Allow unknown tags by default in check-yaml

[j178]

Follow-up to #2674.

Rejecting unknown tags by default causes false positives in ecosystems such as MkDocs and Ansible. A generic YAML checker cannot know the tag vocabulary of the application that consumes a file, so an unrecognized application-specific tag does not make the YAML invalid.

This intentionally differs from upstream check-yaml. prek allows unknown tags by default and provides --disallow-unknown-tags for users who want to enforce a closed tag vocabulary. Only tag rejection changes; all other loading checks remain enabled.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-02T17:25:42.015744Z	834c86d	New commits

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[Copilot]

🟡 Changes recommended

It introduces a CLI breaking change by removing --allow-unknown-tags, and the docs currently understate that --disallow-unknown-tags can provide pinned behavior without disabling the fast path.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Updates the built-in check-yaml hook to allow unknown YAML tags by default (addressing CI failures with tools like MkDocs that use custom tags), while providing an opt-in flag to reject them.

Changes:

• Flip default behavior to allow unrecognized YAML tags when loading YAML.
• Replace the CLI flag with --disallow-unknown-tags (and update hook docs/output accordingly).
• Update docs and tests to reflect the new default and flag behavior.

File summaries

File	Description
docs/builtin.md	Documents the new default behavior and how it relates to the pinned pre-commit-hooks implementation.
crates/prek/tests/list_builtins.rs	Updates the expected list-builtins output to show the new flag name/description.
crates/prek/tests/builtin_hooks.rs	Updates integration-style snapshot coverage for allow-by-default vs reject-with-flag behavior.
crates/prek/src/hooks/pre_commit_hooks/check_yaml.rs	Implements the behavior change and updates unit tests around unknown tag handling.

Review details

Suppressed comments (1)

crates/prek/src/hooks/pre_commit_hooks/check_yaml.rs:49

• If you keep a deprecated --allow-unknown-tags flag for compatibility, it should be incorporated into the runtime behavior (and/or used to avoid "field never read" warnings) so that passing it continues to force-allow tags and it cleanly conflicts with --disallow-unknown-tags.

        CheckMode::Load {
            multiple: args.allow_multiple_documents,
            disallow_unknown_tags: args.disallow_unknown_tags,
        }

• Files reviewed: 4/4 changed files
• Comments generated: 2
• Review effort level: Lite

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.11%. Comparing base (78bb3fe) to head (834c86d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2678   +/-   ##
=======================================
  Coverage   94.11%   94.11%           
=======================================
  Files         141      141           
  Lines       30051    30057    +6     
=======================================
+ Hits        28281    28287    +6     
  Misses       1770     1770           

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[prek-ci-bot]

📦 Cargo Bloat Comparison

.text size change: +0.00% (12.8 MiB → 12.8 MiB)

Expand for cargo-bloat output

Head Branch Results

File  .text     Size             Crate Name
0.1%   2.5% 332.0KiB        aws_lc_sys aws_lc_0_44_0_aes_gcm_encrypt_avx512
0.1%   2.5% 332.0KiB        aws_lc_sys aws_lc_0_44_0_aes_gcm_decrypt_avx512
0.0%   0.7%  96.5KiB              prek <prek::cli::Command as clap_builder::derive::Subcommand>::augment_subcommands
0.0%   0.4%  50.1KiB              prek <<prek::config::hook::HookWire as serde_core::de::Deserialize>::deserialize::__Visitor as serde_core::de::Visitor>::visit_map::<&mut <serde_saphyr::de::deserializer::YamlDeserializer as serde_core::de::Deserializer>::deserialize_map::MA>
0.0%   0.4%  49.7KiB              prek prek::run::{closure#0}
0.0%   0.4%  48.7KiB annotate_snippets annotate_snippets::renderer::render::render
0.0%   0.3%  43.9KiB              prek <<prek::config::Config as serde_core::de::Deserialize>::deserialize::__Visitor as serde_core::de::Visitor>::visit_map::<&mut <serde_saphyr::de::deserializer::YamlDeserializer as serde_core::de::Deserializer>::deserialize_map::MA>
0.0%   0.3%  40.3KiB              prek <prek::cli::RunOptions as clap_builder::derive::Args>::augment_args
0.0%   0.3%  37.0KiB              prek prek::cli::run::run::run::{closure#0}
0.0%   0.2%  28.7KiB      serde_saphyr <granit_parser::scanner::Scanner<granit_parser::input::str::StrInput>>::fetch_more_tokens
0.0%   0.2%  28.6KiB              prek prek::archive::unpack::<fs_err::tokio::file::File, &std::path::PathBuf>::{closure#0}
0.0%   0.2%  28.2KiB      serde_saphyr <granit_parser::scanner::Scanner<granit_parser::input::str::StrInput>>::fetch_more_tokens
0.0%   0.2%  28.0KiB        aws_lc_sys aws_lc_0_44_0_edwards25519_scalarmuldouble_alt
0.0%   0.2%  27.5KiB        aws_lc_sys aws_lc_0_44_0_edwards25519_scalarmuldouble
0.0%   0.2%  27.3KiB              toml <toml::de::deserializer::value::ValueDeserializer as serde_core::de::Deserializer>::deserialize_any::<<prek::config::hook::HookWire as serde_core::de::Deserialize>::deserialize::__Visitor>
0.0%   0.2%  27.1KiB              prek prek::archive::unpack::<fs_err::tokio::file::File, &std::path::PathBuf>::{closure#0}
0.0%   0.2%  27.0KiB              prek prek::archive::unpack::<fs_err::tokio::file::File, &std::path::PathBuf>::{closure#0}
0.0%   0.2%  27.0KiB              prek prek::archive::unpack::<fs_err::tokio::file::File, &std::path::PathBuf>::{closure#0}
0.0%   0.2%  26.9KiB              prek prek::archive::unpack::<fs_err::tokio::file::File, &std::path::PathBuf>::{closure#0}
0.0%   0.2%  26.9KiB              prek prek::archive::unpack::<fs_err::tokio::file::File, &std::path::PathBuf>::{closure#0}
4.2%  87.0%  11.1MiB                   And 20385 smaller methods. Use -n N to show more.
4.8% 100.0%  12.8MiB                   .text section size, the file size is 263.5MiB

Base Branch Results

File  .text     Size             Crate Name
0.1%   2.5% 332.0KiB        aws_lc_sys aws_lc_0_44_0_aes_gcm_encrypt_avx512
0.1%   2.5% 332.0KiB        aws_lc_sys aws_lc_0_44_0_aes_gcm_decrypt_avx512
0.0%   0.7%  96.5KiB              prek <prek::cli::Command as clap_builder::derive::Subcommand>::augment_subcommands
0.0%   0.4%  50.1KiB              prek <<prek::config::hook::HookWire as serde_core::de::Deserialize>::deserialize::__Visitor as serde_core::de::Visitor>::visit_map::<&mut <serde_saphyr::de::deserializer::YamlDeserializer as serde_core::de::Deserializer>::deserialize_map::MA>
0.0%   0.4%  49.7KiB              prek prek::run::{closure#0}
0.0%   0.4%  48.7KiB annotate_snippets annotate_snippets::renderer::render::render
0.0%   0.3%  43.9KiB              prek <<prek::config::Config as serde_core::de::Deserialize>::deserialize::__Visitor as serde_core::de::Visitor>::visit_map::<&mut <serde_saphyr::de::deserializer::YamlDeserializer as serde_core::de::Deserializer>::deserialize_map::MA>
0.0%   0.3%  40.3KiB              prek <prek::cli::RunOptions as clap_builder::derive::Args>::augment_args
0.0%   0.3%  37.0KiB              prek prek::cli::run::run::run::{closure#0}
0.0%   0.2%  28.7KiB      serde_saphyr <granit_parser::scanner::Scanner<granit_parser::input::str::StrInput>>::fetch_more_tokens
0.0%   0.2%  28.6KiB              prek prek::archive::unpack::<fs_err::tokio::file::File, &std::path::PathBuf>::{closure#0}
0.0%   0.2%  28.2KiB      serde_saphyr <granit_parser::scanner::Scanner<granit_parser::input::str::StrInput>>::fetch_more_tokens
0.0%   0.2%  28.0KiB        aws_lc_sys aws_lc_0_44_0_edwards25519_scalarmuldouble_alt
0.0%   0.2%  27.5KiB        aws_lc_sys aws_lc_0_44_0_edwards25519_scalarmuldouble
0.0%   0.2%  27.3KiB              toml <toml::de::deserializer::value::ValueDeserializer as serde_core::de::Deserializer>::deserialize_any::<<prek::config::hook::HookWire as serde_core::de::Deserialize>::deserialize::__Visitor>
0.0%   0.2%  27.1KiB              prek prek::archive::unpack::<fs_err::tokio::file::File, &std::path::PathBuf>::{closure#0}
0.0%   0.2%  27.0KiB              prek prek::archive::unpack::<fs_err::tokio::file::File, &std::path::PathBuf>::{closure#0}
0.0%   0.2%  27.0KiB              prek prek::archive::unpack::<fs_err::tokio::file::File, &std::path::PathBuf>::{closure#0}
0.0%   0.2%  26.9KiB              prek prek::archive::unpack::<fs_err::tokio::file::File, &std::path::PathBuf>::{closure#0}
0.0%   0.2%  26.9KiB              prek prek::archive::unpack::<fs_err::tokio::file::File, &std::path::PathBuf>::{closure#0}
4.2%  87.0%  11.1MiB                   And 20385 smaller methods. Use -n N to show more.
4.8% 100.0%  12.8MiB                   .text section size, the file size is 263.5MiB

[prek-ci-bot]

⚡️ Hyperfine Benchmarks

Summary: 0 regressions, 0 improvements above the 10% threshold.

Environment

• OS: Linux 6.17.0-1022-azure
• CPU: 4 cores
• prek version: prek 0.5.1+8 (0c54469 2026-09-02)
• Rust version: rustc 1.98.0 (88d9e12ae 2026-08-18)
• Hyperfine version: hyperfine 1.20.0

CLI Commands

Benchmarking basic commands in the main repo:

prek --version

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base --version	2.5 ± 0.1	2.4	3.0	1.08 ± 0.04
prek-head --version	2.3 ± 0.0	2.2	2.5	1.00

prek list

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base list	9.6 ± 0.1	9.4	9.9	1.02 ± 0.02
prek-head list	9.4 ± 0.1	9.2	9.7	1.00

prek validate-config .pre-commit-config.yaml

⏭️ Skipped: .pre-commit-config.yaml not found

prek sample-config

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base sample-config	2.8 ± 0.1	2.7	3.0	1.03 ± 0.04
prek-head sample-config	2.7 ± 0.1	2.6	2.9	1.00

Cold vs Warm Runs

Comparing first run (cold) vs subsequent runs (warm cache):

prek run --all-files (cold - no cache)

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run --all-files	38.4 ± 1.5	36.9	41.4	1.01 ± 0.05
prek-head run --all-files	38.0 ± 1.0	36.6	40.1	1.00

prek run --all-files (warm - with cache)

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run --all-files	38.1 ± 1.1	36.4	40.7	1.02 ± 0.04
prek-head run --all-files	37.4 ± 0.9	35.9	39.9	1.00

Full Hook Suite

Running the builtin hook suite on the benchmark workspace:

prek run --all-files (full builtin hook suite)

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run --all-files	37.9 ± 1.1	35.3	42.4	1.00
prek-head run --all-files	38.7 ± 3.5	36.3	54.6	1.02 ± 0.10

Individual Hook Performance

Benchmarking each hook individually on the test repo:

prek run trailing-whitespace --all-files

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run trailing-whitespace --all-files	13.1 ± 0.3	12.6	13.5	1.02 ± 0.03
prek-head run trailing-whitespace --all-files	12.8 ± 0.3	12.2	13.5	1.00

prek run end-of-file-fixer --all-files

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run end-of-file-fixer --all-files	11.5 ± 0.4	10.8	12.2	1.02 ± 0.05
prek-head run end-of-file-fixer --all-files	11.3 ± 0.4	10.8	12.2	1.00

prek run check-json --all-files

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run check-json --all-files	8.4 ± 0.3	7.9	9.3	1.04 ± 0.05
prek-head run check-json --all-files	8.1 ± 0.2	7.6	8.5	1.00

prek run check-yaml --all-files

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run check-yaml --all-files	8.2 ± 0.3	7.8	9.1	1.03 ± 0.04
prek-head run check-yaml --all-files	7.9 ± 0.2	7.6	8.7	1.00

prek run check-toml --all-files

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run check-toml --all-files	8.9 ± 2.2	7.9	20.2	1.07 ± 0.26
prek-head run check-toml --all-files	8.3 ± 0.4	7.7	9.1	1.00

prek run check-xml --all-files

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run check-xml --all-files	8.5 ± 0.5	7.9	9.6	1.06 ± 0.08
prek-head run check-xml --all-files	8.1 ± 0.3	7.7	9.2	1.00

prek run detect-private-key --all-files

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run detect-private-key --all-files	11.7 ± 0.6	10.7	12.7	1.02 ± 0.08
prek-head run detect-private-key --all-files	11.5 ± 0.7	10.5	13.3	1.00

prek run fix-byte-order-marker --all-files

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run fix-byte-order-marker --all-files	13.2 ± 0.8	11.9	14.4	1.01 ± 0.09
prek-head run fix-byte-order-marker --all-files	13.1 ± 0.8	11.9	14.6	1.00

Installation Performance

Benchmarking hook installation (fast path hooks skip Python setup):

prek install-hooks (cold - no cache)

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base install-hooks	5.0 ± 0.1	5.0	5.1	1.03 ± 0.02
prek-head install-hooks	4.9 ± 0.1	4.8	5.0	1.00

prek install-hooks (warm - with cache)

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base install-hooks	5.1 ± 0.1	5.0	5.1	1.00 ± 0.02
prek-head install-hooks	5.1 ± 0.1	5.0	5.1	1.00

File Filtering/Scoping Performance

Testing different file selection modes:

prek run (staged files only)

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run	25.9 ± 0.4	25.4	26.7	1.03 ± 0.02
prek-head run	25.2 ± 0.3	24.5	25.9	1.00

prek run --files '*.json' (specific file type)

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run --files '*.json'	5.7 ± 0.1	5.6	6.0	1.05 ± 0.02
prek-head run --files '*.json'	5.4 ± 0.0	5.3	5.5	1.00

Workspace Discovery & Initialization

Benchmarking hook discovery and initialization overhead:

prek run --dry-run --all-files (measures init overhead)

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run --dry-run --all-files	7.4 ± 0.1	7.3	7.6	1.02 ± 0.02
prek-head run --dry-run --all-files	7.2 ± 0.1	7.1	7.3	1.00

Meta Hooks Performance

Benchmarking meta hooks separately:

prek run check-hooks-apply --all-files

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run check-hooks-apply --all-files	8.8 ± 0.1	8.7	9.2	1.03 ± 0.02
prek-head run check-hooks-apply --all-files	8.6 ± 0.1	8.5	8.8	1.00

prek run check-useless-excludes --all-files

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run check-useless-excludes --all-files	8.8 ± 0.1	8.7	8.9	1.03 ± 0.01
prek-head run check-useless-excludes --all-files	8.5 ± 0.1	8.4	8.7	1.00

prek run identity --all-files

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
prek-base run identity --all-files	7.7 ± 0.1	7.6	8.0	1.02 ± 0.02
prek-head run identity --all-files	7.6 ± 0.1	7.5	7.7	1.00

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: f464973bf7

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[Copilot]

🔵 Needs a closer look

Dropping the previously supported --allow-unknown-tags flag is a breaking CLI/config change that will cause existing configurations to fail to parse.

Review details

Suppressed comments (1)

crates/prek/src/hooks/pre_commit_hooks/check_yaml.rs:23

• Removing --allow-unknown-tags is a breaking change for existing prek configs (especially for users who added it in 0.5.1 to work around default tag rejection). Consider keeping --allow-unknown-tags as a hidden/deprecated no-op flag so older configs continue to parse, while --disallow-unknown-tags controls the new behavior.

    /// Reject unrecognized YAML tags.
    #[arg(long)]
    disallow_unknown_tags: bool,

• Files reviewed: 4/4 changed files
• Comments generated: 0 new
• Review effort level: Lite

[Copilot]

🔵 Needs a closer look

The --allow-unknown-tags → --disallow-unknown-tags rename breaks existing configs that still pass the old flag, and it should be retained as a hidden/deprecated compatibility no-op.

Review details

Suppressed comments (1)

crates/prek/src/hooks/pre_commit_hooks/check_yaml.rs:23

• Renaming the CLI from --allow-unknown-tags to --disallow-unknown-tags is a breaking change for existing prek / pre-commit configs that already pass --allow-unknown-tags (even though it is now the default). Consider keeping --allow-unknown-tags as a hidden/deprecated no-op flag for compatibility, and error if both --allow-unknown-tags and --disallow-unknown-tags are provided.

    /// Reject unrecognized YAML tags.
    #[arg(long, conflicts_with = "unsafe")]
    disallow_unknown_tags: bool,

• Files reviewed: 4/4 changed files
• Comments generated: 0 new
• Review effort level: Lite