transformBody() discards cache_control on system identity block, reducing Anthropic prompt cache efficiency

[ichasco-heytrade]

Description

Summary

The transformBody() function in transforms.ts discards cache_control when splitting the SYSTEM_IDENTITY block from the rest of the system prompt. This prevents Anthropic from caching the full system prompt consistently, reducing cache efficiency from ~66% ~38% in agentic multi-turn sessions.

Offending code

(transforms.ts, equivalent to ~line 569 in compiled output) const { cache_control: _cc, ...identityProps } = entryProps; // ← destructures AND discards cache_control splitSystem.push({ ...identityProps, text: SYSTEM_IDENTITY }); // ← new block has NO cache_control ## Impact (measured empirically, same traffic, claude-sonnet-5)

	With bug	Fixed (preserve cache_control)
Cache efficiency	38.4%	66.4%
Cost/generation	$0.49 FULL 33% 12/20 (60%)

Without fix, system prompt (~230K tokens) only partially cached (tool definitions 32K). With fix, full 262K+ cache_read works from first call.

Proposed fix

const { cache_control, ...identityProps } = entryProps; // preserve const identityBlock = { ...identityProps, text: SYSTEM_IDENTITY }; if (cache_control) identityBlock.cache_control = cache_control; // re-attach splitSystem.push(identityBlock);

Environment

• Plugin: v2.1.6
• Model: anthropic/claude-sonnet-5 via LiteLLM proxy
• pattern: agentic multi-turn 76 tools, ~230K token system prompt

Steps to reproduce

Open OpenCode

Use the plugin to auth with claude

Operating system

Linux

Plugin version

v2.1.6

OpenCode version

1.18.14

Debug logs