diff --git a/AGENTS.md b/AGENTS.md
index 61c9bf1..74f23bc 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -14,11 +14,19 @@
 - **Formatting**: Always run `go fmt` before commits (included in test target)
 - **Types**: Use explicit types (e.g., `int64`, `float64`). Convert numbers appropriately when unmarshaling JSON
 - **Naming**: PascalCase for exported, camelCase for unexported. Use descriptive names
+- **Don't reinvent the wheel**: Use standard library functions (e.g., `slices.Contains` instead of custom contains functions)
 - **Error handling**: Return wrapped errors with context. Define custom errors in models/errors.go. Use `ErrorIs` for error checking
-- **Testing**: Use github.com/nalgeon/be. Table-driven tests with helper functions. Test both marshal/unmarshal for encoding types
+- **Testing**: Use github.com/nalgeon/be. Table-driven tests with helper functions. Test both marshal/unmarshal for encoding types. Use `t.ArtifactDir()` instead of `t.TempDir()` and `t.Context()` instead of `context.Background()`
 - **Concurrency**: Use channels and goroutines with WaitGroups for parallel processing (see entry.go patterns)
 - **Comments**: Include license header on cmd files. Document exported functions and types
 
+## Config System
+- Viper instance is stored in context using a custom key type (`confKeyType`)
+- Use `config.New(path, overrides)` to create a new viper instance
+- Use `config.RetrieveFromContext(ctx)` to get both viper and the unmarshaled Config struct
+- Formatters can use `v.Sub("formatters." + kind)` to get their own sub-config and unmarshal into their specific config struct
+- Test files must create viper instances and add them to context using `config.AddToContext`
+
 ## Git Commit Guidelines
 - **Format**: Prepend commit messages with a gitmoji emoji (see https://gitmoji.dev)
 - **Style**: Write detailed commit messages that explain what changed and why
@@ -39,9 +47,10 @@
 
 ## Project Structure
 - `cmd/my-log/`: Main application entrypoint
-- `internal/cmd/`: Cobra commands (root, drop, config)
+- `internal/testutil/bep`: Reusable test assertions to complement those used by github.com/nalgeon/be
+- `cli/`: Cobra commands (root, drop, config)
 - `models/`: Core types (Entry, Log, Meta) with marshal/unmarshal implementations
-- `config/`: TOML-based configuration with env overrides
-- `formatters/`: Output formatters (plain, json, null)
+- `config/`: TOML-based configuration with viper, env overrides, and context propagation
+- `formatters/`: Output formatters (plain, json, null) with their own sub-configs
 - `files/`: File operations (append)
 - `tools/`: Utilities (date parsing, string parsing, write buffers)