[yafshar]

Summary

Follow-up to #895, as agreed in review.

Generator-peer refit accepted a donor based on rank, worker identity, tensor names, byte sizes and dtypes, but never checked the donor's accelerator family. A quantized CUDA donor could therefore pass validation for an XPU target. Matching tensor metadata does not mean backend-specific quantized layouts are compatible, and the shared accelerators_compatible() policy rejects that combination. RDMA source selection and artifact discovery already applied this policy; the generator-peer path was the one that did not.

RuntimeTensorNixlUpdateMethod._prepare_peer_read() now applies accelerators_compatible() to every acquired lease, using:

• the target engine's accelerator (accelerator_backend_for(device).name)
• the donor's accelerator from the leased GetTensorManifestResponse
• the source type, quantization and dtype of the identity the peer was discovered under
• defer_unknown=False, because the leased manifest is the authoritative post-fetch value

On rejection it raises ManifestMismatchError inside the existing cleanup-protected block, so the lease is closed before any NIXL connection or transfer.

Changes

• GeneratorPeerUpdateSource carries the SourceIdentity the peer was discovered under. GeneratorSourceResolver builds the identity once per version and uses it both to list sources and to describe each candidate.
• RuntimeTensorNixlUpdateMethod takes the target accelerator. runtime.py passes the engine device's backend name, which is the same value donors publish in GetTensorManifestResponse.accelerator.
• The accelerator check runs before the existing manifest and endpoint validation, in the same try that closes the lease.

Behavior

Donor	Target	Weights	Result
same known family	same	quantized or unquantized	accepted
CUDA	XPU, or the reverse	unquantized (MX_SOURCE_TYPE_WEIGHTS)	accepted
CUDA	XPU, or the reverse	quantized (by quantization or by dtype)	rejected
unknown (empty)	any	quantized MX_SOURCE_TYPE_WEIGHTS	rejected
CUDA	XPU, or the reverse	non-hetero source type (e.g. LoRA)	rejected

• During preparation: a rejection lets the session fall back to the next peer or configured source, as it already did for other ManifestMismatchErrors.
• During apply: a pre-transfer retry acquires a fresh lease, and that lease is validated too. A rejection there is raised to the caller, which must release and restage the update. Nothing has been written to live storage at that point.
• Same-family deployments: known same-family donors remain accepted. Quantized weight donors with missing accelerator metadata now fail closed.

Tests

test_refit_runtime_tensor.py:

• Compatible donors are accepted, and the lease is held until release. Cases: same-family quantized on CUDA and XPU, and unquantized CUDA→XPU and XPU→CUDA.
• Incompatible donors are rejected before transfer, with the lease closed and no fetch or receive. Cases: cross-family fp8 in both directions, fp8 expressed only as a dtype, an unknown donor with fp8, and a cross-family LoRA source type.
• After a rejection, the same method prepares and installs the next, compatible peer.
• A reacquired lease whose donor changed accelerator is rejected during apply, with no write and the update not marked as mutated.
• The resolver attaches the discovery identity, and the runtime passes the engine accelerator to the method.

The existing fixtures in test_refit_s3_receiver.py and test_reshard_refit_floor_staged.py now declare the identity and accelerator.

Docs: the generator-peer section of docs/rl/ARCHITECTURE.md, and the accelerators_compatible() docstring, which lists its call sites.

Summary by CodeRabbit

• Bug Fixes
  • Refit transfers now check accelerator compatibility before data is transferred, including when a peer lease is reacquired. Incompatible leases are closed, and the system can try another source during preparation.

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Walkthrough

The refit path now carries generator-peer identity and the engine accelerator into runtime tensor updates. It checks donor accelerator compatibility before validating peer tensor metadata and endpoints, and closes incompatible leases before transfer.

Changes

Generator-peer refit

Layer / File(s)	Summary
Peer identity and accelerator wiring modelexpress_client/python/modelexpress_rl/inference/plan.py, modelexpress_client/python/modelexpress_rl/inference/source/generator.py, modelexpress_client/python/modelexpress_rl/inference/runtime.py, modelexpress_client/python/modelexpress_rl/inference/methods/runtime_tensor.py, modelexpress_client/python/tests/test_refit_runtime_tensor.py	Generator peer candidates carry the discovered source identity. Runtime construction passes the engine device accelerator to the tensor update method.
Lease compatibility checks modelexpress_client/python/modelexpress_rl/inference/methods/runtime_tensor.py, modelexpress_client/python/tests/test_refit_runtime_tensor.py, modelexpress_client/python/tests/test_refit_s3_receiver.py, modelexpress_client/python/tests/test_reshard_refit_floor_staged.py, modelexpress_client/python/modelexpress/metadata/payload.py, docs/rl/ARCHITECTURE.md	Peer-read preparation checks donor accelerator compatibility before tensor metadata and endpoint validation. Tests cover accepted and rejected leases, alternative sources, and compatibility checks on reacquired leases. The architecture documentation describes the compatibility rules and retry behavior.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: Minimal · up to 4f4ca

Incompatible peers are rejected before transfer, and the inspected caller releases a failed update. No issue requiring a fix before merge was established.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 18.18% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly describes the main change: rejecting accelerator-incompatible generator peers before transfer.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 18.18% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 11 functions across 8 files. (1 skipped: 1 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

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_{A rabbit checks the donor’s kind,
Then checks the target, ears inclined.
If weights agree, the tensors fly,
If not, the lease is closed nearby.
A fresh peer waits along the way,
And hops to help another day.}

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Failed pre-merge checks:
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Write docstrings for the functions missing them to satisfy the coverage
threshold.

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Review info

Run configuration

• Source mode: Auto
• Repository: ai-dynamo/modelexpress
• Branch: main
• File: .coderabbit.yaml
• Commit: 9149ac4
• Layers: Repository YAML
• Review profile: CHILL
• Plan: Enterprise
• Run ID: e8479a0d-1afb-437a-9a8e-d5bee9da59aa

Commits

Reviewing files that changed from the base of the PR and between 9149ac4 and 4f4ca1d.

Files selected for processing: 9

• docs/rl/ARCHITECTURE.md
• modelexpress_client/python/modelexpress/metadata/payload.py
• modelexpress_client/python/modelexpress_rl/inference/methods/runtime_tensor.py
• modelexpress_client/python/modelexpress_rl/inference/plan.py
• modelexpress_client/python/modelexpress_rl/inference/runtime.py
• modelexpress_client/python/modelexpress_rl/inference/source/generator.py
• modelexpress_client/python/tests/test_refit_runtime_tensor.py
• modelexpress_client/python/tests/test_refit_s3_receiver.py
• modelexpress_client/python/tests/test_reshard_refit_floor_staged.py

Included review availability: This review used your included allowance. Your plan provides up to 12 included reviews per hour; 11 remain after this review.

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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