diff --git a/packages/contracts/src/orchestration.test.ts b/packages/contracts/src/orchestration.test.ts
index d89e1fb2957c5a0bbfd65f8d53c8f464079bfda5..7c7bade4502e26d1cb2c722b5af42cc3271cd3fd 100644
--- a/packages/contracts/src/orchestration.test.ts
+++ b/packages/contracts/src/orchestration.test.ts
@@ -812,6 +812,37 @@ it.effect("decodes thread pull request links with snapshot and stack", () =>
   }),
 );
 
+// A stored event that fails to decode stops the event store read, and with it
+// server startup, so rows written before `turnId` existed must still load.
+it.effect("decodes a legacy message-sent event persisted without turnId", () =>
+  Effect.gen(function* () {
+    const event = yield* decodeOrchestrationEvent({
+      sequence: 539,
+      eventId: "event-message-legacy-1",
+      aggregateKind: "thread",
+      aggregateId: "thread-1",
+      type: "thread.message-sent",
+      occurredAt: "2026-01-01T00:00:00.000Z",
+      commandId: "cmd-message-legacy-1",
+      causationEventId: null,
+      correlationId: "cmd-message-legacy-1",
+      metadata: {},
+      payload: {
+        threadId: "thread-1",
+        messageId: "message-1",
+        role: "user",
+        text: "written before turn ids were recorded",
+        streaming: false,
+        createdAt: "2026-01-01T00:00:00.000Z",
+        updatedAt: "2026-01-01T00:00:00.000Z",
+      },
+    });
+    assert.strictEqual(event.type, "thread.message-sent");
+    if (event.type !== "thread.message-sent") return;
+    assert.strictEqual(event.payload.turnId, null);
+  }),
+);
+
 it.effect("decodes thread archived and unarchived events", () =>
   Effect.gen(function* () {
     const archived = yield* decodeOrchestrationEvent({
diff --git a/packages/contracts/src/orchestration.ts b/packages/contracts/src/orchestration.ts
index 9a9ed05d4e6d8d1dd46d8b19671cb172e58052d9..3e323e4964d58a1f1088e07335105a9dc0924e1d 100644
--- a/packages/contracts/src/orchestration.ts
+++ b/packages/contracts/src/orchestration.ts
@@ -1893,7 +1893,8 @@ export const ThreadMessageSentPayload = Schema.Struct({
   text: Schema.String,
   attachments: Schema.optional(Schema.Array(ChatAttachment)),
   context: Schema.optional(OrchestrationMessageContext),
-  turnId: Schema.NullOr(TurnId),
+  // Events persisted before the field existed carry no key at all.
+  turnId: Schema.NullOr(TurnId).pipe(Schema.withDecodingDefault(Effect.succeed(null))),
   streaming: Schema.Boolean,
   createdAt: IsoDateTime,
   updatedAt: IsoDateTime,