Types and Errors

Types and errors

The programmatic surface: what a host reads when it wants structured results instead of a printed Display summary. The result enums a host matches on (Outcome, AgentStatus, SkipReason, CheckStatus, Error) are #[non_exhaustive], so give them a wildcard arm. The data types a backend builds or reads by field (Capabilities, AgentResult, the components IR) are not, so you can construct and exhaustively match those.

Outcome

What a lifecycle op actually did.

variant	meaning	Display
NoOp	already converged, nothing changed	no changes needed
Installed	a fresh install	installed
Updated { from: Option<String>, to: String }	version bump; from is None when the prior version could not be read	updated (X -> Y) or updated (to Y)
Repaired	a broken or partial state was reconciled back to healthy	repaired
Adopted	self_heal found a healthy install with no marker and wrote one	adopted existing install
Removed	uninstalled	removed
Cleared	self_heal found a cleanly-uninstalled plugin and cleared the stale marker	cleared stale marker

AgentReport, AgentResult, AgentStatus, SkipReason

Every *_report lifecycle method (install_report, install_into_report, update_report, uninstall_report, self_heal_report) returns Result<AgentReport>. The plain methods (install, update, ...) collapse the same report to one Outcome and surface any per-agent failure as Err(Error::Backend).

pub struct AgentReport {
    pub results: Vec<AgentResult>,
}

pub struct AgentResult {
    pub agent: &'static str,   // the backend id, "claude", "codex", ...
    pub status: AgentStatus,
}

pub enum AgentStatus {
    Converged(Outcome),   // ran; this is its own outcome, not the merged one
    Skipped(SkipReason),  // never got to write anything
    Failed(String),       // failed; rendered for the user, fan-out continued past it
}

pub enum SkipReason {
    NotDetected,        // detect() returned false
    ScopeUnsupported,    // no config surface at the requested scope
    SourceUnsupported,   // a GitHub source has no local tree for a config-merge backend
}

results holds one entry per agent reconcile/remove was asked to run; an agent excluded by an explicit install_into filter gets no entry at all, distinct from being skipped. Read the vector directly to build a per-agent UI, or use the two convenience methods:

• AgentReport::merged() -> Outcome: the first real change across the agents (a change outranks a no-op), Outcome::NoOp when nothing changed. This is what the plain lifecycle methods return.
• AgentReport::is_healthy() -> bool: true when no agent's status is Failed. Skips don't count as unhealthy.

AgentReport and AgentStatus both have a Display impl (one agent: status line per result, using Outcome's or SkipReason's own wording), a ready setup summary when a host just wants to print it.

Capabilities

What one AgentBackend can host. A backend reports these as fixed, compile-time-known facts about the tool it targets, not runtime detection.

field	type	meaning
plugins	bool	native plugin install (copies the whole tree); implies every other surface except instructions
mcp	bool	can host MCP servers
hooks	bool	can host hook bindings
commands	bool	can host slash-command definitions
agents	bool	can host agent (subagent) definitions
skills	bool	can host skill directories
instructions	bool	can receive host-authored always-loaded guidance through its own native context channel (a dedicated file plus any registration); false for a plugin-native backend, which delivers guidance through its own channel instead
statusline	bool	manages the tool's status-line slot from PluginHost::statusline. Not implied by plugins: the slot lives in the tool's own settings file, outside any plugin tree, so a plugin-native backend still writes it (claude does; the other three true today are qwen-code, antigravity-cli, droid)
scopes	&'static [&'static str]	which of "user"/"project" this backend can target

setup reads this to report a partial fit ("this agent hosts MCP servers, not hooks") instead of dropping features silently; it is the runtime truth behind the per-tool coverage table on Harness comparison.

DoctorReport, DoctorCheck, CheckStatus

doctor()'s Display is covered on the Doctor page; this is the shape for a host that wants to walk the checks itself instead of printing them.

pub struct DoctorReport { /* checks: Vec<DoctorCheck>, private */ }

pub struct DoctorCheck {
    pub name: &'static str,
    pub status: CheckStatus,
}

pub enum CheckStatus {
    Ok(String),
    Warn(String),
    Fail { problem: String, fix: String },
}

checks is private so a report is always built whole; read it back with .checks() -> &[DoctorCheck]. .is_healthy() -> bool is true when no check is Fail (a Warn is tolerated).

Building a report is public too, for an external AgentBackend's own report(): DoctorReport::from_checks(checks: Vec<DoctorCheck>) wraps a check list as-is; DoctorReport::from_error(err: Error) collapses an upfront failure (say, an unreadable config) into a single Fail check named "doctor" with fix: "see the error above".

The components IR

Plugin::components(&source) -> Result<PluginComponents> parses a plugin tree once into a harness-agnostic shape every non-CC backend renders from, instead of each one walking the tree by hand. Errors on Source::GitHub: a config-merge backend has no local tree to parse from a github ref.

pub struct PluginComponents {
    pub mcp_servers: Vec<McpServer>,
    pub hooks: Vec<HookBinding>,
    pub commands: Vec<MarkdownDoc>,
    pub agents: Vec<MarkdownDoc>,
    pub skills: Vec<SkillDir>,
}

type	fields	notes
McpServer	name: String, kind: McpKind, command: String, args: Vec<String>, env: BTreeMap<String, String>	.is_portable() is false when command/args carry ${CLAUDE_PLUGIN_ROOT}
McpKind	Stdio, Http { url: String }, Sse { url: String }
HookBinding	event: String, matcher: Option<String>, command: String	.is_portable() mirrors McpServer's, checked against command
MarkdownDoc	name: String (file stem), rel: String (the file's path in the tree), frontmatter: BTreeMap<String, Value>, body: String, raw: Vec<u8> (verbatim file bytes)	backs both commands and agents
SkillDir	name: String, files: Vec<(String, Vec<u8>)> (each file's path under the skill dir, and its bytes)

What gets parsed from where, and the frontmatter parser's limits, are on Plugin tree.

Error

Every fallible call in the crate returns Result<T, Error> (type Result<T> = std::result::Result<T, Error>).

variant	meaning
ClaudeNotFound	claude is not on PATH
ClaudeTooOld { found, floor }	claude is below the required version floor
CopilotNotFound	copilot is not on PATH
CopilotTooOld { found }	copilot is below 1.0.71, the plugin-management floor
Cli { bin, args, code, stderr }	a CLI invocation exited nonzero
Json { what, source }	a JSON parse failed
Io { context, source }	a filesystem operation failed
Tree(String)	the embedded or path plugin tree is malformed (an authoring bug in the host, e.g. missing plugin.json or author)
Config { path, detail }	a harness config file exists but does not parse; the read-modify-write refuses to clobber it rather than risk destroying the user's config
Backend { agent, detail }	one agent's failure, rendered as a string
Verify(String)	a mutating CLI call exited 0 but list --json does not reflect the expected end state
Lock { path, source }	failed to acquire the shared flock

Backend is the one variant an out-of-crate AgentBackend should return for its own failures, since every other variant carries in-crate semantics (CLI orchestration, tree parsing, config merging) that don't fit a tool this crate never shipped a backend for. It does double duty internally too: the plain lifecycle methods collapse an AgentReport to one Result<Outcome>, so any agent's Failed(String), built-in or external, becomes Error::Backend there too. The *_report methods skip that collapse entirely; a per-agent failure stays inside AgentStatus::Failed and the call still returns Ok.