "OXYPACK1" || entry*

entry: type u8 || flags u8 || stored_len u32le || raw_len u32le || ref bytes(32) || body[stored_len]

index: "OXYIDX01" || count u32le || ( ref bytes(32) || offset u64le || stored_len u32le || raw_len u32le || type u8 || flags u8 )*

```

`type` indexes the object types in the order of §5.3 (chunk 0, blob 1, change 2, checkpoint 3, op 4, attest 5, identity 6, policy 7, state 8). Flag bit 0 means the body is zstd-compressed and `raw_len` is its decompressed size; otherwise the body is stored raw and the two lengths are equal. `offset` is the position of the body, not of the entry. Index entries are sorted by `ref`. A reader MUST re-hash every object it reads from a pack and reject a mismatch.

### 13.5 Lazy objects

A blob or chunk may be **absent** in a partial repository (§17). After a fetch with a blob filter, the repository records every blob its new changes reference but did not receive, together with the remote that withheld it (`.oxy/lazy/<ref>`), and fetches it through `objects` on first read, dropping the record once stored. Changes and checkpoints are never lazy: a repository that has a change has all of its inline data.

## 14. Identity, signatures, and trust policy

### 14.1 Algorithms

| Purpose | Algorithm |

|---|---|

| Hash | BLAKE3-256 |

| Signature (composite) | Ed25519 **and** ML-DSA-65 (FIPS 204) |

| Transport AEAD | ChaCha20-Poly1305 |

| Transport key agreement | X25519, with ML-KEM-768 hybrid where the Noise pattern supports it |

| Random | OS CSPRNG |

ML-DSA-65 is chosen over ML-DSA-44 for NIST security category 3, at the cost of signatures of 3309 bytes.

### 14.2 Identity object

```

Identity {

  version    [0]: 1

  ed25519_pk [1]: bytes(32)

  mldsa65_pk [2]: bytes(1952)

  name       [3]: text

  email      [4]: text

  created    [5]: u64 (wall ms)

  expires    [6]: u64 | null

  previous   [7]: KeyId | null        // rotation chain

  delegates  [8]: [KeyId]             // keys allowed to sign or seal on this identity's behalf

}

KeyId = ObjectRef of the Identity

```

The two public keys live in one object, so a `KeyId` names the pair. `delegates` lists keys that may authenticate changes whose `author` is this identity: a proxy acting for a Thin client (§22), or a CI signer. Delegation is one level deep and is not transitive. Rotation is a new identity with `previous` set, plus an attestation of kind `rotate` signed by the previous key. Revocation is an attestation of kind `revoke` signed by the revoked key or by a policy root key (§14.5).

**Lifecycle rules.** Because the `KeyId` is the hash of the whole object, any change to an identity, including its `delegates` or `expires`, is a rotation: a successor object with `previous` set. A successor MAY keep the same key pair (`oxy identity renew`, `oxy identity delegate add|remove`) or carry a fresh one (`oxy identity rotate`); either way earlier signatures remain verifiable under the earlier object, which stays stored. A rotation **supersedes** the previous KeyId: a policy that named it MUST be updated to name the successor (§14.5), and a rule key covers every predecessor in its `previous` chain that is not revoked, so changes signed before a rotation remain authorized. Revocation is for compromised or abandoned keys: a revoked key authorizes nothing, whatever its chain. `expires` bounds the changes a key can authenticate: a signature or seal over a change whose `hlc.wall_ms` is later than the identity's `expires` is invalid. A client records the revocations it makes or verifies under `.oxy/revoked/` and reports signatures by such keys as valid but revoked.

### 14.3 Composite signature (G2, G4)

```

Signature {

  key  [0]: KeyId

  alg  [1]: 1                          // ed25519+mldsa65

  ed   [2]: bytes(64)

  ml   [3]: bytes(3309)

}

message = "oxy1 sig " || domain || 0x00 || KeyId || ObjectRef-being-signed

domain  = "change" | "attest" | "policy"

```

Both signatures are computed over the same `message`, which includes the `KeyId` of the composite pair. Verification MUST require both signatures to verify with the keys of the identity named by `key`. A signature with either half missing, empty, or failing is invalid. This is the same construction as the IETF LAMPS composite signature drafts and prevents stripping the post-quantum half.

Profiles that cannot compute ML-DSA (§22) do not sign; they produce changes that a proxy signs.

### 14.4 Attestations

```

Attest {

  version [0]: 1

  kind    [1]: "ref" | "tag" | "seal" | "rotate" | "revoke" | "lock"

  repo    [2]: bytes(16)               // repository ID, chosen at init

  hlc     [3]: HLC

  body    [4]: map                     // per kind

}

ref:    { name, old_state: ObjectRef, new_state: ObjectRef, serial: u64 }

tag:    { name, state: ObjectRef, message }

seal:   { hashes: [ChangeHash] }        // sorted, unique, 1..4096 entries

rotate: { new: KeyId }

revoke: { key: KeyId, reason }

lock:   { node: NodeId, path, acquire: bool, expires }

```

A **seal** is one signature over a batch of change hashes. A composite signature is 3373 bytes, which is larger than a typical change, so signing every change individually roughly doubles the size of history in high-frequency workflows. A seal replaces that with one signature per push.

**Cadence.** With the default `core.sign = "seal"`, changes are left unsigned locally and `oxy push` creates one seal covering every change in the push that carries no individual signature, immediately before the reference attestation. Seals are created only at push: a seal made earlier would be invalidated by any amend, since rewriting produces a new hash. With `core.sign = "change"`, every change is signed at commit and no seal is needed. Policy (§14.5) decides which the receiving repository accepts.

**Verification.** A sealed change `C` is authenticated iff a seal `S` exists such that `hash(C) ∈ S.body.hashes`, `S` carries a valid signature from key `K`, and `K` equals `C.author` or is listed in `C.author`'s identity as a delegate (§14.2). The same author-or-delegate rule applies to individual change signatures. This rule is what stops a collaborator from sealing a change whose `author` field names someone else (§3.1). A seal's signer must additionally be authorized by policy for the reference being pushed.

**Handling.** A seal is stored and transported with the changes it covers: `fetch` (§16.1) streams the seals covering every change it returns, `push` (§16.4) MUST include a seal or signature for every change, and a seal covering any reachable change is itself reachable (§21). Seals from different keys MAY overlap; a change needs at least one valid authenticator, and policy thresholds count distinct authenticating keys.

### 14.5 Trust policy (G2)

The `policy` reference points to a signed `Policy` object:

```

Policy {

  version   [0]: 1

  repo      [1]: bytes(16)

  serial    [2]: u64

  roots     [3]: { keys: [KeyId], threshold: u32 }

  rules     [4]: [ { pattern: glob over reference names, keys: [KeyId], threshold: u32,

                     require: "signed" | "sealed" | "any" /* default */, allow_conflicts: bool } ]

  identities[5]: [Identity]           // embedded so verification needs no fetch

  revoked   [6]: [KeyId]

  expires   [7]: u64

}

```

- The initial policy is signed by the root keys; every update MUST be signed by `roots.threshold` root keys of the policy it replaces and have a higher `serial`. An update that a single holder cannot complete is a **proposal**: the signed policy (`[policy bytes, [Signature]]`, §16.3) is passed to other root holders, each of whom adds a signature with `oxy policy sign`; whoever completes the threshold applies it. Rotating a root key (§14.2) substitutes the successor in the roots and rules; such a policy is signed by both the previous and the successor key so a fresh clone can bootstrap from it.

- A change's signer is authorized by a rule if the rule lists it, or lists a successor of it, or lists the change's `author` (or a successor) when the signer is that author's delegate.

- A change or attestation is **authorized** for reference `r` if its signer is listed in the first rule whose pattern matches `r`, and the rule's threshold of distinct authorized signers is met across the attestation and each change in the closure of `new_state` not previously in the closure of `old_state`. `require` selects which authenticators count for changes: individual signatures only, seals only, or either. Every change MUST additionally satisfy the author-or-delegate rule of §14.4 regardless of `require`.

- Clients MUST verify policy on fetch, MUST refuse to update a `remotes/` reference whose attestation is unauthorized, and MUST warn loudly on an expired policy. Servers MUST enforce the same rules on push.

- A repository without a `policy` reference is **unverified**; every command that talks to a remote MUST print a one-line warning until one is created. `oxy policy init` creates one with the author's key as the sole root; `oxy policy grant` and `oxy policy revoke` produce successors.

- **Propagation.** The `refs` response carries the current signed policy together with the earlier policies the server still holds, and a push MAY carry a newer policy with the intermediate ones in its bundle, applied before any bookmark update. A receiver behind by several serials verifies each step against the one before it. A receiver accepts a policy only if it is for the same repository, its serial exceeds the current one, and it is signed by the current policy's root threshold; the first policy a repository ever sees vouches for itself, which means at least one valid signature by one of its own roots (trust on first use).

- **Downgrade protection.** A client records the highest policy serial each remote has presented. A remote that later presents none, or a lower serial, is refused. A remote that has never presented a policy is unverified: the client fetches with a warning and cannot verify what it received. Policy signatures use the domain `policy` (§14.3).

- **Existing history.** A policy governs changes added after it. The receiver of a push checks the changes the new state adds beyond the state it already had, so history that predates the policy is not re-verified. A fetching client verifies the bookmark's attestation, which binds the whole state through its head hashes, and does not re-verify individual changes; it therefore trusts what an authorized attester vouched for, which is the point of the policy.

## 15. Transport

Every transport carries the same wire protocol (§16). The transport provides the channel; integrity and authenticity of history come from the object model regardless of channel.

### 15.1 HTTPS (mandatory)

Every server MUST offer HTTPS with TLS 1.3 at `https://<host>/<repo>/oxy/v1/`. Authentication is by bearer token, mutual TLS, or HTTP signature over a server-issued challenge with the client's identity key. HTTPS is required because it traverses corporate proxies, load balancers, and CDNs; it is the interoperability baseline. A reference server MAY speak plain HTTP behind a TLS-terminating proxy; it MUST NOT be exposed without TLS except on a loopback or otherwise trusted network. Reads MAY be open; a push MUST be authenticated when any credential is configured.

### 15.2 Noise (optional)

Peer-to-peer and self-hosted deployments MAY use Noise over TCP or QUIC.

- Pattern `Noise_XXhfs_25519+kyber768_ChaChaPoly_BLAKE2s` (hybrid post-quantum handshake) for first contact, or `IKhfs` when the server's static key is known.

- The client's Noise static key MUST be the X25519 key derived from its Ed25519 identity key (RFC 8032 key clamping via the standard birational map), so the transport identity and the signing identity are the same KeyId.

- Servers publish their static key fingerprint out of band. Clients pin it on first use and MUST refuse a changed key without explicit `oxy remote trust` action.

### 15.3 Zero-Crypto Transport (ZCT, restricted)

ZCT is a plaintext, length-prefixed framing of the wire protocol over TCP or serial, for Thin clients that cannot run ChaCha20 or X25519 at usable speed.

- ZCT is disabled by default on servers and on clients. A remote MUST be explicitly configured with `transport = "zct"` and clients MUST print a warning on every use.

- ZCT provides no confidentiality. Over ZCT the client MUST verify every change signature or seal, every reference attestation, and every ObjectRef it receives; a client incapable of verification (Thin profile without Ed25519) MUST only use ZCT through a trusted proxy on a network it trusts, and this MUST be a documented deployment decision.

- Push over ZCT is permitted only with a reference attestation signed by an authorized key; there is no other authentication. Servers MAY refuse push over ZCT entirely.

- ZCT MUST NOT be offered on a public network interface by default.

### 15.4 Proxy

A **proxy** is a Full-profile `oxy` server that fronts one or more remotes for Thin clients, performing verification, chunking, delta computation, and signing on their behalf. Its security properties are those of the Thin client's trust in it.

## 16. Wire protocol

Messages are deterministic CBOR. Over HTTPS each message is a request body with `Content-Type: application/oxy-cbor` and responses are CBOR sequences (RFC 8742) with `Content-Encoding: zstd` when supported. Over Noise and ZCT, messages are framed as `u32 big-endian length || bytes`.

### 16.1 Endpoints

| Endpoint | Request | Response |

|---|---|---|

| `GET capabilities` | — | `{ versions: [1], transports, features: ["lazy", "checkpoint", "seal", "lock"], max_frame }` |

| `GET refs` | `{ patterns }` | `[ { name, state: ObjectRef, heads, attestation: Signed<Attest> } ]` plus the current `Signed<Policy>` |

| `POST negotiate` | `{ have: [ChangeHash], want: [ChangeHash], filter }` | stream of `ChangeHash` the client lacks, in dependency order |

| `POST fetch` | `{ changes: [ChangeHash], include_blobs: bool, filter }` | stream of `Signed<Change>`, then every `Signed<Attest kind=seal>` covering them, then referenced `blob`/`chunk` objects |

| `POST objects` | `{ refs: [ObjectRef] }` | stream of objects |

| `GET checkpoint` | `{ heads, filter }` | nearest `Checkpoint` and its objects, and the list of changes to replay |

| `POST push` | stream of `Signed<Change>`, objects, seals, then `Signed<Attest kind=ref>` per reference | `{ results: [ { name, ok, reason } ] }` |

| `POST lock` | `Signed<Attest kind=lock>` | `{ ok, holder, expires }` |

### 16.2 Negotiation

The client sends the heads it has for the requested references and the heads it wants. Because dependencies form a DAG and every state is its heads' closure, the server computes `closure(want) \ closure(have)` by a reverse traversal, stopping at any hash in `have`. For very large histories a server MAY ask the client for the Merkle root of its known-hash set in 2^16-hash-prefix buckets and skip buckets whose roots match. The response is ordered so that no change precedes any of its dependencies.

### 16.3 Message encodings

Every message is a deterministic CBOR map. Signatures use the encoding of §5.5; a change or attestation travels as its canonical bytes so the receiver hashes exactly what was signed.

| Message | Encoding |

|---|---|

| Signed attestation | `[attest bytes, [Signature]]` |

| `Bundle` | `{0: [[change bytes, [Signature]]], 1: [signed attestation], 2: [[type name, object bytes]], 3: [signed policy]}`; changes in dependency order; objects are chunks, blobs, identities, and states only; key 3 (optional) holds the policies between the receiver's serial and the one sent with a push, ascending |

| `capabilities` response | `{0: [versions], 1: repo id bytes(16), 2: [feature names]}` |

| Signed policy | `[policy bytes, [Signature]]` |

| `refs` response | `{0: [{0: name, 1: state ref, 2: [heads], 3: signed attestation or null, 4: last serial or null}], 1: repo id, 2: signed policy or null, 3: [tag], 4: [signed policy]}`; key 4 (optional) lists earlier policies ascending by serial; the attestation is present only when it describes the current state; a tag is `{0: name, 1: state ref, 2: [heads], 3: signed attestation or null}` |

| `negotiate` request | `{0: [have], 1: [want]}`; response `{0: [hashes]}` in dependency order |

| `fetch` request | `{0: [hashes], 1: max blob size or null}`; response a `Bundle` without blobs above the size |

| `objects` request | `{0: [refs]}`; response a `Bundle` carrying only objects, a blob accompanied by its chunks |

| `checkpoint` request | `{0: [heads]}`; response a `Bundle` of objects: the checkpoint of exactly those heads, its payload blob, and the blob's chunks |

| `shallow` request | `{0: [heads], 1: depth}`; response `{0: [hashes]}`, the changes within `depth` steps of the heads, dependencies first |

| `push` request | `{0: Bundle, 1: [{0: name, 1: old state, 2: new state, 3: force, 4: signed attestation or null}], 2: signed policy or null, 3: [tag]}`; response `{0: [{0: name, 1: ok, 2: reason}]}`, where a result named `policy` reports the policy update and `tags/<name>` each tag |

The HTTP content type is `application/oxy-cbor`.

### 16.4 Push validation

On push, the server MUST, in order: verify that every change has a valid signature or seal from a key embedded in policy and satisfies the author-or-delegate rule (§14.4), verify the attestation for each reference, check that `old_state` matches its current state (compare-and-swap), check policy rules for the reference including `allow_conflicts`, check that every dependency of every change is either included or already stored, and only then update references atomically. A failure rejects the whole push.

## 17. Partial, sparse, and shallow repositories

A repository records a **filter** in `.oxy/config`:

```

[filter]

paths    = ["src/", "docs/"]      # sparse: only these subtrees are materialized and blobs fetched

max-blob = "1MiB"                 # blobs larger than this are lazy

depth    = 100                    # shallow: history beyond this many changes from heads is not stored

```

- **Sparse** repositories hold the full change graph but only materialize the listed paths: the working copy contains nothing else, files outside the filter are neither reported nor removed, and changes touching other paths are stored and applied to the graph. A sparse clone sets `max-blob` to zero unless a size is given, so blobs outside the filter are fetched only if the filter later widens (§13.5). `oxy sparse PATH..` changes the filter of a clean working copy and re-checks it out.

- **Lazy blobs** are fetched via `objects` on first read. `oxy status` and `oxy commit` MUST NOT need them.

- **Shallow** repositories hold a checkpoint of their bookmark's heads (obtained with the `checkpoint` message) and the changes within `depth` dependency steps of those heads (the `shallow` message lists them). The *shallow boundary* is the set of dependencies of stored changes that are not stored (`.oxy/shallow`). Dependencies into the boundary are accepted as well-formed, negotiation stops at the boundary (the remote is assumed to hold it), and the log shows every change from the checkpoint's metadata. `oxy deepen N` fetches `N` more levels below the boundary; when nothing remains the repository is no longer shallow.

- Thin clients (§22) always use `depth = 0`: a checkpoint plus zero history.

Servers MUST honour `filter` on `negotiate`, `fetch`, and `checkpoint`.