diff --git a/.config/dotnet-tools.json b/.config/dotnet-tools.json
index 5ad47a3..439425f 100644
--- a/.config/dotnet-tools.json
+++ b/.config/dotnet-tools.json
@@ -13,6 +13,12 @@
       "commands": [
         "fsharp-analyzers"
       ]
+    },
+    "woofware.nunittestrunner": {
+      "version": "0.3.4",
+      "commands": [
+        "woofware.nunittestrunner"
+      ]
     }
   }
-}
\ No newline at end of file
+}
diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 0000000..affe265
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,90 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+WoofWare.Expect is an F# expect/snapshot testing library (similar to Jest snapshots). The project consists of two main components:
+
+- **WoofWare.Expect**: Core library that provides the `expect` computation expression and snapshot comparison functionality
+- **WoofWare.Expect.Test**: Test suite using NUnit that demonstrates and validates the library functionality
+
+## Build and Development Commands
+
+This project uses Nix for development environment management. All commands should be run within the Nix development shell:
+
+```bash
+# Restore dependencies
+nix develop --command dotnet restore
+
+# Build the project
+nix develop --command dotnet build --configuration Release
+
+# Run tests
+nix develop --command dotnet test
+
+# Pack NuGet package
+nix develop --command dotnet pack --configuration Release
+```
+
+### Code Formatting and Analysis
+
+```bash
+# Format F# code (via Nix)
+nix run .#fantomas -- .
+
+# Check formatting without modifying files
+nix run .#fantomas -- --check .
+
+# Run F# analyzers
+nix run .#fsharp-analyzers -- --project ./WoofWare.Expect/WoofWare.Expect.fsproj --analyzers-path ./.analyzerpackages/g-research.fsharp.analyzers/*/
+```
+
+### Single Test Execution
+
+NUnit's filtering is pretty borked.
+You can't apply filters that contain special characters in the test name (like a space character).
+You have to do e.g. `FullyQualifiedName~singleword` rather than `FullyQualifiedName~single word test`, but this only works on tests whose names are single words to begin with.
+
+Instead of running `dotnet test`, you can perform a build (`dotnet build`) and then run `dotnet woofware.nunittestrunner WoofWare.Expect.Test/bin/Debug/net9.0/WoofWare.Expect.Test.dll`.
+This is an NUnit test runner which accepts a `--filter` arg that takes the same filter syntax as `dotnet test`, but actually parses it correctly: test names can contain spaces.
+(The most foolproof way to provide test names to WoofWare.NUnitTestRunner is by XML-encoding: e.g. `FullyQualifiedName="MyNamespace.MyTestsClass<ParameterType1%2CParameterType2>.MyTestMethod"`. The `~` query operator is also supported.)
+
+## Architecture
+
+### Core Components
+
+- **Builder.fs**: Contains the `ExpectBuilder` computation expression that powers the `expect` syntax
+- **Domain.fs**: Core types including `CallerInfo`, `ExpectState`, and `SnapshotValue`
+- **SnapshotUpdate.fs**: Handles updating snapshot files when tests fail
+- **AstWalker.fs**: Uses Fantomas.FCS to parse F# source and locate/update snapshot strings
+- **Diff.fs**: Provides Patience and Myers diff algorithms for readable test output
+- **Dot.fs**: ASCII art rendering of dot files (requires `graph-easy`)
+
+### Key Design Patterns
+
+- **Computation Expression**: The library is built around F#'s computation expression syntax with `expect { }` blocks
+- **Source Code Manipulation**: Uses F# compiler services to locate and update snapshot strings in source files
+- **Dual Mode Operation**: Tests can run in assertion mode (normal) or update mode (to fix failing snapshots)
+
+### Snapshot Types
+
+- `snapshot`: Plain text comparison using `ToString()`
+- `snapshotJson`: JSON serialization with configurable options
+- `snapshotList`: Formatted list comparison
+- `snapshotThrows`: Exception expectation testing
+- `withFormat`: Custom formatting functions
+- `withJsonSerializerOptions`: Custom JSON serialization
+- `withJsonDocOptions`: Custom JSON parsing (e.g., for comments)
+
+### Test Snapshot Management
+
+- Individual updates: Add `'` to `expect` (making it `expect'`), run test to update, then remove `'`
+- Bulk updates: Use `GlobalBuilderConfig.enterBulkUpdateMode()` in test setup and `GlobalBuilderConfig.updateAllSnapshots()` in teardown
+
+## Important Notes
+
+- Use verbatim string literals for snapshots - the update mechanism requires this
+- Avoid format strings (`$"..."`) or string concatenation in snapshot definitions
+- The project follows strict F# conventions with warnings as errors
+- All commits go through comprehensive CI including formatting, analysis, and testing
diff --git a/nix/deps.json b/nix/deps.json
index e7715b3..e22c56d 100644
--- a/nix/deps.json
+++ b/nix/deps.json
@@ -333,5 +333,10 @@
     "pname": "Testably.Abstractions.FileSystem.Interface",
     "version": "9.0.0",
     "hash": "sha256-6JW+qDtqQT9StP4oTR7uO0NnmVc2xcjSZ6ds2H71wtg="
+  },
+  {
+    "pname": "WoofWare.NUnitTestRunner",
+    "version": "0.3.4",
+    "hash": "sha256-OaBYMEAXUDiz9ei2/Zg4Q1A8BNDK1oaMB44uVz4UG/0="
   }
 ]