fix(core): send unchanged hooked requests without copying the body (#59097)

packages/core/src/session/model-request.ts

@@ -321,17 +321,20 @@ export const layer = Layer.effect(
       const http: StreamOptions["http"] = hasHttpHooks
         ? (req, handler) =>
             Effect.gen(function* () {
-              const before = yield* hooks.trigger("session", "http.request", {
-                ...scope,
-                request: yield* HttpClientRequest.toWeb(req),
-              })
-              let sent = HttpClientRequest.fromWeb(before.request)
-              if (before.request.body)
-                sent = HttpClientRequest.bodyUint8Array(
-                  sent,
-                  new Uint8Array(yield* Effect.promise(() => before.request.clone().arrayBuffer())),
-                  before.request.headers.get("content-type") ?? undefined,
-                )
+              const request = yield* HttpClientRequest.toWeb(req)
+              const before = yield* hooks.trigger("session", "http.request", { ...scope, request })
+              // A hook that kept the request object cannot have replaced its body, so the original body is sent as is.
+              // Reading it back would hold a second full copy of the request, screenshots included, for the whole call.
+              const sent =
+                before.request === request
+                  ? HttpClientRequest.setBody(HttpClientRequest.fromWeb(request), req.body)
+                  : before.request.body
+                    ? HttpClientRequest.bodyUint8Array(
+                        HttpClientRequest.fromWeb(before.request),
+                        new Uint8Array(yield* Effect.promise(() => before.request.clone().arrayBuffer())),
+                        before.request.headers.get("content-type") ?? undefined,
+                      )
+                    : HttpClientRequest.fromWeb(before.request)
               const res = yield* handler(sent)
               const after = yield* hooks.trigger("session", "http.response", {
                 ...scope,

packages/core/test/session-model-request-hooks.test.ts

@@ -81,6 +81,54 @@ describe("SessionModelRequest HTTP hooks", () => {
     }).pipe(Effect.provideService(SessionModelTransport.Service, transport)),
   )
 
+  it.effect("sends a kept request's original body and a replaced request's new body", () =>
+    Effect.gen(function* () {
+      const hooks = yield* PluginHooks.Service
+      let replace = false
+      yield* hooks.register("session", "http.request", (event) =>
+        Effect.sync(() => {
+          if (replace) {
+            event.request = new Request(event.request.url, { method: "POST", body: '{"replaced":true}' })
+            return
+          }
+          event.request.headers.set("x-plugin", "kept")
+        }),
+      )
+      const requests = yield* SessionModelRequest.Service.pipe(Effect.provide(SessionModelRequest.layer))
+      const prepared = yield* requests.primary({
+        session,
+        agent: Agent.ID.make("build"),
+        model,
+        system: [],
+        messages: [],
+      })
+      const http = prepared.options.http
+      if (!http) throw new Error("Expected HTTP middleware")
+      const original = HttpClientRequest.post("https://example.test/v1/chat/completions").pipe(
+        HttpClientRequest.bodyText('{"messages":[]}', "application/json"),
+      )
+      const send = () => {
+        const sent: HttpClientRequest.HttpClientRequest[] = []
+        return http(original, (request) =>
+          Effect.sync(() => {
+            sent.push(request)
+            return HttpClientResponse.fromWeb(request, new Response("{}", { status: 200 }))
+          }),
+        ).pipe(Effect.map(() => sent[0]!))
+      }
+
+      const kept = yield* send()
+      expect(kept.body).toBe(original.body)
+      expect(kept.headers["x-plugin"]).toBe("kept")
+
+      replace = true
+      const replaced = yield* send()
+      expect(replaced.body._tag).toBe("Uint8Array")
+      if (replaced.body._tag !== "Uint8Array") return
+      expect(new TextDecoder().decode(replaced.body.body)).toBe('{"replaced":true}')
+    }).pipe(Effect.provideService(SessionModelTransport.Service, transport)),
+  )
+
   it.effect("keeps hook edits to a tool schema out of later requests", () =>
     Effect.gen(function* () {
       const hooks = yield* PluginHooks.Service