[Feature] vLLM-Omni RDMA connector

[natureofnature]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

Refer to #955, provide a RDMA based transfer implementation based on mooncake transfer engine for CPU<->CPU and GPU<->GPU.

Progress

1. Added d2d connector
2. Enable rdma test
3. Added cross node testing and benchmark
4. Support Bagel (AR->DIT) disaggregation (relevant modifications will be release in next PR)

TODO

Integrate with Bagel/Qwen2.5/3 model inference

Test Plan

1. Inter node test, 3 modes (serialization/deserialization, cpu pin memory, gpu pin memory for CI)
2. Cross node test, 3 modes. (for performance testing)
3. Inter node model test (bagel, qwen2.5 omni, qwen3 omni etc.)
4. Cross node model test (bagel, qwen2.5 omni, qwen3 omni etc.)

Test Result

Internode functionality

1. test_buffer_management.py passed
2. test_mooncake_rdma.py passed

Cross nodes performance

Case 1: Simulated test

Using 1GB data, repeated 20 times, zero copy (using managed buffer), gpu (gpu direct transfer), copy (data -> buffer -> RDMA). Tested on H800 clusters.

Mode	Throughput	Efficiency (vs 45 GB/s, the maximum bandwidth on tested servers)
zero copy	33.7 GB/s	75%
gpu	25.6 GB/s	57%
copy	13.2 GB/s	29%

Case2: Bagel AR/DIT disaggregation test

Using a text to image with prompt around 3400 tokens, which generates around 190MB KV cache between AR->DIT stages, below is the performance results.

Stage	Mooncake Store	RDMA with serialize/deserialize/memory copy	RDMA CPU zero-copy	RDMA GPU RDMA Direct
AR (Stage-0)	102 ms	99 ms	100 ms	106 ms
AR→DIT Data transmission	810 ms	300 ms	14 ms	14 ms
DIT (Stage-1)	10,083 ms	10,240 ms	10,077 ms	10,039 ms
Other time cost	65 ms	104 ms	103 ms	93 ms
E2E time	11,060 ms	10,743 ms	10,294 ms	10,252 ms
Overal performance gain	baseline	~3%	>7%	>7%

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft.

BEFORE SUBMITTING, PLEASE READ https://github.com/vllm-project/vllm-omni/blob/main/CONTRIBUTING.md (anything written below this line will be removed by GitHub Actions)

[chatgpt-codex-connector]

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[natureofnature]

@codex review

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[hsliuustc0106]

Can we rename it? I do not remember RDMAConnector is used vllm upstream

[natureofnature]

@codex review

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[princepride]

Docs build failed, PTAL

WARNING -  griffe: vllm_omni/distributed/omni_connectors/connectors/mooncake_transfer_engine_connector.py:744: Confusing indentation for continuation line 11 in docstring, should be 4 * 2 = 8 spaces, not 6
WARNING -  griffe: vllm_omni/distributed/omni_connectors/connectors/mooncake_transfer_engine_connector.py:745: Confusing indentation for continuation line 12 in docstring, should be 4 * 2 = 8 spaces, not 6
WARNING -  griffe: vllm_omni/distributed/omni_connectors/connectors/mooncake_transfer_engine_connector.py:746: Confusing indentation for continuation line 13 in docstring, should be 4 * 2 = 8 spaces, not 6
WARNING -  griffe: vllm_omni/distributed/omni_connectors/connectors/mooncake_transfer_engine_connector.py:747: Confusing indentation for continuation line 14 in docstring, should be 4 * 2 = 8 spaces, not 6
WARNING -  griffe: vllm_omni/distributed/omni_connectors/connectors/mooncake_transfer_engine_connector.py:748: Confusing indentation for continuation line 15 in docstring, should be 4 * 2 = 8 spaces, not 6
WARNING -  griffe: vllm_omni/distributed/omni_connectors/connectors/mooncake_transfer_engine_connector.py:750: Confusing indentation for continuation line 17 in docstring, should be 4 * 2 = 8 spaces, not 6
WARNING -  griffe: vllm_omni/distributed/omni_connectors/connectors/mooncake_transfer_engine_connector.py:751: Confusing indentation for continuation line 18 in docstring, should be 4 * 2 = 8 spaces, not 6

[princepride]

Please also add yaml file

[princepride]

@Gaohan123 @ZJY0516 @hsliuustc0106 PTAL

[Gaohan123]

Can we use uv to align with vLLM?

[Copilot]

Pull request overview

Copilot reviewed 20 out of 20 changed files in this pull request and generated 21 comments.

Comments suppressed due to low confidence (1)

vllm_omni/distributed/omni_connectors/connectors/base.py:51

• Overridden method signature does not match call, where it is passed too many arguments. Overriding method method MooncakeTransferEngineConnector.cleanup matches the call.
  Overridden method signature does not match call, where it is passed too many arguments. Overriding method method MooncakeTransferEngineConnector.cleanup matches the call.
  Overridden method signature does not match call, where it is passed too many arguments. Overriding method method MooncakeTransferEngineConnector.cleanup matches the call.
  Overridden method signature does not match call, where it is passed an argument named 'from_stage'. Overriding method method MooncakeTransferEngineConnector.cleanup matches the call.
  Overridden method signature does not match call, where it is passed an argument named 'from_stage'. Overriding method method MooncakeTransferEngineConnector.cleanup matches the call.
  Overridden method signature does not match call, where it is passed an argument named 'from_stage'. Overriding method method MooncakeTransferEngineConnector.cleanup matches the call.
  Overridden method signature does not match call, where it is passed an argument named 'to_stage'. Overriding method method MooncakeTransferEngineConnector.cleanup matches the call.
  Overridden method signature does not match call, where it is passed an argument named 'to_stage'. Overriding method method MooncakeTransferEngineConnector.cleanup matches the call.
  Overridden method signature does not match call, where it is passed an argument named 'to_stage'. Overriding method method MooncakeTransferEngineConnector.cleanup matches the call.

    def cleanup(self, request_id: str) -> None:

---

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[hsliuustc0106]

PR #1019 Review: [Feature] vLLM-Omni RDMA Connector

Overview

This PR adds RDMA-based data transfer support using the Mooncake Transfer Engine for high-performance multi-node communication between vLLM-Omni stages. The implementation includes a new MooncakeTransferEngineConnector that supports both CPU and GPU memory pools with RDMA and TCP protocols.

Summary of Changes

• 20 files changed, +3,645 additions, -63 deletions
• New connector: MooncakeTransferEngineConnector with managed memory pool and zero-copy deserialization
• Documentation: Comprehensive design doc and test configuration guide
• Tests: Integration tests for buffer management and RDMA operations
• Configuration updates: Added connector to factory and initialization utilities

---

✅ Strengths

1. Well-documented architecture: The design doc clearly explains topology limitations, port offset scheme, and troubleshooting. The benchmark results showing 58x speedup over TCP are impressive.

2. Robust memory management:

   • BufferAllocator with thread-safe free list and double-free detection
   • ManagedBuffer with RAII pattern (__del__, context manager support)
   • TTL-based cleanup (5 minutes) for stale buffers

3. Thoughtful thread safety:

   • Thread-local ZMQ REQ socket caching to avoid ordering violations
   • Inproc notification system for listener thread wake-up
   • Proper use of locks around shared state (_local_buffers_lock)

4. Comprehensive error handling:

   • Early validation of ZMQ bind errors propagated to __init__
   • Socket invalidation on timeout/error
   • Graceful shutdown with idempotent close()

5. Good test coverage:

   • Buffer management tests
   • RDMA transfer tests
   • Benchmarking infrastructure included

---

⚠️ Issues & Concerns

P1 - Critical Issues

1. CUDA synchronization before RDMA send (line 582-595)
The code synchronizes on the CUDA stream before RDMA sends, which is critical for correctness. However, the synchronization logic has duplicated code paths and could be simplified:

# Current: Duplicate sync blocks in both branches
if src_view.device != dst_tensor.device:
    # ... sync ...
else:
    # ... sync ...

Suggestion: Factor out the synchronization into a helper method to reduce duplication and ensure consistency.

2. ZMQ socket used across threads (listener thread + worker threads)
The _zmq_listener_loop creates ROUTER socket that is used by the listener thread for recv_multipart() and by worker threads via response_queue.put() then socket.send_multipart(). While response_queue provides thread-safe handoff, the actual send_multipart() call happens in the listener thread which is correct. However, the notify_recv PULL socket is bound in listener thread but accessed by worker threads via _notify_listener()'s PUSH socket connection, which is proper. This appears correct, but the comment at line 1095 about avoiding ZMQ socket across threads suggests this was a concern - verify this is actually an issue.

3. Resource leak on transfer failure (lines 1132-1145)
When RDMA write fails, the code logs a warning and returns TRANS_ERROR but does not release the receive buffer on the sender side:

if ret == 0:
    self.cleanup(meta.request_id)
    response_queue.put((identity, TRANS_DONE))
else:
    # Buffer retained for retry - but what about receiver's buffer?
    response_queue.put((identity, TRANS_ERROR))

The receiver's recv_buffer will be leaked if the transfer fails and times out on the receiver side.

4. put() accepts calls after connector falls back to receiver mode
When ZMQ bind fails, can_put is set to False (line 1034), but put() checks self.can_put before rejecting (line 501). The order of operations in __init__ means _bind_error is checked AFTER can_put is modified. However, there's a race: if put() is called between bind failure and error propagation, it would execute.

P2 - Important Issues

5. Missing optional dependency guard in imports
The connector imports mooncake.engine.TransferEngine at top level with a try/except, but the module is still imported in factory.py without the same guard. This could cause import failures even when not using RDMA:

# factory.py line ~125
from .connectors.mooncake_transfer_engine_connector import MooncakeTransferEngineConnector

This triggers the try: from mooncake.engine... block even if the import fails, causing TransferEngine = None but the module is still loaded.

6. Cleanup signature mismatch
The cleanup() method in MooncakeTransferEngineConnector accepts from_stage and to_stage parameters (line 888), but the base class OmniConnectorBase likely has a different signature. Copilot noted this issue - verify the base class signature and ensure compatibility.

7. Inconsistent error handling in get() for unresolved sender_host
Lines 745-750 raise RuntimeError when sender_host is unresolved, but the metadata query path at line 681 already checks for valid sender_host before constructing the ZMQ address. The validation at line 763 is also checking str(src_host).lower() == "auto". This creates three different error paths for the same condition.

8. Port offset scheme complexity
The port calculation logic is complex and could be error-prone. While documented, the formula involves multiple offsets that are concatenated in runtime. Consider extracting this to a dedicated helper function with unit tests.

---

📋 Code Quality Notes

1. Type hints: Good use of type hints throughout, though some Any types could be more specific.

2. Logging: Extensive logging with appropriate levels (debug/info/warning/error).

3. Naming: Clear, descriptive variable names and function names.

4. Documentation: Inline docstrings are comprehensive, especially the class docstring explaining topology limitations.

---

🔍 Specific Code Observations

Line 1032-1037: When ZMQ bind fails, can_put is set to False and _bind_error is set, then the listener returns. The __init__ at line 378 checks _bind_error and propagates it. However, there's a window where other threads could call put() before the exception is raised.

Line 1132: Variable src_lengths is used but not defined in the function scope (it should be src_lengths from line 1128).

Line 942: if getattr(self, "_closed", True): has inverted logic - it should be if getattr(self, "_closed", False): since the default should be "not closed" to allow first-time execution. Currently it would always return early unless _closed is explicitly False.

---

✅ Test Coverage

The tests look comprehensive:

• Buffer allocation/deallocation with edge cases
• Double-free detection
• RDMA transfer with different modes (serialize, pin_memory, gpu)
• Cross-node benchmarking infrastructure

---

📝 Recommendations

1. Fix the inverted _closed check in close() (line 942)
2. Add receiver buffer cleanup on transfer failure in _handle_pull_request
3. Guard the factory import of RDMA connector behind optional dependency check
4. Consider a helper for CUDA synchronization to reduce duplication
5. Add unit tests for port offset calculation logic
6. Document the race condition in put() after bind failure (or add a lock around the transition)
7. Verify base class signature compatibility for cleanup() method

---

Overall Assessment

Status: Request Changes - This is a significant feature with good design, but has several critical issues that should be addressed before merge.

The RDMA connector implementation is well-architected and thoroughly documented. The performance improvements (58x speedup) are substantial. However, there are 3-4 issues that could lead to memory leaks or race conditions in production, and a potential resource cleanup bug that should be fixed.

[natureofnature]

PR #1019 Review: [Feature] vLLM-Omni RDMA Connector

Overview

This PR adds RDMA-based data transfer support using the Mooncake Transfer Engine for high-performance multi-node communication between vLLM-Omni stages. The implementation includes a new MooncakeTransferEngineConnector that supports both CPU and GPU memory pools with RDMA and TCP protocols.

Summary of Changes

• 20 files changed, +3,645 additions, -63 deletions
• New connector: MooncakeTransferEngineConnector with managed memory pool and zero-copy deserialization
• Documentation: Comprehensive design doc and test configuration guide
• Tests: Integration tests for buffer management and RDMA operations
• Configuration updates: Added connector to factory and initialization utilities

✅ Strengths

1. Well-documented architecture: The design doc clearly explains topology limitations, port offset scheme, and troubleshooting. The benchmark results showing 58x speedup over TCP are impressive.

2. Robust memory management:

   • BufferAllocator with thread-safe free list and double-free detection
   • ManagedBuffer with RAII pattern (__del__, context manager support)
   • TTL-based cleanup (5 minutes) for stale buffers

3. Thoughtful thread safety:

   • Thread-local ZMQ REQ socket caching to avoid ordering violations
   • Inproc notification system for listener thread wake-up
   • Proper use of locks around shared state (_local_buffers_lock)

4. Comprehensive error handling:

   • Early validation of ZMQ bind errors propagated to __init__
   • Socket invalidation on timeout/error
   • Graceful shutdown with idempotent close()

5. Good test coverage:

   • Buffer management tests
   • RDMA transfer tests
   • Benchmarking infrastructure included

⚠️ Issues & Concerns

P1 - Critical Issues

1. CUDA synchronization before RDMA send (line 582-595) The code synchronizes on the CUDA stream before RDMA sends, which is critical for correctness. However, the synchronization logic has duplicated code paths and could be simplified:

# Current: Duplicate sync blocks in both branches
if src_view.device != dst_tensor.device:
    # ... sync ...
else:
    # ... sync ...

Suggestion: Factor out the synchronization into a helper method to reduce duplication and ensure consistency.

2. ZMQ socket used across threads (listener thread + worker threads) The _zmq_listener_loop creates ROUTER socket that is used by the listener thread for recv_multipart() and by worker threads via response_queue.put() then socket.send_multipart(). While response_queue provides thread-safe handoff, the actual send_multipart() call happens in the listener thread which is correct. However, the notify_recv PULL socket is bound in listener thread but accessed by worker threads via _notify_listener()'s PUSH socket connection, which is proper. This appears correct, but the comment at line 1095 about avoiding ZMQ socket across threads suggests this was a concern - verify this is actually an issue.

3. Resource leak on transfer failure (lines 1132-1145) When RDMA write fails, the code logs a warning and returns TRANS_ERROR but does not release the receive buffer on the sender side:

if ret == 0:
    self.cleanup(meta.request_id)
    response_queue.put((identity, TRANS_DONE))
else:
    # Buffer retained for retry - but what about receiver's buffer?
    response_queue.put((identity, TRANS_ERROR))

The receiver's recv_buffer will be leaked if the transfer fails and times out on the receiver side.

4. put() accepts calls after connector falls back to receiver mode When ZMQ bind fails, can_put is set to False (line 1034), but put() checks self.can_put before rejecting (line 501). The order of operations in __init__ means _bind_error is checked AFTER can_put is modified. However, there's a race: if put() is called between bind failure and error propagation, it would execute.

P2 - Important Issues

5. Missing optional dependency guard in imports The connector imports mooncake.engine.TransferEngine at top level with a try/except, but the module is still imported in factory.py without the same guard. This could cause import failures even when not using RDMA:

# factory.py line ~125
from .connectors.mooncake_transfer_engine_connector import MooncakeTransferEngineConnector

This triggers the try: from mooncake.engine... block even if the import fails, causing TransferEngine = None but the module is still loaded.

6. Cleanup signature mismatch The cleanup() method in MooncakeTransferEngineConnector accepts from_stage and to_stage parameters (line 888), but the base class OmniConnectorBase likely has a different signature. Copilot noted this issue - verify the base class signature and ensure compatibility.

7. Inconsistent error handling in get() for unresolved sender_host Lines 745-750 raise RuntimeError when sender_host is unresolved, but the metadata query path at line 681 already checks for valid sender_host before constructing the ZMQ address. The validation at line 763 is also checking str(src_host).lower() == "auto". This creates three different error paths for the same condition.

8. Port offset scheme complexity The port calculation logic is complex and could be error-prone. While documented, the formula involves multiple offsets that are concatenated in runtime. Consider extracting this to a dedicated helper function with unit tests.

📋 Code Quality Notes

1. Type hints: Good use of type hints throughout, though some Any types could be more specific.
2. Logging: Extensive logging with appropriate levels (debug/info/warning/error).
3. Naming: Clear, descriptive variable names and function names.
4. Documentation: Inline docstrings are comprehensive, especially the class docstring explaining topology limitations.

🔍 Specific Code Observations

Line 1032-1037: When ZMQ bind fails, can_put is set to False and _bind_error is set, then the listener returns. The __init__ at line 378 checks _bind_error and propagates it. However, there's a window where other threads could call put() before the exception is raised.

Line 1132: Variable src_lengths is used but not defined in the function scope (it should be src_lengths from line 1128).

Line 942: if getattr(self, "_closed", True): has inverted logic - it should be if getattr(self, "_closed", False): since the default should be "not closed" to allow first-time execution. Currently it would always return early unless _closed is explicitly False.

✅ Test Coverage

The tests look comprehensive:

• Buffer allocation/deallocation with edge cases
• Double-free detection
• RDMA transfer with different modes (serialize, pin_memory, gpu)
• Cross-node benchmarking infrastructure

📝 Recommendations

1. Fix the inverted _closed check in close() (line 942)
2. Add receiver buffer cleanup on transfer failure in _handle_pull_request
3. Guard the factory import of RDMA connector behind optional dependency check
4. Consider a helper for CUDA synchronization to reduce duplication
5. Add unit tests for port offset calculation logic
6. Document the race condition in put() after bind failure (or add a lock around the transition)
7. Verify base class signature compatibility for cleanup() method

Overall Assessment

Status: Request Changes - This is a significant feature with good design, but has several critical issues that should be addressed before merge.

The RDMA connector implementation is well-architected and thoroughly documented. The performance improvements (58x speedup) are substantial. However, there are 3-4 issues that could lead to memory leaks or race conditions in production, and a potential resource cleanup bug that should be fixed.

---

Response to P1 & P2 Review Comments

P1-1: CUDA synchronization before RDMA send (line 582-595)

The synchronization logic is correct — the three branches handle genuinely different device combinations:

• Cross-device copy (line 581-590): Uses non_blocking=True and syncs on the source device for D2H or destination device for H2D.
• Same-device copy (line 591-595): Blocking copy, then sync on CUDA device if applicable.
• bytes→tensor copy (line 596-602): Always CPU source; sync only if destination is CUDA.

Each branch correctly identifies which CUDA device to synchronize on. Factoring into a helper is a valid cleanup suggestion and can be done as a follow-up — it does not affect correctness.

P1-2: ZMQ socket used across threads

This is a false positive. The design specifically avoids cross-thread socket access via a single-writer pattern:

• The ROUTER socket is created, recv_multipart()'d, and send_multipart()'d exclusively in the listener thread (_zmq_listener_loop).
• Worker threads (from _sender_executor thread pool) perform the time-consuming RDMA transfer via engine.batch_transfer_sync_write(), then place the result (identity, response_bytes) into a Python queue.Queue — which is thread-safe by design.
• Workers wake up the listener through a per-thread PUSH socket (stored in threading.local()) connected to an inproc:// PULL socket owned by the listener thread.
• The listener, upon being woken by the PULL notification, drains the queue and calls socket.send_multipart() in its own thread.

At no point does any worker thread touch the ROUTER socket. The response_queue carries plain bytes objects, not socket references.

P1-3: Resource leak on transfer failure (lines 1132-1145)

This is handled on both sides:

• Sender side: The buffer is intentionally retained in _local_buffers to allow receiver retries. It is eventually reclaimed by the TTL mechanism (_cleanup_stale_buffers, runs every ~10s, default TTL = 60s), or by close().
• Receiver side: When get() receives TRANS_ERROR or encounters any exception, the recv_buffer is explicitly released in the except block at line 885 (recv_buffer.release()), so there is no leak on the consumer side.

P1-4: put() race between bind failure and error propagation

There is no race condition. The sequence in __init__ is:

1. self.can_put = (role == "sender") → set to True
2. self._listener_thread.start() → background thread starts
3. self._listener_ready.wait(timeout=1.0) → blocks until listener binds or fails
4. If bind fails → listener sets self._bind_error and signals _listener_ready
5. __init__ resumes → checks self._bind_error is not None → raises RuntimeError

Since __init__ raises before returning, no external code can ever obtain a reference to the connector object. It is impossible to call put() on a connector whose bind failed.

P2-5: Missing optional dependency guard in factory.py

Already handled. The import in factory.py (line 94-102) is a lazy import inside the _create_mooncake_transfer_engine_connector() function body — it only executes when the factory is asked to create this specific connector type, not at module load time. Additionally, __init__.py guards the top-level import with try/except ImportError.

P2-6: Cleanup signature mismatch

This is an intentional backward-compatible extension. The base class defines cleanup(self, request_id: str). The override adds from_stage: str | None = None, to_stage: str | None = None — both with default values. A call like connector.cleanup("r1") still works and matches the base class contract. Copilot itself flagged this as "suppressed due to low confidence." We can update the base class signature in a follow-up if desired.

P2-7: Inconsistent error handling for unresolved sender_host

The three checks serve different entry points and are defense-in-depth, not duplication:

1. Line 745-750 (get() entry guard): Rejects get(metadata=None) early if sender_host is unresolved, forcing callers to call update_sender_info() first.
2. Line 681 (_query_metadata_from_sender internal): Validates before constructing the ZMQ address — this method could theoretically be called from other contexts in the future.
3. Line 758 (get() post-metadata): Validates source_host from externally-provided metadata (e.g., from a queue), which is a different trust boundary.

These are layered checks at different trust levels, not redundant code paths.

P2-8: Port offset scheme complexity

The port offset calculation (zmq_port + purpose_offset + stage_offset + dp_index * tp_size + tp_rank) is implemented in the orchestration layer (PR2 / KV transfer manager), not in this connector PR. The connector simply receives a pre-computed zmq_port via its config. Documentation for the scheme is provided in mooncake_transfer_engine_connector.md. Extracting it into a helper with unit tests is a good suggestion and will be addressed in PR2.

---

Summary & Fix Plan

#	Issue	Severity	Verdict	Action	Priority
P1-1	CUDA sync code duplication	P1	Correct but verbose	Extract helper method to reduce duplication in PR2	Follow-up (non-blocking)
P1-2	ZMQ socket cross-thread	P1	False positive — single-writer pattern is correct	No change needed	N/A
P1-3	Resource leak on RDMA failure	P1	Handled — receiver releases buffer, sender has TTL	No change needed	N/A
P1-4	put() race on bind failure	P1	No race — __init__ blocks and raises	No change needed	N/A
P2-5	Import guard in factory.py	P2	Already handled — lazy import in function body	No change needed	N/A
P2-6	cleanup() signature mismatch	P2	Intentional backward-compatible extension	Update base class signature in PR2	Follow-up (non-blocking)
P2-7	Redundant sender_host checks	P2	Defense-in-depth at different trust boundaries	No change needed	N/A
P2-8	Port offset complexity	P2	Not in this PR scope (PR2 code)	Add helper + unit tests in PR2	PR2

Conclusion: No blocking issues found. All P1 items are either false positives or already handled. Two low-priority cleanups (P1-1, P2-6) can be addressed as follow-ups without blocking merge.