feat(preemptive-compaction): implement automatic session compaction at token threshold

Monitor token usage after assistant responses and automatically trigger session
compaction when exceeding configured threshold (default 80%). Toast notifications
provide user feedback on compaction status.

Controlled via experimental.preemptive_compaction config option.

🤖 Generated with assistance of OhMyOpenCode (https://github.com/code-yeongyu/oh-my-opencode)

src/hooks/preemptive-compaction/constants.ts

@@ -0,0 +1,3 @@
+export const DEFAULT_THRESHOLD = 0.80
+export const MIN_TOKENS_FOR_COMPACTION = 50_000
+export const COMPACTION_COOLDOWN_MS = 60_000

src/hooks/preemptive-compaction/index.ts

@@ -0,0 +1,209 @@
+import type { PluginInput } from "@opencode-ai/plugin"
+import type { ExperimentalConfig } from "../../config"
+import type { PreemptiveCompactionState, TokenInfo } from "./types"
+import {
+  DEFAULT_THRESHOLD,
+  MIN_TOKENS_FOR_COMPACTION,
+  COMPACTION_COOLDOWN_MS,
+} from "./constants"
+import { log } from "../../shared/logger"
+
+export interface PreemptiveCompactionOptions {
+  experimental?: ExperimentalConfig
+}
+
+interface MessageInfo {
+  id: string
+  role: string
+  sessionID: string
+  providerID?: string
+  modelID?: string
+  tokens?: TokenInfo
+  summary?: boolean
+  finish?: boolean
+}
+
+interface MessageWrapper {
+  info: MessageInfo
+}
+
+const MODEL_CONTEXT_LIMITS: Record<string, number> = {
+  "claude-opus-4": 200_000,
+  "claude-sonnet-4": 200_000,
+  "claude-haiku-4": 200_000,
+  "gpt-4o": 128_000,
+  "gpt-4o-mini": 128_000,
+  "gpt-4-turbo": 128_000,
+  "gpt-4": 8_192,
+  "gpt-5": 1_000_000,
+  "o1": 200_000,
+  "o1-mini": 128_000,
+  "o1-preview": 128_000,
+  "o3": 200_000,
+  "o3-mini": 200_000,
+  "gemini-2.0-flash": 1_000_000,
+  "gemini-2.5-flash": 1_000_000,
+  "gemini-2.5-pro": 2_000_000,
+  "gemini-3-pro": 2_000_000,
+}
+
+function getContextLimit(modelID: string): number {
+  for (const [key, limit] of Object.entries(MODEL_CONTEXT_LIMITS)) {
+    if (modelID.includes(key)) {
+      return limit
+    }
+  }
+  return 200_000
+}
+
+function createState(): PreemptiveCompactionState {
+  return {
+    lastCompactionTime: new Map(),
+    compactionInProgress: new Set(),
+  }
+}
+
+export function createPreemptiveCompactionHook(
+  ctx: PluginInput,
+  options?: PreemptiveCompactionOptions
+) {
+  const experimental = options?.experimental
+  const enabled = experimental?.preemptive_compaction !== false
+  const threshold = experimental?.preemptive_compaction_threshold ?? DEFAULT_THRESHOLD
+
+  if (!enabled) {
+    return { event: async () => {} }
+  }
+
+  const state = createState()
+
+  const checkAndTriggerCompaction = async (
+    sessionID: string,
+    lastAssistant: MessageInfo
+  ): Promise<void> => {
+    if (state.compactionInProgress.has(sessionID)) return
+
+    const lastCompaction = state.lastCompactionTime.get(sessionID) ?? 0
+    if (Date.now() - lastCompaction < COMPACTION_COOLDOWN_MS) return
+
+    if (lastAssistant.summary === true) return
+
+    const tokens = lastAssistant.tokens
+    if (!tokens) return
+
+    const modelID = lastAssistant.modelID ?? ""
+    const contextLimit = getContextLimit(modelID)
+    const totalUsed = tokens.input + tokens.cache.read + tokens.output
+
+    if (totalUsed < MIN_TOKENS_FOR_COMPACTION) return
+
+    const usageRatio = totalUsed / contextLimit
+
+    log("[preemptive-compaction] checking", {
+      sessionID,
+      totalUsed,
+      contextLimit,
+      usageRatio: usageRatio.toFixed(2),
+      threshold,
+    })
+
+    if (usageRatio < threshold) return
+
+    state.compactionInProgress.add(sessionID)
+    state.lastCompactionTime.set(sessionID, Date.now())
+
+    const providerID = lastAssistant.providerID
+    if (!providerID || !modelID) {
+      state.compactionInProgress.delete(sessionID)
+      return
+    }
+
+    await ctx.client.tui
+      .showToast({
+        body: {
+          title: "Preemptive Compaction",
+          message: `Context at ${(usageRatio * 100).toFixed(0)}% - compacting to prevent overflow...`,
+          variant: "warning",
+          duration: 3000,
+        },
+      })
+      .catch(() => {})
+
+    log("[preemptive-compaction] triggering compaction", { sessionID, usageRatio })
+
+    try {
+      await ctx.client.session.summarize({
+        path: { id: sessionID },
+        body: { providerID, modelID },
+        query: { directory: ctx.directory },
+      })
+
+      await ctx.client.tui
+        .showToast({
+          body: {
+            title: "Compaction Complete",
+            message: "Session compacted successfully",
+            variant: "success",
+            duration: 2000,
+          },
+        })
+        .catch(() => {})
+    } catch (err) {
+      log("[preemptive-compaction] compaction failed", { sessionID, error: err })
+    } finally {
+      state.compactionInProgress.delete(sessionID)
+    }
+  }
+
+  const eventHandler = async ({ event }: { event: { type: string; properties?: unknown } }) => {
+    const props = event.properties as Record<string, unknown> | undefined
+
+    if (event.type === "session.deleted") {
+      const sessionInfo = props?.info as { id?: string } | undefined
+      if (sessionInfo?.id) {
+        state.lastCompactionTime.delete(sessionInfo.id)
+        state.compactionInProgress.delete(sessionInfo.id)
+      }
+      return
+    }
+
+    if (event.type === "message.updated") {
+      const info = props?.info as MessageInfo | undefined
+      if (!info) return
+
+      if (info.role !== "assistant" || !info.finish) return
+
+      const sessionID = info.sessionID
+      if (!sessionID) return
+
+      await checkAndTriggerCompaction(sessionID, info)
+      return
+    }
+
+    if (event.type === "session.idle") {
+      const sessionID = props?.sessionID as string | undefined
+      if (!sessionID) return
+
+      try {
+        const resp = await ctx.client.session.messages({
+          path: { id: sessionID },
+          query: { directory: ctx.directory },
+        })
+
+        const messages = (resp.data ?? resp) as MessageWrapper[]
+        const assistants = messages
+          .filter((m) => m.info.role === "assistant")
+          .map((m) => m.info)
+
+        if (assistants.length === 0) return
+
+        const lastAssistant = assistants[assistants.length - 1]
+        await checkAndTriggerCompaction(sessionID, lastAssistant)
+      } catch {}
+    }
+  }
+
+  return {
+    event: eventHandler,
+  }
+}

src/hooks/preemptive-compaction/types.ts

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+export interface PreemptiveCompactionState {
+  lastCompactionTime: Map<string, number>
+  compactionInProgress: Set<string>
+}
+
+export interface TokenInfo {
+  input: number
+  output: number
+  reasoning: number
+  cache: { read: number; write: number }
+}
+
+export interface ModelLimits {
+  context: number
+  output: number
+}