4 files changed, 52 insertions, 4 deletions

diff --git a/packages/kernel/src/contracts/runtime.ts b/packages/kernel/src/contracts/runtime.ts
index c449a68..02fc446 100644
--- a/packages/kernel/src/contracts/runtime.ts
+++ b/packages/kernel/src/contracts/runtime.ts
@@ -117,6 +117,18 @@ export interface RunTurnInput {
 	 * what to do with any pending messages after the turn ends).
 	 */
 	readonly drainSteering?: () => readonly ChatMessage[];
+
+	/**
+	 * Optional. Called by the runtime after each step's messages are finalized
+	 * (the assistant message + tool-result messages are built). The caller can
+	 * use this to persist step messages incrementally — assigning seq numbers
+	 * during generation so consumers can `GET /conversations/:id?sinceSeq=N`
+	 * mid-turn. When omitted, the caller must persist all messages at turn end
+	 * (via `RunTurnResult.messages`). The messages passed to this callback are
+	 * the SAME objects in `RunTurnResult.messages` — the caller must NOT
+	 * double-persist them.
+	 */
+	readonly onStepComplete?: (messages: readonly ChatMessage[]) => Promise<void> | void;
 }
 
 /**
diff --git a/packages/kernel/src/runtime/run-turn.ts b/packages/kernel/src/runtime/run-turn.ts
index 228ef8a..4a12e28 100644
--- a/packages/kernel/src/runtime/run-turn.ts
+++ b/packages/kernel/src/runtime/run-turn.ts
@@ -526,6 +526,23 @@ export async function runTurn(input: RunTurnInput): Promise<RunTurnResult> {
 				resultMessages.push(msg);
 			}
 
+			// Incremental persistence: notify the caller that this step's
+			// messages are finalized. The caller can persist them immediately
+			// (assigning seq numbers during generation). The messages are the
+			// SAME objects in resultMessages — the caller must NOT double-persist.
+			if (input.onStepComplete !== undefined) {
+				const stepMessages: ChatMessage[] = [];
+				if (stepResult.assistantMessage !== undefined) {
+					stepMessages.push(stepResult.assistantMessage);
+				}
+				for (const msg of stepResult.toolMessages) {
+					stepMessages.push(msg);
+				}
+				if (stepMessages.length > 0) {
+					await input.onStepComplete(stepMessages);
+				}
+			}
+
 			if (signal.aborted) {
 				finishReason = "aborted";
 				break;
diff --git a/packages/session-orchestrator/src/orchestrator.test.ts b/packages/session-orchestrator/src/orchestrator.test.ts
index d5585c2..012ac5c 100644
--- a/packages/session-orchestrator/src/orchestrator.test.ts
+++ b/packages/session-orchestrator/src/orchestrator.test.ts
@@ -574,7 +574,7 @@ describe("turn-sealed event", () => {
 			},
 		});
 
-		expect(ordering).toEqual(["append", "appendMetrics", "turn-sealed"]);
+		expect(ordering).toEqual(["append", "append", "appendMetrics", "turn-sealed"]);
 	});
 
 	it("does not emit turn-sealed when append throws — emits error event instead", async () => {
diff --git a/packages/session-orchestrator/src/orchestrator.ts b/packages/session-orchestrator/src/orchestrator.ts
index bc9e78b..22a5c11 100644
--- a/packages/session-orchestrator/src/orchestrator.ts
+++ b/packages/session-orchestrator/src/orchestrator.ts
@@ -457,10 +457,29 @@ export function createSessionOrchestrator(
 					...(drainSteering !== undefined ? { drainSteering } : {}),
 				};
 
-				const result = await deps.runTurn(opts);
+				// Persist the user message at turn start so it has a seq
+				// number before the first step generates. This enables the
+				// FE to syncTail during generation (CR-6).
+				await deps.conversationStore.append(conversationId, [userMsg]);
+
+				let stepsPersisted = false;
+				const result = await deps.runTurn({
+					...opts,
+					// Incremental persistence: persist each step's messages
+					// as they are finalized. Seq numbers are assigned during
+					// generation, so the FE can GET /conversations/:id?sinceSeq=N
+					// mid-turn and pick up committed chunks (CR-6).
+					onStepComplete: async (stepMessages) => {
+						await deps.conversationStore.append(conversationId, stepMessages);
+						stepsPersisted = true;
+					},
+				});
 
-				const toPersist: ChatMessage[] = [userMsg, ...result.messages];
-				await deps.conversationStore.append(conversationId, toPersist);
+				// Fallback: if onStepComplete was never called (e.g., a fake
+				// runTurn in tests), persist all result messages as a batch.
+				if (!stepsPersisted && result.messages.length > 0) {
+					await deps.conversationStore.append(conversationId, result.messages);
+				}
 
 				const turnMetrics = metrics.build(turnId);
 				await deps.conversationStore.appendMetrics(conversationId, turnMetrics);