vis-lang-interface

The shared contract for Vis language extensions, written in Python.

Vis knows nothing about programming languages. A language extension runs that language's own toolchain and hands the model back a result; this package is what those results look like, so clj.lint_code and py.lint_code read the same way and one presentation renders both.

Building a language extension on it

Add the package as a dependency of your extension and return its result types:

dependencies = [
  "vis-agent>=0.2.26",
  "vis-lang-interface @ git+https://github.com/Blockether/vis-lang-interface@v2.6.0",
]

from vis_lang_interface import LintResult, process

def lint(paths):
    done = process.run(["ruff", "check", "--output-format=json", *paths])
    return LintResult.of("python", findings_of(done.out), files=len(paths))

Module	What it gives you
vis_lang_interface.results	Diagnostic, FormatResult, LintResult, SyntaxResult, TestResult, TestFailure, BuildResult, BuildArtifact, ReplResult, ReplSession
vis_lang_interface.changes	line_changes — the lines an edit added and removed, as git diff --minimal --numstat counts them
vis_lang_interface.process	run, spawn, tool_path, ToolRun, ToolMissing, ToolTimeout
vis_lang_interface.runtime	start, Runtime, Rendezvous, RuntimeGone — a runtime that stays alive between calls
vis_lang_interface.project	project_root, source_files, string_list — a list argument that arrived as one bare string becomes one entry, not one entry for each character
vis_lang_interface.presentation	Activity rendering every language binding shares: a pass or fail verdict for lint, syntax and test runs, changed-line counts for formatting, and an evaluation's code, repairs, output, error and value
vis_lang_interface.syntax	SyntaxGuard — validates edit previews, applies language repairs and reports files that remain unparseable
vis_lang_interface.prompt	routing — the block that tells the model your verbs exist

Count what formatting changed

FormatResult.lines_added and lines_removed say how much formatting changed. Count them with line_changes, so every extension reports the numbers git diff --minimal --numstat gives:

from vis_lang_interface import FormatResult, line_changes

added, removed = line_changes(before, formatted)
result = FormatResult(
    "python", (path,), (), is_written=True, lines_added=added, lines_removed=removed
)

A line counts with its newline: a changed line counts once as removed and once as added, and a last line that only gains or loses its newline counts as changed. A source file of a few thousand lines takes about a millisecond. A changed region of tens of thousands of lines that exact counting cannot settle in about half a second is matched through the lines that repeat least; its counts can then exceed the fewest, never fall below them.

Tell the model your verbs exist

Vis prints one section per active extension into the system prompt. An extension that contributes none still works, but the model has to discover it first, so it reaches for a shell line it already knows and never asks whether the REPL this session started is still running — nothing else reports that. prompt.routing writes that section for you:

from vis_lang_interface import prompt

PROMPT = prompt.routing(
    "Python",
    "py",
    ("format_code", "lint_code", "run_tests", "repl_start", "repl_eval", "repl_stop"),
    notes=("`py.repl_eval` needs the REPL `py.repl_start` already started.",),
)

vis.register_extension(vis.Extension(..., alias="py", prompt=PROMPT))

Keep notes to routing and policy — when a verb is the right approach and what it refuses. Each method's own docstring already carries its arguments and its result.

Keep source files parseable

SyntaxGuard asks the language's own parser whether changed files still parse. Syntax checks run only through edit hooks. They are not tools available to python_execution or public routing verbs.

The interface has no parser. Supply the file suffixes and a private _check_syntax callback. The callback takes {path: text} and the workspace root. It returns a SyntaxResult whose diagnostics name the supplied paths. Keep the callback outside the extension's exported symbols.

from vis_lang_interface import Diagnostic, SyntaxResult
from vis_lang_interface.syntax import SyntaxGuard

def _check_syntax(sources, root):
    rows = [Diagnostic(path, line, column, "error", message) for ... in parse(sources)]
    return SyntaxResult.of("clojure", rows, files=len(sources))

guard = SyntaxGuard("clojure", (".clj", ".cljs", ".cljc", ".edn"), _check_syntax)

vis.register_extension(vis.Extension(..., op_hooks=guard.op_hooks(), ctx=guard.ctx))

• Before a patch writes, Vis gives the hook the proposed text and changed line spans. If a repair callback returns valid source, the host writes that source once and reports the corrections. Otherwise, a patch that breaks a parseable file is refused. A previously broken file can still be repaired in steps.
• After a patch, the guard checks the changed file again. After a Python block, it can repair changed files before checking them again. This includes writes from Path.write_text(), open(..., "w") and programs started by the block.
• Unresolved errors appear in session["clojure_syntax_errors"]. Completed repairs appear in session["clojure_syntax_repairs"], with notes and diffs. Other language names replace the clojure prefix.
• A patch is atomic for one file. A Python block is not transactional. Post-block repairs happen after the original writes and cannot undo the block's other effects. Each repair checks that the file still matches before replacing it. Symbolic links are not replaced.
• The guard watches tracked and untracked workspace files, but skips ignored files. If Git cannot list files, it walks the workspace and skips hidden and build directories. Between listings, file and directory stat calls find changes without a process.
• If the parser is unavailable, the guard allows the operation, logs the failure and waits two minutes before retrying. It never accepts a repair without successful validation.

To enable repairs, pass repair=repair_source to SyntaxGuard. The callback receives the proposed source and keyword arguments original, spans and parses_clean. Spans are inclusive, one-based line ranges in the proposed text. After a Python block, original is None and the spans cover the whole file.

Return an object with a changed source string and a nonempty list or tuple of notes. Return None when the repair is unsafe or cannot produce valid source. Use parses_clean(candidate) to validate the complete candidate without executing it. Keep language parsing and repair algorithms in the language extension, not this package.

Everything a language tool starts runs in the jail

process.run and runtime.start spawn through vis.jailed_shell, so whatever confinement the person running Vis turned on covers the toolchain as much as the model's own shell. A jailed toolchain still reaches the package indexes it needs: Maven, Clojars and PyPI answer through the egress proxy.

A shell child runs under a pty, which merges stdout with stderr and normalizes what passes through. Tool output is data, so process.run gives each run a private directory, writes the two streams into files there and reads them back — ToolRun.out stays parseable JSON. A runtime that stays alive between calls gets a rendezvous instead: two FIFOs in a private directory, handed to that one child as its stdin and stdout, so it keeps speaking one JSON request per line in and one answer per line out without ever touching the pty.

from vis_lang_interface import runtime

live = runtime.start(["python3", driver_path], cwd=project)
answer = live.request({"op": "eval", "code": "1 + 1"}, timeout_s=30)
live.stop()

The extensions that use it

• vis-lang-clojure — cljfmt, zprint, clj-kondo, Lazytest and an nREPL, through the clojure CLI.
• vis-lang-python — ruff, pytest and a managed project REPL.

Development

vis-agent python -m pytest tests -q