fideus-labs · thewtex · Aug 26, 2026 · Aug 25, 2026 · Aug 25, 2026 · Aug 25, 2026
diff --git a/README.md b/README.md
@@ -45,6 +45,8 @@ The main Python package provides:
 - Optional OME-Zarr data model validation during reading
 - Writes OME-Zarr v0.4 to v0.6
 - v0.6 adds RFC-5 coordinate systems and transformations
+- Reads and writes the opt-in development version `0.9.dev1`, which adopts RFC-3
+  (expanded axis count, names, types and order)
 - [Sharded Zarr] stores
 - Optional writing via zarr-python 2, zarr-python 3, [zarrista] or zarrita (TypeScript)
 - [Anatomical orientation metadata](./docs/rfc4.md) (RFC-4)
@@ -67,7 +69,7 @@ The TypeScript package provides universal OME-Zarr support for modern JavaScript
 - 🦕 **Deno-first** with first-class TypeScript support
 - 📦 **Universal compatibility** - Works in Deno, Node.js, and browsers
 - 🔍 **Type-safe** with Zod schema validation
-- 🗂️ **OME-Zarr v0.4, v0.5, and v0.6** support (v0.6 adds RFC-5 coordinate systems and transformations)
+- 🗂️ **OME-Zarr v0.4, v0.5, and v0.6** support (v0.6 adds RFC-5 coordinate systems and transformations), plus the opt-in development version `0.9.dev1` which adopts RFC-3 (expanded axis count, names, types and order)
 - 🌐 **Web ready** - No filesystem dependencies, works with remote stores
 - 🏗️ **Mirrors Python API** - Familiar interfaces for Python users
 - 📚 **Lazy loading** - Efficient handling of large datasets

diff --git a/docs/cli.md b/docs/cli.md
@@ -196,8 +196,10 @@ ngff-zarr upgrade src.zarr -o dst.zarr --to 0.5
 ```
 
 The target version is selected with `--to` (alias `--version`), one of `0.4`,
-`0.5`, or `0.6` (default `0.6`). Add `--validate` to validate the source
-metadata against the NGFF schema while reading. For the write-to-new-store mode,
+`0.5`, `0.6`, or `0.9.dev1` (default `0.6`). `0.9.dev1` is the development
+version that adopts RFC-3; OME publishes no JSON Schema for it yet, so
+`--validate` cannot check a store at that version. Add `--validate` to validate
+the source metadata against the NGFF schema while reading. For the write-to-new-store mode,
 `--overwrite` (the default) replaces any pre-existing data at the output store,
 while `--no-overwrite` refuses to; both flags are ignored for an in-place
 upgrade, which never overwrites array data.

diff --git a/docs/python.md b/docs/python.md
@@ -213,7 +213,7 @@ To read an OME-Zarr file, use [`from_ngff_zarr`], which returns the
 >>> multiscales = nz.from_ngff_zarr('cthead1.ome.zarr')
 ```
 
-OME-Zarr version 0.1 to 0.6 is supported. Version 0.6 adds RFC-5 coordinate systems and transformations.
+OME-Zarr version 0.1 to 0.6 is supported. Version 0.6 adds RFC-5 coordinate systems and transformations. The opt-in development version `0.9.dev1` additionally adopts RFC-3, that expands supported axis counts, names, types and order.
 
 The `store` argument accepts:
 
@@ -551,8 +551,10 @@ boundary (0.5/0.6 to 0.4) -- cannot preserve chunk keys and raises a
 
 **Write-to-new-store.** When an `output` store distinct from `input` is given,
 the source is read lazily and re-written to `output` at the requested version
-through the standard write pipeline. Every supported transition (0.4, 0.5, 0.6,
-in either direction) works in this mode, and the source store is never erased.
+through the standard write pipeline. Every transition among 0.4, 0.5, 0.6 and
+0.9.dev1 works in this mode whenever the target version can express the axis
+model, and the source store is never erased. An RFC-3 axis model is refused
+below 0.9.dev1, so a store using one only converts upward.
 
 Upgrade a 0.5 store to 0.6 in place, keeping every array chunk:
 

diff --git a/docs/spec_features.md b/docs/spec_features.md
@@ -38,6 +38,11 @@ supported by `ngff-zarr`.
 - **OME-Zarr v0.4 to v0.6**: Writes OME-Zarr versions 0.4 to 0.6, including
   RFC-4 anatomical orientation. v0.6 additionally adds RFC-5 coordinate systems
   and transformations.
+- **OME-Zarr 0.9.dev1**: Reads and writes the development version that adopts
+  RFC-3, which extends support for the number, names, types and order
+  of axes. It is opt-in: pass `version="0.9.dev1"` explicitly. The default
+  target is unchanged, and OME publishes no JSON Schema for it yet, so schema
+  validation is unavailable at that version.
 
 ## High Content Screening (HCS)
 

diff --git a/docs/typescript.md b/docs/typescript.md
@@ -9,7 +9,7 @@ NGFF-Zarr provides a TypeScript implementation for working with OME-Zarr data st
 - 🦕 **Deno-first**: Built for Deno with first-class TypeScript support
 - 📦 **Universal compatibility**: Works in Deno, Node.js, and browsers
 - 🔍 **Type-safe**: Full TypeScript support with Zod schema validation
-- 🗂️ **OME-Zarr support**: Read and write OME-Zarr v0.4, v0.5, and v0.6 (v0.6 adds RFC-5 coordinate systems and transformations)
+- 🗂️ **OME-Zarr support**: Read and write OME-Zarr v0.4, v0.5, and v0.6 (v0.6 adds RFC-5 coordinate systems and transformations), plus the opt-in development version `0.9.dev1` which adopts RFC-3 (extended axis count, names, types and order)
 - 🧪 **Well-tested**: Comprehensive test suite with browser validation
 - 🏗️ **Mirrors Python API**: Familiar interfaces for Python users
 - 📖 **Lazy loading**: Efficient handling of large datasets
@@ -378,7 +378,7 @@ async function fromNgffZarr(
   store: string | MemoryStore | FetchStore,
   options?: {
     validate?: boolean;
-    version?: "0.4" | "0.5" | "0.6";
+    version?: "0.4" | "0.5" | "0.6" | "0.9.dev1";
   }
 ): Promise<NgffMultiscales>
 ```
@@ -414,7 +414,7 @@ async function toNgffZarr(
   store: string,
   multiscales: NgffMultiscales,
   options?: {
-    version?: "0.4" | "0.5" | "0.6";
+    version?: "0.4" | "0.5" | "0.6" | "0.9.dev1";
     chunksPerShard?: number | number[] | Record<string, number>;
   }
 ): Promise<void>
@@ -815,7 +815,7 @@ function upgradeOmeZarr(
   input: string | MemoryStore | FetchStore | Readable,
   options?: {
     output?: string | MemoryStore; // FetchStore is read-only, not a destination
-    version?: "0.4" | "0.5" | "0.6"; // default "0.6"
+    version?: "0.4" | "0.5" | "0.6" | "0.9.dev1"; // default "0.6"
     validate?: boolean;
     overwrite?: boolean; // write-to-new-store only; default true
   },
@@ -870,8 +870,10 @@ await upgradeOmeZarr(store, { version: "0.6" });
 
 **Write-to-new-store.** When `output` is a store distinct from `input`, the
 source is read lazily and re-written to `output` at the requested version
-through the standard write pipeline. Every supported transition (0.4, 0.5, 0.6,
-in either direction) works in this mode, and the source store is never mutated.
+through the standard write pipeline. Every transition among 0.4, 0.5, 0.6 and
+0.9.dev1 works in this mode whenever the target version can express the axis
+model, and the source store is never mutated. An RFC-3 axis model is refused
+below 0.9.dev1, so a store using one only converts upward.
 
 ```typescript
 import { upgradeOmeZarr, type MemoryStore } from "@fideus-labs/ngff-zarr";

diff --git a/docs/validation/api.md b/docs/validation/api.md
@@ -41,11 +41,13 @@ from ngff_zarr import (
 )
 ```
 
-`validate_structural(metadata, options=None)` runs the image/multiscales rules
-against either metadata model — the flat v0.4/v0.5 `Metadata` or the v0.6
-`coordinateSystems` one that `from_ome_zarr` and `to_multiscales` return (see
-[[overview]]). When `options` is `None` it uses `ValidateOptions()`, i.e.
-`ValidationLevel.STRICT`. A `ValidationError` carries `.rule` (a `SpecRule`),
+`validate_structural(metadata, options=None, version=None)` runs the
+image/multiscales rules. When `options` is `None` it uses `ValidateOptions()`,
+i.e. `ValidationLevel.STRICT`. `version` is the OME-Zarr version the metadata
+declares; the axis count, type and order rules are inert for the versions that
+adopt the RFC-3 axis model (see [[parity]]), so omitting it holds every store to
+the v0.4 axis caps. `axis-names-unique` is never inert: RFC-3 states it and no
+released schema carries it. A `ValidationError` carries `.rule` (a `SpecRule`),
 `.message` (str), and `.location` (`str | None`); `str(exc)` is
 `Spec rule [<rule>] violated: <message>`.
 
@@ -99,9 +101,12 @@ import {
 } from "@fideus-labs/ngff-zarr";
 ```
 
-`validateStructural(metadata, options?)` runs the image/multiscales rules. When
-`options` (or its `level`) is omitted, the level resolves to
-`ValidationLevel.Strict`. A `ValidationError` carries a readonly `rule`
+`validateStructural(metadata, options?, version?)` runs the image/multiscales
+rules. When `options` (or its `level`) is omitted, the level resolves to
+`ValidationLevel.Strict`. `version` is the OME-Zarr version the metadata
+declares; the axis rules are inert for the versions that adopt the RFC-3 axis
+model (see [[parity]]), so omitting it holds every store to the v0.4 axis
+caps. A `ValidationError` carries a readonly `rule`
 (`SpecRule`) and an optional `location` (`string`); its `message` is
 `Spec rule [<rule>] violated: <message>`.
 

diff --git a/docs/validation/parity.md b/docs/validation/parity.md
@@ -26,7 +26,7 @@ for usage, see [[api]].
 
 ## The contract
 
-Both ports must agree on four observable dimensions:
+Both ports must agree on five observable dimensions:
 
 1. **Rule identifiers** — the same `SpecRule` string values, in the same
    canonical declaration/iteration order.
@@ -38,6 +38,10 @@ Both ports must agree on four observable dimensions:
    (`validate_structural` / `validateStructural`) evaluates rules in the same
    canonical order, so the same metadata yields the same first violation in
    both languages.
+5. **The RFC-3 version set** — the four axis rules take a `version` and are
+   inert for the versions that adopt the RFC-3 free-form axis model. Both ports
+   must treat exactly the same version strings as RFC-3, or the same metadata
+   validates in one language and not the other.
 
 Because both test suites assert these facts against the **same literal
 identifier list**, adding, removing, renaming, or reordering a rule — or
@@ -53,23 +57,34 @@ The canonical `SpecRule` set, in evaluation order:
 1. `axis-count`
 2. `axis-type`
 3. `axis-order`
-4. `scale-length-mismatch`
-5. `global-coord-transform-after-per-level`
-6. `dataset-order-highest-to-lowest`
-7. `omero-channel-color-format`
-8. `axis-orientation-anatomical-type`
-9. `axis-orientation-on-non-space`
-10. `axis-orientation-unique-axis`
-11. `zarr-format`
-12. `ome-namespace`
-13. `plate-row-index-consistency`
-14. `well-acquisition-missing`
-
-Entries 11–12 are the v0.5 namespacing rules; they fire only for v0.5 metadata
+4. `axis-names-unique`
+5. `scale-length-mismatch`
+6. `global-coord-transform-after-per-level`
+7. `dataset-order-highest-to-lowest`
+8. `omero-channel-color-format`
+9. `axis-orientation-anatomical-type`
+10. `axis-orientation-on-non-space`
+11. `axis-orientation-unique-axis`
+12. `zarr-format`
+13. `ome-namespace`
+14. `plate-row-index-consistency`
+15. `well-acquisition-missing`
+
+Entries 12–13 are the v0.5 namespacing rules; they fire only for v0.5 metadata
 and are inert for v0.4. Each suite pins this list as a `CANONICAL_SPEC_RULE_IDS`
 literal — byte-identical between the two languages so the tests are
 line-for-line comparable.
 
+The versions that adopt the RFC-3 axis model are pinned the same way, as a
+`CANONICAL_RFC3_VERSIONS` literal:
+
+1. `0.9.dev1`
+
+Rules 1–3 (and the spatial arm of 3) are inert at those versions and enforced
+at every other. Rule 4, `axis-names-unique`, is never inert: RFC-3 *adds* it,
+and ngff-zarr applies it at all versions as a strictness choice (see
+[[rule-reference]]).
+
 ## The parity tests
 
 | Language   | Test file |
@@ -93,17 +108,21 @@ Each suite independently locks:
   `strict`, and `strict` is the default.
 - **Fail-fast evaluation order** — driving the orchestrator with v0.4 metadata
   that violates the earliest rule plus every later rule, then repairing exactly
-  one rule per stage across ten stages, the rule raised at each stage builds a
-  sequence equal to a shared `EXPECTED_EVALUATION_ORDER`. This proves every rule
-  is evaluated strictly before all rules after it; a final, fully-repaired
+  one rule per stage across eleven stages, the rule raised at each stage builds
+  a sequence equal to a shared `EXPECTED_EVALUATION_ORDER`. This proves every
+  rule is evaluated strictly before all rules after it; a final, fully-repaired
   metadata is accepted with no violation.
-
-The two v0.5 namespacing rules (`zarr-format`, `ome-namespace`) run last in the
-image orchestrator but are inert for the v0.4 metadata the order test exercises,
-so they never appear in `EXPECTED_EVALUATION_ORDER`. The two HCS rules
-(`plate-row-index-consistency`, `well-acquisition-missing`) are deliberately
-absent from the image orchestrator's evaluation order; they are dispatched by
-the separate plate/well orchestrators (see [[rule-reference]]).
+- **RFC-3 version set** — the axis rules are inert at exactly the versions in
+  `CANONICAL_RFC3_VERSIONS` and enforced at every other supported version and
+  when no version is given, asserted through the public orchestrator rather
+  than through the internal predicate.
+
+Four of the fifteen rules never appear in `EXPECTED_EVALUATION_ORDER`, each for
+its own reason. The two v0.5 namespacing rules (`zarr-format`, `ome-namespace`)
+run last in the image orchestrator but are inert for the v0.4 metadata the order
+test exercises. The two HCS rules (`plate-row-index-consistency`,
+`well-acquisition-missing`) are deliberately absent; they are dispatched by the
+separate plate/well orchestrators (see [[rule-reference]]).
 
 ## Sanctioned TypeScript-only adaptations
 
@@ -115,12 +134,12 @@ departures in the TypeScript suite:
   Python an `UPPER_SNAKE` `str, Enum` (`SpecRule.AXIS_COUNT`,
   `ValidationLevel.STRICT`). The `.value` strings are identical, so the
   cross-language assertions hold.
-- **Axis-name labels and the default check** — TypeScript's ordering fixtures
-  use valid `SupportedDims` members in place of the Python helper's free-form
-  axis names, and assert the `strict` default *observably* (because the
-  TypeScript options type is a bare interface with no constructor) rather than
-  through a constructable options object. Neither alters the observed rule set
-  or evaluation order.
+- **The default check** — TypeScript asserts the `strict` default *observably*,
+  because its options type is a bare interface with no constructor, rather than
+  through a constructable options object. This alters neither the observed rule
+  set nor the evaluation order. The ordering fixtures themselves are identical
+  in both ports: `Axis.name` is `AxisName`, so TypeScript uses the same
+  free-form axis names as Python.
 
 Internal message-fidelity helpers in the TypeScript port (rendering whole-number
 scales with a trailing `.0`, Python-style quoted name lists, and `repr`-style