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OPA’s reference gives conflicting guidance about a leading v: its warning says to remove the prefix because it makes a version invalid, while its example says semver.is_valid("v1.1.12-rc1+foo") returns true. The SemVer 2.0.0 specification is clear that v1.2.3 is not a semantic version. The documentation conflict does not establish what a particular OPA release actually does at runtime.
What OPA’s documentation says
OPA describes semver.is_valid(vsn) as a built-in that accepts an input and returns a boolean: true for a valid semantic version and false otherwise. The reference lists the built-in beginning with OPA v0.22.0 and notes that WebAssembly support depends on the SDK. It gives the version shape as MAJOR.MINOR.PATCH[-PRERELEASE][+METADATA], with the prerelease and build-metadata portions optional. OPA’s Semantic Version Built-ins reference
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The same reference contains the inconsistency. Its warning says, “When working with Go-style semantic versions, remember to remove the leading v character, or the semver string will be marked as invalid!” Yet its displayed examples report true for both semver.is_valid("v1.1.12-rc1+foo") and semver.is_valid("1.1.12-rc1+foo"). The warning and the first example cannot straightforwardly describe the same input behavior.
What SemVer 2.0.0 considers valid
The specification answers the prefix question directly: “No, ‘v1.2.3’ is not a semantic version.” It distinguishes the version string, 1.2.3, from a tag name or notation such as v1.2.3 that people may use to indicate that version. Semantic Versioning 2.0.0
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A SemVer version has three numeric core components—major, minor, and patch—in X.Y.Z form. Those components cannot have leading zeroes. A hyphen may introduce prerelease identifiers, and a plus sign may introduce build metadata. The prefix v is not one of these parts and is not permitted as part of the semantic-version string under the specification.
Does a specific OPA release accept the prefix?
The cited documentation establishes a contradiction, not the runtime result for a named OPA release or evaluator. Do not treat the example as proof that every OPA version accepts the prefix, or the warning as proof that every version rejects it. To make an operational claim, check the exact OPA version and evaluator you use.
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Availability and input validity are separate questions. OPA’s reference identifies when the built-in is listed and qualifies WebAssembly support as SDK-dependent; its operations documentation shows a policy using semver.is_valid(input.version) and describes capability files as a way to check built-in compatibility. A capability check can help establish whether a policy’s built-ins are supported, but it does not resolve how a particular evaluator handles a v-prefixed input. OPA Operations documentation
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How to handle version strings in a policy
First decide what the input represents. A tag such as v1.2.3 may be the convention used by a repository or release process, while a SemVer string under the specification is 1.2.3. If your input contract requires a SemVer string, normalize or reject tag prefixes at the boundary according to that contract, then validate the resulting value. Do not silently strip characters unless your system explicitly defines that normalization.
Before relying on semver.is_valid for this edge case, check behavior with the specific OPA release and evaluator in your deployment. The reference’s displayed example and warning are not enough to settle it. OPA’s operations documentation also explains capability files for checking built-in compatibility when you need to verify policy support across environments.
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