From 9a11901590720f7fea95de383ec28e1ea8f31e4e Mon Sep 17 00:00:00 2001
From: YeonGyu-Kim <code.yeon.gyu@gmail.com>
Date: Wed, 3 Dec 2025 13:28:41 +0900
Subject: [PATCH] feat(hook): add context window monitor for Anthropic token
 usage tracking

---
 src/hooks/context-window-monitor.ts | 101 ++++++++++++++++++++++++++++
 src/hooks/index.ts                  |   1 +
 src/index.ts                        |  10 ++-
 3 files changed, 110 insertions(+), 2 deletions(-)
 create mode 100644 src/hooks/context-window-monitor.ts

diff --git a/src/hooks/context-window-monitor.ts b/src/hooks/context-window-monitor.ts
new file mode 100644
index 0000000..e09fc42
--- /dev/null
+++ b/src/hooks/context-window-monitor.ts
@@ -0,0 +1,101 @@
+import type { PluginInput } from "@opencode-ai/plugin"
+
+const ANTHROPIC_CONTEXT_LIMIT = 1_000_000
+const CONTEXT_WARNING_THRESHOLD = 0.75
+
+const CONTEXT_REMINDER = `[SYSTEM REMINDER - 1M Context Window]
+
+You are using Anthropic Claude with 1M context window.
+Current usage has exceeded 75%.
+
+RECOMMENDATIONS:
+- Consider compacting the session if available
+- Break complex tasks into smaller, focused sessions
+- Be concise in your responses
+- Avoid redundant file reads
+
+You have access to 1M tokens - use them wisely. Do NOT rush or skip tasks.
+Complete your work thoroughly despite the context usage warning.`
+
+interface AssistantMessageInfo {
+  role: "assistant"
+  providerID: string
+  tokens: {
+    input: number
+    output: number
+    reasoning: number
+    cache: { read: number; write: number }
+  }
+}
+
+interface MessageWrapper {
+  info: { role: string } & Partial<AssistantMessageInfo>
+}
+
+export function createContextWindowMonitorHook(ctx: PluginInput) {
+  const remindedSessions = new Set<string>()
+
+  const toolExecuteAfter = async (
+    input: { tool: string; sessionID: string; callID: string },
+    output: { title: string; output: string; metadata: unknown }
+  ) => {
+    const { sessionID } = input
+
+    if (remindedSessions.has(sessionID)) return
+
+    try {
+      const response = await ctx.client.session.messages({
+        path: { id: sessionID },
+      })
+
+      const messages = (response.data ?? response) as MessageWrapper[]
+
+      const assistantMessages = messages
+        .filter((m) => m.info.role === "assistant")
+        .map((m) => m.info as AssistantMessageInfo)
+
+      if (assistantMessages.length === 0) return
+
+      const lastAssistant = assistantMessages[assistantMessages.length - 1]
+      if (lastAssistant.providerID !== "anthropic") return
+
+      const totalInputTokens = assistantMessages.reduce((sum, m) => {
+        const inputTokens = m.tokens?.input ?? 0
+        const cacheReadTokens = m.tokens?.cache?.read ?? 0
+        return sum + inputTokens + cacheReadTokens
+      }, 0)
+
+      const usagePercentage = totalInputTokens / ANTHROPIC_CONTEXT_LIMIT
+
+      if (usagePercentage < CONTEXT_WARNING_THRESHOLD) return
+
+      remindedSessions.add(sessionID)
+
+      const usedPct = (usagePercentage * 100).toFixed(1)
+      const remainingPct = ((1 - usagePercentage) * 100).toFixed(1)
+      const usedTokens = totalInputTokens.toLocaleString()
+      const limitTokens = ANTHROPIC_CONTEXT_LIMIT.toLocaleString()
+
+      output.output += `\n\n${CONTEXT_REMINDER}
+[Context Status: ${usedPct}% used (${usedTokens}/${limitTokens} tokens), ${remainingPct}% remaining]`
+    } catch {
+      // Graceful degradation - do not disrupt tool execution
+    }
+  }
+
+  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) {
+        remindedSessions.delete(sessionInfo.id)
+      }
+    }
+  }
+
+  return {
+    "tool.execute.after": toolExecuteAfter,
+    event: eventHandler,
+  }
+}
diff --git a/src/hooks/index.ts b/src/hooks/index.ts
index beee45f..fcb7db4 100644
--- a/src/hooks/index.ts
+++ b/src/hooks/index.ts
@@ -1 +1,2 @@
 export * from "./todo-continuation-enforcer"
+export * from "./context-window-monitor"
diff --git a/src/index.ts b/src/index.ts
index f6dd662..2481472 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,9 +1,10 @@
 import type { Plugin } from "@opencode-ai/plugin"
 import { builtinAgents } from "./agents"
-import { createTodoContinuationEnforcer } from "./hooks"
+import { createTodoContinuationEnforcer, createContextWindowMonitorHook } from "./hooks"
 
 const OhMyOpenCodePlugin: Plugin = async (ctx) => {
   const todoContinuationEnforcer = createTodoContinuationEnforcer(ctx)
+  const contextWindowMonitor = createContextWindowMonitorHook(ctx)
 
   return {
     config: async (config) => {
@@ -13,7 +14,12 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
       }
     },
 
-    event: todoContinuationEnforcer,
+    event: async (input) => {
+      await todoContinuationEnforcer(input)
+      await contextWindowMonitor.event(input)
+    },
+
+    "tool.execute.after": contextWindowMonitor["tool.execute.after"],
   }
 }