diff --git a/AGENTS.md b/AGENTS.md
index 6484821..0d43834 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -34,7 +34,7 @@ Releases: follow [the release procedure](docs/releasing.md) and `scripts/Publish
 
 - `OpenNest.Core`: domain (`Nest -> Plate -> Part -> Drawing -> CNC.Program`), geometry, cutting strategies and diagnostics. Angles are radians; use `Tolerance.Epsilon` for geometry comparisons. `OpenNest.Math` shadows `System.Math`, so qualify the latter.
 - `OpenNest.Engine`: whole-job API in `Jobs/`, interactive proposals via `PlateFillService`, fill strategies, best-fit pairs, packing, sequencing and rapid planning. `INestingEngine.Solve(NestJob)` returns stock IDs/poses; boundary adapters map drawings and materialize results. `NestJobRunner` validates its candidates before committing demand/stock accounting. Do not assume arbitrary plug-in output or interactive paths received that validation. Job identity is reference-based, not drawing-name-based.
-- Engine plug-ins implement `INestingEngine` with a public parameterless constructor, reference Engine and load from `Engines/` beside the host. Build them outside this repository/solution; do not add their projects here.
+- Built-in whole-job engines live in `OpenNest.Engine/NestingEngines/<Name>/`, named for the jobs they suit; see [nesting engines](docs/nesting-engines.md). A change must beat that engine's current benchmark result with every layout valid. External plug-ins implement `INestingEngine` with a public parameterless constructor and load from `Engines/` beside the host; keep their projects out of this solution.
 - `OpenNest.IO`: ACadSharp import/export and ZIP-based `.nest` persistence. All DXF-to-Drawing conversion goes through `CadImporter`: `Import` + `BuildDrawing` for editable/reporting flows, `ImportDrawing` for headless callers. Preserve source offsets, entity IDs, suppressed entities and bends. Bend repair is opt-in, requires explicit source units and may not alter cut geometry or unrelated marks.
 - `OpenNest`: WinForms UI (`Forms/`, `Controls/PlateView`, `Actions/`). `OpenNest.Data` holds cross-platform persistence; new-nest defaults live in `%APPDATA%\OpenNest\defaults.json`. Posts live in `Posts/OpenNest.Posts.<Name>/` and deploy to the desktop output's `Posts/` directory.
 - `OpenNest.Console`, `OpenNest.Mcp`, `OpenNest.Api`: front ends; `OpenNest.Benchmark`: whole-job engine comparisons; `OpenNest.Gpu`: GPU evaluators; `OpenNest.Training`: ML data collection. Benchmark timing comparisons require `--parallel 1`; validate layouts and fulfillment, not just elapsed time.
diff --git a/README.md b/README.md
index e0fbc03..e291fcf 100644
--- a/README.md
+++ b/README.md
@@ -20,7 +20,7 @@ A Windows desktop application for CNC nesting — imports DXF drawings, arranges
 - Windows 10+ for the desktop app; the console, API, and most test projects build on Linux/macOS too.
 - [.NET 8 SDK](https://dotnet.microsoft.com/download/dotnet/8.0) to build from source.
 
-Windows release ZIPs are self-contained: extract the entire archive into a new folder and run `OpenNest.exe`; no separate .NET installation is needed. The package includes the Gpt6Astra, Opus55, and Qwen38FlashNext engine plug-ins. Use the ZIP and SHA-256 checksum from [GitHub Releases](https://github.com/ajisaacs/OpenNest/releases), not the source-code archives.
+Windows release ZIPs are self-contained: extract the entire archive into a new folder and run `OpenNest.exe`; no separate .NET installation is needed. The Rectangles and Irregular nesting engines are built in; see [nesting engines](docs/nesting-engines.md). Use the ZIP and SHA-256 checksum from [GitHub Releases](https://github.com/ajisaacs/OpenNest/releases), not the source-code archives.
 
 ## Build, Test, Run
 
@@ -67,7 +67,7 @@ dotnet run --project OpenNest.Benchmark -- ./benchmark-jobs \
   --sheet-sizes 48x96,60x120,72x120 --engines Default,StockLadder --csv results.csv
 ```
 
-Layouts are validated (bounds, spacing, quantity, rotation, stock match); invalid runs place nothing and pay the penalty. `--parallel` (default 3) speeds up scoring but inflates `Time(ms)` — use `--parallel 1` when comparing speed. Pass `--sheet-sizes` for an unbiased run; otherwise only each file's original sizes are offered. `--progress` logs each solve's start, the engine's `NestJobProgress` (plate evaluations throttled to one line per 2 s, every plate commit) and its finish. Custom engines drop in as DLLs implementing `INestingEngine` (public parameterless constructor) in an `Engines/` folder next to the benchmark. Community engines live in [OpenNest-Engines](https://git.thecozycat.net/aj/OpenNest-Engines).
+Layouts are validated (bounds, spacing, quantity, rotation, stock match); invalid runs place nothing and pay the penalty. `--parallel` (default 3) speeds up scoring but inflates `Time(ms)` — use `--parallel 1` when comparing speed. Pass `--sheet-sizes` for an unbiased run; otherwise only each file's original sizes are offered. `--progress` logs each solve's start, the engine's `NestJobProgress` (plate evaluations throttled to one line per 2 s, every plate commit) and its finish. Custom engines drop in as DLLs implementing `INestingEngine` (public parameterless constructor) in an `Engines/` folder next to the benchmark. Built-in engines and how to change them: [nesting engines](docs/nesting-engines.md).
 
 ## Project Structure
 
@@ -91,11 +91,15 @@ Engines implement `INestingEngine.Solve(NestJob)`. Desktop Auto Nest, console au
 
 | Engine | Description |
 |--------|-------------|
+| **Rectangles** | Plain and near-rectangular plates: maximal-rectangles box packing |
+| **Irregular** | Irregular profiles: no-fit-polygon frontier packing |
 | **Default** | Multi-phase: linear fill → pairs → rect best-fit → extents |
 | **Strip** | Iterative shrink-fill for mixed-drawing layouts |
 | **Vertical / Horizontal Remnant** | Optimizes a clean remnant drop on one edge |
 | **StockLadder** | Whole-job, stock-constrained baseline with salvage-credit ranking |
 
+Which engine suits which jobs, renamed engine names, and the rules for changing an engine: [nesting engines](docs/nesting-engines.md).
+
 ## File Format
 
 `.nest` files are ZIP archives containing drawing programs, metadata, plates, and placements. Saved nests retain each part's lead-ins, lead-outs, tab gaps, and locks, plus the plate's cutting settings. Changing a drawing's geometry removes obsolete cutting paths from its parts; name, quantity, and color edits preserve them. See the [file-format reference](docs/nest-file-format.md) for compatibility and recovery behavior.
diff --git a/docs/automatic-nesting.md b/docs/automatic-nesting.md
index f994db7..0d18c1f 100644
--- a/docs/automatic-nesting.md
+++ b/docs/automatic-nesting.md
@@ -27,9 +27,10 @@ Every invalid result shows the validation report before any plates are changed:
 Overlap Check displays material overlaps, not every spacing/stock/rotation failure in the report. A layout passing validation may still be incomplete; completeness and stop reason are separate from geometric validity. This check does not replace pre-post CNC verification.
 
 The selected engine is saved in `%APPDATA%\OpenNest\engine-selection.json`.
-Selection is restored after plug-ins load. If the saved engine is unavailable,
-Default is selected and the status bar reports the fallback; startup does not
-replace the saved missing-engine preference.
+Selection is restored after plug-ins load. A saved name from an earlier release
+maps to the engine that replaced it (see [nesting engines](nesting-engines.md)).
+If the saved engine is unavailable, Default is selected and the status bar
+reports the fallback; startup does not replace the saved missing-engine preference.
 
 ## Integration constraints
 
diff --git a/docs/nesting-engines.md b/docs/nesting-engines.md
new file mode 100644
index 0000000..e8c77a9
--- /dev/null
+++ b/docs/nesting-engines.md
@@ -0,0 +1,54 @@
+# Nesting engines
+
+Whole-job engines implement `INestingEngine.Solve(NestJob)` and are selected by name through
+`NestingEngineRegistry`. Every automatic nesting front end validates engine output the same way;
+see [automatic nesting and validation](automatic-nesting.md).
+
+## Built-in engines
+
+Engines are named for the jobs they suit, not for how or by whom they were built. Engine code lives
+in `OpenNest.Engine/NestingEngines/<Name>/`, its tests in `OpenNest.Engine.Tests/NestingEngines/`.
+
+| Engine | Best for | Method |
+|---|---|---|
+| Rectangles | Plain and near-rectangular plates | Each part packed as the box of its material at its minimum-area rotation, using a maximal-rectangles free list; stock chosen sheet by sheet by salvage-credited look-ahead cost |
+| Irregular | Irregular profiles | No-fit-polygon frontier packing with gap filling, six whole-job strategy variants and a tail re-plan |
+| StockLadder | Caller-supplied stock ladders | Constrained-first fill with equivalent-demand area repacking |
+| Default, Strip, Vertical Remnant, Horizontal Remnant | Single-strategy fills | The fixed placement strategies behind interactive fill |
+
+Rectangles places irregular parts validly, but only as their bounding boxes; it never nests into a
+notch or hole. Box sides account for how the layout check flattens arcs, so round-edged parts stay
+valid at box contact.
+
+## Renamed engines
+
+Earlier releases shipped these as plug-ins under other names. The registry maps the old names so
+saved desktop selections, scripts and API requests keep working:
+
+| Old name | Now |
+|---|---|
+| `Opus55NestingEngine` | Irregular |
+| `RectanglesNestingEngine` | Rectangles |
+
+Gpt6Astra and Qwen38FlashNext are no longer shipped and have no alias. A saved selection of either
+falls back to Default with the usual status-bar warning.
+
+## Changing an engine
+
+- A change lands only when it beats the engine's current result on `OpenNest.Benchmark` for the
+  jobs that engine targets, with every layout valid. Report cost, validity, unplaced parts and time.
+- Placement must be deterministic: no clocks, unseeded randomness or environment variables. Budget
+  work by counting it; wall time may stop work only through the cancellation token.
+- Keep each engine's tests passing, including `EngineContractTests<TEngine>` (quadrants, overflow,
+  priority, cancellation, stock and plate limits, determinism). Every layout in those tests is
+  checked with `NestLayoutCheck`, the benchmark's validator.
+- Engines may share code. Move a helper into shared Engine code when a second engine needs it,
+  rather than copying it.
+
+## Plug-ins
+
+External engines still load from an `Engines/` folder beside the desktop, console, MCP or benchmark
+executable. A plug-in implements `INestingEngine` with a public parameterless constructor and
+registers under its CLR type name. A plug-in whose name matches a built-in engine, or a renamed
+engine's old name, is skipped: a leftover `OpenNest.Engine.Opus55.dll` cannot shadow Irregular.
+Leftover Gpt6Astra or Qwen38FlashNext DLLs still load as ordinary plug-ins until deleted.
diff --git a/docs/releasing.md b/docs/releasing.md
index a154f9d..f16c54f 100644
--- a/docs/releasing.md
+++ b/docs/releasing.md
@@ -19,13 +19,12 @@ Git refs are owned by Gitea (`aj/OpenNest`) and push-mirrored to GitHub
 4. Download the `OpenNest-X.Y.Z-win-x64` artifact and verify its `.sha256`.
    It contains `OpenNest.vX.Y.Z.win-x64.zip`, with the .NET runtime, native
    dependencies, shipped configurations, all three post-processors, license,
-   and `build-info.json` identifying the exact source commit. Gpt6Astra, Opus55,
-   and Qwen38FlashNext are bundled in `Engines/` with their MIT license and source
-   manifest. `scripts/external-engines.json` pins the external repository revision;
-   no moving branch or prebuilt third-party DLL is used. The script tests and
-   builds each engine against this host, then exercises actual packaged registry
-   discovery and an intentionally missing-DLL failure case. Keep the explicit
-   engine allowlist; never package the template, shared test kit, or test DLLs.
+   and `build-info.json` identifying the exact source commit. The nesting engines
+   are built into `OpenNest.Engine.dll`; no external engine repository or plug-in
+   DLL is packaged. `scripts/ReleaseSmoke` loads the packaged engine assembly and
+   checks that every built-in engine instantiates from it, that the renamed
+   `Opus55NestingEngine` selection resolves to Irregular, and that an unknown
+   engine name is rejected.
 
 The workflow has read-only repository permissions and does **not** publish
 releases. Once present on the default branch, it can also be dispatched manually