feat: add anthropic-server-compaction hook for Claude 4.6 server-side context compaction

[janghoon-ju]

Summary

Add server-side compaction support for Claude 4.6 models (Opus and Sonnet) via Anthropic's compact_20260112 API. This hook injects context_management into the API request through the chat.params hook, enabling the Anthropic API to handle context compression automatically — no client-side compaction logic needed.

Problem

Long-running sessions with Claude 4.6 currently rely on client-side compaction (preemptive-compaction hook), which:

• Requires a separate LLM call to generate the summary (extra cost + latency)
• Can lose critical context during summarization (known issue: [Feature]: Proactive handoff + session rotation to prevent quality degradation from auto-compaction #1346, [Bug]: Claude models keeps repeatedly performing compaction in a loop. #2225)
• Doesn't leverage Anthropic's optimized server-side implementation

Anthropic released server-side compaction as a beta feature with Claude 4.6 (compact-2026-01-12), which handles context management automatically with minimal integration work. This is the same approach used by Claude Code internally.

Solution

New anthropic-server-compaction hook that:

1. Detects Claude 4.6 models (Opus + Sonnet) on Anthropic-compatible providers (anthropic, google-vertex-anthropic, opencode, github-copilot)
2. Injects contextManagement into output.options via chat.params hook
3. AI SDK automatically adds required beta headers (compact-2026-01-12, context-management-2025-06-27)
4. Anthropic API handles compaction server-side — returns compaction content blocks with conversation summary

How it works

chat.params hook → output.options.contextManagement = { edits: [{ type: "compact_20260112", ... }] }
         ↓
opencode ProviderTransform.providerOptions() → { anthropic: { contextManagement: { ... } } }
         ↓
AI SDK @ai-sdk/anthropic → API request with context_management + beta headers
         ↓
Anthropic API → auto-compacts when input tokens exceed trigger threshold

Configuration

Opt-in via experimental.server_compaction:

// oh-my-opencode.jsonc
{
  "experimental": {
    // Simple: enable with defaults (128K token trigger)
    "server_compaction": true,

    // Advanced: custom trigger threshold and instructions
    "server_compaction": {
      "enabled": true,
      "trigger_tokens": 100000,
      "instructions": "Preserve code snippets, file paths, and technical decisions."
    }
  }
}

Coexistence with existing hooks

• preemptive-compaction: Both can be enabled. Server-side compaction triggers at the API level, while preemptive-compaction triggers client-side. In practice, server-side fires first, making preemptive-compaction redundant. Users may want to disable preemptive-compaction when using server-side.
• compaction-context-injector: Still works — it handles opencode's client-side compaction events
• compaction-todo-preserver: Still works — watches for compaction events

Provider compatibility

Provider	Supported	Notes
anthropic	✅	Direct API
google-vertex-anthropic	✅	Uses same AnthropicMessagesLanguageModel from @ai-sdk/anthropic/internal
opencode	✅	Anthropic via OpenCode proxy
github-copilot (Claude models)	✅	Claude models on Copilot
Non-Claude models	⏭️ Skipped	Hook is a no-op for non-4.6 models

What's included

Path	Description
src/hooks/anthropic-server-compaction/hook.ts	Hook implementation (74 LOC)
src/hooks/anthropic-server-compaction/index.ts	Barrel export
src/hooks/anthropic-server-compaction/index.test.ts	15 tests covering positive/negative/edge cases
src/config/schema/experimental.ts	server_compaction config schema
src/config/schema/hooks.ts	Hook name registration
src/plugin/chat-params.ts	Handler wiring
src/plugin/hooks/create-session-hooks.ts	Hook instantiation
src/plugin-interface.ts	Hook passing to handler

Testing

bun test src/hooks/anthropic-server-compaction/index.test.ts
# 15 pass, 0 fail

bun run typecheck  # ✅
bun run build      # ✅

Current limitations

• Beta API: Requires compact-2026-01-12 beta header (handled automatically by AI SDK)
• Same model for summarization: Anthropic uses the same model for the compaction summary — no option to use a cheaper model
• Claude 4.6 only: claude-opus-4-6 and claude-sonnet-4-6 — older models are not supported

References

• Anthropic Compaction API docs
• Vercel AI SDK compact_20260112 support
• Related opencode issues: #2871, #14233

---

Summary by cubic

Adds server-side context compaction for Claude 4.6 (Opus, Sonnet) using Anthropic’s compact_20260112 to reduce extra calls and keep more context. The new hook injects contextManagement via chat.params and activates only for supported Claude models.

• New Features

  • New anthropic-server-compaction hook injects contextManagement for Claude 4.6 on Anthropic, Google Vertex Anthropic, OpenCode, and GitHub Copilot (Claude).
  • Opt-in config: experimental.server_compaction true or object with trigger_tokens (default 128k) and optional instructions.
  • No-op for non-4.6 or non-Claude models; preserves existing options and won’t overwrite existing contextManagement.
  • SDK sends required beta headers automatically; tests added to cover positive/negative cases.

• Migration

  • Enable with experimental.server_compaction; optionally set trigger_tokens and instructions.
  • Consider disabling preemptive-compaction; server-side compaction typically triggers first.

^{Written for commit 78cec1a. Summary will update on new commits.}

[github-actions]

All contributors have signed the CLA. Thank you! ✅
_{Posted by the CLA Assistant Lite bot.}

[janghoon-ju]

I have read the CLA Document and I hereby sign the CLA

[cubic-dev-ai]

No issues found across 10 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: The feature is opt-in via an experimental flag, targeted specifically at Claude 4.6 models, and includes defensive checks to avoid interfering with existing configurations.}

[ualtinok]

This is really useful thanks!
How is the speed difference between server side compaction and current, were you able to test?