agentHost: coordinate overlapping sessions

[Megan Rogge (meganrogge)]

Fixes #330292

Summary

• prompt Agent Host models to identify the current session, exclude it, and check other active sessions across the same project or repository before changing code or creating another session
• direct agents to inspect and message potentially overlapping sessions, dividing or sequencing work to avoid duplicate effort, conflicting edits, and unnecessary merge conflicts
• clarify cross-worktree overlap discovery in list_sessions and keep a short check-first reminder in create_session

Testing

• npm run transpile-client
• ./scripts/test.sh --run src/vs/platform/agentHost/test/node/agentHostPromptRegistry.test.ts
• ./scripts/test.sh --run src/vs/platform/agentHost/test/node/toolInstructions.test.ts
• ./scripts/test.sh --run src/vs/platform/agentHost/test/node/sessionServerTools.test.ts
• ./scripts/test.sh --run src/vs/platform/agentHost/test/node/copilotAgent.test.ts --grep "materialization passes VS Code-specific system message to the SDK"
• ./scripts/test-integration.sh --run src/vs/platform/agentHost/test/node/e2e/providers/copilotPromptsE2E.integrationTest.ts
• targeted ESLint
• npm run hygiene

[Copilot]

Pull request overview

Adds cross-session coordination guidance to prevent overlapping Agent Host work.

Changes:

• Adds universal session-coordination prompt guidance.
• Expands session tool descriptions.
• Updates prompt composition tests.

Show a summary per file

File	Description
toolInstructions.test.ts	Tests coordination prompt composition.
sessionServerTools.ts	Adds overlap checks to session tools.
toolInstructions.ts	Registers universal coordination guidance.

Review details

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• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Balanced

[Megan Rogge (meganrogge)]

/requires-eval-assessment vscbench gpt-5.5,claude-opus-4.7,claude-opus-4.8

sessionTarget: agent-host-copilotcli

[vs-code-engineering]

⏳ Queued vscode build for 635ebb8f6a11f82da5a95b43a0ed68898778baa3 (step 1/2).

• Build: https://dev.azure.com/monacotools/Monaco/_build/results?buildId=463721
• When this succeeds, the eval-assessment publish build will be queued automatically.

[vs-code-engineering]

🚀 Queued eval-assessment publish build for ff9e69f270673cb44bd28ccdab494e0f23e62088 (step 2/2).

• Pipeline run: https://dev.azure.com/monacotools/Monaco/_build/results?buildId=463741
• On success, publishes @vscode/vscode-copilot-evaluation-agent@0.0.0-dev.ff9e69f270 on the dev tag.

[vs-code-engineering]

❌ Eval-assessment build did not succeed (result: failed).

• Pipeline run: https://dev.azure.com/monacotools/Monaco/_build/results?buildId=463741