[Config] Add field validators for attention, KV/EC transfer, and KV events configs

[joinalahmed]

Summary

Add @field_validator decorators to reject invalid configuration values at construction time, preventing silent failures and confusing runtime errors.

Includes comprehensive test coverage (32 tests) following the pattern from PR #44070.

Changes

vllm/config/attention.py - AttentionConfig

• flash_attn_max_num_splits_for_cuda_graph: reject non-positive values
• tq_max_kv_splits_for_cuda_graph: reject non-positive values
• flex_attn_block_m, flex_attn_block_n: validate >= 16 and power of 2
• flex_attn_q_block_size, flex_attn_kv_block_size: validate power of 2

vllm/config/kv_events.py - KVEventsConfig

• buffer_steps: reject non-positive values
• hwm: reject non-positive values
• max_queue_size: reject non-positive values

vllm/config/kv_transfer.py - KVTransferConfig

• kv_buffer_size: reject non-positive values
• kv_rank: reject negative values when set
• kv_parallel_size: reject non-positive values
• kv_port: validate port range [1, 65535]

vllm/config/ec_transfer.py - ECTransferConfig

• ec_buffer_size: reject non-positive values
• ec_rank: reject negative values when set
• ec_parallel_size: reject non-positive values
• ec_port: validate port range [1, 65535]

tests/test_config.py - Test Coverage

• 32 test cases covering all 17 validators
• Each validator tested with both valid and invalid inputs
• Uses pytest.mark.parametrize for comprehensive coverage
• Clear docstrings explaining what/why

Why this is not duplicating existing work

• Checked open PRs for overlaps
• PR Validate num_gpu_blocks_override in CacheConfig #43900 validates num_gpu_blocks_override in cache.py (different file)
• PR fix(config): reject negative max_logprobs (except -1) and long_prefill_token_threshold #44070 validates max_logprobs and long_prefill_token_threshold in model.py and scheduler.py (different files)
• No open PRs touch these specific fields in these files

Problem

These fields currently accept invalid values that downstream code doesn't expect, leading to:

1. Silent no-ops: Negative values ignored by conditionals (e.g., if 0 < threshold < num_tokens is False for negatives)
2. Runtime errors: Invalid ports, buffer sizes cause errors with confusing messages
3. Undefined behavior: Power-of-2 assumptions violated

Examples

Before (silent failure):

config = KVTransferConfig(kv_port=-1)  # Accepted
# Later: connection fails with cryptic socket error

After (clear error):

config = KVTransferConfig(kv_port=-1)
# ValueError: kv_port must be in valid port range [1, 65535], got -1.

Before (silent failure):

config = AttentionConfig(flex_attn_block_m=15)  # Accepted
# Later: kernel launch fails or produces wrong results

After (clear error):

config = AttentionConfig(flex_attn_block_m=15)
# ValueError: flex_attn block size must be >= 16, got 15.

Pattern

Follows the @field_validator(mode="after") pattern established in:

• Validate against some config fields being set to 0 #43794
• [Bugfix] Reject negative values for max_logprobs and long_prefill_token_threshold #44002
• fix(config): reject negative max_logprobs (except -1) and long_prefill_token_threshold #44070
• Reject negative config values for logprobs and prefill threshold #44093
• fix(scheduler): reject non-positive max_num_scheduled_tokens at construction time #44125

All validators:

• Use mode="after" for validation after type coercion
• Provide clear, actionable error messages
• Follow naming convention _check_<field_name>

Tests run

.venv/bin/pre-commit run --all-files

All hooks passed:

• ✅ ruff check
• ✅ ruff format
• ✅ mypy
• ✅ SPDX headers
• ✅ All other pre-commit checks

Comprehensive test suite added:

• 32 test functions covering all validators
• Both positive (should accept) and negative (should reject) test cases
• Will run in CI once PR is labeled ready

Impact

• 398 lines added (126 validator code + 272 test code)
• 17 validators, 32 tests across 4 config files + test file
• No breaking changes - only rejects values that would fail at runtime anyway
• Better user experience - early, clear error messages
• Prevents bugs - catches config errors before deployment

[github-actions]

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🚀

[joinalahmed]

Hi maintainers! 👋

This is my first contribution to vLLM. I've added field validators to prevent invalid config values, following the pattern from recent PRs (#43794, #44070, #44093, #44125).

What was done:

• Added 17 decorators across 4 config files
• All pre-commit hooks passed locally
• No duplicate work (checked open PRs)
• Clear error messages for each validation

Testing:

.venv/bin/pre-commit run --all-files  # All checks passed

The validators only reject values that would fail at runtime anyway, so there are no breaking changes.

Could a maintainer please add the ready label to trigger CI tests? Happy to address any feedback!

Thanks!

[joinalahmed]

Update: Added Comprehensive Tests

I've added 32 test cases to tests/test_config.py covering all 17 validators, following the pattern from PR #44070.

Test coverage:

• ✅ AttentionConfig: 8 tests
• ✅ KVEventsConfig: 6 tests
• ✅ KVTransferConfig: 9 tests
• ✅ ECTransferConfig: 9 tests

Each validator is tested with:

• Multiple invalid values that should be rejected
• Multiple valid values that should be accepted
• Clear docstrings explaining the validation

All pre-commit hooks passed (ruff, mypy, etc.).