From 2f010a4a8e98e596c7270ff2faaac588715ec790 Mon Sep 17 00:00:00 2001
From: Andrew <andrewwang298@gmail.com>
Date: Sun, 23 Aug 2026 19:44:50 -0700
Subject: [PATCH] perf(opencode): overlap the pre-turn snapshot with the
 provider request

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
---
 packages/opencode/src/session/processor.ts    | 30 +++++++++++++------
 packages/opencode/src/session/tools.ts        |  4 ++-
 .../opencode/test/session/compaction.test.ts  |  1 +
 3 files changed, 25 insertions(+), 10 deletions(-)

diff --git a/packages/opencode/src/session/processor.ts b/packages/opencode/src/session/processor.ts
index c6689252cd51..aba2b621daf8 100644
--- a/packages/opencode/src/session/processor.ts
+++ b/packages/opencode/src/session/processor.ts
@@ -2,7 +2,7 @@ import { LayerNode } from "@opencode-ai/core/effect/layer-node"
 import { PermissionV1 } from "@opencode-ai/core/v1/permission"
 import { Image } from "@/image/image"
 import { SessionV1 } from "@opencode-ai/core/v1/session"
-import { Cause, Deferred, Effect, Exit, Layer, Context, Scope, Schema } from "effect"
+import { Cause, Deferred, Effect, Exit, Fiber, Layer, Context, Scope, Schema } from "effect"
 import * as Stream from "effect/Stream"
 import { Agent } from "@/agent/agent"
 import { Config } from "@/config/config"
@@ -35,6 +35,7 @@ export interface Handle {
   readonly requestStartAt: number | undefined
   readonly firstRequestStartAt: number | undefined
   readonly snapshotMs: number
+  readonly awaitSnapshot: Effect.Effect<string | undefined>
   readonly updateToolCall: (
     toolCallID: string,
     update: (part: SessionV1.ToolPart) => SessionV1.ToolPart,
@@ -104,19 +105,14 @@ const layer = Layer.effect(
     const database = yield* Database.Service
 
     const create = Effect.fn("SessionProcessor.create")(function* (input: Input) {
-      // Pre-capture snapshot before the LLM stream starts. The AI SDK
-      // may execute tools internally before emitting start-step events,
-      // so capturing inside the event handler can be too late.
-      const snapshotStart = Date.now()
-      const initialSnapshot = yield* snapshot.track()
       const ctx: ProcessorContext = {
         assistantMessage: input.assistantMessage,
         sessionID: input.sessionID,
         model: input.model,
         toolcalls: {},
         shouldBreak: false,
-        snapshot: initialSnapshot,
-        snapshotMs: Date.now() - snapshotStart,
+        snapshot: undefined,
+        snapshotMs: 0,
         blocked: false,
         needsCompaction: false,
         currentText: undefined,
@@ -127,6 +123,21 @@ const layer = Layer.effect(
       }
       let aborted = false
 
+      // Runs concurrently with the provider request; awaitSnapshot is the barrier
+      // every tool execution and step event takes before touching the result.
+      const snapshotStart = Date.now()
+      const snapshotFiber = yield* snapshot.track().pipe(
+        Effect.tap(() => Effect.sync(() => (ctx.snapshotMs = Date.now() - snapshotStart))),
+        Effect.orDie,
+        Effect.forkIn(scope),
+      )
+      const awaitSnapshot = Effect.gen(function* () {
+        if (ctx.snapshot === undefined) {
+          ctx.snapshot = yield* Fiber.join(snapshotFiber)
+        }
+        return ctx.snapshot
+      })
+
       const parse = (e: unknown) =>
         MessageV2.fromError(e, {
           providerID: input.model.providerID,
@@ -436,7 +447,7 @@ const layer = Layer.effect(
             throw new Error(value.message)
 
           case "step-start":
-            if (!ctx.snapshot) ctx.snapshot = yield* snapshot.track()
+            yield* awaitSnapshot
             yield* session.updatePart({
               id: PartID.ascending(),
               messageID: ctx.assistantMessage.id,
@@ -716,6 +727,7 @@ const layer = Layer.effect(
         get snapshotMs() {
           return ctx.snapshotMs
         },
+        awaitSnapshot,
         updateToolCall,
         completeToolCall,
         process,
diff --git a/packages/opencode/src/session/tools.ts b/packages/opencode/src/session/tools.ts
index 805d2253b8b6..e2f78fc00f2f 100644
--- a/packages/opencode/src/session/tools.ts
+++ b/packages/opencode/src/session/tools.ts
@@ -43,7 +43,7 @@ export const resolve = Effect.fn("SessionTools.resolve")(function* (input: {
   agent: Agent.Info
   model: Provider.Model
   session: Session.Info
-  processor: Pick<SessionProcessor.Handle, "message" | "updateToolCall" | "completeToolCall">
+  processor: Pick<SessionProcessor.Handle, "message" | "updateToolCall" | "completeToolCall" | "awaitSnapshot">
   bypassAgentCheck: boolean
   messages: SessionV1.WithParts[]
   promptOps: TaskPromptOps
@@ -104,6 +104,8 @@ export const resolve = Effect.fn("SessionTools.resolve")(function* (input: {
         return run.promise(
           Effect.gen(function* () {
             const ctx = context(args, options)
+            // Tools can run before any step event; the pre-turn snapshot must land before a tool mutates files.
+            yield* input.processor.awaitSnapshot
             yield* plugin.trigger(
               "tool.execute.before",
               { tool: item.id, sessionID: ctx.sessionID, callID: ctx.callID },
diff --git a/packages/opencode/test/session/compaction.test.ts b/packages/opencode/test/session/compaction.test.ts
index 1878cec50fc9..008f230a2272 100644
--- a/packages/opencode/test/session/compaction.test.ts
+++ b/packages/opencode/test/session/compaction.test.ts
@@ -206,6 +206,7 @@ function fake(
     requestStartAt: undefined,
     firstRequestStartAt: undefined,
     snapshotMs: 0,
+    awaitSnapshot: Effect.succeed(undefined),
     updateToolCall: Effect.fn("TestSessionProcessor.updateToolCall")(() => Effect.succeed(undefined)),
     completeToolCall: Effect.fn("TestSessionProcessor.completeToolCall")(() => Effect.void),
     process: Effect.fn("TestSessionProcessor.process")(() => Effect.succeed(result)),