Implement subagent default setting to Auto without overriding models by sbatten · Pull Request #333663 · microsoft/vscode

SteVen Batten (sbatten) · 2026-08-31T22:21:27Z

This pull request introduces a new setting, chat.subagents.defaultToAuto, which allows local subagents to default to the Auto model when no specific model is specified. Key changes include:

New Setting Implementation: The setting is off by default and only affects unspecified non-BYOK subagents, allowing them to use the eligible Copilot Auto model.
Model Resolution Logic: The logic for model resolution has been updated to ensure that explicit and agent-configured models take precedence over the Auto model.
Cache-First Lookup: The Auto model lookup is now cache-first, activating the provider only when necessary.
Testing: Comprehensive tests have been added to validate participant routing, cache behavior, eligibility, precedence, and fallback scenarios. All tests have passed successfully.

This change enhances the flexibility of subagent behavior while maintaining existing configurations and ensuring backward compatibility.

Copilot

Copilot review overview

Review tier: Balanced
Findings: 1 · 1

New issues introduced by this change (2)

Severity	Finding
	`src/vs/workbench/contrib/chat/common/tools/builtinTools/runSubagentTool.ts` — When `agentName` is omitted, `subagent` is always `undefined`, even though this tool inherits the…
	`src/vs/workbench/contrib/chat/test/common/tools/builtinTools/runSubagentTool.test.ts` — This and the following direct-invocation Auto test are inside the `nested subagent depth tracking`…

What changed in this PR

Adds an experimental setting to default eligible local subagents to Copilot Auto while preserving explicit, agent-configured, and BYOK model selections.

Changes:

Registers chat.subagents.defaultToAuto.
Adds cache-first Auto model resolution and eligibility checks.
Expands model precedence, fallback, and routing tests.

File	Description
`chat.shared.contribution.ts`	Registers the setting.
`constants.ts`	Adds its configuration key.
`runSubagentTool.ts`	Implements Auto model resolution.
`runSubagentTool.test.ts`	Adds routing and fallback coverage.

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Bhavya U (bhavyaus) · 2026-09-03T17:49:13Z

This PR has now been rebased onto the latest main and updated with the follow-up implementation.

In addition to preserving explicit, agent-configured, inherited, and BYOK model behavior while allowing eligible subagents to default to Auto, the PR now adds product telemetry for the model-selection source. The chat.subagentModelSelection event is emitted when the subagent is invoked and contains no model IDs, agent names, prompts, paths, or other user-authored content.

`selectionSource` value	Meaning
`explicitModel`	The `runSubagent` tool call explicitly supplied its `model` argument, and that model resolved successfully.
`agentModel`	An explicitly named subagent supplied a model through its `model:` configuration, and that model resolved successfully.
`autoDefault`	`chat.subagents.defaultToAuto` successfully replaced the inherited model with Auto. This proves the new feature wired Auto in.
`mainModel`	The subagent kept the parent/main request model. This includes the setting being disabled, BYOK preservation, Auto being unavailable or ineligible, an unavailable configured agent model, or inheriting the current agent’s already-selected model.

Selection precedence:

explicitModel → agentModel → autoDefault → mainModel

The event records the source of the model assigned to the subagent, not the concrete backend to which Auto later routes.

Validation completed: 48/48 targeted tests passed, full client and Copilot extension compilation passed, ESLint and pre-commit hygiene passed, and live Code OSS validation confirmed that the parent remained on Claude Opus 5 while the Auto-routed subagent ran on MAI Code 1.1 Flash.

github-actions · 2026-09-03T18:06:24Z

Screenshot Changes

Base: 1e1ee361 Current: 78e63f3e

2 insignificant change(s) omitted (≤20 px, Δ≤2). See CI logs for details.

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Copilot

Copilot review overview

🟡 Changes recommended

The new picker guard mishandles defined empty-string subagent invocation IDs.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review tier: Balanced
Findings: 1

New issues introduced by this change (1)

Severity	Finding
	`extensions/copilot/src/extension/conversation/vscode-node/chatParticipants.ts` — Use a presence check here rather than truthiness. An empty-string `subAgentInvocationId` is still…

Issues resolved since last review (2)

Severity	Finding
	`src/vs/workbench/contrib/chat/test/common/tools/builtinTools/runSubagentTool.test.ts` — This and the following direct-invocation Auto test are inside the `nested subagent depth tracking`… View resolved comment
	`src/vs/workbench/contrib/chat/common/tools/builtinTools/runSubagentTool.ts` — When `agentName` is omitted, `subagent` is always `undefined`, even though this tool inherits the… View resolved comment

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Copilot

Copilot review overview

🟡 Changes recommended

Automatic rate-limit switching can still change the global model picker from a subagent request.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review tier: Balanced
Findings: 1

New issues introduced by this change (1)

Severity	Finding
	`extensions/copilot/src/extension/conversation/vscode-node/chatParticipants.ts` — The picker guard is incomplete: `switchToAutoModel` still unconditionally executes…

Issues resolved since last review (1)

Severity	Finding
	`extensions/copilot/src/extension/conversation/vscode-node/chatParticipants.ts` — Use a presence check here rather than truthiness. An empty-string `subAgentInvocationId` is still… View resolved comment

Copilot AI balanced review requested due to automatic review settings August 31, 2026 22:21

Copilot started reviewing on behalf of SteVen Batten (sbatten) August 31, 2026 22:22 View session

Copilot AI reviewed Aug 31, 2026

View reviewed changes

Comment thread src/vs/workbench/contrib/chat/common/tools/builtinTools/runSubagentTool.ts

Comment thread src/vs/workbench/contrib/chat/test/common/tools/builtinTools/runSubagentTool.test.ts

vs-code-engineering Bot assigned SteVen Batten (sbatten) Sep 1, 2026

SteVen Batten (sbatten) and others added 3 commits September 2, 2026 22:12

Agent Host changes for sbatten/agents/subagent-default-auto-setting

bd23379

Fix subagent Auto model selection

1e63f57

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Add subagent model selection telemetry

f000994

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Bhavya U (bhavyaus) requested a review from Logan Ramos (lramos15) September 3, 2026 17:58

Bhavya U (bhavyaus) force-pushed the sbatten/agents/subagent-default-auto-setting branch from 0586fa1 to f000994 Compare September 3, 2026 17:58

Bhavya U (bhavyaus) self-assigned this Sep 3, 2026

Bhavya U (bhavyaus) marked this pull request as ready for review September 3, 2026 17:58

Bhavya U (bhavyaus) requested a balanced review from Copilot September 3, 2026 18:03

Copilot started reviewing on behalf of Bhavya U (bhavyaus) September 3, 2026 18:04 View session

Copilot AI reviewed Sep 3, 2026

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Comment thread extensions/copilot/src/extension/conversation/vscode-node/chatParticipants.ts Outdated

Change subAgentInvocationId check to undefined

b7222f6

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Bhavya U (bhavyaus) requested a balanced review from Copilot September 3, 2026 18:30

Copilot started reviewing on behalf of Bhavya U (bhavyaus) September 3, 2026 18:31 View session

Copilot AI reviewed Sep 3, 2026

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Comment thread extensions/copilot/src/extension/conversation/vscode-node/chatParticipants.ts

Logan Ramos (lramos15) approved these changes Sep 3, 2026

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Martin Aeschlimann (aeschli) approved these changes Sep 3, 2026

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Bhavya U (bhavyaus) merged commit cec8ce7 into main Sep 3, 2026
41 checks passed

Bhavya U (bhavyaus) deleted the sbatten/agents/subagent-default-auto-setting branch September 3, 2026 19:49

vs-code-engineering Bot added this to the 1.137.0 milestone Sep 3, 2026

Bhavya U (bhavyaus) mentioned this pull request Sep 3, 2026

Avoid changing model picker for subagent Auto retries #334357

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Implement subagent default setting to Auto without overriding models - #333663

Implement subagent default setting to Auto without overriding models#333663
Bhavya U (bhavyaus) merged 4 commits into
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