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:, or failed:

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.md session memory template.
• Agent thread notes: Instructs agent threads to return reports/summaries/findings directly in the final message instead of writing .md files.
• 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.