feat: Send x-session-affinity and x-parent-session-id headers (#20744)

Author: kflansburg

packages/opencode/src/session/llm.ts

@@ -25,6 +25,7 @@ export namespace LLM {
   export type StreamInput = {
     user: MessageV2.User
     sessionID: string
+    parentSessionID?: string
     model: Provider.Model
     agent: Agent.Info
     permission?: Permission.Ruleset
@@ -301,6 +302,8 @@ export namespace LLM {
               "x-opencode-client": Flag.OPENCODE_CLIENT,
             }
           : {
+              "x-session-affinity": input.sessionID,
+              ...(input.parentSessionID ? { "x-parent-session-id": input.parentSessionID } : {}),
               "User-Agent": `opencode/${Installation.VERSION}`,
             }),
         ...input.model.headers,

packages/opencode/src/session/prompt.ts

@@ -1512,6 +1512,7 @@ NOTE: At any point in time through this workflow you should feel free to ask the
                   agent,
                   permission: session.permission,
                   sessionID,
+                  parentSessionID: session.parentID,
                   system,
                   messages: [...modelMsgs, ...(isLastStep ? [{ role: "assistant" as const, content: MAX_STEPS }] : [])],
                   tools,