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Anthropic's Opus 5.5 and Fable 5.1 block forced tool use for JSON output

By PulseAugur Editorial · [1 sources] ·

Anthropic's latest models, Opus 5.5 and Fable 5.1, have changed how they handle tool use, breaking a common workaround for generating structured JSON output. Previously, developers could force models to output JSON by creating a dummy tool and compelling the model to call it, with the API validating the output against a schema. This method is no longer supported by the newest Opus and Fable versions, which now return an API error for forced tool calls. AI

IMPACT Developers using Anthropic models for structured JSON output may need to adapt their workflows due to changes in tool-use handling.

RANK_REASON The item discusses a change in API behavior for existing models that affects a specific developer workaround, rather than a new model release or significant research.

Read on dev.to — LLM tag →

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

Anthropic's Opus 5.5 and Fable 5.1 block forced tool use for JSON output

How we ranked this

Signal score
9 / 100
Composite score across the factors below. Higher = stronger signal that this story matters right now.
Newsworthiness bucket
Tool
The item discusses a change in API behavior for existing models that affects a specific developer workaround, rather than a new model release or significant research.
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
Editorial topic classification. Feeds into how the story surfaces on /topic/<slug> hub pages and into the per-entity coverage mix.
AI-industry relevance
High
Clearly on-topic for AI-industry coverage.
Story freshness
Same-day
Cluster formed today. Ranking reflects the current source set at time of score.

Full methodology in our editorial standards.

COVERAGE [1]

  1. dev.to — LLM tag TIER_1 English(EN) · Matt Cockayne ·

    The model that wouldn't be forced

    <p>When Anthropic released Opus 5.5 I was massively looking forward to it. It's a big step up on the Opus line, and the estate's chat adapters were going to get it the moment I could wire it in.</p> <p>Then I read the release notes, and there it was, a few paragraphs down: the mo…

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