**Overall Verdict:** Accept — The submission is complete, internally consistent, objectively gradable, and reachable with the specified Wolfram toolchain.

**Completeness Check:**
- Instruction: Complete — The interaction, contractions, spin averaging, Mandelstam reduction, required artifacts, and end state are explicit.
- Inputs: Complete — The notebook provides a structured starter workflow, while the package supplies massless on-shell and scalar-product rules.
- Toolchain: Complete — Linux and compatible Mathematica, FeynCalc, and FeynArts versions are identified, with a clear noninteractive execution path.
- Reference output: Complete — Both references are present; the scalar output is exactly `8 lambda^2 (t^2 + u^2)`, and the derivation covers both contractions, exchange sign, scalar and pseudoscalar traces, interference, spin averaging, validation, and export.
- Rubric: Complete — It defines exact syntax, structure, symbol, arithmetic, unresolved-object, symbolic-equivalence, and independent polynomial checks.

**File and Task Reachability:**
The attachment set is `reachable`, and the end-to-end task is `reachable`. All four files were inspected: `mandelstam_defs.wl` is a 586-byte ASCII package with 16 scalar-product rules and the massless constraint; `njl_model.nb` is a 7,060-byte textual Wolfram notebook expression with no embedded binary payload; `njl_scattering_complete.wl` is a 5,879-byte ASCII derivation script; and `amplitude_squared.wl` is a 23-byte ASCII file containing one exact scalar expression. No attachment content remained unreviewed.

The intended path is to open the starter notebook, load the Mandelstam package after FeynCalc, implement the standalone derivation, run it under the specified stack, export the scalar expression, and apply the stated polynomial and equivalence checks. Submitted code was not executed under safe-review rules, and no Wolfram runtime was available for independent runtime validation; static structure, content, and cross-file consistency were checked instead. 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:**
None — no affirmative evidence supports a screening label; public identifiability was not independently checked beyond the submitted evidence.

**Recommended Changes:**
The starter materials, reference derivation, final expression, and grading contract form a coherent package. No material changes are required.