Subagent spawned via Task tool without its own variant inherits parent's variant concatenated onto model ID (model#variant), causing "Model not found" errors #50698

Description

Description (the bug description + error, same as before):
When a subagent is spawned via the Task tool and the target agent has no variant of its own defined, it appears to inherit the parent session's active variant — but instead of applying it as a separate parameter (as the config schema and task.ts both do correctly), something downstream joins it onto the model ID as a single #-delimited string before the request reaches the provider.

Our agent's model field is a clean "litellm/" — no # suffix anywhere in our own config. specs/v2/config.md explicitly documents model and variant as separate fields specifically to avoid ambiguity, since model IDs can contain slash-delimited segments. The SessionModel schema ({ id, providerID, variant: optional(Schema.String) }) and task.ts (const variant = msg.info.variant; const model = next.model ?? { modelID: msg.info.modelID, providerID: msg.info.providerID }) both keep model and variant distinct at that point — something downstream rejoins them into one invalid string.

Error observed:

Model not found: litellm/us-gov.anthropic.claude-sonnet-5#medium
The configured model ID (litellm/us-gov.anthropic.claude-sonnet-5#medium) is invalid. Suggested valid alternatives: us-gov.anthropic.claude-sonnet-5, us-gov.anthropic.claude-opus-4-8, us-gov.anthropic.claude-opus-5.

Plugins

none

OpenCode version

1.18.30

Steps to reproduce

1. Select a non-default reasoning-effort variant on a session (e.g., cycle via the variant keybind to "medium").
2. Spawn a subagent via the Task tool, targeting an agent definition that has no variant of its own.
3. Observe the request fails with: Model not found: /#

Screenshot and/or share link

Operating System

Ubuntu 24.04

Terminal

linux terminal