[BUG] A byo runtime loses the conversation at every suspend: the byo compiler never sets session_db_url

[djannot]

Disclosure: this issue was drafted by an AI agent (Claude) while testing kagent 1.x on Agent
Substrate, and reviewed by a human before filing.

Severity: High · default byo setup (kagent's own Go ADK image) · workaround: override the Harness command (below)

What happens

As a user I run an agent on a byo Harness with kagent's Go ADK image. I tell it my project name in
turn 1. The AgentInstance suspends after the turn, as every instance does. In turn 2 the agent has
forgotten everything (3 of 3 attempts):

turn 1: "My project is named zebra42. Reply with OK."          -> [COMPLETED] OK.
        actor state between turns: ACTOR_STATE_SUSPENDED
turn 2: "What is my project named? Reply with just the name."  -> [INPUT_REQUIRED] What is the name of your project?

The same two turns on a kagent Harness with the same image and model answer zebra42, and so do a
checkpoint fork and a later turn of the original. On byo, every conversation, checkpoint and fork
loses its history after one turn.

Reproduce

kagent v1.0.0-alpha2 on Agent Substrate, with an OpenAI ModelConfig default-model-config:

apiVersion: kagent.dev/v1alpha3
kind: Harness
metadata: {name: byo-harness, namespace: kagent}
spec:
  byo: {}
  workload:
    image: ghcr.io/kagent-dev/kagent/golang-adk@sha256:0ef67974d39518b18de074bd9762fb4aea9764fc8836eeb45d7868b358eecb30
    command: [/app]
  env:
  - {name: PORT, value: "80"}
  substrate:
    workerPoolRef: {name: kagent-default}
    snapshotPolicy: {location: "s3://ate-snapshots/kagent/"}
  allowedAgentTemplates:
    selector: {matchLabels: {kagent.dev/harness: byo-harness}}
---
apiVersion: kagent.dev/v1alpha3
kind: AgentTemplate
metadata: {name: byo-agent, namespace: kagent, labels: {kagent.dev/harness: byo-harness}}
spec:
  description: An agent served by a byo runtime
  modelConfig: {name: default-model-config}
  systemPrompt: You are a concise assistant.

Create an AgentInstance of byo-agent and send the two turns above over
lf.a2a.v1.A2AService/SendMessage.

Expected vs actual

• Expected: turn 2 answers zebra42, as on the kagent runtime.
• Actual: turn 2 has no history; the agent asks for the name.

Ask

Set session_db_url in the byo compiler as the kagent compiler does, or document that a byo runtime
must keep its state under /data and give it that location. Related: #2884 asks for the BYO contract
to be documented.

Root cause: the byo compiler never sets session_db_url, so the Go ADK keeps sessions in memory, and a snapshot keeps only /data

• Every actor gets /data as its durable directory (go/core/internal/substrate/actor_template.go:
  durableDataMount = "/data", a DurableDir volume), and a snapshot keeps only that directory
  (checkpoint tags report SNAPSHOT_CONTENT_SCOPE_DATA). go/harness/README.md: "DurableDir
  contains private state that must survive Actor replacement and snapshot restore".
• The kagent compiler points its runtime there:
  compiled.Config.SessionDBURL = "sqlite+aiosqlite:////data/sessions.db"
  (go/core/internal/translator/kagent/compiler.go).
• The byo compiler (go/core/internal/translator/byo/compiler.go) builds the same ADK-shaped config
  with adkconfig.Build, but never sets SessionDBURL.
• Without it the Go ADK uses an in-memory session store (go/adk/cmd/main.go: "no session DB
  configured, using in-memory session"), which a restore does not bring back.
• go/adk/pkg/session/local.go expects a BYO image to receive the URL: it accepts the Python
  dialect "so a BYO image built with this SDK works regardless of which dialect it was handed".

Workaround, which also confirms the fix

A byo Harness whose command adds the field before it starts the runtime keeps the conversation
through the suspend, a checkpoint fork, and a later turn of the original:

    command:
    - bash
    - -c
    - |
      export KAGENT_CONFIG_JSON="{\"session_db_url\":\"sqlite:////data/sessions.db\",${KAGENT_CONFIG_JSON#\{}"
      exec /app

Scope

• Reproduced on kind with kagent v1.0.0-alpha2 and Agent Substrate 0.2.0-beta5, 3 of 3
  attempts. Source-confirmed on main at c61fad8b (2026-09-23).
• Unaffected: the kagent runtime (its sessions are in /data/sessions.db). Single-turn byo agents
  work, and so do tools and tool approval.