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    "story_id": "mp-2026-09-20-004",
    "source_story_id": "tmp-story-closed-world-tool-hallucination",
    "edition_id": "mp-2026-09-20-morning-0073",
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    "position": 4,
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    "section": "safety",
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    "headline": "The Agent Called Tools That Did Not Exist",
    "slug": "the-agent-called-tools-that-did-not-exist",
    "dek": "A benchmark counted 322 tool hallucinations, then found 154 more on a live merged MCP surface.",
    "summary": "A benchmark counted 322 tool hallucinations, then found 154 more on a live merged MCP surface.",
    "body_text": "A new benchmark separates fabricated tools, invalid schemas and other hallucinated calls from ordinary tool selection and authorization errors. Across ten hosted models and two invocation surfaces, the study measured 322 genuine hallucinations; fabricated-tool calls were far more common with unconstrained raw JSON than with a structured surface. A live Model Context Protocol test found 154 more hallucinations after multiple servers were merged, including collisions and shadowing that a single registry cannot represent. The proposed reference resolver checks membership and signatures before any causal gate, but borrowed arguments that fit a real schema remain irreducible.",
    "why_it_matters": "A benchmark counted 322 tool hallucinations, then found 154 more on a live merged MCP surface.",
    "limitations": [
      "A live Model Context Protocol test found 154 more hallucinations after multiple servers were merged, including collisions and shadowing that a single registry cannot represent.",
      "The proposed reference resolver checks membership and signatures before any causal gate, but borrowed arguments that fit a real schema remain irreducible."
    ],
    "importance": 9,
    "canonical_url": "https://themachinepress.com/story/mp-2026-09-20-004/the-agent-called-tools-that-did-not-exist",
    "json_url": "https://themachinepress.com/story/mp-2026-09-20-004.json",
    "first_published_at": "2026-09-20T09:00:00.000-04:00",
    "modified_at": "2026-09-20T09:00:00.000-04:00",
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        "text": "A benchmark counted 322 tool hallucinations, then found 154 more on a live merged MCP surface.",
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        "qualification": "A live Model Context Protocol test found 154 more hallucinations after multiple servers were merged, including collisions and shadowing that a single registry cannot represent."
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    "source_ids": [
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    "tags": [
      "tool use",
      "MCP",
      "agent reliability"
    ],
    "image_url": null,
    "corrections": []
  },
  "sources": [
    {
      "source_id": "source-2026-09-20-004",
      "title": "arXiv preprint 2609.19425",
      "publisher": "arXiv",
      "url": "https://arxiv.org/abs/2609.19425",
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      "source_type": "primary_research",
      "is_primary_source": true,
      "published_at": "2026-09-16T17:02:31.000-04:00",
      "accessed_at": "2026-09-20T08:23:59.913-04:00",
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  "publisher": {
    "name": "The Machine Press",
    "url": "https://themachinepress.com",
    "description": "A daily newspaper for the age of artificial intelligence."
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  "cite_this_report": {
    "title": "The Agent Called Tools That Did Not Exist",
    "publisher": "The Machine Press",
    "published_at": "2026-09-20T09:00:00.000-04:00",
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