feat(web): send only the trailing user message per agent turn when enabled

Behind NEXT_PUBLIC_SESSIONS_LAST_MESSAGE_ONLY (default off; MUST pair with the backend's
AGENTA_SESSIONS_RECONSTRUCT), buildAgentRequest sends just the newest user message and lets the
runner rebuild prior turns from the durable record log — shrinking request and trace payloads
(the AGE-3970 trace-drawer OOM driver). Only a fresh user turn is trimmed: a HITL resume, whose
trailing turn carries the settled answer, keeps the full history so the answer still binds to its
tool call, as does any run without a session id. New isSessionsLastMessageOnlyEnabled helper.

Author: ardaerzin

web/packages/agenta-playground/src/state/execution/agentRequest.ts

@@ -30,6 +30,7 @@ import {
     workflowMolecule,
     type AgentTemplate,
 } from "@agenta/entities/workflow"
+import {isSessionsLastMessageOnlyEnabled} from "@agenta/shared/api"
 import {projectIdAtom} from "@agenta/shared/state"
 import {getDefaultStore} from "jotai"
 
@@ -400,13 +401,24 @@ export async function buildAgentRequest(
     // Strip answer-less assistant turns so a "no response" turn can't poison the next request.
     const history = messages.filter(hasAnswer)
 
+    // Last-message-only (flag-gated; MUST pair with the backend's AGENTA_SESSIONS_RECONSTRUCT).
+    // On a fresh user turn, send just the trailing user message and let the runner rebuild prior
+    // turns from the durable record log — smaller request + trace payloads. A HITL resume, whose
+    // trailing turn carries the settled answer (not a user turn), keeps the full history so the
+    // answer still binds to its tool call.
+    const lastMessage = history[history.length - 1] as {role?: unknown} | undefined
+    const outboundMessages =
+        isSessionsLastMessageOnlyEnabled() && opts.sessionId && lastMessage?.role === "user"
+            ? [lastMessage]
+            : history
+
     return {
         invocationUrl: url,
         headers,
         requestBody: {
             session_id: opts.sessionId,
             references,
-            data: {inputs: {messages: history}, parameters},
+            data: {inputs: {messages: outboundMessages}, parameters},
         },
     }
 }

web/packages/agenta-playground/tests/unit/agentRequest.test.ts

@@ -13,7 +13,7 @@
  * headers + project come from the real `executionHeadersAtom` / `projectIdAtom`.
  */
 import {createStore, type PrimitiveAtom} from "jotai"
-import {describe, expect, it, beforeEach, vi} from "vitest"
+import {describe, expect, it, beforeEach, afterEach, vi} from "vitest"
 
 vi.mock("@agenta/entities/workflow", async (importOriginal) => {
     const actual = (await importOriginal()) as any
@@ -177,6 +177,48 @@ describe("buildAgentRequest", () => {
         expect(await buildAgentRequest("e", [], {sessionId: "s1", store})).toBeNull()
     })
 
+    describe("last-message-only (NEXT_PUBLIC_SESSIONS_LAST_MESSAGE_ONLY)", () => {
+        const u1 = {role: "user", parts: [{type: "text", text: "q1"}]}
+        const a1 = {role: "assistant", parts: [{type: "text", text: "a1"}]}
+        const u2 = {role: "user", parts: [{type: "text", text: "q2"}]}
+
+        // Runtime override path (getEnv checks globalThis.__env before the build-time snapshot).
+        const enableFlag = () => {
+            ;(globalThis as any).__env = {NEXT_PUBLIC_SESSIONS_LAST_MESSAGE_ONLY: "true"}
+        }
+        afterEach(() => {
+            delete (globalThis as any).__env
+        })
+
+        const outMessages = async (msgs: unknown[], sessionId = "s1") => {
+            seed(store, "e", {})
+            const req = await buildAgentRequest("e", msgs, {sessionId, store})
+            return (req!.requestBody.data as any).inputs.messages
+        }
+
+        it("sends only the trailing user message when enabled", async () => {
+            enableFlag()
+            expect(await outMessages([u1, a1, u2])).toEqual([u2])
+        })
+
+        it("sends the full history by default (flag off)", async () => {
+            const out = await outMessages([u1, a1, u2])
+            expect(out).toEqual([u1, a1, u2])
+        })
+
+        it("keeps the full history on a resume (trailing assistant) even when enabled", async () => {
+            enableFlag()
+            const out = await outMessages([u1, a1])
+            expect(out.length).toBeGreaterThan(1)
+            expect(out[out.length - 1].role).toBe("assistant")
+        })
+
+        it("sends the full history when there is no session id", async () => {
+            enableFlag()
+            expect(await outMessages([u1, a1, u2], "")).toEqual([u1, a1, u2])
+        })
+    })
+
     it("nests messages under data.inputs + draft-aware parameters under data, with session_id", async () => {
         seed(store, "e", {config: {temperature: 0.9, prompt: {x: 1}}})
         const req = await buildAgentRequest("e", [{role: "user"}], {sessionId: "s1", store})

web/packages/agenta-shared/src/api/env.ts

@@ -39,6 +39,7 @@ export const processEnv = {
     NEXT_PUBLIC_AGENTA_SANDBOX_LOCAL_ENABLED: process.env.NEXT_PUBLIC_AGENTA_SANDBOX_LOCAL_ENABLED,
     NEXT_PUBLIC_AGENTA_ENABLED_SANDBOX_PROVIDERS:
         process.env.NEXT_PUBLIC_AGENTA_ENABLED_SANDBOX_PROVIDERS,
+    NEXT_PUBLIC_SESSIONS_LAST_MESSAGE_ONLY: process.env.NEXT_PUBLIC_SESSIONS_LAST_MESSAGE_ONLY,
 }
 
 /**
@@ -84,6 +85,17 @@ export const isSandboxLocalEnabled = (): boolean => {
     return SANDBOX_LOCAL_TRUTHY.has(raw.trim().toLowerCase())
 }
 
+/**
+ * Send only the trailing user message per agent turn and let the runner rebuild prior history
+ * from the durable record log. Default OFF — enable ONLY where the backend runs with
+ * `AGENTA_SESSIONS_RECONSTRUCT=true` (they must be flipped together), or a cold turn loses its
+ * context.
+ */
+export const isSessionsLastMessageOnlyEnabled = (): boolean => {
+    const raw = getEnv("NEXT_PUBLIC_SESSIONS_LAST_MESSAGE_ONLY") || "false"
+    return SANDBOX_LOCAL_TRUTHY.has(raw.trim().toLowerCase())
+}
+
 /** The sandbox providers this deployment enabled, normalized to lowercase ids. Unset/empty
  * falls back to `["local"]` so the picker never hides every option. */
 export const getEnabledSandboxProviders = (): string[] => {

web/packages/agenta-shared/src/api/index.ts

@@ -7,6 +7,7 @@ export {
     getAgentaApiUrl,
     getAgentaWebUrl,
     isSandboxLocalEnabled,
+    isSessionsLastMessageOnlyEnabled,
     getEnabledSandboxProviders,
     processEnv,
 } from "./env"