{
  "id": "P298",
  "slug": "anthropic-claude-api-assistant-prefill-json-prefix-400-migrated-model-assistant-message-must-not",
  "status": "search-observed",
  "product": "Claude API",
  "family": "LLM model/API behavior drift",
  "canonical_url": "https://gitdocs.org/fix/anthropic-claude-api-assistant-prefill-json-prefix-400-migrated-model-assistant-message-must-not",
  "query_alias_url": "https://gitdocs.org/q/anthropic-claude-api-assistant-prefill-json-prefix-400-migrated-model-assistant-message-must-not",
  "markdown_url": "https://gitdocs.org/fix/anthropic-claude-api-assistant-prefill-json-prefix-400-migrated-model-assistant-message-must-not.md",
  "json_url": "https://gitdocs.org/api/fixes/anthropic-claude-api-assistant-prefill-json-prefix-400-migrated-model-assistant-message-must-not.json",
  "canonical_query": "Anthropic Claude API assistant prefill JSON prefix 400 migrated model assistant message must not end with trailing whitespace",
  "observed_codex_queries": [
    "Anthropic Claude API assistant prefill JSON prefix 400 migrated model assistant message must not end with trailing whitespace",
    "Claude API 400 prefill assistant message JSON prefix",
    "site:docs.anthropic.com assistant prefill Claude Messages API",
    "Anthropic docs prefill Claude response assistant role",
    "https://docs.anthropic.com/en/docs/build-with-claude/prompt-engineering/prefill-claudes-response",
    "site:platform.claude.com/docs prefill response assistant message",
    "https://platform.claude.com/docs/llms.txt",
    "\"assistant\" \"prefill\" \"400\" \"Claude\"",
    "site:docs.anthropic.com \"prefill\" \"extended thinking\"",
    "Claude extended thinking prefill not supported",
    "https://docs.anthropic.com/en/api/messages",
    "https://raw.githubusercontent.com/anthropics/anthropic-sdk-typescript/main/README.md",
    "platform.claude.com/docs/en/api/messages.md",
    "https://platform.claude.com/docs/en/docs/test-and-evaluate/strengthen-guardrails/increase-consistency"
  ],
  "organic_dependency_search_count": 14,
  "affected_versions": "",
  "exact_errors": [],
  "diagnosis": "SDK types may still accept fields that the selected hosted model rejects; model retirement, tokenizer, thinking, and tool behavior are external and fast-moving.",
  "before": "",
  "after": "",
  "verification": "Verifier checks current model ID, accepted payload, structured output/tool parsing, and behavior under retries.",
  "unsafe_fixes": [
    "Changing local code without checking the current external contract.",
    "Retrying the same install, build, or API call with no version/source change.",
    "Applying a broad unsafe bypass when a narrow compatibility fix is available."
  ],
  "last_verified_at": "2026-06-25",
  "neutral_validation": {
    "organic_search_observed": true,
    "organic_strict_pass": true,
    "no_web_counterfactual_run": false,
    "no_web_strict_pass": null,
    "search_necessity_under_fixture": null,
    "no_web_oracle_note": "Observed query evidence is separated from necessity proof; no-web and stale-contract counterfactuals should be tracked separately."
  },
  "source_trail": [
    {
      "url": "https://platform.claude.com/docs/en/about-claude/models/migration-guide",
      "label": "Claude model migration guide"
    },
    {
      "url": "https://docs.anthropic.com/en/docs/build-with-claude/prompt-engineering/prefill-claudes-response",
      "label": "Official reference opened by Codex"
    },
    {
      "url": "https://platform.claude.com/docs/llms.txt",
      "label": "Official reference opened by Codex"
    },
    {
      "url": "https://docs.anthropic.com/en/api/messages",
      "label": "Official reference opened by Codex"
    },
    {
      "url": "https://raw.githubusercontent.com/anthropics/anthropic-sdk-typescript/main/README.md",
      "label": "Official reference opened by Codex"
    },
    {
      "url": "https://platform.claude.com/docs/en/docs/test-and-evaluate/strengthen-guardrails/increase-consistency",
      "label": "Official reference opened by Codex"
    }
  ],
  "source_github_links": [
    {
      "url": "https://raw.githubusercontent.com/anthropics/anthropic-sdk-typescript/main/README.md",
      "label": "Official reference opened by Codex"
    }
  ]
}