Every text below is quoted as the binary prints it, so you can search this page for the line you got. Errors go to stderr, prefixed error:, and exit 1. A report that says a test failed or a target was refused is also exit 1, but it is a document, not an error: jess did what you asked, and this is the answer.
Running a script§
error: 'nope.jsl' isn't a file
jess run takes a path, not a target name and not a module name. Check the path, and remember that jess run fix.jsl --verbose hands --verbose to the script, not to jess, so a flag you meant for jess goes before the script.
error: 'script.lua' isn't a .jsl file: jess runs .jsl sources, and nothing else
Sources are .jsl files, everywhere: jess run, jess test, jess check, a manifest's modules, and require. The language is Luau; only the extension is jess's. Rename the file. Under jess test the same text appears as the module's error, since a test module that didn't run is reported, not fatal.
error: 2 type errors, after a report from jess check
jess check found what strict mode would show in your editor, in the scripts given and the files their ./ requires reach. Each error names the file, line, and column. A script that passes jess check can still fail at run time for reasons types don't cover, a missing file or a denied capability; a script that fails it has a bug the checker can prove.
error: module not found: ./nothing
A relative require looked for nothing.jsl and nothing/init.jsl beside the requiring file and found neither. The extension is .jsl, not .luau or .lua, and there is no fallback. A bare name that isn't @jess/, @dream/, or an export of an activated package is also not found: require doesn't search the current folder, the data folder, or anything else.
error: module not found: @jess/whatever: @jess/ is jess's own namespace (its members, such as @jess/shader, @jess/log, @jess/test, @jess/jpk, @jess/package, and the built-in package's exports, such as @jess/style), never a file
@jess/ names are jess's: a member of the jess global, a built-in module, or an export of the built-in package. No file, alias, or other package can stand in for one. The message lists where the real ones come from; the API reference lists them all.
error: dream.fs.readFileString: needs the 'filesystem.read' capability, which this runtime does not grant
The function names itself and the capability. Under jess run and jess test, add --capability filesystem.read. In a command target, declare it in the target's capabilities and run with --capability or --trusted. Capabilities has the whole table. If you'd rather degrade than fail, ask jess.granted first.
error: unknown capability 'filesystem.raed'; a jess runtime can be granted filesystem.read, filesystem.write, ...
A --capability name, or a target's declared one, that no part of jess knows. The message lists every name that exists. A misspelled grant is refused rather than granting nothing, which is the behaviour you want from a thing called a capability.
error: worker.spawn: needs a ScriptDomain, and none owns this runtime; jess run, jess test, and command targets run scripts outside one
jess.worker.spawn and jess.context.bind only mean something inside a script domain, which an application target or an embedding host provides. A plain script, a test module, and a command target run in a single VM with no domain around it. Make it an application target, or do the work in the script.
Manifests§
Every manifest error starts with package: and names the table and key. The Manifest lists the full set under each key; these are the ones people hit first.
error: package: jess.toml [package] has an unknown key 'author'
Jess rejects keys it doesn't know, at every level. Check the spelling against the manifest reference; there is no author, description, or license key, since jess doesn't read them.
error: package: jess.toml has no [package] table
The file parsed but the [package] table is missing, usually because it was written [Package] or the file is the wrong one.
error: toml: expected ']' at line 2, column 9
The file didn't parse as TOML at all. The line and column are the decoder's; the usual cause is an unclosed [table header or a string without its closing quote.
error: package: jess.toml: module '@greeter' is neither exported nor an entrypoint of any layer
A [modules] entry no [[layer]] exports or runs. Add it to a layer's exports, make it an entrypoint, or delete it. A module nobody can reach ships for nothing.
error: package: jess.toml: layer 'example.greetings' exports '@greeter', which [modules] doesn't name
The layer exports a name that [modules] doesn't map to a file. Add the module, or fix the name on one side.
error: package: jess.toml module '@jess/greeter': @jess/ is jess's own namespace, open only to its built-in package (org.dreamweave.jess)
Pick a scope of your own. @jess/ is reserved, whatever the package is called.
error: package: bm/missing-entry: target 'greet' entry is 'tools/run.jsl', which isn't in the payload
The file a target, module, or managed entry names isn't under any include root, or isn't there. Either add its folder to [package] include or fix the path. The same shape of message, module '...' is '...', which isn't in the payload and managed '...' is '...', which isn't in the payload, means the same thing for those.
error: package: 'greeter': [package] include names 'assets', which isn't there
An include root that doesn't exist on disk. Remove it or create it; an empty folder contributes nothing, so you may as well remove it.
error: package: jess.toml dependency 'com.example.words' 'latest' is not a version requirement (^1.2, =1.2.3, =1.2.3-dev.4)
Requirements are caret ranges or exact versions. There is no latest, *, or >=. A pre-release can only be named exactly.
Resolution§
error: package: example.greetings requires example.words: no provider found for example.words
A layer requires another that nothing offered provides. Offer the package that has it with --package or put it in a --package-path folder; jess package graph shows what was found.
error: package: com.example.greeter 1.0.0 requires com.example.words ^1.2 followed by resolution failed: com.example.words is not available
A [dependencies] entry nothing offered meets. Offer the package at a version the requirement accepts. If the only version is a pre-release, the requirement must name it exactly: no range selects a pre-release.
error: package: layer example.greetings has more than one candidate: com.example.greeter 1.0.0 (source pp/one), com.example.greeter 1.0.0 (source pp/two); offer one of them outright
Two packages in the offered set provide the same layer, typically a Package Source and a JPK of the same package in one --package-path folder, or two checkouts. Jess won't pick. Name the one you mean with --package, which is chosen over anything found in a path, or remove the other.
Packages and targets§
error: cannot run target greet followed by required capability denied: and the names
The target declares a capability you didn't grant. Grant each with --capability <name>, or everything the target declares with --trusted, after jess package inspect has shown you what that is. This happens before anything is activated, so nothing ran.
jess: greeter.jpk: tests run from a Package Source; run jess package test on its source
Not an error, and exit 0: a JPK ships its tests but they only run from the folder they were written in, so jess package test on a .jpk reports no providers. Run it on the Package Source. That run packs the source into a temporary JPK and tests both, comparing their digests, which is the check you wanted.
error: 2 failed at the end of a jess package test report
Each test module ran twice, from the source and from it packed, so one failing case counts twice. Read the error: under the failing case: an expectation failure quotes both values and the file and line, as in expected "Hello, world." to be "Hi, world." and bad/tests/run.jsl:8.
Two digests for what you thought was one package
jess package inspect on a source and on a .jpk disagree on digest after an edit. The digest covers the whole payload, so the JPK is now a different package from the folder: re-pack it. A workspace whose jess.lock pins the old digest reports the kept copy as not the one jess.lock pins until you jess workspace sync again.
error: 1 type error from jess package check, with a list above it
The strict type checker found something, listed as file, line, column, and message, with each require resolved as the runtime resolves it. Key 'gret' not found in table '{ read greet: (string) -> string }' is a misspelled field; unknown where you expected a type usually means a value the checker can't see through, which you annotate. These are errors in your package, not in jess, and the check is a gate: fix them.
Editor types§
The editor approves a call the runtime rejects, or vice versa
types/ describes one jess version. After upgrading jess, run jess init in the folder again (it refreshes types/ and leaves your files alone) or jess luau-types types. If the editor shows no jess types at all, it isn't reading .jessrc: the alias file jess init writes is .jessrc, and luau-lsp needs the definitions flag, which Editor Setup spells out per editor.
None of these§
If jess printed something this page doesn't have, and the CLI and API references don't either, that is a documentation bug as well as whatever else it is. File it with the exact text.