diff --git a/README.md b/README.md
index 67e5d86..9cd9d70 100644
--- a/README.md
+++ b/README.md
@@ -4,5 +4,8 @@ Shared versioned HTTP and authenticated WebSocket contracts for `trixie-server`
 
 This repository contains schemas, shared enums, error codes, compatibility fixtures, contract tests, and API documentation only.
 
-The first contract slice is under `contracts/v1/` and `fixtures/v1/`. It covers server readiness and health so the client and server can be developed and tested together before Twitch integration.
+The first contract slice is under `contracts/v1/` and `fixtures/v1/`. It covers
+server readiness, health, errors, and the transport-neutral realtime event
+envelope so the client and server can be developed and tested together before
+Twitch integration.
 
diff --git a/contracts/v1/error.json b/contracts/v1/error.json
new file mode 100644
index 0000000..9cba1d5
--- /dev/null
+++ b/contracts/v1/error.json
@@ -0,0 +1,13 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://trixie.invalid/contracts/v1/error.json",
+  "title": "Trixie API Error",
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["code", "message", "request_id"],
+  "properties": {
+    "code": { "type": "string", "pattern": "^[a-z0-9_]+$" },
+    "message": { "type": "string", "minLength": 1 },
+    "request_id": { "type": "string", "minLength": 1 }
+  }
+}
diff --git a/contracts/v1/realtime-event.json b/contracts/v1/realtime-event.json
new file mode 100644
index 0000000..087b235
--- /dev/null
+++ b/contracts/v1/realtime-event.json
@@ -0,0 +1,14 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://trixie.invalid/contracts/v1/realtime-event.json",
+  "title": "Trixie Realtime Event",
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["event_id", "event_type", "occurred_at", "payload"],
+  "properties": {
+    "event_id": { "type": "string", "minLength": 1 },
+    "event_type": { "type": "string", "pattern": "^[a-z0-9_.-]+$" },
+    "occurred_at": { "type": "string", "format": "date-time" },
+    "payload": { "type": "object" }
+  }
+}
diff --git a/docs/contract-testing.md b/docs/contract-testing.md
index a1653da..bb87af3 100644
--- a/docs/contract-testing.md
+++ b/docs/contract-testing.md
@@ -1,4 +1,12 @@
 # Contract Testing
 
-Fixtures cover HTTP responses, authenticated WebSocket events, errors, permissions, version negotiation, reconnects, and server-owned state transitions. Fixtures are deterministic, synthetic, and secret-free.
+Fixtures cover HTTP responses, authenticated WebSocket events, errors, permissions,
+version negotiation, reconnects, and server-owned state transitions. Fixtures are
+deterministic, synthetic, and secret-free. Every new contract must include at
+least one valid fixture and a negative case where validation rules are important.
+
+The current realtime envelope is deliberately transport-neutral: the server may
+deliver it over an authenticated WebSocket or a future compatible transport, but
+the event identity and payload rules remain the same. `event_id` is the stable
+deduplication key for at-least-once delivery.
 
diff --git a/fixtures/v1/error.unauthorized.json b/fixtures/v1/error.unauthorized.json
new file mode 100644
index 0000000..9d15817
--- /dev/null
+++ b/fixtures/v1/error.unauthorized.json
@@ -0,0 +1,5 @@
+{
+  "code": "unauthorized",
+  "message": "Authentication is required.",
+  "request_id": "req_fixture_unauthorized"
+}
diff --git a/fixtures/v1/realtime.activity.json b/fixtures/v1/realtime.activity.json
new file mode 100644
index 0000000..1664247
--- /dev/null
+++ b/fixtures/v1/realtime.activity.json
@@ -0,0 +1,9 @@
+{
+  "event_id": "evt_fixture_activity_001",
+  "event_type": "activity.follow",
+  "occurred_at": "2026-01-01T00:00:00Z",
+  "payload": {
+    "username": "fixture_viewer",
+    "display_name": "Fixture Viewer"
+  }
+}