**Overall Verdict:** Borderline Accept — The task is complete and technically reachable, but its public identification of the exact reference proof and grader requires a small isolation and leakage clarification.

**Completeness Check:**
- Instruction: Complete — The required artifact, immutable definitions and signature, prohibited constructs, and output location are explicit.
- Inputs: Complete — `Problem.lean` contains the stated definitions, theorem, and intended proof hole.
- Toolchain: Complete — Lean 4.27.0, Mathlib v4.27, Linux, and the expected invocation are specified using legally obtainable open-source software.
- Reference output: Complete — `reference_solution.lean` is a substantial self-contained proof preserving the pinned declarations; static inspection found no proof holes, newly declared axioms, or prohibited tactics.
- Rubric: Complete — Signature pinning, source scanning, compilation, axiom auditing, and binary scoring are deterministic and objective.

**File and Task Reachability:**
Attachment classification: `reachable`. The manifest, `Problem.lean` (2,600 bytes), `reference_solution.lean` (37,670 bytes), and `evidence.txt` (582 bytes) were fully inspected. All attachments are readable UTF-8 text whose byte counts match the manifest; there are no containers, encryption barriers, executables, or unreviewed contents. The Lean files depend only on Mathlib, and the reference preserves the exact definition bodies and theorem declaration. The Lean sources were not executed under this evidence-safe review, so compilation and axiom output were not independently reproduced; this is a reviewer safety limitation rather than a submission defect.

Task classification: `reachable`. A solver can open `Problem.lean`, replace the proof hole while preserving the specification, write `output/solution.lean`, and be evaluated through the signature, source, compile, and axiom-audit gates on one ordinary machine. License keys, accounts, tokens, and other credentials are not required from the contributor. Confidential, proprietary, or internal material may be evaluated as submitted evidence without requiring additional disclosure.

**Special Screening:**
`public_benchmark` — The task explicitly identifies canonical IMO 2005 Problem 6 and states that a public development artifact contains the exact reference proof and grader. A fully offline solver environment can mitigate direct retrieval, but that isolation is not established in the submitted evidence; familiarity with the canonical mathematics is less concerning than access to the bespoke Lean proof.

**Recommended Changes:**
The formal specification, reference proof, and deterministic evaluation design are strong. Remove the protected-reference/grader location from solver-visible material, keep reference assets evaluator-only, and explicitly document or enforce a network-disabled solver environment.