diff --git a/.github/workflows/test-coverage.yaml b/.github/workflows/test-coverage.yaml
new file mode 100644
index 0000000..5ef7c2c
--- /dev/null
+++ b/.github/workflows/test-coverage.yaml
@@ -0,0 +1,33 @@
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+    branches: [main, master]
+
+name: test-coverage
+
+jobs:
+  test-coverage:
+    runs-on: ubuntu-24.04
+    permissions:
+      contents: read
+    env:
+      GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: r-lib/actions/setup-r@v2
+        with:
+          use-public-rspm: true
+
+      - uses: r-lib/actions/setup-r-dependencies@v2
+        with:
+          extra-packages: any::covr, any::xml2
+          needs: coverage
+
+      - name: Test coverage
+        run: |
+          cov <- covr::package_coverage(quiet = FALSE)
+          print(cov)
+          covr::to_cobertura(cov)
+        shell: Rscript {0}
diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 0000000..6820133
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,28 @@
+# AGENTS.md
+
+Guidance for AI agents and contributors working on `magicaxis` (R package for base-graphics scientific plotting with minor ticks and log axes).
+
+## Layout
+- `/R` one file per main function (`magaxis.R`, `magplot.R`, `magimage.R`, `magmap.R`, ...). `NAMESPACE` uses `exportPattern("^[[:alpha:]]+")`, so any function whose name starts with a letter is exported; prefix internal helpers with `.`.
+- `/man` hand-written Rd files (no roxygen) — update the matching `.Rd` when changing arguments.
+- `/tests/testthat` testthat (edition 3) tests; `/vignettes` knitr vignettes; `/data`, `/inst` supporting data.
+- Extra `...` arguments are routed to base graphics via `ParmOff` (a GitHub-only dependency, `asgr/ParmOff`, see `Remotes` in `DESCRIPTION`).
+
+## Commands
+```r
+devtools::load_all(); devtools::test()      # run tests
+devtools::check()                            # or R CMD check
+covr::package_coverage()                     # coverage
+```
+
+## Testing conventions
+- Tests that draw must open a null device (`pdf(nullfile())` + `on.exit(dev.off())`), as in the `with_null_dev` helper in `tests/testthat/test-functions.R`; never rely on an interactive device.
+- Prefer asserting on return values/structure; for pure plotting use `expect_no_error`.
+- Add a test with any bug fix or new argument.
+
+## CI
+- `.github/workflows/R-CMD-check.yaml`: R CMD check on Windows/macOS/Ubuntu.
+- `.github/workflows/test-coverage.yaml`: covr coverage on Ubuntu.
+
+## Style
+Keep the existing terse base-R style; do not reformat unrelated code. Bump `Version`/`Date` in `DESCRIPTION` for releases.
diff --git a/tests/testthat/test-maglab-magmap.R b/tests/testthat/test-maglab-magmap.R
new file mode 100644
index 0000000..2b433ec
--- /dev/null
+++ b/tests/testthat/test-maglab-magmap.R
@@ -0,0 +1,38 @@
+## Tests for maglab and magmap (pure computation, no plotting).
+
+library(magicaxis)
+
+test_that("maglab returns tickat, labat and exp", {
+  out <- maglab(c(0, 10))
+  expect_named(out, c("tickat", "labat", "exp"))
+  expect_true(all(is.finite(out$labat)))
+  expect_equal(out$tickat, out$labat)
+})
+
+test_that("maglab trim=TRUE keeps labels within limits", {
+  out <- maglab(c(0.5, 9.5), trim = TRUE)
+  expect_true(all(out$labat >= 0.5 & out$labat <= 9.5))
+})
+
+test_that("maglab log=TRUE gives power-of-ten label locations", {
+  out <- maglab(c(1, 1000), log = TRUE)
+  expect_true(all(abs(log10(out$labat) - round(log10(out$labat))) < 1e-8))
+  expect_true(min(out$labat) <= 1 && max(out$labat) >= 1000)
+})
+
+test_that("maglab usemultloc requires log=TRUE", {
+  expect_error(maglab(c(1, 100), usemultloc = TRUE, log = FALSE))
+})
+
+test_that("magmap output lies within the requested range", {
+  out <- magmap(seq(1, 100), range = c(0, 1))
+  expect_true(all(out$map >= 0 & out$map <= 1))
+  expect_length(out$map, 100)
+})
+
+test_that("magmap flip=TRUE reverses a linear mapping", {
+  x <- seq(1, 10)
+  a <- magmap(x, type = "num", locut = 1, hicut = 10, range = c(0, 1))$map
+  b <- magmap(x, type = "num", locut = 1, hicut = 10, range = c(0, 1), flip = TRUE)$map
+  expect_equal(a, 1 - b)
+})