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JSON Schema Breaks Tool Calls in 7 Open AI Models

By PulseAugur Editorial · [1 sources] ·

A recent paper from Focus Technology's AI lab reveals that attaching a JSON schema to a request can cause open-weight models to completely fail at tool invocation. This issue, which affects seven different open-weight models, results in a 100% suppression rate for tool calls when a JSON schema is present. The paper explains this is due to the `response_format={"type":"json_schema"}` setting, which compiles the schema into a token bitmask that effectively prevents the model from selecting tool options before sampling even begins. AI

IMPACT This discovery highlights a critical flaw in how some open AI models handle tool invocation when JSON schemas are involved, potentially impacting their reliability in complex task execution.

RANK_REASON Research paper detailing a functional flaw in open models. [lever_c_demoted from research: ic=1 ai=1.0]

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

JSON Schema Breaks Tool Calls in 7 Open AI Models

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Signal score
0 / 100
Composite score across the factors below. Higher = stronger signal that this story matters right now.
Newsworthiness bucket
Tool
Research paper detailing a functional flaw in open models. [lever_c_demoted from research: ic=1 ai=1.0]
Source corroboration
Single-source cluster
Only one publisher covered this so far. Single-source stories can still rank when the publisher is high-authority, but they lack cross-source corroboration.
Topics
model release, product
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AI-industry relevance
High
Clearly on-topic for AI-industry coverage.
Story freshness
56 days old
Aged out of breaking-news scoring windows; ranking reflects the durable signal from the full source set.

Full methodology in our editorial standards.

COVERAGE [1]

  1. Towards AI TIER_1 English(EN) · Chew Loong Nian - AI ENGINEER ·

    JSON Mode Silently Stopped All 7 Open Models From Calling a Single Tool

    <div class="medium-feed-item"><p class="medium-feed-image"><a href="https://pub.towardsai.net/json-mode-silently-stopped-all-7-open-models-from-calling-a-single-tool-a4f1c8d4731b?source=rss----98111c9905da---4"><img src="https://cdn-images-1.medium.com/max/1400/1*Sk7gZoTvXt1-6s6o…

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