2 files changed, 29 insertions, 0 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;