TagLock

Website · Rules · Contributing · Security

Compile-time confidence for Go's runtime metadata — and compatibility intelligence for the contracts that metadata creates.

TagLock statically analyzes Go struct tags as one serialized contract. It finds today's collisions, unsafe exposure, type errors, and namespace drift, then can snapshot those external representations and detect changes that may break APIs, stored JSON, configuration, or downstream consumers tomorrow.

TagLock never executes analyzed project code during static analysis, snapshot, schema, or comparison commands.

Install

TagLock requires Go 1.24 or newer.

go install github.com/theworker02/taglock/cmd/taglock@latest

Source-contract analysis

taglock check ./...
taglock check --format json --fail-on error ./...
taglock check --format sarif ./... > taglock.sarif
taglock check --json-semantics v2 ./...

Diagnostics have stable identifiers such as TAG104 (duplicate external field), TAG301 (naming drift), and TAG401 (sensitive exposure). Text output uses ordinary Go source locations; JSON and SARIF include related collision locations, fix metadata, and semantic-profile properties.

Safe fixes are the default:

taglock fix ./...
taglock fix --rule TAG003 ./...
taglock fix --diff ./...
taglock fix --review ./... # explicitly include wire-changing suggestions

Review-required changes are never silently applied.

Suppress a precise finding on the next declaration:

type LegacyPayload struct {
    //taglock:ignore TAG301 -- public API retains its legacy YAML name
    Name string `json:"display_name" yaml:"displayName"`
}

Malformed and unused suppressions are reported as TAG901 and TAG902.

Contract snapshots

A serialized contract is the externally observable field surface after tag naming, omission, ignore, embedding, promotion, and implementation-specific resolution rules have been applied.

taglock snapshot ./...
taglock snapshot --semantics both --reproducible --output .taglock/contracts.json ./...

Snapshots select exported contracts by default and can use //taglock:contract public, storage, message, configuration, internal, or experimental annotations. Output is canonical, path-normalized, sorted, fingerprinted, and free of runtime field values or secrets.

Compare evolution

Compare checked-in snapshots:

taglock compare --format markdown old.json new.json

Or compare Git revisions in isolated temporary worktrees:

taglock compare --base main --head HEAD --format markdown ./...
taglock compare --base v1.4.0 --head v1.5.0 --format sarif --output taglock.sarif ./...

The current worktree and uncommitted changes are not checked out, reset, or modified. Evolution findings distinguish producer, consumer, bidirectional, storage, configuration, message, and advisory compatibility. Stable EVOL rules cover removal, rename, type and requiredness changes, exposure, embedding, custom marshalers, certainty reduction, deprecation, and storage migration.

Intentional breaks can be acknowledged narrowly in .taglock/changes.yml. Approvals remain visible in machine output and require a reason and migration note. Wildcard, expired, invalid, and unused approvals are rejected or reported.

Deprecation windows can be proven from explicit project release history:

evolution:
  require_deprecation_before_removal: true
  minimum_deprecation_releases: 2
  release_history: [1.4.0, 1.5.0, 2.0.0]

The history must be unique and strictly increasing. Replacement annotations can confirm a rename across changed Go field names; reports expose rename_confidence: confirmed. TagLock does not promote weak similarity into a definitive rename.

JSON v1 and v2

json/v1 and json/v2 are separate semantic profiles. TagLock records the Go toolchain and whether the experimental v2 package is active. Custom MarshalJSON/UnmarshalJSON methods reduce certainty to opaque; text marshalers reduce it to partial. TagLock never claims to know arbitrary custom wire behavior.

taglock migrate json-v2 ./...
taglock migrate json-v2 --format json ./...

Migration findings use JSONMIG001–JSONMIG010 for option, field resolution, embedding, omission, name matching, custom method, ambiguity, and explicit compatibility differences. On toolchains where encoding/json/v2 is present but GOEXPERIMENT=jsonv2 is inactive, TagLock can model the known profile but reports that runtime availability distinction.

Schema and manifests

taglock schema --format json-schema ./...
taglock schema --format openapi --output openapi-components.json ./api/...
taglock schema check ./...
taglock manifest module --output taglock.manifest.json ./...

Schema export supports primitives, aliases through their underlying wire type, pointers/nullability, slices, arrays, maps, nested references, recursion, time.Time, byte slices, required/optional fields, and schema annotations. Opaque custom marshalers produce explicit x-taglock-certainty metadata rather than a guessed schema. OpenAPI output contains component schemas only; TagLock does not claim to understand routes or generate a complete OpenAPI document.

Explicit runtime verification

taglock verify generate ./api/...
taglock verify run ./api/...

Generation writes clearly marked Go tests using zero values and configured fixtures. Only verify run executes project tests; the normal analyzer remains fully static.

Generic declarations are generated only when the project supplies a valid package-scope instantiation:

verification:
  fixtures:
    example/api.Page:
      file: testdata/contracts/page.json
      type_arguments: [string]

Type arguments are parsed and checked against the declaration's constraints before any test file is written. Configured fixtures are also decoded, re-encoded, and compared as normalized JSON, providing an explicit runtime oracle for custom marshalers that static analysis correctly treats as opaque.

Configuration and adoption

taglock init
taglock config validate
taglock config print
taglock baseline create ./...
taglock check --baseline .taglock-baseline.json ./...

TagLock discovers .taglock.yml, .taglock.yaml, taglock.yml, or taglock.yaml by walking upward. The versioned schema validates unknown fields, rules, severities, semantic profiles, compatibility modes, contract levels, and glob syntax. Legacy Phase 1 list-style configuration remains readable.

See .taglock.example.yaml, the generated rule reference, the snapshot format, and the JSON API evolution tutorial.

Embed TagLock

package main

import (
    "github.com/theworker02/taglock/analyzer"
    "golang.org/x/tools/go/analysis/singlechecker"
)

func main() { singlechecker.Main(analyzer.Analyzer) }

analyzer.New(config.Config) remains available for Phase 1 compatibility. analyzer.NewWithOptions provides validated in-memory or fixed-path policies. The analyzer exports compact, versioned contract facts for exported types and is compatible with go vet -vettool invocation.

Development

go test ./...
go test -race ./...
go vet ./...
go build ./cmd/taglock
go run ./cmd/taglock check ./...
go test -bench . ./...

Project documentation

• Architecture
• Contribution guide
• Development guide
• Compatibility policy
• Security policy
• Support
• Code of conduct
• Governance
• Release checklist
• Troubleshooting
• GitHub Pages setup

License

TagLock is available under the MIT License.

Known limitations are documented in SECURITY.md and docs/SNAPSHOT_FORMAT.md. Reproducible performance measurements are documented in docs/BENCHMARKS.md. JSON v2 remains experimental and arbitrary custom serializer behavior cannot be proven statically; CI runs the version-gated differential suite with GOEXPERIMENT=jsonv2 on Go 1.26.