Closes #4105

Description

Problem

emitTodoState emits a synthetic todo_write event after each complete_step sign-off to refresh the frontend TodoPanel. Every event used the static tool ID "host-advance":

// Before — all sign-offs shared the same ID
t := event.Tool{ID: "host-advance", Name: "todo_write", ...}

The frontend (useController.ts) resolves tool items by ID: when it finds an existing item, it updates it in place instead of creating a new one. Multiple complete_step calls within a single turn therefore merged into one item, and the TodoPanel only rendered the first sign-off. Subsequent advances were silently dropped.

Reproduction (reported in #4105):

1. Model sets up todo list: [A:in_progress, B:pending, C:pending]
2. Model signs off A → host advances → emit host-advance → panel shows 1/3 ✓
3. Model signs off B → host advances → emit host-advance → ID collision, merged → panel still 1/3 ✗
4. Model signs off C → host advances → emit host-advance → ID collision, merged → panel still 1/3 ✗

Impact: the model follows the system prompt ("you don't need a separate todo_write") but the panel never catches up. The model's observed workaround is to manually re-send todo_write after every sign-off, adding turns and token overhead.

Design

Add a monotonic atomic.Int64 counter (hostAdvanceSeq) to the Agent struct, and pass the matched todo item index from advanceCanonicalTodo into emitTodoState to form a unique, self-documenting tool ID:

// After
id := fmt.Sprintf("host-advance-%d-%d", a.hostAdvanceSeq.Add(1), itemIndex)

Examples:

host-advance-1-1  → 1st advance, panel item 1
host-advance-2-3  → 2nd advance, panel item 3
host-advance-3-2  → 3rd advance, panel item 2

Preserved (unchanged)

• verifyTodoCompletionTransitions (feat(agent): guard todo completion transitions #2576: blocks unverified completions)
• recordTodoState (fix(agent): close complete_step cross-turn evidence + loop gaps #4014: synthetic receipt for cross-turn gate)
• Canonical todo state, rebuildTodoState, incompleteCanonicalTodos
• Frontend matching (keys on name === "todo_write", not ID)
• plan-seed synthetic events (separate ID namespace)
• No frontend changes required

Related

• Closes [Feature]: 代办任务更新不同步 #4105 (todo panel desync)
• Preserves guards from feat(agent): guard todo completion transitions #2576, fix(agent): block final answers with incomplete todos #2920, fix(agent): close complete_step cross-turn evidence + loop gaps #4014

Files changed

internal/agent/agent.go                  | 15 ++++++++++++---
internal/agent/complete_step_e2e_test.go |  2 +-
2 files changed, 13 insertions(+), 4 deletions(-)

Tests

All passing:

• complete_step_e2e_test.go — 5 E2E tests (helper updated for prefix match)
• canonical_todo_test.go — TestAdvanceCanonicalTodoCompletesAndPromotes etc.
• evidence_flow_test.go — 10 evidence flow tests
• final_readiness_test.go — cross-turn gate behaviour
• internal/evidence — receipt matching
• internal/tool/builtin — complete_step / todo_write execution