Stability

What this project will not change without a major version, what it does not promise to keep, the rules for changing either, and the record of the times it broke its own rule.

What semantic versioning covers #

A change to any of these is a breaking change, and ships only in a major version.

The language: its syntax and semantics as SPEC.md states them. A program that compiles and runs under 7.x compiles, runs and means the same under every later 7.x, with the one exception under The prover's reach below. What a run is allowed to do is the operator's budget, not the language, and the budget a run gets when nobody writes one changed in 5.0 - see Breaks we have made.
The error codes in velaris.ERROR_TABLE, and what each means. The errors page is built from that table.
velaris.audit/1: the fields and their meanings, as EMBEDDING.md and velaris-spec section 8 state them. Within version 1 fields may be added; none changes meaning or disappears without the schema value changing. The other versioned documents Velaris writes follow the same rule within their version: velaris.capabilities/1, velaris.capabilities-check/1, velaris.review/1, velaris.invocation/1, velaris.mcp-tools/1, from 4.1 velaris.conformance/1, and from 8.1 velaris.receipt/1 and the receipt Statement that carries it (velaris-spec section 8.7).
The library API: velaris.check, velaris.audit, velaris.run, velaris.Pool (with Pool.check and Pool.audit from 8.1), velaris.card and, from 4.2, velaris.attest - their names, their parameters, and the fields of what they return (CheckResult, AuditResult, RunResult, Problem, and the in-toto Statements attest returns, whose predicate velaris-spec section 8.5 defines). A new optional parameter or a new field is an addition, not a break.
The budget grammar: SPEC.md section 7.1, stated in full in velaris-spec sections 4 and 5. A budget that parses keeps parsing, and grants the same thing.
The command line: the command names, the flags documented for them (velaris with no arguments prints them, and README and EMBEDDING.md describe them), and the exit codes those documents give. The budget a command line with no --allow installs is part of this, and 5.0 changed it.

What it does not cover #

Anything else in the velaris package (velaris.py until 8.2): every function, class and module-level name not listed above, including those the doors and suites use (Budget, InvocationLog, inspect_source, load_program, ...). They may change in any release.
The wording of messages. Codes are stable; prose is not. That covers error messages, their suggested fixes, an audit's warnings, and every command's text output - read --json, not the text.
Which promises happen to prove. A new version may prove a promise an older one left to runtime, or leave to runtime one an older one proved.
The canonical style velaris fmt writes. A change to it is named in the CHANGELOG, because fmt --check in CI will see it.
Anything marked provisional, in SPEC.md or in velaris-spec. From 7.1 that includes velaris.deps-diff/1 and velaris.deps-diff-lockfiles/1, the JSON velaris deps-diff writes: their fields may change in a minor release, named in the CHANGELOG, until a release says they are covered. The command, its flags and its exit codes are covered like any other command's. From 8.3 the same holds for velaris.eval/1, velaris.receipt-stream/1, velaris.receipts-diff/1, velaris.replay/1, velaris.responses/1, velaris.from-contracts/1, velaris.verify/1 and velaris.permissions-ratchet/1. velaris.receipt/1 gains stop and run_parameters.profile within version 1 (velaris-spec section 8.7).

The rules #

A breaking change ships only in a major version. That includes a security fix that refuses something that used to work: if it breaks something covered above, it is a major version. Version numbers cost nothing, and a major version is how a user learns to read the CHANGELOG before upgrading.
A deprecation is announced in a minor version, warns when the deprecated thing is used for at least one minor version after that, and is removed no sooner than the next major version.
An error code is never reused for a different meaning. A code that is no longer given stays listed as removed (REMOVED_ERRORS in velaris/errors.py, and the errors page), with what it meant and the version that removed it.
Every major version's CHANGELOG entry says what a user of the previous major has to change, item by item.
A minor or patch release that adds an error code, removes a flag or changes a default says why that is not a break (8.2), in its CHANGELOG entry, on a line beginning compatibility:. A new code can be an addition - given only where a Python error used to escape - or a refusal of something that used to run, which is a break; the line is where the difference is argued, in writing, before the release. The release gate reads the compiler's source against the previous tag and refuses a minor or patch release that changes one of the three without the line - a line of prose that says something, not one in a code block or a comment, and not a placeholder (RELEASING.md). It cannot tell whether the argument is right; it makes sure one is made.

Deprecations in force #

Deprecated	Since	What it does now	Removed in
`--no-cache`	8.2	nothing: Velaris keeps no proofs between runs. It is accepted wherever it was, and says once on stderr that it does nothing	9.0
`velaris clean`	8.2	nothing, exit 0, with the same notice	9.0

The prover's reach. A prover that settles more is not a breaking change, even when it refuses a program that compiled before: a promise it now shows false (E700), a call it shows can break a requires (E701), a list read it shows can pass the end (E705), a divisor it shows can be zero (E706). Each such program could already fail while running, with the matching runtime error, for the input the prover names. This is the one way a program that compiled under 4.x can be refused by a later 5.x, and the CHANGELOG names each release that does it. 4.3 widened it once: a division whose divisor mentions a loop's values is translated when the loop's condition and invariants show the divisor positive. 4.3.1 widened it again, not by translating more but by allowing more time: a float proof gets 120 seconds where it had 30, so a refutation a slower machine used to abandon now lands.

A new builtin gives way to your own function. A builtin added in 4.3 or later is not reached in a program that defines a function of the same name (SPEC.md 10.1). Without that rule, adding a builtin would break every program that had already used the name - examples/ledger.vel has had a function called money since 1.13 - and adding one would be a major version. The builtins that existed before 4.3 are unchanged: a function named like one of those is still never reached.

Breaks we have made #

The README has promised semantic versioning since 2.2. 2.0, 3.0, 4.0, 5.0, 6.0, 7.0 and 8.0 broke things in major versions, as promised; the rest below did not. None of them is being undone - the versions are published - and this section exists so the record is whole and so the rules above are applied from 4.0 on.

2.0 (major). to_int, get on a map, read_file and fetch became fallible: a call to one of them not handled with check or try was refused with E520, and the compiler pointed at each.

3.0 (major). env became its own effect. A program that called env() under uses io alone was refused at compile time, and an io-only budget no longer let a program read the environment.

3.4 (minor) shipped three breaking changes it named, and three it did not. Named in its CHANGELOG, which said they shipped in a minor version because each closed an open door: a client of velaris serve had to send a bearer token; the MCP server granted io only unless started with --max-allow; and the GitHub Action, with sarif on by default, needed permissions: security-events: write. Not named: velaris serve refused an argument it did not know, where it had ignored it; the MCP server did the same; and GET /health stopped naming the ceiling to a caller without the token. 3.4 should have been 4.0: closing an open door is a reason to release soon, not a reason to call a break something else. It was not retagged, because 3.4.0 was already published to PyPI and GitHub, and moving a published version breaks everyone who pinned it. This policy exists so that it does not happen again.

3.3 (minor) shipped five breaking changes as fixes, and named none as breaking. A uses clause naming anything but the seven effects was refused (E300). The budget parser became strict: a count in non-ASCII digits, ffi:M@N, ffi: with no module and an unbracketed IPv6 address were refused where they had parsed. ffi,ffi:math came to grant every module, where it had granted math alone - the same budget text granting more. A call through a granted module into a module that was not granted was refused (E311) where it had run. % followed by 2C, 40, 5B, 5D or 25 in a path or host began to be decoded. And velaris audit --json changed shape to velaris.audit/1. Each was a correct fix; together they were a major version.

Earlier minor releases that broke something covered above:

Release	What broke
2.20	Whole numbers became 64-bit: arithmetic past that range stopped with E407, where the interpreter had kept counting
2.42	`velaris fmt` changed where it puts `requires`, `ensures` and `invariant`, so `fmt --check` failed on files the previous version had formatted
2.44	`pop`, `slice` and `set_at` had to be handled with `check` or `try` (E520); a missing `main`, a `main` with parameters and a `main` marked `or fail` became compile-time errors (E400, E401, E523); Python calls began to receive arguments that read as numbers as numbers
2.47	`http.vel`'s `call` returned an `Answer` record instead of text
2.56	`audit().problems` held `Problem` objects instead of dictionaries
2.59	The MCP server and the HTTP door stopped a run at 30 seconds and 512 MB, where it had had no limit
2.62	`args()` stopped including `--allow`, `--deny`, `--timeout` and their values; `check --strict` began refusing a loop not shown to end (E612)
3.1	The memory cap began to hold on Windows, so a run past it stopped with E611 where it had continued; `velaris add` refused to replace a vendored library with different bytes without `--force`
8.2	Each named on a `compatibility:` line of its entry, where rule 5 now requires it. On a run, `--` ends Velaris's flags, so `args()` no longer holds `--` and a flag written after it no longer applies (a Goal C fix); a local or parameter named like a builtin, or like one of the program's functions, no longer hides a call from the effect check (E300) or the Secret check (E560); unary minus of the smallest whole number, and that number divided by -1, stop with E407; blocks nested more than 4,000 deep are E102, where more than some thousands were a Python traceback; `velaris check` gives a `main` marked `or fail` E524, as a run always did, where it gave E523; `--no-cache` and `velaris clean` do nothing

Refusals from a stronger prover, which the rule above does not count as breaks, in 2.6, 2.9, 2.12, 2.18, 2.41.2, 2.43, 2.44 and 2.45: each refused before running a program that had compiled, for a division, a map promise, a nested-list read, a record promise, a call against a requires the prover had been dropping, or a list read the prover could now show wrong.

A reused error code. E610 meant a missing map key from 1.4; 2.0 made that lookup a failure instead, and 2.59 gave E610 to the run's time limit. Rule 3 forbids that from 4.0 on. No code has been removed since.

4.0 (major). Listed item by item in the CHANGELOG under "What a 3.4 user has to change": velaris serve without --max-allow grants io only; on both doors a request's timeout and max_memory_mb may not exceed the operator's --max-timeout and --max-memory-mb, 30 seconds and 512 MB by default, and must be numbers; and velaris serve --max-memory-mb, which on Linux and macOS capped the door's own process, is now the most each run may have.

5.0 (major), 2026-09-12. A run given no budget gets io - the console, and nothing else - where it got all seven effects. velaris file.vel, velaris.run(source) with no allow, and velaris.Pool(...) with no allow are all affected; the two doors were already io (the MCP server in 3.4, the HTTP door in 4.0), and this is the release that makes every place a budget comes from answer the same way. --deny now narrows what --allow gave rather than starting from all seven, so --deny net alone leaves io where it left six effects. The reason: the one question a capability language has to get right is what an operator gets when they say nothing, and until 5.0 the answer was "everything". Every claim this project makes about running code you have not read depended on the operator having written a budget, and THREAT_MODEL.md, the README's related-work paragraph and the paper all conceded the point against WASI, whose modules reach nothing unless handed something. Prior art made it plain: Boruna's default policy grants nothing.

A 4.x user has to change: every command, script, CI step, notebook cell and library call that runs a program needing more than io and did not say so. velaris migrate --to 5.0 [path] reads a program or a tree, works out the narrowest budget each program needs from its own audit, and prints the command to run it under 5.0; --write updates the shell scripts and CI files it can parse and names the lines it left alone. --allow all is the explicit way to ask for what a run used to get, and writes one line to standard error when it is used. The refusal a program now meets names the effect, what the run does allow, and the flag that would grant it.

6.0 (major), 2026-09-12. env() returns Secret of Text where it returned Text, and a Secret cannot be printed, written, sent or passed to Python (E560). Three shapes of program stop compiling: one that emits what env() returned; one whose signature says it returns a Text and returns what env() gave it (E503); and one that puts it in a Text variable, field or parameter (E501). declassify is an eighth effect, so --allow all now grants eight, a uses clause may name it, and velaris.audit/1's effects may hold it - a change to a list STABILITY.md's budget grammar clause covers. stdlib/env_tools.vel changed with the language: setting returns Secret of Text, number_setting declares uses env, declassify, and public_setting is new. The reason: an effect says a program printed something; it does not say whether what it printed was the secret, and a language whose whole claim is about running code you have not read had nothing at all to say about that. It was the one capability the peer-reviewed design beside this one had and this did not, and prior art said so in writing.

This one was written as 5.1 first. It is 6.0 because rule 1 says a break ships in a major version - including a security fix that refuses something that used to work - and because this file already records two releases that got that wrong. A version number costs nothing.

A 5.x user has to change: every use of env()'s result as a Text, which the compiler points at one by one; and a repository with a committed velaris.capabilities has to record declassify the first time one of its programs needs it, in review, which is the ratchet working rather than a break.

7.0 (major), 2026-09-12. 6.0.0 stood for one day. 6.0 let a comparison over a secret give an ordinary Bool, and said in writing why: a comparison is one bit, if key == "" has to be writable, and refusing the Bool while allowing the branch would stop nothing. That is true of one comparison and false of a loop - with length and code_at, key == c reads the key out a character at a time, and the program prints what it read. A second route was found the same way: a failure's reason is text the program can print and the runtime builds it from the values it was given, so to_int(key) printed the key when it failed. 7.0 makes a comparison over a secret a Secret of Bool, refuses any if or while on a value that carries a secret (E563), and refuses a secret to every fallible builtin (E560). Programs that compiled under 6.0 are refused, so it is a major version by rule 1.

6.0.0 is yanked on PyPI, not deleted. It was published, tagged and signed, and deleting or moving a published version breaks whoever pinned it - the same reason 3.4 was not retagged. A yank does neither: an install pinned to ==6.0.0 still resolves and gets the same file, and an install that asks for anything looser no longer reaches it. That is the distinction that lets a bad release stay honest without breaking anyone who committed to it - it is still there, and listed here as what it was: a release whose central feature had a hole, closed the same day it was found. What the rule bought is that the fix arrived as 7.0 rather than as 6.0.1, so nobody upgrades into a refusal without reading why.

8.0 (major). Four changes refuse programs that ran under 7.x, each a security fix, each therefore a major by rule 1. Listed item by item in the CHANGELOG under "What a user of 7.x has to change":

A proxy the net budget does not cover is refused (E317). Through 7.x a net: grant checked the URL's host, but an ambient HTTP_PROXY / HTTPS_PROXY routed the request to a proxy that need not be a granted host - the grant bounded a string, not the socket's peer. This was the open gap THREAT_MODEL.md recorded. 8.0 disables ambient proxies unless the proxy's own host:port is inside the net budget; velaris add no longer defers to a proxy either. With no proxy set, behaviour is unchanged.
A function named like a built-in is refused (E204). fn print, fn env, fn read_file and the like were silently shadowed by the built-in through 7.x; now they are an error. The 4.3-and-later give-way rule (SPEC.md 10.1, the Money and Secret builtins) is unchanged, and the shipped standard library is exempt.
read_file on a documented credential location is refused (E318), pointed at read_file_secret; and such a location is not covered by a broad fs:read: grant unless a path within it is named explicitly.
velaris add refuses a redirect from https to http, and a redirect to a host outside the URL's origin.

Three new error codes, E204, E317 and E318; none reuses a meaning (rule 3). velaris.audit/1 gains ffi_native within version 1 (an addition, not a break), and the invocation log gains an optional run_params. Nothing published before 8.0 is moved.

8.4 (minor): a default that refuses nothing a budget allows. From 8.4 a run in a process of its own asks the operating system to hold its budget (docs/confinement.md). It is recorded here because it is a default that changed, and a reader of this file should find every one. It is not a break by this file's rules: a program that compiles and runs under 8.3.1 inside its budget compiles and runs the same way, with the same output and the same exit status, because what the system is asked to refuse is only what the budget already refused - and a granted ffi module the table does not know widens the OS policy to nothing rather than risk refusing what it needs. --no-confine and confine=False restore 8.3.1 exactly. Two things a consumer of documents can see: a receipt's run_parameters.confinement, "none" in 8.3 for every run but velaris eval's, is now the level (full, partial, none) with three fields beside it, and under velaris eval the mechanism names 8.3 wrote there (landlock-net, landlock, job-one-process, sandbox-exec) moved to confinement_layers; and velaris eval, which 8.3 ran under the budget alone where the system offered nothing, now refuses to run there, which is the one refusal 8.4 adds and is of a command documented as provisional.

8.1 (minor), and two refusals it adds. Both are listed in its CHANGELOG under "What 8.1 refuses that 8.0 did not", and are recorded here so the record stays whole.

The HTTP door and the MCP server refuse an import from outside the directory they serve (E515), where 8.0 compiled one. The list above names the command line's commands, flags and exit codes, and the library; what a door does with a request is not on it. But this file counted 3.4's door that began refusing requests it had accepted as a break made in a minor version, and by that reading this is one too, and is recorded as one. It shipped in 8.1 because it closes a read of the door host's files by any caller - a token holder, or whatever an MCP client let a model send, under the io ceiling (advisory-import-read.md) - and because a door whose programs import from elsewhere has one flag to set, --root. The library, which the list does cover, refuses nothing new: import_root= is an added parameter.
velaris.check(), velaris.audit() and velaris.attest() stop at 60 seconds and 2048 MB unless raised, where 8.0 waited. The command line has had the same ceiling since 8.0, whose CHANGELOG counted it as an addition (its memory cap took hold only on Windows until 8.1, and now holds on Linux as documented); this applies that reading to the library. A check that needs longer passes timeout=, and None restores 8.0's call exactly.