Ensemble is the coding agent built from The Art of Building AI Coding Agents, Bill Cox's executable teaching book. Edition 3 is the current edition: the application is C# 14 / .NET 10, developed inside Ensemble with Cadence.

Build the code and the specification together. Development earns the spec. The goal is not to write a detailed plan up front and hand it to an AI, or to vibe-code a system and document its source afterward. The goal is to become the expert by pair-programming with the AI, then preserve that expertise in a form another agent can execute.

Bill's workflow is concrete:

1. Discuss the design, then draft just the next chapter's TL;DR and exercises.
2. Pair-program the implementation. Steer the AI in real time, try ideas, correct mistakes, and record what was unclear and why each important decision was made.
3. Once the implementation passes the exercise graders, use those notes to write the chapter's body: the construction process, lessons, tradeoffs and reasons, not a tour of the finished source.
4. Give a fresh student agent the chapter and graders to test whether the teaching is sufficient to reconstruct the implementation. Feed what that attempt reveals back into the chapter.

The result is an executable specification: a codebook that transfers the judgment needed to build the system, not merely a list of desired features. Ensemble V2 executed this book to build V3; Bill and Cadence then worked inside V3 to improve both the agent and the teaching. Individual chapters retain their documented verification limits.

The human's expertise grows during development instead of depending on a final inspection of unfamiliar generated code. The code is an output; the reusable asset is the knowledge needed to build it again. Don't just document what to build. Teach how and why to build it that way. You cannot learn all of that up front.

Use an existing .NET 10 SDK (the runtime alone cannot build the application). Nothing here installs an SDK, reads credentials for a build, starts a server automatically, or replaces an installed application.

First check that your shell can find the SDK:

dotnet --list-sdks           # should include a 10.x SDK

If dotnet is not found, or make reports dotnet: No such file or directory, the build has not reached compilation: the SDK executable is missing from your shell's PATH. If you already have the SDK, either add its directory to PATH for the current shell (replace the example path with your actual SDK directory):

export PATH="/absolute/path/to/dotnet-sdk:$PATH"
dotnet --list-sdks

Or leave PATH unchanged and give Make the executable's full path:

make DOTNET=/absolute/path/to/dotnet
make cli DOTNET=/absolute/path/to/dotnet
# Use the same override for other targets, such as make test.

For example, if you installed the SDK in $HOME/.local/share/ensemble-dotnet10, use make DOTNET="$HOME/.local/share/ensemble-dotnet10/dotnet". This is an example location, not a required layout. If no .NET 10 SDK is installed, install one from Microsoft's .NET 10 downloads, then open a new terminal and check again.

With dotnet on PATH:

make                         # builds the C# GUI and writes ./ensemble launcher
make cli                     # builds the C# terminal client
./ensemble --help            # explicit launch; inspect options before use
# Terminal client after make cli:
dotnet agent/Cli/bin/Debug/net10.0/ch04.dll --help

If you used the explicit SDK override instead, use that full executable path in place of dotnet when launching the terminal client too.

Choose your own data directory, workspace, funding configuration and unused port for a real launch. Do not replace a running host just to refresh source. The GUI embeds its assets at build time. See the crossover chapter and the source README for application behavior and qualified test evidence.

make test runs the C# regression executable and root Go tooling tests; make test-go tests only the latter. Go remains at the root for reusable graders, local fake providers and publication tooling—not as the active runtime.

Use each chapter's documented executable command and scope. C# must not be sent to a Go-source consumer grader. For example, Chapter 5 documents the explicit grade-edition3 -ch 5 -executable-only route and its 65 scoped points. make grade CH=5 DIR=/path/to/CLI-wrapper GRADEFLAGS='-executable-only ...' requires an explicit target and forwards to that Edition 3 command.

Executable grader numbers and preserved chNN snapshots retain their original meanings; the chapter mapping explains later manuscript renumbering. Historical Edition 2 grading can be run from edition-2/; Edition 1 remains in edition-1/. Neither archive is rewritten for the current layout.

make book assembles the current manuscript; make ebook builds its EPUB; make print builds its print and reading PDFs. These use existing local publishing tools, never install them. See book build instructions for prerequisites and isolated output. Generated publications are distinct from source edits; a source promotion alone does not publish a new book or upload it.

The Puffin book is the working home for Gateway, Connections, Agenda, Puffin and the related Watchtower/security work. Its notes follow Development-Driven Specs; outlines are provisional, not completed features. The coding-agent book remains under book/.