CC 2.1.128 Release: New Built-in Background Agent, C# Beta Support, and Model Deprecations
Claude Code (CC) 2.1.128 ships with +1406 tokens, focusing on agent prompt restructures, API reference updates, and model deprecations.
New Agent Instructions: Background Jobs & Onboarding
The old background-job behavior system prompt is replaced with built-in background-agent instructions. Key behaviors:
- Progress narration during background execution
- Tool-result restatement after each step
- Delegation of noisy investigation subtasks
- Explicit status signals:
result:,needs input:, orfailed:
New onboarding-guide closing instructions upload the finalized ONBOARDING.md via ShareOnboardingGuide, handle existing-guide and unavailable-tool cases, and return the team share link.
New Tool Description: RemoteTrigger
The RemoteTrigger tool provides a claude.ai remote-trigger API for listing, reading, creating, updating, and running scheduled remote agent routines — without exposing OAuth tokens.
C# Beta Support
The Claude API reference now documents beta C# tool-runner and Managed Agents support via BetaToolRunner and client.Beta.Agents/Sessions/Environments.
Go & Java SDK Updates
- Go: Adds typed model constants, updates adaptive thinking syntax, and documents the beta advisor tool parameter.
- Java: SDK version updated from 2.17.0 to 2.27.0; added beta advisor tool guidance.
Model Catalog Changes
Claude Sonnet 4 and Claude Opus 4 are deprecated. Recommended replacements:
- Opus 4 → Opus 4.7 or Sonnet 4.6
- Older Sonnet models → Sonnet 4.6
The skill guide Building LLM-powered applications with Claude refreshes the current-model table to Opus 4.7, Opus 4.6, Sonnet 4.6, and Haiku 4.5.
Other Notable Changes
- Removed: Session memory update instructions and the structured
summary.mdsession memory template. - Agent thread notes: Instructs agent threads to return reports/summaries/findings directly in the final message instead of writing
.mdfiles. - Edit tool: Hardcodes the Read-output line-number prefix format as "line number + tab" in indentation-preservation guidance.
- ReadFile tool: Always appends the additional read note placeholder at the end of empty-file warnings.
- Claude API SDK references: Added structured refusal stop-details guidance across Python, TypeScript, C#, Go, Java, PHP, Ruby; added programmatic API error type guidance for Java, PHP, Ruby.
- Model migration guide: Adds Opus 4.7 as the recommended target and adds a tuning check to parse tool inputs as JSON instead of matching serialized raw strings.
📖 Read the full source: r/ClaudeAI
👀 See Also
Why One Developer Leaves AI Co-Author Tags in Commits
A developer explains why they deliberately include 'Co-Authored-by: Claude' in their Git commits, comparing it to EXIF data in photography and discussing the challenges of surgical AI-assisted code changes.
Tensions Escalate Between The Pentagon and AI Company Anthropic
The Pentagon's use of Anthropic's AI in classified operations, such as a raid in Venezuela, has created tension over the company's AI safety policies.
Claude Opus 4.7 Regresses in Reasoning and Conversation, Users Report
Opus 4.7 introduces a new tokenizer costing 30-50% more, exhibits meta-narration, position instability, and planning without execution—making it worse for technical collaboration than 4.6.
Trading Strategy Benchmark: Cheaper AI Models Outperform Claude Opus 4.6
A benchmark tested 10 LLMs on developing trading strategies, with cheaper models like Minimax 2.5 and Gemini 3.1 outperforming Claude Opus 4.6 despite its 10x higher cost. The experiment was run three times with consistent results.