Handoff Generation Pipeline

/handoff generation pipeline

This document describes how the coding-agent implements /handoff today: trigger path, generation prompt, completion capture, session switch, and context reinjection.

Scope

Covers:

• Interactive /handoff command dispatch
• AgentSession.handoff() lifecycle and state transitions
• How handoff output is captured from assistant output
• How old/new sessions persist handoff data differently
• UI behavior for success, cancel, and failure

Does not cover:

• Generic tree navigation/branch internals
• Non-handoff session commands (/new, /fork, /resume)

Implementation files

• ../src/modes/controllers/input-controller.ts
• ../src/modes/controllers/command-controller.ts
• ../src/session/agent-session.ts
• ../src/session/session-manager.ts
• ../src/extensibility/slash-commands.ts

Trigger path

1. /handoff is declared in builtin slash command metadata (slash-commands.ts) with optional inline hint: [focus instructions].
2. In interactive input handling (InputController), submit text matching /handoff or /handoff ... is intercepted before normal prompt submission.
3. The editor is cleared and handleHandoffCommand(customInstructions?) is called.
4. CommandController.handleHandoffCommand performs a preflight guard using current entries:
   • Counts type === "message" entries.
   • If < 2, it warns: Nothing to hand off (no messages yet) and returns.

The same minimum-content guard exists again inside AgentSession.handoff() and throws if violated. This duplicates safety at both UI and session layers.

End-to-end lifecycle

1) Start handoff generation

AgentSession.handoff(customInstructions?):

• Reads current branch entries (sessionManager.getBranch())
• Validates minimum message count (>= 2)
• Creates #handoffAbortController
• Builds a fixed, inline prompt requesting a structured handoff document (Goal, Constraints & Preferences, Progress, Key Decisions, Critical Context, Next Steps)
• Appends Additional focus: ... if custom instructions are provided

Prompt is sent via:

await this.prompt(handoffPrompt, { expandPromptTemplates: false });

expandPromptTemplates: false prevents slash/prompt-template expansion of this internal instruction payload.

2) Capture completion

Before sending prompt, handoff() subscribes to session events and waits for agent_end.

On agent_end, it extracts handoff text from agent state by scanning backward for the most recent assistant message, then concatenating all content blocks where type === "text" with \n.

Important extraction assumptions:

• Only text blocks are used; non-text content is ignored.
• It assumes the latest assistant message corresponds to handoff generation.
• It does not parse markdown sections or validate format compliance.
• If assistant output has no text blocks, handoff is treated as missing.

3) Cancellation checks

handoff() returns undefined when either condition holds:

• no captured handoff text, or
• #handoffAbortController.signal.aborted is true

It always clears #handoffAbortController in finally.

4) New session creation

If text was captured and not aborted:

1. Flush current session writer (sessionManager.flush())
2. Start a brand-new session (sessionManager.newSession())
3. Reset in-memory agent state (agent.reset())
4. Rebind agent.sessionId to new session id
5. Clear queued context arrays (#steeringMessages, #followUpMessages, #pendingNextTurnMessages)
6. Reset todo reminder counter

newSession() creates a fresh header and empty entry list (leaf reset to null). In the handoff path, no parentSession is passed.

5) Handoff-context injection

The generated handoff document is wrapped and appended to the new session as a custom_message entry:

<handoff-context>
...handoff text...
</handoff-context>

The above is a handoff document from a previous session. Use this context to continue the work seamlessly.

Insertion call:

this.sessionManager.appendCustomMessageEntry("handoff", handoffContent, true);

Semantics:

• customType: "handoff"
• display: true (visible in TUI rebuild)
• Entry type: custom_message (participates in LLM context)

6) Rebuild active agent context

After injection:

1. sessionManager.buildSessionContext() resolves message list for current leaf
2. agent.replaceMessages(sessionContext.messages) makes the injected handoff message active context
3. Method returns { document: handoffText }

At this point, the active LLM context in the new session contains the injected handoff message, not the old transcript.

Persistence model: old session vs new session

Old session

During generation, normal message persistence remains active. The assistant handoff response is persisted as a regular message entry on message_end.

Result: the original session contains the visible generated handoff as part of historical transcript.

New session

After session reset, handoff is persisted as custom_message with customType: "handoff".

buildSessionContext() converts this entry into a runtime custom/user-context message via createCustomMessage(...), so it is included in future prompts from the new session.

Controller/UI behavior

CommandController.handleHandoffCommand behavior:

• Calls await session.handoff(customInstructions)
• If result is undefined: showError("Handoff cancelled")
• On success:
  • rebuildChatFromMessages() (loads new session context, including injected handoff)
  • invalidates status line and editor top border
  • reloads todos
  • appends success chat line: New session started with handoff context
• On exception:
  • if message is "Handoff cancelled" or error name is AbortError: showError("Handoff cancelled")
  • otherwise: showError("Handoff failed: <message>")
• Requests render at end

Cancellation semantics (current behavior)

Session-level cancellation primitive

AgentSession exposes:

• abortHandoff() → aborts #handoffAbortController
• isGeneratingHandoff → true while controller exists

When this abort path is used, the handoff subscriber rejects with Error("Handoff cancelled"), and command controller maps it to cancellation UI.

Interactive /handoff path limitation

In current interactive controller wiring, /handoff does not install a dedicated Escape handler that calls abortHandoff() (unlike compaction/branch-summary paths that temporarily override editor.onEscape).

Practical impact:

• There is session-level cancellation support, but no handoff-specific keybinding hook in the /handoff command path.
• User interruption may still occur through broader agent abort paths, but that is not the same explicit cancellation channel used by abortHandoff().

Aborted vs failed handoff

Current UI classification:

• Aborted/cancelled

  • abortHandoff() path triggers "Handoff cancelled", or
  • thrown AbortError
  • UI shows Handoff cancelled

• Failed

  • any other thrown error from handoff() / prompt pipeline (model/API validation errors, runtime exceptions, etc.)
  • UI shows Handoff failed: ...

Additional nuance: if generation completes but no text is extracted, handoff() returns undefined and controller currently reports cancelled, not failed.

Short-session and minimum-content guardrails

Two guards prevent low-signal handoffs:

• UI layer (handleHandoffCommand): warns and returns early for < 2 message entries
• Session layer (handoff()): throws the same condition as an error

This avoids creating a new session with empty/near-empty handoff context.

State transition summary

High-level state flow:

1. Interactive slash command intercepted
2. Preflight message-count guard
3. #handoffAbortController created (isGeneratingHandoff = true)
4. Internal handoff prompt submitted (visible in chat as normal assistant generation)
5. On agent_end, last assistant text extracted
6. If missing/aborted → return undefined or cancellation error path
7. If present:
   • flush old session
   • create new empty session
   • reset runtime queues/counters
   • append custom_message(handoff)
   • rebuild and replace active agent messages
8. Controller rebuilds chat UI and announces success
9. #handoffAbortController cleared (isGeneratingHandoff = false)

Known assumptions and limitations

• Handoff extraction is heuristic: “last assistant text blocks”; no structural validation.
• No hard check that generated markdown follows requested section format.
• Missing extracted text is reported as cancellation in controller UX.
• /handoff interactive flow currently lacks a dedicated Escape→abortHandoff() binding.
• New session lineage metadata (parentSession) is not set by this path.