MCP: Fetch — deterministic MCP env with single fetch tool

[aibozo]

Summary:

• Adds environments/mcp_fetch/, an MCPEnv-based environment exposing the fetch MCP tool.
• Default mode auto-starts a deterministic local fixture server covering redirects, auth, headers, query params, gzipped/
  large payloads, hash/truncation checks, pointer/workflow chains, and rubric-graded summaries.
• Agents must reply with ANSWER: <value>; host allow-list, timeouts, and max-bytes caps enforce safety.

Design:

• FetchEnv(MCPEnv) wires load_environment(...), dataset builder, rubric, and simulator lifecycle management.
• fetch tool schema: inputs (url, method, headers, params, timeout_s, max_bytes); outputs (status, headers,
  body_text/body_json, hash, final_url, content_type, bytes, truncated, etc.). Host allow-list defaults to
  127.0.0.1:<fixture_port> with allow_online overrides.
• Verifiers handle exact-match, JSON paths, header/status checks, hash digit sums, char counts, and JudgeRubric delegation.

Data:

• tasks/qa.jsonl has 84 tasks (≥20 enforced) spanning planner/workflow combos, pointer lookups, ledger math, header
  challenges, query summaries, truncation/hash probes, etc.
• Offline fixtures (fixtures/html|json|text/*) plus helper scripts keep runs deterministic.

Evaluation:

uv run pytest tests/environments/test_mcp_fetch.py -q
uv run ruff check environments/mcp_fetch

• Deterministic rubric defaults to normalized exact-match accuracy; JudgeRubric handles summary tasks via tasks/ judge_rubrics.yaml.

Difficulty calibration:

Model	Correct/Total	Accuracy
gpt-4.1-mini	46 / 84	54.8%
gpt-5	68 / 84	81.00%

Notes:

• README documents offline vs Hub commands, PYTHONPATH requirement, and calibration table.
• .gitignore blocks nested .venv/ directories; committed envs were removed.

Run gh pr create --title "MCP: Fetch — deterministic MCP env with single fetch tool" --body-file --base
main --head aibozo:main --repo PrimeIntellect-ai/prime-environments once authenticated, or paste the text into the compare
page noted above.

[CLAassistant]

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[aibozo]

I screwed up the commit format, sorry. if you want me to close and create another pull request with the proper format i can, but all of the appropriate things have been done. The new and old test suite pass, I reviewed the code (i can remove the extra bits but they seemed helpful). Let me know what you want me to do formatting issue wont happen again, I'm new at this.