Struct LastErrorFrame 

pub struct LastErrorFrame {
    pub message: String,
    pub source: &'static str,
    pub written_ms: u64,
}

The most recent persisted error_during_execution frame at the tail of a block’s own output file — populated ONLY when the very last non-blank line IS one (a block that errored once and then kept producing normal output afterward must NOT be flagged; only “the last thing that happened to this block was an unrecovered error” counts).

This reads a signal that already exists and is already durable: per agent_handlers/input.rs’s own comment, this exact frame shape is deliberately persisted to the block file (not just live-broadcast) so it survives pane reload — the same source the frontend itself renders as the pane’s error bubble. Composing a second reader over that existing store, rather than adding a new independent tracker, follows this module’s own design constraint (spec §5.1/§3 pt. 8).

See docs/reports/REPORT_MUXSPECT_SPAWN_REFUSAL_DIAGNOSIS_EXTENSION_2026_08_03.md for the incident this closes the diagnostic gap on: a block whose spawn was refused before any controller/process ever existed showed up as featureless “no controller, no processes” — indistinguishable from a healthy idle block — even though the actual, actionable reason was already sitting in this exact file.

Fields

message: String

The frame’s error.message verbatim (includes the [AgentMux] prefix every construction site already writes).

source: &'static str

Best-effort tag for which pre-spawn failure path wrote the frame — see classify_last_error_source. NOT a stable/tested taxonomy like agents::failure::FailureClass (which only classifies POST-spawn exit failures); building that properly for pre-spawn refusals is real, separate follow-up work (report §3.3), not attempted here.

written_ms: u64

Epoch ms the output file was last written — i.e. when this frame (being the file’s last line) was appended. Deliberately a raw timestamp, not a precomputed “age,” matching last_computed_ms’s existing convention elsewhere in this response so --json consumers get a stable value and the CLI renderer computes age the same way it already does for every other timestamp.

Trait Implementations

impl Serialize for LastErrorFrame

fn serialize<__S>(&self, __serializer: __S) -> Result<__S::Ok, __S::Error>

where __S: Serializer,

Serialize this value into the given Serde serializer.