diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 46a4d1e28..99a3fd449 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -98,6 +98,10 @@ implementation can be found in `internal/compiler-interface` and `internal/compi
 while general infrastructure, sbt internal APIs and high-level compiler APIs for
 Zinc are available in the rest of projects inside `internal`.
 
+Incremental-compilation behaviour is covered end to end by the scripted tests in
+`zinc/src/sbt-test`. See [Scripted tests](contributing-docs/scripted_tests.md) for the test layout,
+the `test` script syntax, and the `incOptions.properties` file used to configure Zinc per test.
+
 Zinc also has a JMH benchmark suite. This benchmark suite can benchmark
 any project that runs on 2.12.x/2.11.x. The Zinc team uses it
 to make sure that there's not a performance regression in the Zinc compiler phases.
diff --git a/contributing-docs/scripted_tests.md b/contributing-docs/scripted_tests.md
new file mode 100644
index 000000000..894d30f7f
--- /dev/null
+++ b/contributing-docs/scripted_tests.md
@@ -0,0 +1,137 @@
+Scripted tests
+==============
+
+End-to-end incremental-compilation scenarios. Each test drives a real compile, edits sources, and
+recompiles, asserting on what Zinc decided to recompile. The engine is a custom re-implementation in
+[`internal/zinc-scripted`](../internal/zinc-scripted), not sbt's `scripted` plugin.
+
+Layout
+------
+
+```
+zinc/src/sbt-test/<group>/<name>/
+  build.json               # optional; project list for multi-project tests
+  *.scala, *.java          # the sources being compiled
+  changes/                 # edited versions, copied over the originals mid-test
+  incOptions.properties    # optional; see below
+  test                     # the script of steps
+```
+
+Naming the script `pending` instead of `test` marks the scenario as known-failing: it still runs, but
+a failure is tolerated, and *passing* is what fails the build (a reminder to rename it back). A
+`pending` file takes precedence over a `test` file in the same directory.
+
+Groups: `source-dependencies`, `apiinfo`, `macros`, `pipelining`, `profiler`, `reporter`, `general`.
+
+Without a `build.json` the test is a single project named `root` rooted at the test directory. With
+one, each entry declares a `name` and optional `dependsOn`, `in`, and `scalaVersion`. A project's
+base directory is `in` when given, otherwise the subdirectory named after the project
+([IncHandler.scala:124](../internal/zinc-scripted/src/test/scala/sbt/internal/inc/IncHandler.scala#L124)).
+
+The `test` script is one step per line:
+
+- `> compile` runs a task on the root project, `> use/compile` on the `use` project.
+- `-> compile` expects the task to fail.
+- `$ copy-file changes/A.scala A.scala` runs a file command (`copy-file`, `delete`, `exists`,
+  `absent`, `newer`, `touch`, `sleep`, `pause`).
+- Assertions are tasks too: `checkRecompilations`, `checkIterations`, `checkProducts`,
+  `checkDependencies`, `checkClasses`, `checkWarnings`, `checkErrors`, `checkSame`, and others.
+
+The full task list is the `commands` map in
+[`IncHandler.scala`](../internal/zinc-scripted/src/test/scala/sbt/internal/inc/IncHandler.scala#L229);
+file commands come from `ZincFileCommands` and sbt's `FileCommands`.
+
+Running
+-------
+
+```bash
+sbt scripted                                                # all tests
+sbt "scripted source-dependencies/abstract-class-to-trait"   # one test
+sbt "scripted source-dependencies/*"                         # one group
+```
+
+`incOptions.properties`
+-----------------------
+
+Scripted tests have no build definition rich enough to configure Zinc, so this file is how a test
+sets a few [`IncOptions`](../internal/compiler-interface/src/main/contraband-java/xsbti/compile/IncOptions.java)
+fields and the scalac options for a project. It is read by
+[`IncHandler.loadIncProperties`](../internal/zinc-scripted/src/test/scala/sbt/internal/inc/IncHandler.scala#L809)
+and parsed by
+[`IncOptionsUtil.fromStringMap`](../internal/zinc-core/src/main/java/xsbti/compile/IncOptionsUtil.java#L78).
+
+Place it in the **project** base directory: the test root for a single-project test, or each
+subproject directory (`dep/`, `use/`) for a multi-project one. Both `incoptions.properties` and
+`incOptions.properties` are accepted, lowercase taking precedence.
+
+Standard `java.util.Properties` syntax (`#` comments, `key = value`). Omitting a key leaves the
+option at the effective scripted default shown below, which for some keys is *not* the `IncOptions`
+default.
+
+```properties
+# source-dependencies/same-source-transitive-invalidation
+transitiveStep = 1
+```
+
+### Supported keys
+
+These take effect. "Default" is the value you get when the key is absent from a scripted test, which
+is what a test author needs; where that differs from the `IncOptions` default, the difference is
+noted.
+
+| Key | Value | Default |
+| --- | --- | --- |
+| `transitiveStep` | int: invalidation cycles before falling back to the transitive closure | 3 |
+| `recompileAllFraction` | double: fraction of sources invalidated that triggers a full recompile | **1.0** (`IncOptions` uses 0.5) |
+| `relationsDebug` | boolean: verbose invalidation diagnostics, including the pruned relations and detected API changes | false |
+| `apiDebug` | boolean: log API diffs | **true** (`IncOptions` uses false) |
+| `apiDiffContextSize` | int: context lines in API diffs | 5 |
+| `recompileOnMacroDef` | boolean, or `NOTHING` to leave unset | unset, which behaves as true |
+| `logRecompileOnMacro` | boolean | true |
+| `useOptimizedSealed` | boolean: use the optimized sealed-children invalidation | false |
+| `storeApis` | boolean: persist extracted APIs in the analysis | true |
+| `pipelining` | boolean: pipelined compilation | **true** (`IncOptions` uses false) |
+| `scalac.options` | **space-separated** options passed to the compiler; `[basedir]` expands to the project's absolute base directory. Handled by `IncHandler`, not `IncOptionsUtil` | none |
+| `incOptions.storeApis` | boolean; same as `storeApis`, applied after parsing. Handled by `IncHandler` | true |
+
+Two wrinkles in this table:
+
+- `recompileAllFraction = 0.5` cannot be expressed. The engine detects "not set" by comparing the
+  parsed value against `IncOptions.defaultRecompileAllFraction()` rather than checking whether the
+  key is present
+  ([IncHandler.scala:837](../internal/zinc-scripted/src/test/scala/sbt/internal/inc/IncHandler.scala#L837)),
+  so writing the default explicitly still yields 1.0. Any other value is honoured.
+- With `pipelining` on, `-Ypickle-java -Ypickle-write <earlyOutput>` is appended to `scalac.options`
+  automatically. Java-heavy tests set `pipelining = false` to opt out.
+
+### Recognized but ineffective keys
+
+`IncOptionsUtil` parses these, so they look supported, but nothing observes them in a scripted run.
+
+| Key | Why |
+| --- | --- |
+| `classfileManagerType` | Always overwritten with a transactional manager rooted at `target/classes.bak` ([IncHandler.scala:363](../internal/zinc-scripted/src/test/scala/sbt/internal/inc/IncHandler.scala#L363)) |
+| `transactionalManagerBaseDirectory` | Only feeds the manager built from `classfileManagerType`, which is then discarded |
+| `allowMachinePath` | Not consulted; the engine hardcodes `true` when it builds the `MappedFileConverter` ([IncHandler.scala:117](../internal/zinc-scripted/src/test/scala/sbt/internal/inc/IncHandler.scala#L117)) |
+| `apiDumpDirectory` | Unimplemented in Zinc itself, as `incremental.contra` notes |
+| `ignoredScalacOptions` | Only affects whether a *change* in scalac options forces a full recompile ([MiniSetupUtil.scala:156](../internal/zinc-core/src/main/scala/sbt/internal/inc/MiniSetupUtil.scala#L156)), and scalac options cannot change during a scripted run, see caveats |
+
+### Unsupported `IncOptions` fields
+
+`IncOptionsUtil` recognizes only the keys listed above. Other `IncOptions` fields, including
+`strictMode`, `enabled`, `useCustomizedFileManager`, `auxiliaryClassFiles`, `extra`, and
+`externalHooks`, have no property key and cannot be set from this file.
+
+### Caveats
+
+1. **The file is read once, at project initialization.** `incOptions` and `scalacOptions` are `val`s
+   on `ProjectStructure`
+   ([IncHandler.scala:350](../internal/zinc-scripted/src/test/scala/sbt/internal/inc/IncHandler.scala#L350))
+   and every compilation reuses them
+   ([IncHandler.scala:723](../internal/zinc-scripted/src/test/scala/sbt/internal/inc/IncHandler.scala#L723)),
+   so copying `changes/incOptions.properties` over the live file partway through a `test` script has
+   no effect. This is also why `ignoredScalacOptions` is inert: a test cannot vary its scalac options
+   between compilations, so the comparison that key relaxes never sees a difference.
+   `source-dependencies/scalac-options` is written as if it did work, and passes for other reasons.
+2. **Unknown keys are silently ignored.** There is no validation, so a typo, or one of the
+   unsupported fields above, looks exactly like a working setting.
diff --git a/zinc/src/sbt-test/README.md b/zinc/src/sbt-test/README.md
new file mode 100644
index 000000000..545bff204
--- /dev/null
+++ b/zinc/src/sbt-test/README.md
@@ -0,0 +1,7 @@
+Scripted tests
+==============
+
+End-to-end incremental-compilation scenarios, run with `sbt scripted`.
+
+See [contributing-docs/scripted_tests.md](../../../contributing-docs/scripted_tests.md) for the test
+layout, the `test` script syntax, and the `incOptions.properties` format.