feat(durable): read a conversation's context as of an earlier entry

closes #10512

packages/durable/CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Added
 
 - `ToolExecutionApi.models` and `HookApi.models`: the Harness's `models`, so tools and hooks can resolve models and make requests with the same catalog, credentials, and request transforms as generation ([#10395](https://github.com/earendil-works/pi/issues/10395)).
+- `Conversation.context(context, { at })` returns the model context as of a visible earlier entry, the same view `fork(at)` would start with, without creating a conversation ([#10512](https://github.com/earendil-works/pi/issues/10512)).
 
 ### Changed
 

packages/durable/docs/spec.md

@@ -539,7 +539,8 @@ interface Conversation {
     change: (tx: Tx) => T | Promise<T>,
     context: Context,
   ): Promise<T>;
-  context(context: Context): Promise<ContextView>;
+  /** With `at`, the context as of that visible entry, as `fork(at)` would start with. */
+  context(context: Context, options?: { readonly at?: EntryId }): Promise<ContextView>;
   entries(
     query: Omit<EntryQuery, "conversationId">,
     limit: number,

packages/durable/src/harness/harness.ts

@@ -120,8 +120,8 @@ class ConversationImpl<Tool extends ToolRegistration> implements Conversation {
 		return this.#host.harness.commitWith(change, context, { conversationId: this.id });
 	}
 
-	context(context: Context): Promise<ContextView> {
-		return readContext(this.#host.harness, this.#host.storage, this.id, context);
+	context(context: Context, options?: { readonly at?: EntryId }): Promise<ContextView> {
+		return readContext(this.#host.harness, this.#host.storage, this.id, context, options?.at);
 	}
 
 	entries(

packages/durable/src/harness/types.ts

@@ -531,7 +531,11 @@ export interface Conversation {
 
 	/** Session commit whose `tx.createTask()` defaults to this conversation. */
 	commit<T>(change: (tx: Tx) => T | Promise<T>, context: Context): Promise<T>;
-	context(context: Context): Promise<ContextView>;
+	/**
+	 * Committed raw active transcript and model context. With `at`, the context as of that visible entry: the same view
+	 * `fork(at)` would start with, without creating a conversation.
+	 */
+	context(context: Context, options?: { readonly at?: EntryId }): Promise<ContextView>;
 	/** Newest-first fork-aware history of this conversation. */
 	entries(
 		query: Omit<EntryQuery, "conversationId">,

packages/durable/test/harness-context.test.ts

@@ -142,6 +142,38 @@ describe("conversation context", () => {
 		expect(parentView.entries.map((entry) => entry.kind)).toEqual(["reset", "message"]);
 	});
 
+	// #10512
+	it("reads the context as of an earlier entry, as a fork at that entry starts", async () => {
+		const { root, append, message } = await setup();
+		const first = await message(user("first"));
+		const call = await message(assistant("calling", { calls: ["x", "y"] }));
+		const result = await message(toolResult("x"));
+		await message(toolResult("y"));
+		const edit = await append({
+			kind: "edit",
+			edits: [{ target: first.id, action: "replace", messages: [user("first v2")] }],
+		});
+		const reset = await append({ kind: "reset", head: "self", model: [user("fresh start")] });
+		const tail = await message(assistant("after reset"));
+
+		for (const at of [first, call, result, edit, reset, tail]) {
+			const fork = await root.fork(at.id, { ownership: { kind: "ownerless" } }, context);
+			expect(await root.context(context, { at: at.id })).toEqual(await fork.context(context));
+		}
+		// Stepped back before the edit and the reset: neither applies, and a cut call gets synthesized results.
+		expect((await root.context(context, { at: call.id })).messages.map(describeMessage)).toEqual([
+			"user:first",
+			"assistant:calling",
+			"result:x:error",
+			"result:y:error",
+		]);
+		expect(await root.context(context, { at: tail.id })).toEqual(await root.context(context));
+		expect(await root.context(context, {})).toEqual(await root.context(context));
+
+		const other = await root.fork(first.id, { ownership: { kind: "ownerless" } }, context);
+		await expect(other.context(context, { at: tail.id })).rejects.toThrow("is not visible");
+	});
+
 	it("extends a task invocation's context read with only newer entries", async () => {
 		let scanned = 0;
 		const memory = new MemoryStorage();