agentHost: separate agent selection and execution mode telemetry

[Aaron Munger (amunger)]

Overview

This follows up #328504 by keeping two distinct telemetry concepts separate:

• chat.modeChange tracks user selections of chat modes and custom agents.
• agentHost.executionModeChanged tracks Agent Host execution modes: interactive, plan, and autopilot.

The SDK-native session.mode.changed event remains unchanged as a peer signal.

Before

Event	Coverage
chat.modeChange	Regular workbench chat-mode and custom-agent picker selections
SDK session.mode.changed	Copilot SDK execution-mode transitions

Agent Host custom-agent selections made through the Agents Window picker were not reported by chat.modeChange. Agent Host execution-mode transitions also lacked a consistent VS Code-owned event.

After

Event	Coverage
chat.modeChange	Chat-mode and custom-agent selections from both regular workbench chat and the Agents Window
agentHost.executionModeChanged	Effective Agent Host execution-mode transitions
SDK session.mode.changed	Existing SDK-native execution-mode transitions, unchanged

chat.modeChange

Retains the picker-oriented shape:

• fromMode
• mode
• requestCount
• storage
• extensionId
• toolsCount
• handoffsCount
• isClaudeAgent

Custom-agent names retain their existing privacy treatment.

agentHost.executionModeChanged

Reports:

• provider
• agentSessionId
• isSubagentSession
• previousMode
• newMode
• turnCount

Repeated SDK echoes of an already-applied mode transition remain suppressed.

Changes

• Extract the existing chat.modeChange reporting into a shared helper.
• Use the helper from both the regular chat picker and Agents Window picker.
• Move Agent Host execution-mode reporting to agentHost.executionModeChanged.
• Add focused coverage for custom-agent selections, metadata/privacy handling, execution-mode transitions, default resets, and SDK-echo deduplication.

Dashboard implications

• chat.modeChange remains a coherent user-selection event instead of mixing custom-agent and execution-mode value domains.
• Agents Window custom-agent selections add new chat.modeChange rows.
• Agent Host execution transitions move to agentHost.executionModeChanged; they remain distinguishable by provider, agentSessionId, and isSubagentSession.

Validation

• Pre-commit hygiene passes.
• Four focused unit tests pass.
• Full typecheck and layer validation are currently blocked by unrelated errors on main in copilotAgentSession.ts, copilotSystemNotification.ts, and fixtureUtils.ts.

[Copilot]

🟡 Not ready to approve

Agents Window telemetry can attribute mode selections to the wrong session or peer chat.

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Pull request overview

Separates Agent Host execution-mode telemetry from user-selected chat modes while extending chat.modeChange to the Agents Window.

Changes:

• Extracts shared chat-mode telemetry reporting.
• Adds Agents Window picker telemetry.
• Renames Agent Host execution telemetry and updates tests/docs.

File summaries

File	Description
chatModeTelemetry.test.ts	Tests shared telemetry behavior.
chatModeTelemetry.ts	Adds shared reporting helper.
chatExecuteActions.ts	Uses the shared helper.
SESSIONS.md	Documents telemetry separation.
modePicker.test.ts	Tests Agents Window reporting.
modePicker.ts	Reports picker selections.
agentSideEffects.test.ts	Tests execution-mode telemetry.
agentSideEffects.ts	Calls the renamed reporter method.
agentHostTelemetryReporter.ts	Defines the separate execution event.
sessions/SKILL.md	Records the telemetry distinction.

Review details

• Files reviewed: 10/10 changed files
• Comments generated: 1
• Review effort level: Balanced

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