protokit 0.8.0

Python toolkit for Protocol Buffers: structural message diffing and schema compatibility checking.

protokit

Python toolkit for Protocol Buffers: message diffing and schema compatibility checking.

protokit diff — structural, filterable message diffs with cross-descriptor-pool comparison, schema evolution detection, and a pytest hook.

protokit compat — descriptor-level schema compatibility checks with 17 built-in rules, four profiles, and a pluggable rule API.

Installation

pip install protokit

Message Diffing

Library

from protokit.message import diff_messages, ChangeType

result = diff_messages(msg1, msg2)

if result.has_changes():
    for diff in result:
        print(f"{diff.path}: {diff.old_value} -> {diff.new_value}")

# Filter by path prefix
user_changes = result.filter(path="user.address")

# Filter by change type
additions = result.filter(change_type=ChangeType.ADDED)

# Combine filters
new_address_fields = result.filter(path="user.address", change_type=ChangeType.ADDED)

CLI

Compare two binary protobuf messages:

# Same-schema mode
protokit diff left.pb right.pb --desc schema.descriptor_set --message-type myapp.User

# Cross-schema mode (schema evolution)
protokit diff left.pb right.pb \
  --left-desc v1.descriptor_set --right-desc v2.descriptor_set \
  --left-type v1.User --right-type v2.User

# JSON input
protokit diff left.json right.json --desc schema.descriptor_set --message-type myapp.User --json

# Text format input
protokit diff left.textproto right.textproto --desc schema.descriptor_set --message-type myapp.User --text-format

# From .proto files (requires protoc on PATH)
protokit diff left.pb right.pb --proto schema.proto --message-type myapp.User

Example output:

Found 3 differences:

  ~ user.name: 'Alice' -> 'Bob'
  + user.address.city: 'New York'
  - user.phone: '[phone]'

JSON output for scripting:

protokit diff left.pb right.pb --desc schema.descriptor_set --message-type myapp.User --format json

{
  "equal": false,
  "differences": [
    {
      "path": "user.name",
      "change_type": "MODIFIED",
      "old_value": "Alice",
      "new_value": "Bob",
      "field_type": "TYPE_STRING"
    }
  ],
  "warnings": []
}

Quiet mode for CI (exit code only):

protokit diff left.pb right.pb --desc schema.descriptor_set --message-type myapp.User --quiet
echo $?  # 0 = equal, 1 = different, 2 = error

pytest Integration

Add the hook to your conftest.py:

from protokit.message.pytest_plugin import pytest_assertrepr_compare  # noqa: F401

Now assert msg1 == msg2 shows a structured diff on failure:

assert msg1 == msg2
E     myapp.User != myapp.User
E       2 difference(s):
E       ~ name: 'Alice' -> 'Bob'
E       ~ address.city: 'SF' -> 'NYC'

Features

Cross-descriptor-pool comparison — Compare messages from different .descriptor_set files. Field matching is name-based, not descriptor-identity-based.

Schema evolution detection — Detects field number changes, type changes, and cardinality changes across schema versions. Reported as first-class diff entries.

Queryable diff objects — DiffResult is immutable and filterable. Filter by path prefix, exact path, or change type. Chain filters freely.

treat_as_map — Match repeated message fields by a key field instead of index, ignoring order.

from protokit.message import MessageDifferencer

differ = MessageDifferencer()
differ.treat_as_map("items", key="id")
result = differ.compare(msg1, msg2)
# Paths use key notation: items[id="abc"].name

Float comparison — Exact (IEEE 754) or approximate (fraction + margin) modes.

from protokit.message import MessageDifferencer, FloatComparison

differ = MessageDifferencer()
differ.set_float_comparison(FloatComparison.APPROXIMATE, fraction=1e-6, margin=1e-9)

Ignore fields

differ = MessageDifferencer()
differ.ignore_fields("timestamp", "request_id")  # bare name = global
differ.ignore_fields("user.internal_id")          # dotted path = scoped

Max depth

differ = MessageDifferencer()
differ.max_depth = 3
result = differ.compare(msg1, msg2)
assert not result.is_complete  # truncated subtrees exist

CLI Options (protokit diff)

Flag	Description
--desc FILE	Descriptor set file (same-schema mode)
--message-type NAME	Fully-qualified message type name
--left-desc FILE	Left descriptor set (cross-schema mode)
--right-desc FILE	Right descriptor set (cross-schema mode)
--left-type NAME	Left message type (cross-schema mode)
--right-type NAME	Right message type (cross-schema mode)
--proto FILE	.proto file (requires protoc on PATH)
--proto-path DIR	Import path for protoc. Repeatable.
--text-format	Parse input as protobuf text format
--json	Parse input as JSON-encoded protobuf
--format NAME	Output format (default: human). Built-in for diff: human, json, junit. See Output Formatters.
--formatter-module MODULE	Python module exposing a FORMATTERS = [(name, fn, kind), ...] list (repeatable). See Output Formatters.
--quiet	Suppress output, exit code only. Mutually exclusive with any non-human --format.
--verbose	Show warnings even when equal
--filter PATH	Filter diffs by path prefix
--ignore FIELD	Ignore field. Repeatable.
--treat-as-map FIELD KEY	Treat repeated field as map with key
--float-mode exact|approximate	Float comparison mode
--max-depth N	Maximum comparison depth
--strict-schema	Warn on message type name changes

Schema Compatibility

Check whether a .proto schema change is safe before you merge it. Catches three kinds of breakage:

• Wire breaks — field-number reuse, cardinality flips, incompatible encodings.
• Semantic breaks — removed fields, added enum values, presence-semantics changes.
• Policy breaks — custom-option changes and org-specific rules (via plugins).

Library

from protokit.schema import check_compatibility, CompatibilityLevel

report = check_compatibility(
    old_pool, "acme.User",
    new_pool, "acme.User",
    level=CompatibilityLevel.CONSUMER_SAFE,
)

if not report.is_compatible:
    for f in report.findings:
        print(f"{f.severity.value}/{f.direction.value} {f.path}: {f.message}")

# Per-severity buckets
print(len(report.wire_breaks), len(report.semantic_breaks), len(report.policy_breaks))

Cross-type comparisons (renamed messages) pass different type names for each side:

report = check_compatibility(old_pool, "acme.UserV1", new_pool, "acme.UserV2")

CLI

Compare two descriptor sets:

protokit compat old.descriptor_set new.descriptor_set \
  --type acme.User \
  --level consumer-safe

From .proto sources:

protokit compat old.proto new.proto --proto --type acme.User

Cross-type comparison:

protokit compat old.descriptor_set new.descriptor_set \
  --old-type acme.UserV1 --new-type acme.UserV2

JSON output for CI:

protokit compat old.descriptor_set new.descriptor_set --type acme.User --format json --quiet
echo $?  # 0 = compatible, 1 = incompatible, 2 = error

Compatibility Profiles

Four profiles control which findings surface. Each is a pair of filters: a severity threshold and a direction filter.

Profile	Question Answered	Surfaces
WIRE	Will deserialization crash?	Wire-level breaks only, any direction.
CONSUMER_SAFE	Can old consumers safely read new messages?	Wire + semantic with BACKWARD or BOTH direction. Excludes FORWARD-only findings like new-field-added.
PRODUCER_SAFE	Can new consumers safely read old messages?	Wire + semantic with FORWARD or BOTH direction. Excludes BACKWARD-only findings like field-removed.
STRICT	Any compatibility concern at all?	All severities (including POLICY) in all directions.

Built-in Rules

Rule	Severity	Direction	Detects
field_removed	SEMANTIC	BACKWARD	Field present in old, absent in new.
field_added	SEMANTIC	BACKWARD	New field (non-required, not in a oneof). Old consumer sees unknown data.
field_number_changed	WIRE	BOTH	Same name, different number.
field_type_wire_incompatible	WIRE	BOTH	Scalar type change across wire groups (e.g. int32 ↔ sint32).
field_type_semantic_change	SEMANTIC	BOTH	Type change within a wire group (e.g. string ↔ bytes).
field_type_name_changed	POLICY	BOTH	Message/enum field points at a renamed type (shape may still match).
repeated_to_singular	WIRE	BOTH	Cardinality flip between singular and repeated.
map_to_repeated	WIRE	BOTH	Map ↔ repeated conversion.
oneof_membership_changed	SEMANTIC	BOTH	Field moved in/out of a real oneof.
oneof_field_added	SEMANTIC	BACKWARD	New alternative in a real oneof — old exhaustive switches break.
required_field_added	WIRE	FORWARD	New proto2 required field — old producers can't satisfy.
options_changed	POLICY	BOTH	Any serialized-options change.
presence_changed	SEMANTIC	BOTH	has_presence differs across schemas.
enum_value_removed	SEMANTIC	FORWARD	Enum value deleted — new consumer sees unknown number in old data.
enum_value_added	SEMANTIC	BACKWARD	Enum value added — old consumer sees unknown number in new data.
enum_number_reused	WIRE	BOTH	Enum number now binds a different name.
reserved_field_reused	WIRE / SEMANTIC	BOTH	Reserved number reused → WIRE; reserved name reused → SEMANTIC.

Note: Directions indicate which reader is at risk, not which side of the schema changed. BACKWARD = old consumer fails on new data (breaks forward compatibility); FORWARD = new consumer fails on old data (breaks backward compatibility). This keeps profile names aligned with what they filter: CONSUMER_SAFE = BACKWARD + BOTH protects old consumers; PRODUCER_SAFE = FORWARD + BOTH protects against old producers.

Custom Rules (Plugins)

Plugins inspect descriptors and call ctx.emit(...) to record findings. Register on a SchemaChecker:

from protokit.schema import (
    CompatibilityLevel,
    FieldRuleContext,
    SchemaChecker,
    Severity,
)

def no_newly_deprecated_fields(ctx: FieldRuleContext) -> None:
    """Flag fields that gained a `deprecated = true` option."""
    if ctx.old_field is None or ctx.new_field is None:
        return
    old_dep = ctx.old_field.GetOptions().deprecated
    new_dep = ctx.new_field.GetOptions().deprecated
    if not old_dep and new_dep:
        ctx.emit(
            severity=Severity.POLICY,
            message="field newly marked deprecated",
        )

checker = SchemaChecker(level=CompatibilityLevel.STRICT)
checker.register_field_rule("no_newly_deprecated", no_newly_deprecated_fields)
report = checker.check(old_pool, "acme.User", new_pool, "acme.User")

Message-level plugins fire once per visited message:

from protokit.schema import MessageRuleContext

def require_docs(ctx: MessageRuleContext) -> None:
    # Example: enforce that new messages carry docstring comments.
    ...

checker.register_message_rule("require_docs", require_docs)

Plugin exceptions (and misuse like returning an awaitable) are caught — the engine records a Warning entry in report.warnings and continues with subsequent plugins. No single bad plugin can take down a compatibility check. When any report.warnings are present, protokit compat exits with code 2 so CI never silently passes a broken custom policy.

Rule Packs

A rule pack is any Python module exposing a RULES list of (rule_id, plugin_fn) pairs:

# myorg/proto_rules.py
RULES = [
    ("no_newly_deprecated", no_newly_deprecated_fields),
    ("require_docs", require_docs_on_messages),
]

Load via CLI:

protokit compat old.descriptor_set new.descriptor_set \
  --type acme.User \
  --compat-rule-pack myorg.proto_rules

Or programmatically:

import myorg.proto_rules
checker.load_rule_pack(myorg.proto_rules)

Note: Rule packs are ordinary Python modules. load_rule_pack runs importlib.import_module(...), which executes the module's top-level code. Only load rule packs from sources you trust — the same bar you'd apply to pip install.

Composing a CompatibilityPolicy

Bundle a profile with custom rules and ignore paths for reuse across type pairs:

from protokit.schema import CompatibilityPolicy, CompatibilityLevel

policy = CompatibilityPolicy(
    base=CompatibilityLevel.CONSUMER_SAFE,
    custom_rules=(("no_newly_deprecated", no_newly_deprecated_fields),),
    ignore_paths=("internal_debug",),
)

report = policy.check(old_pool, "acme.User", new_pool, "acme.User")

CLI Options (protokit compat)

Flag	Description
positional OLD_INPUT NEW_INPUT	Two descriptor sets, or two .proto files with --proto.
--type NAME	Fully-qualified type name (same on both sides).
--old-type NAME	Old-side type name (cross-type mode).
--new-type NAME	New-side type name (cross-type mode).
--proto	Treat OLD_INPUT / NEW_INPUT as .proto source. Requires protoc.
-I, --proto-path DIR	Import path for protoc (repeatable, with --proto).
--level LEVEL	wire | consumer-safe (default) | producer-safe | strict.
--format NAME	Output format (default: human). Built-in for compat: human, json, junit, sarif. See Output Formatters.
--formatter-module MODULE	Python module exposing a FORMATTERS = [(name, fn, kind), ...] list (repeatable). See Output Formatters.
--compat-rule-pack MODULE	Dotted module name exposing a RULES list. Repeatable. Renamed in 0.8.0 (D7); the old name --rule-pack is accepted as a deprecation alias and will be removed in protokit 1.0.
--ignore PATH	Suppress findings at this dotted path prefix. Repeatable.
--dedupe-by-type	Emit findings for each shared nested type only once (original behavior). Default is path-complete: findings appear at every path where the type is referenced.
--quiet	Suppress output; return exit code only. Mutually exclusive with any non-human --format.

Stability. --compat-rule-pack is part of protokit compat's 0.8.0+ public CLI surface; the legacy --rule-pack alias is accepted as a deprecation path and removed in protokit 1.0. The Python API entry point (SchemaChecker.load_rule_pack) is unchanged. See CHANGELOG D7 for the migration path.

Git-integrated subcommands

protokit compat also exposes three git-aware subcommands for Phase 2 workflows:

• protokit compat check --since REF --proto-file PATH --type X — compare HEAD against a prior ref.
• protokit compat check --against-base [BRANCH] --proto-file PATH — compare HEAD against the merge-base with BRANCH (auto-resolves @{upstream} → origin/main → origin/master when the argument is omitted).
• protokit compat history --range OLD..NEW --proto-file PATH --type X — walk the commits in the range that affect the proto's compatibility and emit per-pair findings.
• protokit compat bisect --old REF --new REF --proto-file PATH --type X — find the earliest commit in the range that broke compatibility.
• protokit compat ci [--base BRANCH] --proto-file PATH --type X — CI gate, same semantics as check --against-base with a distinct name for pipeline yaml.

All five support --format json (bisect's shape carries resolved old / new SHAs, commits_walked, and aggregated per-commit diagnostics), and history / bisect / ci accept the same --compat-rule-pack / --ignore / --dedupe-by-type options as check. bisect additionally accepts --keep-going, which walks every commit in the range even after the first break — one CI run surfaces everything rather than forcing multiple "fix-rerun" cycles.

history / bisect enumeration accuracy (--fast tradeoff)

history and bisect walk a range of commits and determine which of them affected the root proto's compatibility. Because proto compatibility depends on the transitive import graph (not just the root file), the enumeration has to look beyond commits that touched the root itself.

Two modes:

Default (exact, 10/10 correctness). Walks every commit in the range that touched any .proto file. For each candidate commit, parses the root's dep graph at that ref (no compilation — just import-statement scanning) and keeps the commit only if its changed files intersect the dep graph. Catches every real break, including those introduced via dependencies that existed only at intermediate refs. This mode is the default because a bisect that silently misses a break is worse than a bisect that took a few extra seconds — the hardest bug to fix is the one the tool doesn't show you.

--fast (E+, ~9/10 correctness, ~3x faster on monorepos). Unions the dep graph at the range's OLD and NEW endpoints and issues one git log --follow -- PATH per file in the union, merging results. Preserves rename tracking per-path. Misses commits that modified a dependency which was live only mid-range — e.g. if the root swapped its import from date.proto to calendar.proto between OLD and NEW, a commit that broke date.proto while it was still a dep won't appear. This failure mode is rare in practice (dep swaps aren't a hot path in most proto repos) but real. Use --fast for tight interactive loops; stay on the default for CI gates.

In both modes, commits that touched .proto files outside the root's dep tree are always excluded — unrelated schema churn never inflates a bisect range.

Known limitation — rename without importer update. If a commit renames a dependency (e.g. date.proto → calendar.proto) without updating the root proto's import statement in the same commit, the rename commit can be invisible to the walk even in exact mode. The root's dep graph at the rename commit can no longer resolve the old dependency (it's gone) and doesn't yet know about the new one (import line still says the old name), so the filter has nothing to intersect with. This is a rare pattern in practice — most teams rename a file and its importers in the same commit — but worth flagging. The workaround is to rerun the walk against a ref where the importer has been updated.

Schema Linting

Positioning: protokit targets buf BASIC coverage; defaults reflect Python-protobuf-developer ergonomics, not buf's defaults (see proto2-strict for opt-in proto2 strictness).

protokit lint runs descriptor-level lint rules against one or more .proto files (or pre-built FileDescriptorSet binaries). As of protokit 0.6.0, protokit lint covers 26 of 26 buf v1.69.0 BASIC rules. The 26th rule, package/no-import-cycle, uses a Tarjan SCC pre-walk accumulator to detect package-level cycles where individual file imports are acyclic (file-level cycles are caught at the protobuf COMPILE phase by both buf and protokit's compiler). The proto2-only buf BASIC rule FIELD_NOT_REQUIRED ships in the opt-in proto2-strict profile as of 0.6.0 — outside the 26-rule baseline (which is proto-syntax-agnostic) but available to proto2 shops via --profile proto2-strict or pyproject profile = ["default", "proto2-strict"]. The built-in packs span single-language style + cross-language namespace consistency + cross-file directory/package layout + AIP-203 well-formedness: naming (AIP-122 + PascalCase/snake_case/UPPER_SNAKE conventions for messages, enums, services, RPCs, oneofs, files, and packages), enum (no-allow-alias, first-value-zero), imports (no-public, no-weak, unused), package (defined, directory-match, same-directory, directory-same-package, no-import-cycle), file (syntax-specified), package_same (go-package, java-package, csharp-namespace, php-namespace, ruby-package, swift-prefix, java-multiple-files), (new in 0.5.0) options/field-behavior-consistent (AIP-203 well-formedness), and (new in 0.6.0) package/no-import-cycle (Tarjan SCC pre-walk). 27 rules across 6 packs in the recommended profile, 33 rules in default (adds the deprecated-replacement 5-rule family + options/field-behavior-consistent). Plus, as of 0.5.0, users may declare custom/<user-suffix> synthetic rules in pyproject.toml to enforce option-aware annotation requirements without writing Python (see Custom annotation rules below). Lint is intentionally orthogonal to protokit compat — compat answers "is this schema change safe for consumers?", lint answers "does this schema follow our style conventions?".

Quick Start

A typical pyproject.toml configuration:

[tool.protokit.lint]
profile = "default"
exclude = ["third_party/**", "vendor/**"]
min_severity = "warning"

A typical invocation:

# Lint every .proto file in the project (walks pyproject.toml from CWD)
protokit lint protos/**/*.proto

# Lint a pre-built descriptor set
protokit lint schema.descriptor_set

# Override the pyproject min_severity for one run
protokit lint --min-severity error protos/**/*.proto

# Run without any pyproject configuration (use built-in defaults only)
protokit lint --no-config protos/**/*.proto

# Show the running version + the pinned buf parity reference
protokit lint --version

Profiles

protokit lint exposes three protokit-native profile names plus two buf-compatibility aliases. Aliases resolve at the config-load input boundary, so user rule packs declaring an alias name (e.g., profiles=("basic",)) will never match — the alias resolves to its target before rule-pack profile-name lookup.

Profile	Rules	Purpose
essentials	0 (forward-placeholder)	Light-touch tier reserved for a future curation pass; no rules ship in this profile as of 0.6.0.
recommended	27	Buf BASIC parity (26 of 26 buf v1.69.0 BASIC rules, complete as of 0.6.0). naming (9), enum (2), imports (3), package (5; includes package/no-import-cycle via Tarjan SCC pre-walk), file (1; file/syntax-specified demoted to WARNING in 0.6.0 — pragmatic-not-dogmatic about proto2), package_same (7).
default	33	Buf BASIC parity (recommended's 27 rules) + the deprecated-replacement family (5 error-severity option-aware rules in options/deprecated_replacement — promoted from WARNING in 0.7.0; demotable via [severities] / disabled_rules / --disable-rule) + AIP-203 well-formedness (1 warning-severity rule in options/field_behavior: options/field-behavior-consistent).
proto2-strict (0.6.0+)	1	Opt-in proto2-specific strictness. Currently ships field/not-required (the proto2-only buf:FIELD_NOT_REQUIRED rule at ERROR severity). Activate via --profile proto2-strict or pyproject profile = ["default", "proto2-strict"]. Proto2-specific anti-pattern rules ship here rather than in recommended/default so proto2 shops opt in explicitly.
minimal (alias)	→ essentials	Buf-compatibility alias resolved at _coerce_profile.
basic (alias)	→ recommended	Buf-compatibility alias resolved at _coerce_profile.

The buf-parity rule library ships at the error severity floor (matching buf's BASIC severity posture), with one deliberate divergence: file/syntax-specified is demoted to warning in recommended + default as of 0.6.0 under the pragmatic-not-dogmatic UX philosophy — proto3-only shops who relied on the prior ERROR enforcement can re-promote via [tool.protokit.lint.severities] "file/syntax-specified" = "error". The deprecated-replacement family in default originally shipped at warning to bound the leading-comment-regex heuristic's blast radius; the 0.7.0 release flips it to error after empirical validation confirmed a 0.0% noisy hit-rate on a googleapis sample (see the 0.7.0 entry in the CHANGELOG). To soften the floor without dropping rules: use --min-severity=warning globally, or [tool.protokit.lint.severities] per-rule (see below). To suppress one or more rules entirely, see the new Disabling and re-enabling rules section.

Disabling and re-enabling rules

As of 0.7.0, protokit lint exposes a full per-rule disable / enable surface across three interfaces — pyproject, CLI, and programmatic from_dict. Five mechanisms total; all unified at the config-resolution layer so the engine hot path sees only an effective rule set.

Disable mechanisms:

Mechanism	Where	Example
"off" severity (sentinel)	[tool.protokit.lint.severities]	"naming/snake-case-fields" = "off"
disabled_rules list	[tool.protokit.lint]	disabled_rules = ["naming/snake-case-fields"]
--disable-rule flag	CLI (repeatable; env-var PROTOKIT_DISABLE_RULE)	--disable-rule naming/snake-case-fields

Enable mechanisms:

Mechanism	Where	Example
enabled_rules list	[tool.protokit.lint]	enabled_rules = ["package/no-import-cycle"]
--enable-rule flag	CLI (repeatable; env-var PROTOKIT_ENABLE_RULE)	--enable-rule package/no-import-cycle

Composition precedence (polarity-first / tier-second):

1. Any disable at any tier wins over any enable (polarity-first). --enable-rule R does NOT override pyproject disabled_rules ⊇ R — a LintRuntimeWarning(category="contradictory_disable_config") fires on the contradiction.
2. Within the same polarity, CLI overrides pyproject (tier-second). --disable-rule R wins over pyproject enabled_rules ⊇ R.

Custom-rule prefix expansion: for user-declared [[custom_annotation_rules]] entries, the bare form disabled_rules = ["custom/<suffix>"] suppresses every kind of <suffix> (multi-kind expansion at config-resolution). Per-kind disable still works via the explicit mangled form: disabled_rules = ["custom/<suffix>__method"].

Escape hatch: --no-config bypasses the entire pyproject table (profile, exclude, severities, custom_annotation_rules, AND disabled_rules / enabled_rules). Users who want to override ONE disabled rule without losing the rest of their pyproject config MUST edit the pyproject directly. The contradictory_disable_config warning text names --no-config as the blunt-instrument escape hatch with this caveat.

Severity filtering interaction: --min-severity is a display filter, NOT a disable mechanism. A rule at --min-severity warning still LOADS and runs, but its INFO findings are dropped post-engine.run. Use one of the disable mechanisms above to skip loading the rule entirely.

Unknown rule_ids: entries in disabled_rules / enabled_rules that don't match any loaded rule_id fire one LintRuntimeWarning(category="unknown_rule_id") per id (lenient-with-warning; the rest of the config still applies). Carries the normalized rule_id so case-sensitivity / typo issues are visible.

Upgrade notes (0.4.x → 0.5.0)

0.5.0 ships option-aware pack expansion as the strategic- differentiator headline: users now declare option-aware annotation requirements via [[tool.protokit.lint.custom_annotation_rules]] in pyproject.toml without writing Python (synthetic custom/<user-suffix> rules). 0.5.0 also adds the first AIP-203 well-formedness validator (options/field-behavior-consistent) to the default profile.

Migration impact:

• recommended users — zero new findings on upgrade.
• default users without (google.api.field_behavior) — zero new findings on upgrade.
• default users consuming (google.api.field_behavior) — may see new warning-severity findings on duplicate values, the FIELD_BEHAVIOR_UNSPECIFIED zero value, or 5 curated contradictory pairs. Demote to info via [severities] or fix the schema per AIP-203 guidance.

The buf BASIC parity numerator at 0.5.0 ship time was 25 of 26 + 1 scheduled (the +1 scheduled rule was FIELD_NOT_REQUIRED, originally scoped for 0.5.0 but deferred to a later release). 0.6.0 closes both: FIELD_NOT_REQUIRED lands in the opt-in proto2-strict profile, and PACKAGE_NO_IMPORT_CYCLE (the 26th) lands in recommended + default — see the 0.5.x → 0.6.0 upgrade notes below.

See the 0.5.0 entry in CHANGELOG.md for:

• Full additions enumeration (custom/<suffix> synthetic rule infrastructure + options/field-behavior-consistent 3-arm dict-template rule + dynamic-pool extension-access helper + worked-example integration fixture).
• Wire-format changes (schema_version 0.3 → 0.5 via two closed-Literal LintRuntimeWarning.category additions: custom_annotation_extension_unresolved + extension_unresolved).
• Pre-upgrade migration recipe (2 numbered demotion paths; schema-fix preferred).
• Worked-example walkthrough (synthetic custom/<suffix>).
• Consumer migration (Python API audit for LintRuntimeWarning. category switch tables; _extension_access + _custom_rules
  • CustomAnnotationRuleSpec INTERNAL classifications).

Upgrade notes (0.5.x → 0.6.0)

0.6.0 closes the buf-parity arc: protokit lint now covers 26 of 26 buf v1.69.0 BASIC rules + ships the proto2-strict opt-in profile + revises the UX philosophy.

New rules:

• package/no-import-cycle (the 26th buf BASIC rule). ERROR severity in recommended + default profiles. Detects package-level import cycles where individual file imports are acyclic (file-level cycles are caught at the protobuf COMPILE phase). Emits one finding per cycle-closing import statement at the import's line/column.
• field/not-required (the proto2-only buf:FIELD_NOT_REQUIRED rule). ERROR severity in the new opt-in proto2-strict profile only. Activate via --profile proto2-strict or pyproject profile = ["default", "proto2-strict"].

Behavior changes:

• file/syntax-specified demoted from ERROR to WARNING in recommended + default profiles under the pragmatic-not-dogmatic-about-proto2 UX philosophy. Re-promote via [tool.protokit.lint.severities] "file/syntax-specified" = "error" if your project is proto3-only.

Migration impact by --max-warnings posture:

Posture	Pre-0.6.0	Post-0.6.0
--max-warnings unset	proto2 file: exit 1 (ERROR)	proto2 file: exit 0 (WARNING; not counted) — silent CI-pass regression risk
--max-warnings 0	proto2 file: exit 1	proto2 file: exit 1 (counted as warning instead of error)
--min-severity error	proto2 file: exit 1 (ERROR passes severity floor)	proto2 file: exit 0 (WARNING filtered by severity floor)

Pre-upgrade migration recipe (full text in the 0.6.0 entry of CHANGELOG.md):

• Want explicit ERROR enforcement of file/syntax-specified? [tool.protokit.lint.severities] "file/syntax-specified" = "error"
• Want proto2-strict checks? [tool.protokit.lint] profile = ["default", "proto2-strict"]
• Have package-level import cycles you're not ready to fix? [tool.protokit.lint.severities] "package/no-import-cycle" = "warning"
• Want to demote field/not-required after opting in? [tool.protokit.lint.severities] "field/not-required" = "warning"
• Pin to 0.5.0 indefinitely? pip install protokit==0.5.0

Upgrade notes (0.6.x → 0.7.0)

0.7.0 ships two paired changes: a deprecated-replacement promotion that flips all 5 rules in options/deprecated_replacement from WARNING to ERROR in the default profile only, and a per-rule disable surface (see the new Disabling and re-enabling rules section above). The per-rule disable surface shipped first as the safety net so the migration recipe is real on day one.

Behavior change — deprecated-replacement promotion:

All 5 rules in options/deprecated_replacement now fire at error severity in the default profile. Deprecated elements MUST carry a replacement reference in their leading comment OR be explicitly suppressed via one of the disable mechanisms above. The heuristic regex is UNCHANGED — only the severity flips. recommended is unaffected (the deprecated-replacement family has no buf BASIC analogue and ships default-only).

Migration impact by --max-warnings posture:

Posture	Pre-0.7.0	Post-0.7.0
--max-warnings unset	finding: exit 0 (WARNING; not counted)	finding: exit 1 (ERROR; has_error short-circuits) — silent CI-pass regression risk
--max-warnings 0	finding: exit 1 (counted as warning)	finding: exit 1 (ERROR; has_error short-circuits before max_warnings gate)
--min-severity error	finding: exit 0 (WARNING filtered by floor)	finding: exit 1 (ERROR passes floor)

The posture-1 row is the dominant concern: projects that previously ignored deprecated-replacement WARNINGs will see CI flip from green to red on upgrade.

Empirical validation (hard gate before promotion): 200 random .proto files from googleapis (random.seed(42)) returned 19 deprecated-replacement findings; manual classification per a documented noisy-vs-load-bearing rubric returned 0 noisy hits (0.0%). Gate threshold was >10% OR >5 absolute noisy hits → STOP. Result: gate passed with substantial margin. Full audit trail in the 0.7.0 CHANGELOG entry.

Pre-upgrade migration recipe (full text in the 0.7.0 entry of CHANGELOG.md):

1. Fix the schema (recommended). Add a replacement reference to the leading comment of every deprecated element.
2. Demote one rule back to WARNING: [tool.protokit.lint.severities] "options/deprecated-field-must-have-replacement-comment" = "warning"
3. Disable one rule via "off" (new in 0.7.0): [tool.protokit.lint.severities] "options/deprecated-field-must-have-replacement-comment" = "off"
4. Disable the whole deprecated-replacement family via disabled_rules (new in 0.7.0): [tool.protokit.lint] disabled_rules = [...] with the 5 rule_ids. See the CHANGELOG for the full 5-rule family-list form.
5. Pin to 0.6.0 indefinitely: pip install protokit==0.6.0.

Wire-format change: _LINT_JSON_SCHEMA_VERSION bumps "0.5" → "0.6" for the two new LintRuntimeWarning.category Literal values ("contradictory_disable_config" + "unknown_rule_id"). Consumers parsing the schema against "0.5" MUST update. The pyproject [project] version bumps 0.6.0 → 0.7.0 independently per the version-bump communication contract.

Custom annotation rules

Declare option-aware annotation requirements in pyproject.toml via the [[tool.protokit.lint.custom_annotation_rules]] array-of- tables. Each entry materializes a synthetic custom/<rule_suffix> rule that participates in profile composition + [severities] overlay exactly like a built-in rule.

[[tool.protokit.lint.custom_annotation_rules]]
rule_suffix    = "audit-required"
option         = "example.audit_level"
element_kinds  = ["method"]
allowed_values = ["LOW", "HIGH", "CRITICAL"]
severity       = "error"

Fields:

• rule_suffix (required) — kebab-case identifier matching [a-z][a-z0-9]*(-[a-z0-9]+)*. The synthetic rule_id is custom/<rule_suffix>. Must NOT collide with another entry or with a built-in custom/* rule_id (none ship today; the prefix is reserved for user declarations).
• option (required) — fully-qualified extension name in descriptor-pool form (bare; e.g., example.audit_level). NOT the parenthesized proto-source syntax ((example.audit_level)); pool.FindExtensionByName accepts only the bare form, and passing the parenthesized form silently emits one LintRuntimeWarning(category="custom_annotation_extension_unresolved") per file instead of firing the rule. Duplicate rule_suffix across entries is rejected at config-load with exit code 2 (error[lint-pyproject-config-invalid]:).
• element_kinds (required) — non-empty subset of ElementKind values: "field", "method", "message", "enum", "enum_value", "service", "file", "oneof".
• allowed_values (optional) — homogeneous scalar list (all strings OR all ints OR all bools). When present, the rule fires both on presence absence AND on values outside the set. Floats and mixed-type lists are rejected at config-load.
• severity (optional) — "error" / "warning" / "info"; defaults to "warning". As of 0.7.0, "off" is also accepted at [tool.protokit.lint.severities] and unloads the rule entirely; equivalent to disabled_rules = ["custom/<rule_suffix>"]. See the Disabling and re-enabling rules section for multi-kind prefix-expansion semantics.

Behavior:

• The rule fires when the option is absent OR (when allowed_values is set) when its value is outside the set.
• Each finding's violation_kind is one of "custom-annotation-absent" (option not present) or "custom-annotation-value-mismatch" (value not in allowed_values). params carries "option" + "rule_id" on every finding, plus "actual_value" on the value-mismatch arm (string-coerced enum identifier or raw scalar).
• When pool.FindExtensionByName raises KeyError (the extension is not registered in any input proto), the rule emits one structured LintRuntimeWarning(category="custom_annotation_extension_unresolved") per (rule_id, file) pair and skips firing.

A CI-runnable worked example lives at tests/schema/lint/cli/test_d6d_custom_annotation_example.py (with fixtures under tests/schema/lint/cli/cli_fixtures/d6d_custom_annotation/).

Upgrade notes (0.3.x → 0.4.0)

0.4.0 adds the first cross-file lint dispatch infrastructure (a pre-walk package-options accumulator) and two rules to consume it: package/same-directory and package/directory-same-package. Combined with the audit-trail correction of the inherited "buf BASIC = 18 rules" claim (actual: 26 rules), protokit lint now covers 25 of 26 buf BASIC rules. Teams with cross-directory package scattering or mixed-package directories will see new error-severity findings on first 0.4.0 invocation.

See the 0.4.0 entry in CHANGELOG.md for:

• Full additions enumeration (the two new rules + the pre-walk accumulator + 9-fixture parity gate + three-arm assert_parity_multi_file).
• Audit-trail correction (17 of 18 → 25 of 26; the 0.3.0 CHANGELOG retains its original numerator framing as audit trail).
• Behavior changes (the two new rules firing default-on as error severity; wire format unchanged at schema_version: "0.3").
• Pre-upgrade migration recipe with 5 numbered TOML demotion paths (path 5 covers Python API consumers via LintProfile.rule_severity_overrides).
• Upgrade-notes triage recipe (5-step adoption walkthrough, including co-fire-resolution-order guidance).
• Consumer migration (FileLintContext.directory_packages + directory_packages_by_dir + LintEngine._build_directory_package_accumulator INTERNAL classifications).

Upgrade notes (0.2.x → 0.3.0)

0.3.0 adds the first option-aware rules (deprecated-replacement family) + cross-language buf-BASIC parity (PACKAGE_SAME_* family). Multi-language teams will see new error-severity findings on cross-file option disagreement.

See the 0.3.0 entry in CHANGELOG.md for:

• Full additions enumeration (the two new rule families + a runtime-warning addition + parity gate + multi-file harness).
• Wire-format changes (schema_version 0.2 → 0.3).
• Behavior changes (PACKAGE_SAME_* firing default-on as error severity).
• Pre-upgrade migration recipe with 4 numbered TOML demotion paths + worst-case adoption math (up to 140 findings on a 20-file no-package legacy corpus) + 3 accepted-tradeoff scenarios (""-package aggregation, transitive-import supply chain, WKT enforcement).
• Upgrade-notes triage recipe (5-step adoption walkthrough).
• Consumer migration (Python API audit for LintRuntimeWarning. category switch tables; CompileResult.source_info_descriptors INTERNAL classification).

Upgrade notes (0.1.x → 0.2.0)

Upgrading from protokit 0.1.x to 0.2.0 expands BUILTIN_PACKS from 1 pack (naming, 9 rules) to 5 packs (17 rules total). Existing users will see new ERROR-severity findings on previously-green CI.

Triage path:

1. Upgrade protokit (pip install -U protokit or equivalent).

2. Enumerate the new findings:

   protokit lint --format=json <inputs> | jq '.findings[] | {rule_id, severity, location}'
   

3. Decide per finding: fix the schema, or demote the rule (next section). Per-rule demotion in pyproject.toml is the lowest-cost option for category-wide noise (e.g., imports/unused on vendored protos — pair with exclude for the vendored paths themselves).

4. For an emergency revert, pin protokit~=0.1.0 (which means >=0.1.0, <0.2.0) and file an issue describing any false-positives. Pre-1.0 is the right time to surface rule heuristic gaps.

Demotion paths

The 0.2.0 release ships rules at error severity (buf BASIC parity). Four demotion paths are available, in increasing specificity:

1. Pin to 0.1.x (protokit~=0.1.0) — defers the upgrade entirely.
2. Full opt-out — --no-builtin-rules (CLI) or [tool.protokit.lint] no_builtin_rules = true (pyproject) skips BUILTIN_PACKS entirely. Pair with --rule-pack MODULE to provide a custom rule set; an empty rule set exits 2 via the no-rules error code.
3. Global severity floor — --min-severity=warning or [tool.protokit.lint] min_severity = "warning" raises the floor across every rule. Cheapest if you want to keep visibility without blocking CI on the new categories.
4. Per-rule severity overrides — [tool.protokit.lint.severities] "imports/unused" = "warning" demotes one rule without touching the rest. Multiple keys compose; user overrides always win on collision with profile defaults. Unknown rule_ids fire a severities_unloaded_rule runtime warning naming each id (typo surfacing without blocking).

[tool.protokit.lint]
profile = "recommended"

[tool.protokit.lint.severities]
"imports/unused" = "warning"
"file/syntax-specified" = "info"

[tool.protokit.lint] configuration

protokit lint discovers pyproject.toml by walking up from the current working directory until it reaches the first .git directory or file (worktree-safe — both .git/ directories and .git pointer files terminate the walk-up). The first pyproject.toml encountered is used; if it lacks a [tool.protokit.lint] table, built-in defaults apply silently.

Recognized keys (every key is optional):

Key	Type	Description
profile	string or list of strings	Profile name(s) to compose. Single profile is the common case; multi-profile composition lifts the strictest floor and union-merges rule_ids. Buf aliases (minimal → essentials, basic → recommended) resolve at the input boundary.
exclude	list of strings	Gitignore-style globs matched against FileDescriptorProto.name. Patterns are additive with CLI --exclude.
min_severity	string ("info", "warning", "error")	Minimum severity to emit. Relaxing the composed profile floor fires a min_severity_relaxed runtime warning.
max_warnings	integer	Non-error exit threshold for warning-level findings.
format	string	Default output formatter ("human", "json", "junit", "sarif", or a --formatter-module name).
no_builtin_rules	boolean	When true, skip loading BUILTIN_PACKS (the auto-loaded naming / enum / imports / package / file packs). User packs supplied via --rule-pack MODULE become load-bearing; an empty rule set exits 2 via the no-rules error code.
disabled_rules (0.7.0+)	list of strings	Per-rule disable directives. Accepts canonical pack/rule-suffix, bare custom/<suffix>, or mangled custom/<suffix>__<kind> forms. Bare custom suffixes prefix-expand to every kind of the matching rule. See Disabling and re-enabling rules.
enabled_rules (0.7.0+)	list of strings	Per-rule enable directives. Same accepted formats as disabled_rules. Disable wins across all tiers (polarity-first precedence); a contradictory disable+enable fires a contradictory_disable_config runtime warning.
[tool.protokit.lint.severities]	table (rule_id → severity string)	Per-rule severity overrides applied AFTER profile composition. Accepted values: "error", "warning", "info", and (0.7.0+) "off" (unloads the rule, equivalent to disabled_rules). User overrides always win on collision via post-compose dict-spread. Unknown rule_ids fire a severities_unloaded_rule runtime warning (typo surfacing without blocking the run).

Unknown keys and type mismatches produce a hard error (exit 2) that names the recognized keys and offending field. List-valued keys also reject heterogeneous arrays — exclude = ["a", 1, "b"] fails at the element-type check, not silently coerced.

CLI flags

In addition to the pyproject keys, the CLI carries:

Flag	Purpose
--config PATH	Use a pinned config file; bypasses CWD walk-up. Strict mode: missing/unreadable/table-absent/invalid-TOML all exit 2.
--no-config	Skip the [tool.protokit.lint] table entirely; built-in defaults apply. Mutually exclusive with --config.
--exclude PATTERN	Append a gitignore-style glob to the resolved exclude list (repeatable).
--no-exclude	Override every pyproject + CLI exclude pattern; lint every input file. Wins at apply-time over --exclude.
--profile NAME	Override the pyproject profile key for one run.
--min-severity LEVEL	Override the pyproject min_severity key for one run.
--max-warnings N	Override the pyproject max_warnings key for one run.
--format NAME	Override the pyproject format key for one run. Also reads PROTOKIT_FORMAT envvar.
--rule-pack MODULE	Load a user rule pack on top of the built-ins (repeatable).
--no-builtin-rules	Skip BUILTIN_PACKS for this run. Pair with --rule-pack MODULE to supply a custom rule set; empty rule sets exit 2 via the no-rules error code. Mirrors [tool.protokit.lint] no_builtin_rules = true.
--disable-rule RULE_ID (0.7.0+)	Per-rule disable directive (repeatable; env-var PROTOKIT_DISABLE_RULE uses space-separated values per Click multiple=True semantics — PROTOKIT_DISABLE_RULE="naming/snake-case-fields imports/unused" — comma-separation is NOT supported). Wins over pyproject enabled_rules within polarity (CLI > pyproject); always wins over any enable (polarity-first). Bad values exit 2 via lint-cli-option-invalid.
--enable-rule RULE_ID (0.7.0+)	Per-rule enable directive (repeatable; env-var PROTOKIT_ENABLE_RULE uses space-separated values; comma-separation is NOT supported). Same precedence rules apply — --enable-rule R does NOT override pyproject disabled_rules ⊇ R; a contradictory_disable_config warning fires on the contradiction. Use --no-config to bypass the entire pyproject (with the caveat that this drops every other pyproject key too).
--version	Print protokit <version> (parity: buf <pin>) and exit. The pinned buf version is _BUF_PARITY_PIN in src/protokit/schema/lint/cli.py; the parity CI job uses the same pin.
--proto	Treat inputs as .proto source files instead of pre-built descriptor sets; invokes the in-process compile path.
--proto-path DIR / -I DIR	Add an include directory to the --proto compile path (repeatable).
--statistics / --no-statistics	Show / suppress the trailing statistics line (filtered count, runtime warnings).
--quiet	Suppress findings on stdout; structured stderr warnings remain visible. Mutually exclusive with non-human --format.

CLI flags replace the pyproject value for their key, except --exclude, which appends. --no-exclude clears the resolved exclude list (CLI + pyproject) entirely.

JSON output shape (--format=json)

The --format=json output is a stable wire format for CI integrations and agents. Top-level keys:

Key	Type	Description
schema_version	string	Wire-format version (currently "0.6" as of 0.7.0; bumped from "0.5" for two new LintRuntimeWarning.category Literal values per the closed-Literal-discriminator bump policy). Bumps any time JSON/SARIF wire shapes change in a consumer-detectable way. Absence of the key (output from protokit < 0.2.0) is the implicit "0.1". The matching SARIF field is runs[].properties.lint_schema_version.
findings	list of objects	One per emitted finding. Per-finding keys: rule_id, severity ("error" / "warning" / "info"), location (rendered string), location_file, location_kind (lowercased LintLocation variant — "field", "message", "enum", etc.), violation_kind, message.
filtered_count	int	Findings dropped by --min-severity filtering. Mirrored in summary.filtered_count for convenience.
runtime_warnings	list of objects	One per LintRuntimeWarning. Per-warning keys: category ("rule_exception" / "unloaded_rule" / "severities_unloaded_rule" / "min_severity_relaxed" / "all_files_excluded" / "custom_annotation_extension_unresolved" / "extension_unresolved" / "contradictory_disable_config" (0.7.0+) / "unknown_rule_id" (0.7.0+)), rule_id (populated for rule-scoped categories — rule_exception, unloaded_rule, severities_unloaded_rule, custom_annotation_extension_unresolved, extension_unresolved, contradictory_disable_config, unknown_rule_id — and null for non-rule-scoped categories — min_severity_relaxed, all_files_excluded), message, exception_type (string or null), descriptor_path (string or null).
diagnostics	list of objects	Compile-time diagnostics surfaced by --proto mode (level, category, message). Empty for --input descriptor-set mode.
summary	object	Aggregate counts. Keys: errors, warnings, info, total, filtered_count, runtime_warning_count.

A non-JSON-serializable rule param (e.g., a pathlib.Path) renders via repr() rather than failing the entire emission — this guarantees one broken param value never suppresses every other finding.

Multi-profile attribution note

When profile = ["a", "b"] composes multiple profiles, the resolved profile floor reported in min_severity_relaxed messages is the composed floor — a single value after the composition step. The message does not name which contributing profile set the relaxed floor. If attribution matters, consult the composed-profile result via the public API rather than reading it out of the warning message text.

Security Considerations

protokit lint reads pyproject.toml files discovered via CWD walk-up. The walk-up terminates at the first .git directory or file encountered, which is the typical project-root boundary for any code-bearing repository.

Bypass channels. The following configuration keys can relax lint policy and therefore should be reviewed alongside any other policy-affecting change:

• exclude — drops files from the lint pool.
• min_severity — raises the emission floor (hides findings).
• max_warnings — raises the non-error threshold (turns failures into passes).
• profile — switches the active rule set; a less-strict profile exercises fewer rules.

Changes to these keys should go through the same code-review discipline as source-level changes; CI gates that enforce lint policy should be aware that [tool.protokit.lint] edits are policy-affecting.

Walk-up trust assumptions. The walk-up uses Path.exists() on the .git candidate (not Path.is_dir()), which covers standard checkouts AND git worktrees / submodules. The .git path is checked for existence only; its contents (the gitdir: ... pointer in worktree .git files) are NEVER read, parsed, or followed by protokit lint.

No-.git CI caveat. If the working tree is not a git checkout — e.g., a shallow-clone-replacement that strips .git, or a CI environment that materializes sources outside any repository — the walk-up runs to the filesystem root. In that configuration, an attacker who controls a parent directory of the CWD can plant a pyproject.toml containing [tool.protokit.lint] keys that relax the lint policy. For untrusted-parent-CWD environments, use --no-config (to disable pyproject reading entirely) or --config <pinned-path> (to read a specific, vetted config) instead of the default walk-up.

Pre-1.0 stability disclaimer

protokit is pre-1.0. Minor-version releases may include breaking changes to public Python APIs and machine output formats (JSON, JUnit, SARIF). Breaking changes are documented in the CHANGELOG — historically via BREAKING:-prefixed section headings (pre-0.2.0), and from 0.2.0 onward via plain delivery-named sections that describe the user-visible impact without a ceremonial prefix. The version bump itself is the authoritative signal; the CHANGELOG section is the communication contract. Consumers should pin to a specific minor version (e.g., protokit~=0.5.0) until 1.0 ships. The 1.0 release will define the stable public surface and commit to semver compatibility for that surface.

Public Surface (DRAFT — frozen at 1.0)

The candidate stable surface, listed here so consumers can anticipate what 1.0 will commit to. Each row is marked tentatively IN (under consideration for the stable surface) or INTERNAL (deliberately not under consideration; subject to change without notice). This appendix is maintained each delivery so 1.0 inherits a defined surface rather than discovering it via accumulation.

Surface	Element	Status
Python dataclass	LintReport (fields, ordering, frozen-ness)	IN
Python dataclass	LintRuntimeWarning (category: Literal["rule_exception", "unloaded_rule", "severities_unloaded_rule", "min_severity_relaxed", "all_files_excluded", "custom_annotation_extension_unresolved", "extension_unresolved", "contradictory_disable_config", "unknown_rule_id"] — CLOSED DISCRIMINATOR: consumer switch statements should be exhaustive; additions trigger a _LINT_JSON_SCHEMA_VERSION minor bump per the bump-contract at _builtin_lint.py:227-312. Last two values added in 0.7.0. Contrast with LintSeverity open ladder), rule_id: str | None, message, exception_type, descriptor_path	IN
Python module	BUILTIN_PACKS (auto-loaded rule packs; includes package_same as of 0.3.0 → 7 PACKAGE_SAME_* rules default-on under recommended + default profiles)	IN
Python function	leading_comment(source_info_descriptors, file_name, path) (free function in protokit.schema.lint.rules.options._comments; reads [replaced-by: <X>] and similar leading-comment annotations from the indexed source-info descriptors)	IN
Python class field	CompileResult.source_info_descriptors: Mapping[str, FileDescriptorProto] | None (the source-locations index built from FileDescriptorSet before pool.Add() discards source_code_info; consumed by leading-comment introspection)	INTERNAL
Python class field	FileLintContext.package_options: Mapping[str, Mapping[str, Mapping[str, str | None]]] | None (the pre-walk accumulator for cross-file PACKAGE_SAME_* option-consistency rules; outer key package_name, second-level key option_attr, inner map {file_name: value})	INTERNAL
Python method	LintEngine._build_package_options_accumulator (single-pass file-scan over compile_result.pool_file_names producing the per-package option-value view; threaded into FileLintContext.package_options)	INTERNAL
Python class field	FileLintContext.directory_packages: Mapping[str, Mapping[str, str]] | None (per-package view of the pre-walk accumulator; outer key package_name, inner map {file_name: dirname}; sibling-pattern reference to FileLintContext.package_options)	INTERNAL
Python class field	FileLintContext.directory_packages_by_dir: Mapping[str, Mapping[str, frozenset[str]]] | None (inverted per-directory view of the pre-walk accumulator; outer key dirname, inner map {package_name: frozenset(file_names)}; provides O(1) lookup for package/directory-same-package)	INTERNAL
Python method	LintEngine._build_directory_package_accumulator (single-pass file-scan over compile_result.root_files; dual-view return shape may extend pre-1.0)	INTERNAL
Python dataclass	LintFinding (rule_id, severity, location, violation_kind, params)	IN
Python dataclass	LintProfile (name, rule_ids, min_severity, rule_severity_overrides)	IN
Python dataclass	LintRuleSpec (rule_id, severity, profiles, source_spec, element, message_template, fn)	IN
Python class	LintEngine.run(compile_result, *, profile) signature	IN
Python helper	LintProfile.compose(*profiles), LintProfile.from_pack(module, profile_name)	IN
JSON wire	lint_json output shape (top-level keys + per-finding/per-warning shapes)	IN
JSON wire	lint_json["schema_version"]: "0.6" (top-level wire-format version; absence → implicit "0.1"; bumped from "0.5" in 0.7.0 for two new LintRuntimeWarning.category Literal values)	IN
SARIF wire	runs[].properties.runtime_warnings shape (level, message, properties.category, properties.subcategory; 0.7.0 adds properties.rule_id for contradictory_disable_config + unknown_rule_id categories only — pre-existing rule-scoped categories (rule_exception, unloaded_rule, severities_unloaded_rule, custom_annotation_extension_unresolved, extension_unresolved) do NOT carry rule_id in the SARIF propertyBag despite being rule-scoped; SARIF consumers needing complete rule_id attribution should use --format=json where rule_id is populated uniformly)	IN
SARIF wire	runs[].invocations[].toolExecutionNotifications (compile-stage diagnostics)	IN
SARIF wire	runs[].properties.lint_schema_version: "0.6" (parity with lint_json["schema_version"])	IN
SARIF wire	tool.driver.rules[].defaultConfiguration.level (added in 0.7.0; pre-flight rule severity for IDE consumers)	IN
JUnit wire	<system-out> dual line format (compile diagnostics, then runtime warnings)	IN
Profile names	essentials / recommended / default (protokit-native names; default extends recommended with the deprecated-replacement family (5 error-severity option-aware rules as of 0.7.0 — promoted from warning) + options/field-behavior-consistent)	IN
Profile aliases	minimal → essentials, basic → recommended (resolved at _coerce_profile input boundary)	IN
CLI flags	--config, --no-config, --exclude, --no-exclude, --profile, --min-severity, --max-warnings, --format, --rule-pack, --no-builtin-rules, --disable-rule (0.7.0+), --enable-rule (0.7.0+), --version	IN
Exit codes	0 (clean), 1 (findings exceeded threshold), 2 (configuration/setup error)	IN
Error codes (stderr error[lint-<code>]: prefix)	no-rules, unknown-profile, format-unavailable, compile-failed, formatter-exception, bad-input, pool-conflict, missing-imports, rule-collision, rule-pack-load, pyproject-config-load, pyproject-config-invalid, exclude-pattern-invalid, no-rules-after-disable (0.7.0+), cli-option-invalid (0.7.0+) (full set in _LINT_ERROR_CODES)	IN
Stderr formatter envelopes	protokit lint: warning [<category>]: <message> (human format)	IN
Internal module	protokit.schema.lint._config (loader + ResolvedLintConfig)	INTERNAL
Internal module	protokit.schema.lint._cli_utils	INTERNAL
Threshold constants	_LINT_HUMAN_SUMMARIZATION_THRESHOLD (per-category human-stderr summarization)	INTERNAL

The surface above is a working draft. Names and signatures may shift before 1.0; the version bump + CHANGELOG section for each delivery is the authoritative signal for any individual change. Historical BREAKING:-prefixed sections (pre-0.2.0) carry the same weight as plain delivery sections (0.2.0 onward).

Output Formatters

--format NAME selects how protokit diff and every protokit compat subcommand render their output. Built-in names cover the common CI-integration formats; the --formatter-module flag loads user-supplied packs for anything else.

Built-in formatters

Kind	Names	Notes
DIFF	human, json, junit	junit uses a binary-result single-testcase pattern (one assertion per comparison); per-difference detail goes in the failure body. SARIF intentionally omitted — message diffs don't fit SARIF's rule/result model.
COMPAT	human, json, junit, sarif	junit is per-finding; empty checks emit a synthetic passing testcase so CI doesn't read the suite as "no tests ran." sarif is a single SARIF 2.1.0 run with one result per finding; tool.driver.rules declares every fired rule_id.
COMPAT_HISTORY	human, json, junit, sarif	junit wraps per-commit suites under <testsuites>; each suite carries the commit subject as package and a sequential id. sarif aggregates results into one run with partialFingerprints.commit per result.
COMPAT_BISECT	human, json, junit, sarif	junit carries range_spec, old_sha, new_sha, and breaking_commit in a <properties> block. sarif exposes the same in run.properties.

15 built-in formatters in total. Built-in names are reserved — third-party packs cannot register under (kind, "human"), (kind, "json"), (COMPAT, "junit"), (COMPAT, "sarif"), etc. (See Trust model.)

JUnit example

protokit compat ci --base origin/main \
  --proto-file acme/user.proto --type acme.User \
  --format junit

Produces output that validates against the Apache Ant JUnit XML reference (the format Jenkins, GitLab CI, GitHub Actions test result actions, CircleCI, and TeamCity all consume):

<?xml version='1.0' encoding='utf-8'?>
<testsuite name="protokit-compat-acme.User"
           tests="1" failures="1" errors="0"
           timestamp="1970-01-01T00:00:00" hostname="localhost" time="0">
  <properties/>
  <testcase classname="field_removed" name="user.email" time="0">
    <failure type="SEMANTIC/BACKWARD"
             message="field present in old, absent in new">field present in old, absent in new</failure>
  </testcase>
  <system-out/>
  <system-err/>
</testsuite>

The vendored xsd lives at tests/fixtures/junit-xml/JUnit.xsd (Windy Road's Apache Ant reference, Apache 2.0 licensed).

SARIF example

protokit compat ci --base origin/main \
  --proto-file acme/user.proto --type acme.User \
  --format sarif > findings.sarif

Produces a SARIF 2.1.0 document consumable by GitHub Code Scanning, GitLab security dashboards, and any OASIS SARIF consumer:

{
  "version": "2.1.0",
  "$schema": "https://json.schemastore.org/sarif-2.1.0.json",
  "runs": [{
    "tool": {
      "driver": {
        "name": "protokit",
        "version": "0.1.0",
        "rules": [
          {"id": "field_removed", "name": "field_removed",
           "shortDescription": {"text": "Field present in old, absent in new."}}
        ]
      }
    },
    "results": [
      {
        "ruleId": "field_removed",
        "level": "error",
        "message": {"text": "field present in old, absent in new"},
        "locations": [{
          "logicalLocations": [{"fullyQualifiedName": "user.email"}],
          "physicalLocation": {"artifactLocation": {"uri": "acme/user.proto"}}
        }]
      }
    ],
    "invocations": [{"executionSuccessful": true}]
  }]
}

Severity mapping: WIRE and SEMANTIC findings map to SARIF "error"; POLICY findings map to "warning". The vendored schema lives at tests/fixtures/sarif/sarif-2.1.0.json (OASIS 2.1.0 via SchemaStore).

Custom formatters via --formatter-module

A formatter pack is any Python module exposing a FORMATTERS list of (name, fn, kind) tuples. The function signature is (report, FormatterContext) -> str:

# myorg/formatters.py
from protokit.formatters import FormatterContext, FormatterKind
from protokit.schema import CompatibilityReport


def slack_summary(report: CompatibilityReport, ctx: FormatterContext) -> str:
    # ctx.target_type is None on cross-type runs (--old-type X
    # --new-type Y); fall back to old->new so the suite still
    # identifies what's being checked.
    if ctx.target_type is not None:
        target = ctx.target_type
    elif ctx.old_target_type or ctx.new_target_type:
        target = f"{ctx.old_target_type}->{ctx.new_target_type}"
    else:
        target = "(unknown type)"
    verdict = "COMPATIBLE" if report.is_compatible else "INCOMPATIBLE"
    lines = [f"*protokit compat — {target}*", f"{len(report)} finding(s) · {verdict}"]
    for f in report:
        lines.append(f"• [{f.severity.value}] {f.path}: {f.message}")
    return "\n".join(lines)


FORMATTERS = [
    ("slack", slack_summary, FormatterKind.COMPAT),
]

Load it via the CLI:

protokit compat check old.descriptor_set new.descriptor_set \
  --type acme.User \
  --formatter-module myorg.formatters --format slack

Or programmatically:

from protokit.formatters import register_formatter, FormatterKind
register_formatter("slack", slack_summary, kind=FormatterKind.COMPAT)

A complete runnable example lives at examples/custom_formatter.py.

Trust model

--formatter-module follows the same trust model as the rule-pack flags (protokit lint --rule-pack and protokit compat --compat-rule-pack): protokit imports the named module and reads its FORMATTERS attribute. A formatter pack runs with your full process privileges. It can:

• Read environment variables (including GITHUB_TOKEN, AWS credentials, anything in the CI environment).
• Make network calls — exfiltrate data or fetch second-stage payloads.
• Read or modify any file the invoking user has access to, including .git/ and source code.
• Spawn subprocesses.

Treat formatter packs as pip install-grade trust. Only load packs from sources you already audit for pip install. Do not load a pack just because a GitHub Action config suggests it.

Within that trust model, three things protokit enforces:

1. Exit code stays the report's verdict. The CLI exit code (0 / 1 / 2) is determined by the compat report itself (compatibility verdict + diagnostic levels), not by formatter output. A buggy formatter can corrupt the rendered document but cannot flip CI gating. A formatter that calls sys.exit() is caught and routed through the contract-violation error path.
2. Built-in names are reserved. Third-party packs cannot register under (kind, "human"), (kind, "json"), (COMPAT, "junit"), (COMPAT, "sarif"), etc. Attempts to shadow a built-in fail with conflicts with a reserved built-in name at registration time, regardless of replace=True.
3. Best-effort stdout-write guard. The CLI redirects sys.stdout to an in-memory buffer for the duration of each formatter call and exits 2 if any bytes land there. This catches the common accidental footgun — a forgotten print() or sys.stdout.write() in a debug statement. Limitations: os.write(1, ...), C-extension stdio, sys.__stdout__.write, and a sys.stdout reference captured at module-import time all bypass the guard. It is a bug-catcher for honest formatters, not a sandbox against hostile ones.

Note: Pack import side-effects persist beyond the two-phase registry rollback. A pack module that mutates sys.path, pokes sys.modules, or calls register_formatter at import time leaves those mutations in place even if a later entry in its FORMATTERS list is malformed and the registry rolls back. protokit.formatters cannot undo arbitrary Python state. Pack authors should keep module-import-time code to a minimum and put all registrations in the FORMATTERS list; mixing the two is undefined behavior. See Trust model for the broader point: treat packs as pip install-grade trust.

Diagnostics from a custom formatter

Formatters are pure (report, ctx) -> str functions — the returned string is the entire output. If your formatter needs to emit progress notes, debug lines, or non-fatal warnings, use Python's standard logging module rather than print():

import logging

logger = logging.getLogger("protokit.formatters.my_pack")

def my_formatter(report, ctx):
    logger.info("rendering %d findings", len(report))
    # ... build output ...
    return output

Python's logging defaults to stderr when basicConfig is called, which keeps debug output off the stdout stream the CLI uses for structured output. It never interacts with the stdout-write guard. The protokit-namespaced logger root protokit.formatters is a convention — name your sub-logger whatever helps downstream filtering.

Note: register_formatter rejects re-registration of an existing non-built-in name unless replace=True is passed explicitly. This makes accidental name collisions loud rather than silent.

Supported Field Types

• Scalars (int32/64, uint32/64, sint32/64, fixed32/64, sfixed32/64, float, double, bool, string, bytes)
• Nested messages (arbitrary depth)
• Repeated fields (index-based or key-based via treat_as_map)
• Map fields (native protobuf maps)
• Oneof fields (including proto3 optional)
• Enum fields (same-pool and cross-pool with wire-compatibility)

Requirements

• Python 3.10+
• protobuf >= 4.21.0
• click >= 8.0

Acknowledgments

protokit lint tracks rule-set parity with buf lint, the lint subcommand of the buf CLI by Buf Technologies, Inc. — a comprehensive protobuf tooling suite covering lint, formatting, breaking-change detection, code generation, the Buf Schema Registry, and the Connect RPC framework. protokit is an independent project, not affiliated with or endorsed by Buf Technologies.

The functional overlap is intentionally narrow:

• protokit lint ↔ buf lint: closely tracked. protokit lint matches 26 of 26 buf v1.69.0 BASIC rules, with deliberate divergences where Python-protobuf-developer ergonomics differ (see the Schema Linting section's positioning statement).
• protokit compat ↔ buf breaking: both detect schema compatibility breaks, with different framing — protokit ships four named profiles (WIRE, CONSUMER_SAFE, PRODUCER_SAFE, STRICT) and a pluggable Python rule-pack API.
• protokit diff: binary protobuf message diffing — no equivalent in buf.
• Everything else buf provides (buf format, buf generate, buf push, the Buf Schema Registry, Connect, protovalidate, etc.): protokit does not replicate.

protokit uses Google's official protobuf Python library at runtime and does not depend on any Buf-authored Python package. The buf CLI itself is optional — install via brew install buf to cross-verify protokit's lint output against buf's reference implementation. The parity test suite (tests/parity/) uses an installed buf binary when available and skips cleanly when not.

buf is open source under Apache 2.0.

License

MIT