From f0182cdc9eead4d6c5894fb342bcbb64f603a208 Mon Sep 17 00:00:00 2001
From: David Brailovsky <davidbrai@gmail.com>
Date: Fri, 14 Aug 2026 10:54:00 +0200
Subject: [PATCH] fix(agent): make fork parent source-derived

Move optional fork destination id into ForkOptions and stop allowing fork callers to pass a parentSessionId override. Update the harness spec to state that fork destination parentSessionId is always the source session id.
---
 packages/agent/docs/harness.md               | 9 +++++----
 packages/agent/src/harness/session/memory.ts | 2 +-
 packages/agent/src/harness/session/types.ts  | 6 ++++--
 3 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/packages/agent/docs/harness.md b/packages/agent/docs/harness.md
index 80858b5ee..bad807dc0 100644
--- a/packages/agent/docs/harness.md
+++ b/packages/agent/docs/harness.md
@@ -802,8 +802,8 @@ A fork is a repository operation over one coherent source-session snapshot. It c
 
 ```ts
 type ForkOptions =
-  | { scope?: "branch"; entryId?: string; position?: "before" | "at" }
-  | { scope: "tree" };
+  | { scope?: "branch"; entryId?: string; position?: "before" | "at"; id?: string }
+  | { scope: "tree"; id?: string };
 ```
 
 - Memory and JSONL obtain the snapshot as one job on the source storage queue. SQLite uses one read transaction.
@@ -812,7 +812,8 @@ type ForkOptions =
 - Facts follow the selected scope: name/custom facts always copy; labels copy only when their target copies unless tree scope copies all targets.
 - Any message may be the fork point. Request construction heals orphaned tool calls.
 - Copied entries keep their ids.
-- The destination metadata records `parentSessionId`.
+- `id` optionally supplies the destination session id.
+- The destination metadata records `parentSessionId` as the source session id; callers cannot override it through fork options.
 
 A source with only fresh/unconfigured `main`—new format 4 or read-only normalized v3—may have no configuration. Either fork scope then creates one unconfigured destination `main`, which first harness attachment seeds normally. Every configured format-4 lane copied by a fork keeps its current total configuration.
 
@@ -840,7 +841,7 @@ interface SessionRepo<M extends SessionMetadata = SessionMetadata,
   open(metadata: M): Promise<Session<M>>;
   list(options?: L): Promise<M[]>;
   delete(metadata: M): Promise<void>;
-  fork(source: M, options: ForkOptions & C): Promise<Session<M>>;
+  fork(source: M, options: ForkOptions): Promise<Session<M>>;
 }
 
 interface SessionReader {
diff --git a/packages/agent/src/harness/session/memory.ts b/packages/agent/src/harness/session/memory.ts
index 3cc560332..aebf2d378 100644
--- a/packages/agent/src/harness/session/memory.ts
+++ b/packages/agent/src/harness/session/memory.ts
@@ -394,7 +394,7 @@ export class MemorySessionRepo implements SessionRepo {
 		this.sessions.delete(metadata.id);
 	}
 
-	async fork(source: SessionMetadata, options: ForkOptions & SessionCreateOptions): Promise<Session> {
+	async fork(source: SessionMetadata, options: ForkOptions): Promise<Session> {
 		this.assertOpen();
 		const sourceRecord = this.sessions.get(source.id);
 		if (sourceRecord === undefined) throw new Error(`Unknown session: ${source.id}`);
diff --git a/packages/agent/src/harness/session/types.ts b/packages/agent/src/harness/session/types.ts
index a7bf2116c..92237c383 100644
--- a/packages/agent/src/harness/session/types.ts
+++ b/packages/agent/src/harness/session/types.ts
@@ -533,7 +533,9 @@ export interface SessionCreateOptions {
 	parentSessionId?: string;
 }
 
-export type ForkOptions = { scope?: "branch"; entryId?: string; position?: "before" | "at" } | { scope: "tree" };
+export type ForkOptions =
+	| { scope?: "branch"; entryId?: string; position?: "before" | "at"; id?: string }
+	| { scope: "tree"; id?: string };
 
 export interface SessionRepo<
 	TMetadata extends SessionMetadata = SessionMetadata,
@@ -544,5 +546,5 @@ export interface SessionRepo<
 	open(metadata: TMetadata): Promise<Session<TMetadata>>;
 	list(options?: TListOptions): Promise<TMetadata[]>;
 	delete(metadata: TMetadata): Promise<void>;
-	fork(source: TMetadata, options: ForkOptions & TCreateOptions): Promise<Session<TMetadata>>;
+	fork(source: TMetadata, options: ForkOptions): Promise<Session<TMetadata>>;
 }