diff --git a/cmd/mothd/testdata/testpiler.sh b/cmd/mothd/testdata/testpiler.sh
index 2450d96..60da536 100755
--- a/cmd/mothd/testdata/testpiler.sh
+++ b/cmd/mothd/testdata/testpiler.sh
@@ -6,31 +6,35 @@ fail () {
 }
 
 case "$ACTION:$CAT:$POINTS" in
-inventory::)
-    echo "pategory 1 2 3 4 5 10 20 300"
-    echo "nealegory 1 3 2"
-    ;;
-open:*:*)
-    case "$CAT:$POINTS:$FILENAME" in
-    *:*:moo.txt)
-        echo "Moo."
+    inventory::)
+        cat <<EOT
+{
+    "pategory": [1, 2, 3, 4, 5, 10, 20, 300],
+    "nealegory": [1, 3, 2]
+}
+EOT
+        ;;
+    open:*:*)
+        case "$CAT:$POINTS:$FILENAME" in
+            *:*:moo.txt)
+                echo "Moo."
+                ;;
+            *)
+                fail "Cannot open: $FILENAME"
+                ;;
+        esac
+        ;;
+    answer:pategory:1)
+        if [ "$ANSWER" = "answer" ]; then
+            echo "correct"
+        else
+            echo "Sorry, wrong answer."
+        fi
+        ;;
+    answer:pategory:2)
+        fail "Internal error"
         ;;
     *)
-        fail "Cannot open: $FILENAME"
+        fail "ERROR: Unknown action: $action"
         ;;
-    esac
-    ;;
-answer:pategory:1)
-    if [ "$ANSWER" = "answer" ]; then
-        echo "correct"
-    else
-        echo "Sorry, wrong answer."
-    fi
-    ;;
-answer:pategory:2)
-    fail "Internal error"
-    ;;
-*)
-    fail "ERROR: Unknown action: $action"
-    ;;
 esac
diff --git a/cmd/transpile/category.go b/cmd/transpile/category.go
index f11198f..9e8ebda 100644
--- a/cmd/transpile/category.go
+++ b/cmd/transpile/category.go
@@ -1,10 +1,16 @@
 package main
 
 import (
-	"fmt"
+	"bytes"
+	"context"
+	"encoding/json"
 	"io"
+	"io/ioutil"
 	"log"
+	"os/exec"
 	"strconv"
+	"strings"
+	"time"
 
 	"github.com/spf13/afero"
 )
@@ -26,11 +32,20 @@ func (n NopReadCloser) Close() error {
 // NewFsCategory returns a Category based on which files are present.
 // If 'mkcategory' is present and executable, an FsCommandCategory is returned.
 // Otherwise, FsCategory is returned.
-func NewFsCategory(fs afero.Fs) Category {
-	if info, err := fs.Stat("mkcategory"); (err == nil) && (info.Mode()&0100 != 0) {
-		return FsCommandCategory{fs: fs}
+func NewFsCategory(fs afero.Fs, cat string) Category {
+	bfs := NewRecursiveBasePathFs(fs, cat)
+	if info, err := bfs.Stat("mkcategory"); (err == nil) && (info.Mode()&0100 != 0) {
+		if command, err := bfs.RealPath(info.Name()); err != nil {
+			log.Println("Unable to resolve full path to", info.Name(), bfs)
+		} else {
+			return FsCommandCategory{
+				fs:      bfs,
+				command: command,
+				timeout: 2 * time.Second,
+			}
+		}
 	}
-	return FsCategory{fs: fs}
+	return FsCategory{fs: bfs}
 }
 
 // FsCategory provides a category backed by a .md file.
@@ -70,7 +85,7 @@ func (c FsCategory) Open(points int, filename string) (io.ReadCloser, error) {
 	return NewFsPuzzle(c.fs, points).Open(filename)
 }
 
-// Answer check whether an answer is correct.
+// Answer checks whether an answer is correct.
 func (c FsCategory) Answer(points int, answer string) bool {
 	// BUG(neale): FsCategory.Answer should probably always return false, to prevent you from running uncompiled puzzles with participants.
 	p, err := c.Puzzle(points)
@@ -87,25 +102,81 @@ func (c FsCategory) Answer(points int, answer string) bool {
 
 // FsCommandCategory provides a category backed by running an external command.
 type FsCommandCategory struct {
-	fs afero.Fs
+	fs      afero.Fs
+	command string
+	timeout time.Duration
 }
 
 // Inventory returns a list of point values for this category.
 func (c FsCommandCategory) Inventory() ([]int, error) {
-	return nil, fmt.Errorf("Not implemented")
+	ctx, cancel := context.WithTimeout(context.Background(), c.timeout)
+	defer cancel()
+
+	cmd := exec.CommandContext(ctx, c.command, "inventory")
+	stdout, err := cmd.Output()
+	if err != nil {
+		return nil, err
+	}
+
+	ret := make([]int, 0)
+	if err := json.Unmarshal(stdout, &ret); err != nil {
+		return nil, err
+	}
+
+	return ret, nil
 }
 
 // Puzzle returns a Puzzle structure for the given point value.
 func (c FsCommandCategory) Puzzle(points int) (Puzzle, error) {
-	return Puzzle{}, fmt.Errorf("Not implemented")
+	var p Puzzle
+
+	ctx, cancel := context.WithTimeout(context.Background(), c.timeout)
+	defer cancel()
+
+	cmd := exec.CommandContext(ctx, c.command, "puzzle", strconv.Itoa(points))
+	stdout, err := cmd.Output()
+	if err != nil {
+		return p, err
+	}
+
+	if err := json.Unmarshal(stdout, &p); err != nil {
+		return p, err
+	}
+
+	return p, nil
 }
 
 // Open returns an io.ReadCloser for the given filename.
 func (c FsCommandCategory) Open(points int, filename string) (io.ReadCloser, error) {
-	return NopReadCloser{}, fmt.Errorf("Not implemented")
+	ctx, cancel := context.WithTimeout(context.Background(), c.timeout)
+	defer cancel()
+
+	cmd := exec.CommandContext(ctx, c.command, "file", strconv.Itoa(points), filename)
+	stdout, err := cmd.Output()
+	buf := ioutil.NopCloser(bytes.NewBuffer(stdout))
+	if err != nil {
+		return buf, err
+	}
+
+	return buf, nil
 }
 
-// Answer check whether an answer is correct.
+// Answer checks whether an answer is correct.
 func (c FsCommandCategory) Answer(points int, answer string) bool {
+	ctx, cancel := context.WithTimeout(context.Background(), c.timeout)
+	defer cancel()
+
+	cmd := exec.CommandContext(ctx, c.command, "answer", strconv.Itoa(points), answer)
+	stdout, err := cmd.Output()
+	if err != nil {
+		log.Printf("ERROR: Answering %d points: %s", points, err)
+		return false
+	}
+
+	switch strings.TrimSpace(string(stdout)) {
+	case "correct":
+		return true
+	}
+
 	return false
 }
diff --git a/cmd/transpile/category_test.go b/cmd/transpile/category_test.go
new file mode 100644
index 0000000..025e4d5
--- /dev/null
+++ b/cmd/transpile/category_test.go
@@ -0,0 +1,110 @@
+package main
+
+import (
+	"bytes"
+	"io"
+	"testing"
+
+	"github.com/spf13/afero"
+)
+
+func TestFsCategory(t *testing.T) {
+	c := NewFsCategory(newTestFs(), "cat0")
+
+	if inv, err := c.Inventory(); err != nil {
+		t.Error(err)
+	} else if len(inv) != 9 {
+		t.Error("Inventory wrong length", inv)
+	}
+
+	if p, err := c.Puzzle(1); err != nil {
+		t.Error(err)
+	} else if len(p.Answers) != 1 {
+		t.Error("Wrong length for answers", p.Answers)
+	} else if p.Answers[0] != "YAML answer" {
+		t.Error("Wrong answer list", p.Answers)
+	} else if !c.Answer(1, p.Answers[0]) {
+		t.Error("Correct answer not accepted")
+	}
+
+	if c.Answer(1, "incorrect answer") {
+		t.Error("Incorrect answer accepted as correct")
+	}
+
+	if r, err := c.Open(1, "moo.txt"); err != nil {
+		t.Error(err)
+	} else {
+		defer r.Close()
+		buf := new(bytes.Buffer)
+		if _, err := io.Copy(buf, r); err != nil {
+			t.Error(err)
+		}
+		if buf.String() != "Moo." {
+			t.Error("Opened file contents wrong")
+		}
+	}
+
+	if r, err := c.Open(1, "error"); err == nil {
+		r.Close()
+		t.Error("File wasn't supposed to exist")
+	}
+}
+
+func TestOsFsCategory(t *testing.T) {
+	fs := NewRecursiveBasePathFs(afero.NewOsFs(), "testdata")
+	static := NewFsCategory(fs, "static")
+
+	if p, err := static.Puzzle(1); err != nil {
+		t.Error(err)
+	} else if len(p.Pre.Authors) != 1 {
+		t.Error("Wrong authors list", p.Pre.Authors)
+	} else if p.Pre.Authors[0] != "neale" {
+		t.Error("Wrong authors", p.Pre.Authors)
+	}
+
+	generated := NewFsCategory(fs, "generated")
+
+	if inv, err := generated.Inventory(); err != nil {
+		t.Error(err)
+	} else if len(inv) != 5 {
+		t.Error("Wrong inventory", inv)
+	}
+
+	if p, err := generated.Puzzle(1); err != nil {
+		t.Error(err)
+	} else if len(p.Answers) != 1 {
+		t.Error("Wrong answers", p.Answers)
+	} else if p.Answers[0] != "answer1.0" {
+		t.Error("Wrong answers:", p.Answers)
+	}
+	if _, err := generated.Puzzle(20); err == nil {
+		t.Error("Puzzle shouldn't exist")
+	}
+
+	if r, err := generated.Open(1, "moo.txt"); err != nil {
+		t.Error(err)
+	} else {
+		defer r.Close()
+		buf := new(bytes.Buffer)
+		if _, err := io.Copy(buf, r); err != nil {
+			t.Error(err)
+		}
+		if buf.String() != "Moo.\n" {
+			t.Errorf("Wrong body: %#v", buf.String())
+		}
+	}
+	if r, err := generated.Open(1, "fail"); err == nil {
+		r.Close()
+		t.Error("File shouldn't exist")
+	}
+
+	if !generated.Answer(1, "answer1.0") {
+		t.Error("Correct answer failed")
+	}
+	if generated.Answer(1, "wrong") {
+		t.Error("Incorrect answer didn't fail")
+	}
+	if generated.Answer(2, "error") {
+		t.Error("Error answer didn't fail")
+	}
+}
diff --git a/cmd/transpile/main.go b/cmd/transpile/main.go
index b40bb7e..ef46a61 100644
--- a/cmd/transpile/main.go
+++ b/cmd/transpile/main.go
@@ -102,6 +102,8 @@ func (t *T) Handle(action string) error {
 
 // PrintInventory prints a puzzle inventory to stdout
 func (t *T) PrintInventory() error {
+	inv := make(map[string][]int)
+
 	dirEnts, err := afero.ReadDir(t.Fs, ".")
 	if err != nil {
 		return err
@@ -113,16 +115,16 @@ func (t *T) PrintInventory() error {
 				log.Print(err)
 				continue
 			} else {
-				fmt.Fprint(t.w, ent.Name())
 				sort.Ints(puzzles)
-				for _, points := range puzzles {
-					fmt.Fprint(t.w, " ")
-					fmt.Fprint(t.w, points)
-				}
-				fmt.Fprintln(t.w)
+				inv[ent.Name()] = puzzles
 			}
 		}
 	}
+
+	m := json.NewEncoder(t.w)
+	if err := m.Encode(inv); err != nil {
+		return err
+	}
 	return nil
 }
 
@@ -157,7 +159,7 @@ func (t *T) Open() error {
 
 // NewCategory returns a new Fs-backed category.
 func (t *T) NewCategory(name string) Category {
-	return NewFsCategory(NewRecursiveBasePathFs(t.Fs, name))
+	return NewFsCategory(t.Fs, name)
 }
 
 func main() {
diff --git a/cmd/transpile/main_test.go b/cmd/transpile/main_test.go
index 25b304a..76a0f8a 100644
--- a/cmd/transpile/main_test.go
+++ b/cmd/transpile/main_test.go
@@ -3,6 +3,7 @@ package main
 import (
 	"bytes"
 	"encoding/json"
+	"strings"
 	"testing"
 
 	"github.com/spf13/afero"
@@ -61,7 +62,7 @@ func TestEverything(t *testing.T) {
 	if err := tp.Handle("inventory"); err != nil {
 		t.Error(err)
 	}
-	if stdout.String() != "cat0 1 2 3 4 5 10 20 21 22\ncat1 93\n" {
+	if strings.TrimSpace(stdout.String()) != `{"cat0":[1,2,3,4,5,10,20,21,22],"cat1":[93]}` {
 		t.Errorf("Bad inventory: %#v", stdout.String())
 	}
 
diff --git a/cmd/transpile/puzzle.go b/cmd/transpile/puzzle.go
index 50c9d28..ea88c25 100644
--- a/cmd/transpile/puzzle.go
+++ b/cmd/transpile/puzzle.go
@@ -227,12 +227,12 @@ func (fp FsCommandPuzzle) Open(filename string) (io.ReadCloser, error) {
 	cmd := exec.CommandContext(ctx, fp.command, "-file", filename)
 	// BUG(neale): FsCommandPuzzle.Open() reads everything into memory, and will suck for large files.
 	out, err := cmd.Output()
+	buf := ioutil.NopCloser(bytes.NewBuffer(out))
 	if err != nil {
-		return NopReadCloser{}, err
+		return buf, err
 	}
-	buf := bytes.NewBuffer(out)
 
-	return ioutil.NopCloser(buf), nil
+	return buf, nil
 }
 
 // Answer checks whether the given answer is correct.
@@ -243,7 +243,7 @@ func (fp FsCommandPuzzle) Answer(answer string) bool {
 	cmd := exec.CommandContext(ctx, fp.command, "-answer", answer)
 	out, err := cmd.Output()
 	if err != nil {
-		log.Print("ERROR", err)
+		log.Printf("ERROR: checking answer: %s", err)
 		return false
 	}
 
diff --git a/cmd/transpile/puzzle_test.go b/cmd/transpile/puzzle_test.go
index c0bd933..ca58662 100644
--- a/cmd/transpile/puzzle_test.go
+++ b/cmd/transpile/puzzle_test.go
@@ -87,7 +87,7 @@ func TestPuzzle(t *testing.T) {
 }
 
 func TestFsPuzzle(t *testing.T) {
-	catFs := NewRecursiveBasePathFs(afero.NewOsFs(), "testdata")
+	catFs := NewRecursiveBasePathFs(NewRecursiveBasePathFs(afero.NewOsFs(), "testdata"), "static")
 
 	if _, err := NewFsPuzzle(catFs, 1).Puzzle(); err != nil {
 		t.Error(err)
diff --git a/cmd/transpile/testdata/generated/mkcategory b/cmd/transpile/testdata/generated/mkcategory
new file mode 100755
index 0000000..cfa4892
--- /dev/null
+++ b/cmd/transpile/testdata/generated/mkcategory
@@ -0,0 +1,45 @@
+#! /bin/sh -e
+
+fail () {
+    echo "ERROR: $*" 1>&2
+    exit 1
+}
+
+case $1:$2:$3 in
+    inventory::)
+        echo "[1,2,3,"
+        echo "4,5]"
+        ;;
+    puzzle:1:)
+        cat <<EOT
+{
+    "Answers": ["answer1.0"],
+    "Pre": {
+        "Authors": ["author1.0"],
+        "Body": "<h1>moo.</h1>"
+    }
+}
+EOT
+                ;;
+    puzzle:*:)
+        fail "No such puzzle"
+        ;;
+    file:1:moo.txt)
+        echo "Moo."
+        ;;
+    file:*:*)
+        fail "No such file: $2"
+        ;;
+    answer:1:answer1.0)
+        echo "correct"
+        ;;
+    answer:1:*)
+        echo "incorrect"
+        ;;
+    answer:*:*)
+        fail "Fail answer"
+        ;;
+    *)
+        fail "What is $1" 1>&2
+        ;;
+esac
diff --git a/cmd/transpile/testdata/1/puzzle.md b/cmd/transpile/testdata/static/1/puzzle.md
similarity index 100%
rename from cmd/transpile/testdata/1/puzzle.md
rename to cmd/transpile/testdata/static/1/puzzle.md
diff --git a/cmd/transpile/testdata/2/moo.js b/cmd/transpile/testdata/static/2/moo.js
similarity index 100%
rename from cmd/transpile/testdata/2/moo.js
rename to cmd/transpile/testdata/static/2/moo.js
diff --git a/cmd/transpile/testdata/2/moo.txt b/cmd/transpile/testdata/static/2/moo.txt
similarity index 100%
rename from cmd/transpile/testdata/2/moo.txt
rename to cmd/transpile/testdata/static/2/moo.txt
diff --git a/cmd/transpile/testdata/2/puzzle.md b/cmd/transpile/testdata/static/2/puzzle.md
similarity index 100%
rename from cmd/transpile/testdata/2/puzzle.md
rename to cmd/transpile/testdata/static/2/puzzle.md
diff --git a/cmd/transpile/testdata/3/mkpuzzle b/cmd/transpile/testdata/static/3/mkpuzzle
similarity index 100%
rename from cmd/transpile/testdata/3/mkpuzzle
rename to cmd/transpile/testdata/static/3/mkpuzzle