Struct BufferingSink 

pub struct BufferingSink { /* private fields */ }

A sink that buffers a narrative trace of bg-agent activity.

#1022 B9: pre-fix, bg agents ran with NullSink so every event inside them — tool calls, info lines, approval requests, errors — was silently dropped. The user only saw two lines: the spawn message and the completion message. The model only saw the final output. What the bg agent actually did was opaque.

BufferingSink records short, human-readable lines for events that matter for traceability:

• ToolCallStart → " 🔧 ToolName"
• Info → forwarded as-is (sub-agent emits info for things like nested spawn / cache hit)
• ApprovalRequest / AskUserRequest → short auto-reject note (they auto-reject on closed channel — see B10)
• Streaming text (TextDelta/TextDone) is not recorded — the final output already crosses the result oneshot, so capturing text here would duplicate it.

Drained at result-injection time and emitted as a multi-line Info event so the user sees ✅ bg agent X completed\n 🔧 Read\n 🔧 Bash\n … instead of just ✅ bg agent X completed.

Cap is intentionally generous (256 lines): a runaway bg agent could otherwise grow this unboundedly. After the cap we record a single … (trace truncated at N lines) marker and stop.

Implementations

impl BufferingSink

pub fn new() -> Self

Create a buffering sink with the default 256-line cap.

pub fn with_cap(cap: usize) -> Self

Create a buffering sink with a custom cap (mainly for tests).

pub fn take_lines(&self) -> Vec<String>

Drain and return all buffered lines. The sink is empty after this returns.

Trait Implementations

impl Default for BufferingSink

fn default() -> Self

Returns the “default value” for a type.

impl EngineSink for BufferingSink

fn emit(&self, event: EngineEvent)

Emit an engine event to the client.