agentHost: report chat mode change telemetry

[Aaron Munger (amunger)]

Overview

What

Emit chat.modeChange when the effective Agent Host execution mode changes at the session-config boundary. Agent Host values remain native (interactive, plan, and autopilot); they are intentionally not translated to workbench chat/custom-agent mode names. The existing workbench emitter and SDK-native session.mode.changed event are unchanged.

The compatibility row includes standard Agent Host discriminators and suppresses duplicate SDK echoes. Workbench-only custom-agent fields (extensionId, isClaudeAgent, toolsCount, and handoffsCount) are deliberately omitted rather than populated with misleading analogues.

Why

Tracked in microsoft/vscode-internalbacklog#8247. In the sampled 30-day window, chat.modeChange had about 14.4M workbench rows but no Agent Host-discriminated rows in the VS Code telemetry store. This preserves the existing query surface for Agent Host execution-mode transitions while keeping the semantic difference visible in the values themselves.

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Property-by-property parity

Legend: ✅ = exact / verbatim match, ≈ = close analogue, ⊗ = omitted.

Field	Workbench source	Agent Host source	Parity
fromMode	Previous chat/custom-agent mode name	Previous native session execution mode	≈ Different mode axis; native value is preserved
mode	Selected chat/custom-agent mode name	New native session execution mode	≈ Different mode axis; native value is preserved
storage	Selected mode storage	Constant builtin	✅ Agent Host execution modes are built in
extensionId	Selected extension-contributed mode	—	⊗ Custom-agent-only field
isClaudeAgent	Selected Claude custom agent	—	⊗ Custom-agent-only field
provider	—	Agent provider id	Agent Host discriminator
agentSessionId	—	Agent Host session id	Agent Host discriminator
isSubagentSession	—	Session URI classification	Agent Host discriminator

Measurement-by-measurement parity

Metric	Workbench source	Agent Host source	Parity
requestCount	Chat model request count	Completed Agent Host turn count	≈ Closest lifecycle count available at the transition boundary
toolsCount	Selected custom-agent tool count	—	⊗ Custom-agent-only field
handoffsCount	Selected custom-agent handoff count	—	⊗ Custom-agent-only field

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Row-count and dashboard implications

• Historical rows: workbench chat/custom-agent picker transitions only.
• New rows: Agent Host execution-mode transitions add rows under the same event kind; repeated SDK echoes do not add duplicates.
• Discriminator: queries can split Agent Host rows using the presence of provider / agentSessionId.
• Field coverage: custom-agent-only fields remain absent on Agent Host rows.

Semantic-shift ledger

Field	Shift	Impact
fromMode, mode	Agent Host rows describe execution mode, not chat/custom-agent selection	Queries should retain native values and split populations with Agent Host discriminators
requestCount	Completed turns rather than workbench request-array length	Trend-compatible count, but not definitionally identical

[Copilot]

Pull request overview

Adds telemetry for effective Agent Host execution-mode transitions while avoiding duplicate SDK echoes.

Changes:

• Detects interactive, plan, autopilot, and default-reset transitions.
• Emits chat.modeChange with Agent Host session context.
• Adds coverage for transition mapping and deduplication.

Show a summary per file

File	Description
agentSideEffects.test.ts	Tests mode telemetry transitions and deduplication.
agentSideEffects.ts	Detects effective session-mode changes.
agentHostTelemetryReporter.ts	Defines and emits mode-change telemetry.
agentHostStateManager.ts	Publishes post-reducer session-config changes.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Balanced