diff --git a/examples/tracing/README.md b/examples/tracing/README.md
index f30ca64..4156677 100644
--- a/examples/tracing/README.md
+++ b/examples/tracing/README.md
@@ -2,8 +2,9 @@
 
 End-to-end OpenTelemetry: every envelope on the wire (in either
 direction) emits a span, and W3C trace context (`traceparent` /
-`tracestate`) rides along inside `envelope.extensions["x.otel"]` so
-the spans link into one distributed trace.
+`tracestate`) rides along inside
+`envelope.extensions["x-vendor.opentelemetry.tracecontext"]` so the
+spans link into one distributed trace.
 
 The middleware MUST be wired on **both** sides. If only one side wires
 it, that side's spans are correct in isolation, but the peer creates
@@ -70,5 +71,6 @@ Tempo, Honeycomb, or any OTLP receiver.
 
 ## Spec sections
 
-- §11 Trace propagation (W3C context via `extensions["x.otel"]`)
+- §11 Trace propagation (W3C context via
+  `extensions["x-vendor.opentelemetry.tracecontext"]`)
 - §10.3 trace inheritance on delegation
diff --git a/examples/tracing/client.ts b/examples/tracing/client.ts
index e0b6d22..040b87e 100644
--- a/examples/tracing/client.ts
+++ b/examples/tracing/client.ts
@@ -3,7 +3,7 @@
  *
  * Wires `@agentruntimecontrolprotocol/middleware-otel` into the client transport so spans on
  * the submit path link end-to-end with the server's spans via W3C trace
- * context carried in `extensions["x.otel"]`.
+ * context carried in `extensions["x-vendor.opentelemetry.tracecontext"]`.
  *
  * Run after `server.ts`:
  *   pnpm tsx examples/tracing/client.ts
diff --git a/examples/tracing/server.ts b/examples/tracing/server.ts
index 226f6a9..c2a4d2e 100644
--- a/examples/tracing/server.ts
+++ b/examples/tracing/server.ts
@@ -2,9 +2,10 @@
  * Tracing example — server.
  *
  * Wires `@agentruntimecontrolprotocol/middleware-otel` into the runtime side so every inbound
- * frame extracts the W3C trace context from `extensions["x.otel"]` and
- * every outbound frame injects one. Spans are exported to the console
- * via `ConsoleSpanExporter` — no collector required.
+ * frame extracts the W3C trace context from
+ * `extensions["x-vendor.opentelemetry.tracecontext"]` and every outbound
+ * frame injects one. Spans are exported to the console via
+ * `ConsoleSpanExporter` — no collector required.
  */
 
 import {
diff --git a/packages/middleware/otel/src/index.ts b/packages/middleware/otel/src/index.ts
index 3ddf541..d9a7dd8 100644
--- a/packages/middleware/otel/src/index.ts
+++ b/packages/middleware/otel/src/index.ts
@@ -6,11 +6,13 @@
  *
  *   - Outbound `send` calls produce a `arcp.send` span with the envelope
  *     type/id/session_id as attributes, and the active trace context is
- *     injected into the envelope's `extensions["x.otel"]` field (W3C
- *     traceparent + tracestate) so the peer can continue the trace.
+ *     injected into the envelope's
+ *     `extensions["x-vendor.opentelemetry.tracecontext"]` field (W3C
+ *     `traceparent` + `tracestate`) so the peer can continue the trace.
  *   - Inbound frames produce a `arcp.recv` span, extracting any
- *     `extensions["x.otel"]` trace context so the inbound frame appears
- *     as a child of the originating span on the remote end.
+ *     `extensions["x-vendor.opentelemetry.tracecontext"]` trace context so
+ *     the inbound frame appears as a child of the originating span on the
+ *     remote end.
  *
  * Wire it on either side:
  * ```ts
@@ -50,7 +52,8 @@ const OTEL_EXTENSION_NAME = "x-vendor.opentelemetry.tracecontext" as const;
 
 /**
  * Wrap a {@link Transport} so each frame produces a span and W3C trace
- * context is propagated through `envelope.extensions["x.otel"]`.
+ * context is propagated through
+ * `envelope.extensions["x-vendor.opentelemetry.tracecontext"]`.
  *
  * The returned transport satisfies the same interface, so it is a drop-in
  * for `ARCPServer.accept(...)` / `ARCPClient.connect(...)`.