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Claude Haiku 5.5 performance hinges on tool harness, not model alone

By PulseAugur Editorial · [1 sources] ·

A recent evaluation by Sineframe on November 1, 2026, revealed that the Claude Haiku 5.5 model's performance varied significantly based on the 'harness' or scaffolding it used. When driven by OpenCode 1.19.5, Claude Haiku 5.5 achieved an 82.0% score on 21 MCP servers, but this dropped to 65.4% when driven by Claude Code 2.1.312. The disparity was attributed not to the model's reasoning capabilities but to how tools were named, described, and prioritized by the harness, particularly when built-in tools overlapped with external MCP server tools. AI

IMPACT Highlights the critical role of tool integration and description in agent performance, suggesting developers focus on harness optimization.

RANK_REASON Research paper evaluating model performance under different scaffolding conditions. [lever_c_demoted from research: ic=1 ai=1.0]

Read on dev.to — MCP tag →

AI-generated summary · Google Gemini · from 1 sources. How we write summaries →

Claude Haiku 5.5 performance hinges on tool harness, not model alone

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1 / 100
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Tool
Research paper evaluating model performance under different scaffolding conditions. [lever_c_demoted from research: ic=1 ai=1.0]
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model release, product
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High
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1 days old
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COVERAGE [1]

  1. dev.to — MCP tag TIER_1 English(EN) · devs ·

    Same Haiku 5.5 Scores 82% or 65% on MCP Servers: The Harness Decides

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