[bug] continuity.reason is computed, phrased for a human, returned on every session open, and displayed nowhere

[serge-ivo]

resolveSessionContinuity goes to deliberate trouble to produce a sentence a human can read, the API returns it on every session open, and no surface shows it.

What is computed

lib/coding-session-continuity.ts:59-63 — the field exists specifically to be shown:

/**
 * Why, phrased so a reply can use it verbatim after "started a fresh conversation — ".
 * Empty-ish reasons are not allowed: an unexplained fresh start is the bug report #408 predicts.
 */
reason: string;

Every branch fills it: "you asked for a clean slate", "the previous session on this repo ran a different engine", "the previous session never got its engine running", "the previous conversation on this repo was last touched 3 days ago". describeAge (:78) exists solely to round that age to a unit that reads as a fact.

What is returned

Observed 2026-08-17, POST /v1/instances/bd43f4de-…/coding/sessions:

{
  "session": { "id": "csess_7d077f96-…", "status": "active", … },
  "runnerConnected": true,
  "resumed": true,
  "continuity": {
    "mode": "resume",
    "resumeFrom": "csess_21b8d1b9-…",
    "reason": "the previous conversation on this repo was last touched 11 hours ago"
  }
}

What is displayed

Nothing. grep -c continuity agents/coder/web/src/CodingTab.tsx → 0. That file is the only CodingTab — the console imports it (store/console/src/lib/surfaces.tsx:2, import { CodingTab } from "@proagentstore/coder-web"), so this is not a two-implementations problem. The MCP side does not surface it either (#696).

So the platform knows whether it kept your conversation, knows why, has phrased it for you, and says nothing.

Why it matters here specifically

This is the sentence that makes seamlessness legible. Without it the two outcomes are indistinguishable at a glance: a resumed session and a cold one both open on a blank pane. The owner hit exactly this on 2026-08-17 — the work had been preserved and the resume worked, and the reasonable conclusion from the screen was that everything was lost.

It is also the honest counterpart to #695. Removing "your session ended, start a new one" leaves a gap; this is what belongs in it. The two should ship together, or the first one alone just makes the platform quieter rather than clearer.

Fix

Render continuity.reason on open, prefixed by mode:

• resume → "Picking up where you left off — the previous conversation on this repo was last touched 11 hours ago."
• fresh → "Started a fresh conversation — {reason}." (the doc comment already phrases the field for exactly this concatenation)

openNotice already exists as state in CodingTab.tsx:128 and is already used for the reused-engine notice (reusedEngineNotice, #549), so there is a place to put it and a precedent for the shape.

Show it once per open, not persistently. A fresh outcome deserves more prominence than a resume one — resume is the expected case and should be reassuring rather than loud, while an unexpected cold start is the thing the user needs to know before they start talking to an agent that has forgotten them.

[serge-ivo]

Shipped in 77e8e4eb (+ 5212948c, the 320px measurement). CI green, Host Worker deployed.

Verified live, not just in CI — both strings are present in the console bundle served from https://proagentstore.online/console/:

Picking up where you left off    => 1
Started a fresh conversation     => 1

What was built. agents/coder/web/src/open-notice.ts (new, pure): continuityNotice(continuity) → {id, tone, text} | null. resume → "Picking up where you left off — {reason}." (tone quiet); fresh → "Started a fresh conversation — {reason}." (tone warn), honouring the asymmetry the issue asks for — resume is the expected case and reads as reassurance; a cold start is the loud one, because that is the thing the user needs to know before they start talking to an agent that has forgotten them. Shown once per open (cleared on close), rendered by OpenNoticeBanners.tsx at both session views.

Composition with #549. openNotices returns a list, not a slot. The reused-engine notice and the continuity notice answer different questions — which engine you got and what it remembers — so neither may overwrite the other.

It takes unknown. The reuse path, the create-race loser and any older API all answer without a continuity block, so absent is ordinary rather than an error: it renders nothing, and a missing reason keeps the headline without a dangling em dash.

Vocabulary. The openers say "conversation", never "session" — and #695 (shipped alongside) reworded four of the seven server-side reason branches for the same reason, since it is this rendering that made them user-facing. The unit test asserts the no-"session" property with reason absent, so it measures our prose rather than the server's.

File-size ratchet: CodingTab.tsx had one line of headroom against its pin. The pin was not raised — the decision (open-notice.ts) and the rendering (OpenNoticeBanners.tsx) were extracted, landing the file at 1466.

Verified: pnpm typecheck, pnpm vitest run (9 new unit cases), biome, all nine script gates, docs:drift, docs:build, console vite build, and pnpm test:e2e 423/423 across chromium + webkit — the new #697 specs run in the WebKit project and assert overflow at 320px and 390px with the longest reason the policy can emit.