diff --git a/internal/connector/handoff.go b/internal/connector/handoff.go
index b48a869c4..8accc92ba 100644
--- a/internal/connector/handoff.go
+++ b/internal/connector/handoff.go
@@ -23,6 +23,11 @@ import (
 // HandoffLine is one trusted request, as the reader gets it. It is the
 // instruction a worker used to pull, written out: the agent's own mention
 // stripped, nothing more than the record carries.
+//
+// Its keys are a contract with whoever reads the lines, the basecamp-connect
+// skill first: renaming or dropping one breaks a reader nothing here runs.
+// requestLineContract holds the names, and the skill's example is held to
+// the line exactly, so a key added here is documented where readers learn it.
 type HandoffLine struct {
 	Type      string `json:"type"` // "request"
 	EventID   int64  `json:"event_id"`
diff --git a/internal/connector/request_contract_test.go b/internal/connector/request_contract_test.go
new file mode 100644
index 000000000..d21d3ac58
--- /dev/null
+++ b/internal/connector/request_contract_test.go
@@ -0,0 +1,141 @@
+package connector
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"reflect"
+	"regexp"
+	"slices"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// requestLineContract is every key a "type":"request" line carries, nested
+// keys as parent.child. The basecamp-connect skill, and any session or
+// practice that drives the connector, keys on these names: renaming or
+// dropping one breaks a reader that nothing here runs, so it is a breaking
+// change made on purpose, here, or not at all. Adding a key is not.
+var requestLineContract = []string{
+	"acknowledge",
+	"content",
+	"content_updated_at",
+	"event_id",
+	"event_type",
+	"recording.bucket_id",
+	"recording.project_name",
+	"recording.recording_id",
+	"recording.title",
+	"recording.type",
+	"recording.url",
+	"reply_to.kind",
+	"reply_to.recording_id",
+	"requester_id",
+	"requester_name",
+	"role",
+	"trigger",
+	"type",
+}
+
+// optionalKeys are the contract's keys a line may leave out: the ones whose
+// value is not always known.
+var optionalKeys = []string{"recording.project_name", "requester_name"}
+
+// The line the connector writes carries exactly the keys the contract names,
+// under those names: a key dropped or renamed breaks a reader, and a key
+// added is added to the contract on purpose.
+func TestTheRequestLineKeepsItsContract(t *testing.T) {
+	assert.Equal(t, requestLineContract, writtenLineKeys(t))
+}
+
+// Every key but the optional ones is there at its zero value too. A false
+// acknowledge or an empty title is a real line, and a key that omitempty
+// drops there is gone from the line its readers get.
+func TestARequestLineAtItsZeroValuesKeepsItsRequiredKeys(t *testing.T) {
+	raw, err := json.Marshal(HandoffLine{})
+	require.NoError(t, err)
+	want := slices.DeleteFunc(slices.Clone(requestLineContract), func(k string) bool { return slices.Contains(optionalKeys, k) })
+	assert.Equal(t, want, lineKeys(t, raw))
+}
+
+// skillExample finds the request line example in the skill.
+var skillExample = regexp.MustCompile("(?s)### 2\\. The request line\\s*```json\\r?\\n(.*?)```")
+
+// The skill's own example of the line names exactly the keys the line
+// carries, so the session reading the line is taught the names it will see,
+// and a key added to the line is documented where the reader learns it.
+func TestTheSkillDocumentsTheRequestLineItGets(t *testing.T) {
+	skill, err := os.ReadFile(filepath.Join("..", "..", "skills", "basecamp-connect", "SKILL.md"))
+	require.NoError(t, err)
+	example := skillExample.FindSubmatch(skill)
+	require.NotNil(t, example, "the skill's request line example is where this test looks for it")
+	assert.Equal(t, writtenLineKeys(t), lineKeys(t, example[1]))
+}
+
+// writtenLineKeys is the keys of a request line as the connector writes one,
+// with every field set, so a key that omitempty would drop is still seen —
+// including one added after this test was written.
+func writtenLineKeys(t *testing.T) []string {
+	t.Helper()
+	var line HandoffLine
+	fill(t, reflect.ValueOf(&line).Elem())
+	raw, err := json.Marshal(line)
+	require.NoError(t, err)
+	return lineKeys(t, raw)
+}
+
+// fill sets every field of v to a value that is not its zero.
+func fill(t *testing.T, v reflect.Value) {
+	t.Helper()
+	if v.Type() == reflect.TypeFor[time.Time]() {
+		v.Set(reflect.ValueOf(time.Date(2026, 10, 6, 0, 0, 0, 0, time.UTC)))
+		return
+	}
+	switch v.Kind() {
+	case reflect.Struct:
+		for i := range v.NumField() {
+			if v.Type().Field(i).IsExported() {
+				fill(t, v.Field(i))
+			}
+		}
+	case reflect.String:
+		v.SetString("x")
+	case reflect.Int, reflect.Int64, reflect.Int32:
+		v.SetInt(1)
+	case reflect.Bool:
+		v.SetBool(true)
+	default:
+		t.Fatalf("the request line has a %s field; teach fill to set one", v.Kind())
+	}
+}
+
+// lineKeys is a line's keys, nested ones as their full dotted path, sorted.
+func lineKeys(t *testing.T, raw []byte) []string {
+	t.Helper()
+	var obj map[string]any
+	require.NoError(t, json.Unmarshal(raw, &obj))
+	var keys []string
+	var walk func(prefix string, obj map[string]any)
+	walk = func(prefix string, obj map[string]any) {
+		for k, v := range obj {
+			if nested, ok := v.(map[string]any); ok && len(nested) > 0 {
+				walk(prefix+k+".", nested)
+				continue
+			}
+			keys = append(keys, prefix+k)
+		}
+	}
+	walk("", obj)
+	slices.Sort(keys)
+	return keys
+}
+
+// An empty object is a key like any other, and the skill example is found
+// whatever line endings the checkout gave the file.
+func TestTheContractWalkSeesEmptyObjectsAndCRLF(t *testing.T) {
+	assert.Equal(t, []string{"a.b", "metadata"}, lineKeys(t, []byte(`{"metadata":{},"a":{"b":1}}`)))
+	assert.NotNil(t, skillExample.FindSubmatch([]byte("### 2. The request line\r\n\r\n```json\r\n{}\r\n```")))
+}