feat(hooks): add grep-output-truncator for context-aware output limiting

src/hooks/grep-output-truncator.ts

@@ -0,0 +1,131 @@
+import type { PluginInput } from "@opencode-ai/plugin"
+
+const ANTHROPIC_ACTUAL_LIMIT = 200_000
+const CHARS_PER_TOKEN_ESTIMATE = 4
+const TARGET_MAX_TOKENS = 50_000
+
+interface AssistantMessageInfo {
+  role: "assistant"
+  tokens: {
+    input: number
+    output: number
+    reasoning: number
+    cache: { read: number; write: number }
+  }
+}
+
+interface MessageWrapper {
+  info: { role: string } & Partial<AssistantMessageInfo>
+}
+
+function estimateTokens(text: string): number {
+  return Math.ceil(text.length / CHARS_PER_TOKEN_ESTIMATE)
+}
+
+function truncateToTokenLimit(output: string, maxTokens: number): { result: string; truncated: boolean } {
+  const currentTokens = estimateTokens(output)
+
+  if (currentTokens <= maxTokens) {
+    return { result: output, truncated: false }
+  }
+
+  const lines = output.split("\n")
+
+  if (lines.length <= 3) {
+    const maxChars = maxTokens * CHARS_PER_TOKEN_ESTIMATE
+    return {
+      result: output.slice(0, maxChars) + "\n\n[Output truncated due to context window limit]",
+      truncated: true,
+    }
+  }
+
+  const headerLines = lines.slice(0, 3)
+  const contentLines = lines.slice(3)
+
+  const headerText = headerLines.join("\n")
+  const headerTokens = estimateTokens(headerText)
+  const availableTokens = maxTokens - headerTokens - 50
+
+  if (availableTokens <= 0) {
+    return {
+      result: headerText + "\n\n[Content truncated due to context window limit]",
+      truncated: true,
+    }
+  }
+
+  let resultLines: string[] = []
+  let currentTokenCount = 0
+
+  for (const line of contentLines) {
+    const lineTokens = estimateTokens(line + "\n")
+    if (currentTokenCount + lineTokens > availableTokens) {
+      break
+    }
+    resultLines.push(line)
+    currentTokenCount += lineTokens
+  }
+
+  const truncatedContent = [...headerLines, ...resultLines].join("\n")
+  const removedCount = contentLines.length - resultLines.length
+
+  return {
+    result: truncatedContent + `\n\n[${removedCount} more lines truncated due to context window limit]`,
+    truncated: true,
+  }
+}
+
+export function createGrepOutputTruncatorHook(ctx: PluginInput) {
+  const GREP_TOOLS = ["safe_grep", "Grep"]
+
+  const toolExecuteAfter = async (
+    input: { tool: string; sessionID: string; callID: string },
+    output: { title: string; output: string; metadata: unknown }
+  ) => {
+    if (!GREP_TOOLS.includes(input.tool)) return
+
+    const { sessionID } = input
+
+    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 totalInputTokens = assistantMessages.reduce((sum, m) => {
+        const inputTokens = m.tokens?.input ?? 0
+        const cacheReadTokens = m.tokens?.cache?.read ?? 0
+        return sum + inputTokens + cacheReadTokens
+      }, 0)
+
+      const remainingTokens = ANTHROPIC_ACTUAL_LIMIT - totalInputTokens
+
+      const maxOutputTokens = Math.min(
+        remainingTokens * 0.5,
+        TARGET_MAX_TOKENS
+      )
+
+      if (maxOutputTokens <= 0) {
+        output.output = "[Output suppressed - context window exhausted]"
+        return
+      }
+
+      const { result, truncated } = truncateToTokenLimit(output.output, maxOutputTokens)
+      if (truncated) {
+        output.output = result
+      }
+    } catch {
+      // Graceful degradation
+    }
+  }
+
+  return {
+    "tool.execute.after": toolExecuteAfter,
+  }
+}

src/hooks/index.ts

@@ -3,3 +3,4 @@ export { createContextWindowMonitorHook } from "./context-window-monitor"
 export { createSessionNotification } from "./session-notification"
 export { createSessionRecoveryHook } from "./session-recovery"
 export { createCommentCheckerHooks } from "./comment-checker"
+export { createGrepOutputTruncatorHook } from "./grep-output-truncator"

src/index.ts

@@ -1,6 +1,6 @@
 import type { Plugin } from "@opencode-ai/plugin"
 import { createBuiltinAgents } from "./agents"
-import { createTodoContinuationEnforcer, createContextWindowMonitorHook, createSessionRecoveryHook, createCommentCheckerHooks } from "./hooks"
+import { createTodoContinuationEnforcer, createContextWindowMonitorHook, createSessionRecoveryHook, createCommentCheckerHooks, createGrepOutputTruncatorHook } from "./hooks"
 import { updateTerminalTitle } from "./features/terminal"
 import { builtinTools } from "./tools"
 import { createBuiltinMcps } from "./mcp"
@@ -44,6 +44,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
   const contextWindowMonitor = createContextWindowMonitorHook(ctx)
   const sessionRecovery = createSessionRecoveryHook(ctx)
   const commentChecker = createCommentCheckerHooks()
+  const grepOutputTruncator = createGrepOutputTruncatorHook(ctx)
 
   updateTerminalTitle({ sessionId: "main" })
 
@@ -185,6 +186,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
     },
 
     "tool.execute.after": async (input, output) => {
+      await grepOutputTruncator["tool.execute.after"](input, output)
       await contextWindowMonitor["tool.execute.after"](input, output)
       await commentChecker["tool.execute.after"](input, output)