feat(slack): add onEmpty delivery policy for proactive sessions

[vanessaxteo]

Problem

A proactive session started via receive(slack, { message, target, auth }) — most commonly a scheduled "check" task — delivers its final assistant message like any turn. So a run that finds nothing to report either posts noise, or (if it emits an empty message) goes completely silent with no way to tell it actually ran. There's no first-class way for a run to signal "I executed, there's nothing to deliver," and no opt-in liveness signal.

This came up building scheduled tasks for an internal agent (v): a daily "check for X" task posts a full response even when there's nothing new, and suppressing it entirely leaves users unsure the schedule is alive. Today the only workaround is a per-agent magic-string sentinel that the channel's message.completed override has to recognize.

Change

Add an onEmpty delivery policy to the Slack receive target, threaded into session state and honored by the default message.completed handler:

await receive(slack, {
  message: "Check for new incidents; report only if there are any.",
  target: { channelId, onEmpty: "heartbeat" },
  auth,
});

• "suppress" (default) — post nothing, identical to today's behavior for an empty turn.
• "heartbeat" — post a short built-in confirmation line (✓ Ran — nothing new to report.).
• { heartbeat: "…" } — post a caller-supplied line instead (e.g. with the schedule's name).

The agent still decides when there's nothing to report (instruct it to end such a turn with an empty message); the channel just applies the policy. No sentinel string required.

Notes

• Scoped to the Slack channel. onEmpty rides on the channel-specific receive target, so the generic cross-channel receive path is untouched. Other channels can adopt the same shape later.
• Backward compatible. Interactive (mention/DM) sessions carry no onEmpty and continue to suppress empty output. Only addition: a whitespace-only final message is now treated as empty (previously it would post blank text).
• New SlackEmptyDeliveryPolicy type exported from eve/channels/slack.

Tests / checks

• defaults.test.ts: posts real content; suppresses empty/whitespace by default; built-in heartbeat; custom heartbeat; real content wins over heartbeat; tool-call narration still buffers without posting.
• slackChannel.test.ts: receive threads target.onEmpty into seeded session state.
• Full eve unit tier green (3839 tests), typecheck, lint, fmt, guard:invariants, and docs:check all pass. Changeset added (patch). Docs updated (channels/slack → Proactive sessions).

Context

Companion to the agent-side workaround in our internal repo (a prompt sentinel + a one-line message.completed guard); this lands the capability in the framework so agents don't each reinvent it.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
eve-docs	Ready	Preview, Comment, Open in v0	Jun 23, 2026 1:36am

[github-actions]

Bundle + Package Summary: apps/fixtures/weather-agent

Key takeaways

• No notable deltas vs main (b4fc7ac).

Delta vs main (b4fc7ac)

Area	Metric	Baseline	Current	Delta
Package	Packed tarball	3.29 MB	3.29 MB	+916 B ⚠️
Package	Unpacked publish size	11.85 MB	11.86 MB	+2.5 kB ⚠️
Package	Installed footprint	52.06 MB	52.06 MB	+2.5 kB ⚠️
Package	Published files	2239	2239	0
Package	Installed files	5444	5444	0
Runtime	Unique function payloads	2	2	0
Runtime	Total function bytes	9.35 MB	9.35 MB	-176 B ✅
Runtime	Public routes	9	9	0

Changed function payloads vs main (b4fc7ac) (2)

Function	Status	Baseline	Current	Delta	Route changes
functions/__server.func	changed	3.87 MB	3.87 MB	-144 B ✅	none
functions/.well-known/workflow/v1/flow.func	changed	5.48 MB	5.48 MB	-32 B ✅	none

Build Metadata

• Preset: vercel
• Nitro: nitro@3.0.260610-beta
• Output directory: apps/fixtures/weather-agent/.vercel/output
• Build metadata timestamp: 2026-06-23T01:36:57.591Z
• Route aliases: 9 public, 1 internal (10 total aliases)
• Vercel routes in config: 10
• Severity legend: 🔴 dominant/large, 🟠 notable, 🟡 watch, ⚪ small

Package Drill-Down

Package Details

• Package: eve@0.13.0
• Package directory: packages/eve
• Tarball: 3.29 MB (eve-0.13.0.tgz)
• Unpacked payload: 11.86 MB across 2239 published files
• Installed footprint: 52.06 MB across 5444 installed files
• Installed root package: 10.74 MB
• Installed dependencies: 41.32 MB
• Runtime dependencies: 1
• Peer dependencies: 12 (11 optional)

Installed footprint is measured from an isolated temporary npm install of the packed tarball.

Heavy installed dependencies

• @rolldown/binding-linux-x64-gnu: 20.61 MB (39.6%)
• eve: 10.74 MB (20.6%)
• ai: 6.20 MB (11.9%)
• zod: 4.97 MB (9.5%)
• nitro: 2.41 MB (4.6%)

Publish payload breakdown

Published file size
🟠 dist/src/compiled/experimental-ai-sdk-code-mo... [####....................] 1.51 MB 12.7%
🟡 dist/src/compiled/@workflow/core/runtime.js      [##......................] 775.4 kB 6.5%
🟡 dist/src/compiled/@vercel/sandbox/index.js       [##......................] 632.0 kB 5.3%
🟡 dist/src/compiled/@chat-adapter/slack/index.js   [#.......................] 436.9 kB 3.7%
🟡 dist/src/compiled/_chunks/workflow/attribute-... [#.......................] 370.9 kB 3.1%
🔴 Other published files                            [########################] 8.14 MB 68.6%

Installed footprint breakdown

Installed package size
🔴 @rolldown/binding-linux-x64-gnu [########################] 20.61 MB 39.6%
🔴 eve                             [#############...........] 10.74 MB 20.6%
🔴 ai                              [#######.................] 6.20 MB 11.9%
🟠 zod                             [######..................] 4.97 MB 9.5%
🟠 nitro                           [###.....................] 2.41 MB 4.6%
🟡 rolldown                        [#.......................] 771.0 kB 1.5%
🔴 Other installed packages        [#######.................] 6.37 MB 12.2%

Runtime dependencies (1)

Package	Range	Notes
nitro	3.0.260610-beta

Peer dependencies (12)

Package	Range	Notes
@opentelemetry/api	^1.0.0	optional peer
@sveltejs/kit	^2.0.0	optional peer
ai	catalog:
braintrust	^3.0.0	optional peer
just-bash	^3.0.0	optional peer
microsandbox	^0.5.0	optional peer
next	^16.0.0	optional peer
nuxt	^4.0.0	optional peer
react	^19.0.0	optional peer
svelte	^5.0.0	optional peer
vite	^8.0.0	optional peer
vue	^3.5.0	optional peer

Function Drill-Down

Payload Size Graph

Unique function payload size and share of total
🔴 functions/.well-known/workflow/v1/flow.func     [########################] 5.48 MB 58.6%
🔴 functions/__server.func                         [#################.......] 3.87 MB 41.4%

Top Function Payloads

🟠 functions/.well-known/workflow/v1/flow.func • 1 public route • 5.48 MB

Metric	Value
Public routes	/.well-known/workflow/v1/flow
Runtime	nodejs24.x
Handler	index.mjs
Payload	5.48 MB
Function files	5.48 MB across 27 files
Traced dependencies	0 B
Signal	🟠 Bundled file __eve_nitro_handler__.mjs is 1.51 MB (27.6%)

🟠 🔎 Dependency Analysis

📦 Bundled files:

Bundled file size
🟠 __eve_nitro_handler__.mjs              [########################] 1.51 MB 27.6%
🟠 _chunks/runtime.mjs                    [###############.........] 958.4 kB 17.5%
🟡 _chunks/sandbox.mjs                    [############............] 766.0 kB 14.0%
🟡 _chunks/attribute-changes-Bi5DLT8S.mjs [########................] 472.2 kB 8.6%
🟡 _chunks/dist-DTchiX0N.mjs              [#######.................] 460.6 kB 8.4%
🟠 Other bundled files                    [#####################...] 1.31 MB 24.0%

🧾 Vercel Config

{
  "handler": "index.mjs",
  "launcherType": "Nodejs",
  "shouldAddHelpers": false,
  "supportsResponseStreaming": true,
  "runtime": "nodejs24.x",
  "environment": {
    "NODE_OPTIONS": "--experimental-require-module",
    "WORKFLOW_QUEUE_NAMESPACE": "eve"
  },
  "maxDuration": "max",
  "experimentalTriggers": [
    {
      "type": "queue/v2beta",
      "topic": "__eve_wkf_workflow_*",
      "consumer": "default",
      "retryAfterSeconds": 5,
      "initialDelaySeconds": 0
    }
  ]
}

---

🟠 functions/__server.func • 8 public routes, 1 internal alias • 3.87 MB

Metric	Value
Public routes	/ /eve/v1/callback/[token] /eve/v1/connections/[name]/callback/[token] /eve/v1/health /eve/v1/info /eve/v1/session /eve/v1/session/[sessionId] /eve/v1/session/[sessionId]/stream
Internal aliases	/__server
Runtime	nodejs24.x
Handler	index.mjs
Payload	3.87 MB
Function files	3.87 MB across 21 files
Traced dependencies	0 B
Signal	🟠 Bundled file index.mjs is 1.40 MB (36.3%)

🟠 🔎 Dependency Analysis

📦 Bundled files:

Bundled file size
🟠 index.mjs                              [########################] 1.40 MB 36.3%
🟠 _chunks/runtime.mjs                    [###############.........] 875.8 kB 22.6%
🟠 _chunks/sandbox.mjs                    [#############...........] 766.0 kB 19.8%
🟡 _chunks/attribute-changes-Bi5DLT8S.mjs [########................] 448.5 kB 11.6%
⚪ _libs/zod.mjs                          [##......................] 114.2 kB 3.0%
🟡 Other bundled files                    [####....................] 258.8 kB 6.7%

🧾 Vercel Config

{
  "handler": "index.mjs",
  "launcherType": "Nodejs",
  "shouldAddHelpers": false,
  "supportsResponseStreaming": true,
  "runtime": "nodejs24.x"
}