fix(core): share affinity in provider session headers (#51931)

packages/core/src/plugin/provider/github-copilot.ts

@@ -11,6 +11,7 @@ import { Model } from "../../model.js"
 import { Agent } from "../../agent.js"
 import { define } from "@opencode/plugin/effect/plugin"
 import { Provider } from "../../provider.js"
+import { SessionAffinity } from "../../session/affinity.js"
 import type { PluginInternal } from "../internal.js"
 
 const clientID = "Ov23li8tweQw6odWQebz"
@@ -272,7 +273,7 @@ export const GithubCopilotPlugin = define({
             .pipe(Effect.orElseSucceed(() => undefined))
           const interaction = interactionType(evt.kind, session?.parentID !== undefined)
           evt.headers["X-Interaction-Type"] = interaction
-          evt.headers["X-Interaction-Id"] = evt.sessionID
+          evt.headers["X-Interaction-Id"] = session ? SessionAffinity.get(session) : evt.sessionID
           if (interaction !== "conversation-agent") evt.headers["x-initiator"] = "agent"
         }),
       { providerID: Provider.ID.githubCopilot },

packages/core/src/plugin/provider/openai.ts

@@ -9,6 +9,7 @@ import { Bus } from "../../bus.js"
 import { Integration } from "../../integration.js"
 import { OauthCallbackPage } from "../../oauth/page.js"
 import { Provider } from "../../provider.js"
+import { SessionAffinity } from "../../session/affinity.js"
 import type { PluginInternal } from "../internal.js"
 
 const clientID = "app_EMoamEEZ73f0CkXaXp7hrann"
@@ -299,12 +300,16 @@ export const OpenAIPlugin = define({
     yield* ctx.session.hook(
       "model.request",
       (evt) =>
-        Effect.sync(() => {
+        Effect.gen(function* () {
           if (!chatgpt) return
           if (evt.baseURL && URL.canParse(evt.baseURL) && new URL(evt.baseURL).origin === "https://api.openai.com")
             evt.baseURL = codexBaseURL
+          const session = yield* ctx.session
+            .get({ sessionID: evt.sessionID })
+            .pipe(Effect.orElseSucceed(() => undefined))
           evt.headers.originator = "opencode"
-          evt.headers["session-id"] = evt.sessionID
+          // ChatGPT routes its prompt cache on this header, so children share the parent's.
+          evt.headers["session-id"] = session ? SessionAffinity.get(session) : evt.sessionID
         }),
       { providerID: Provider.ID.openai },
     )

packages/core/src/session/affinity.ts

@@ -0,0 +1,8 @@
+export * as SessionAffinity from "./affinity.js"
+
+import type { SessionSchema } from "./schema.js"
+
+// TODO: Should the `model.request` hook expose affinity so plugins stop deriving it from `ctx.session.get`?
+/** The Session ID that groups model requests for provider cache routing: children share the parent's, forks the fork source's. */
+export const get = (session: Pick<SessionSchema.Info, "id" | "parentID" | "fork">) =>
+  session.parentID ?? session.fork?.sessionID ?? session.id

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

@@ -31,6 +31,7 @@ import { Permission } from "../permission.js"
 import { PluginHooks } from "../plugin/hooks.js"
 import { QuestionTool } from "../tool/plugin/question.js"
 import { Tool } from "../tool.js"
+import { SessionAffinity } from "./affinity.js"
 import { SessionModelTransport } from "./model-transport.js"
 import { SessionProviderContext } from "./provider-context.js"
 import { SessionRunnerModel } from "./runner/model.js"
@@ -270,7 +271,7 @@ export const layer = Layer.effect(
       const entries = Object.entries(shaped.options)
       const generation = Object.fromEntries(entries.filter(([k]) => GENERATION_KEYS.has(k))) as GenerationOptionsFields
       const providerOptions = Object.fromEntries(entries.filter(([k]) => !GENERATION_KEYS.has(k)))
-      const affinity = session.parentID ?? session.fork?.sessionID ?? session.id
+      const affinity = SessionAffinity.get(session)
       const base = LLM.request({
         model: model.model,
         http: {

packages/core/test/plugin/provider-github-copilot.test.ts

@@ -183,10 +183,11 @@ describe("GithubCopilotPlugin", () => {
   it.effect("classifies child-session steps as subagent interactions", () =>
     Effect.gen(function* () {
       yield* addPlugin()
-      const event = yield* modelRequest((yield* sessions()).child, "primary")
+      const ids = yield* sessions()
+      const event = yield* modelRequest(ids.child, "primary")
       expect(event.headers).toEqual({
         "X-Interaction-Type": "conversation-subagent",
-        "X-Interaction-Id": event.sessionID,
+        "X-Interaction-Id": ids.parent,
         "x-initiator": "agent",
       })
     }),
@@ -207,10 +208,11 @@ describe("GithubCopilotPlugin", () => {
   it.effect("classifies compaction requests by kind rather than agent", () =>
     Effect.gen(function* () {
       yield* addPlugin()
-      const event = yield* modelRequest((yield* sessions()).child, "compaction", "build")
+      const ids = yield* sessions()
+      const event = yield* modelRequest(ids.child, "compaction", "build")
       expect(event.headers).toEqual({
         "X-Interaction-Type": "conversation-compaction",
-        "X-Interaction-Id": event.sessionID,
+        "X-Interaction-Id": ids.parent,
         "x-initiator": "agent",
       })
     }),

packages/core/test/plugin/provider-openai.test.ts

@@ -1,6 +1,6 @@
 import { Money } from "@opencode/schema/money"
 import { Agent } from "@opencode/schema/agent"
-import { Session } from "@opencode/schema/session"
+import { Session } from "@opencode/core/session"
 import { OpenAIResponses } from "@opencode/ai/protocols/openai-responses"
 import { describe, expect } from "bun:test"
 import { ConfigProvider, DateTime, Effect } from "effect"
@@ -41,10 +41,14 @@ function required<T>(value: T | undefined): T {
   return value
 }
 
-const request = Effect.fn(function* (providerID: Provider.ID, baseURL: string) {
+const request = Effect.fn(function* (
+  providerID: Provider.ID,
+  baseURL: string,
+  sessionID = Session.ID.make("ses_test"),
+) {
   const hooks = yield* PluginHooks.Service
   const event = yield* hooks.trigger("session", "model.request", {
-    sessionID: Session.ID.make("ses_test"),
+    sessionID,
     agent: Agent.ID.make("build"),
     model: Model.Ref.make({ providerID, id: Model.ID.make("gpt-5.5") }),
     kind: "primary",
@@ -156,6 +160,12 @@ describe("OpenAIPlugin", () => {
       expect(custom.headers).not.toHaveProperty("originator")
       expect(proxy.baseURL).toBe("https://proxy.example/v1?region=us")
       expect(proxy.headers).toMatchObject({ originator: "opencode", "session-id": "ses_test" })
+      const sessions = yield* Session.Service
+      const location = yield* Location.Service
+      const parent = yield* sessions.create({ location: { directory: location.directory } })
+      const child = yield* sessions.create({ parentID: parent.id })
+      const childRequest = yield* request(Provider.ID.openai, "https://api.openai.com/v1", child.id)
+      expect(childRequest.headers).toMatchObject({ "session-id": parent.id })
       const eligible = required(yield* models.get(Provider.ID.openai, Model.ID.make("gpt-5.5")))
       expect(eligible.package).toBe("@opencode/ai/providers/openai")
       expect(eligible.headers).toMatchObject({ originator: "opencode", "chatgpt-account-id": "acct_123" })