Refactor enableThinking/reasoningEffort into IModelCapabilityOptions

[bhavyaus]

Summary

Refactors how tool search, context editing, thinking, and reasoning effort are controlled in chat requests. Instead of checking ChatLocation to decide whether to enable these features, callers now explicitly opt in via modelCapabilities on IMakeChatRequestOptions.

This should prevent issues such as #298471 where the claude agent is opted into TST because messagesAPI enables it but was not adding in the required headers since it wasn't aware it was opting into TST enablement.

Port of microsoft/vscode-copilot-chat#5026.

Key Changes

New IModelCapabilityOptions interface

Groups per-request model capability opt-ins:

• enableThinking - opt-in flag for thinking
• reasoningEffort - effort level from model picker
• enableToolSearch - opt-in for tool search tool
• enableContextEditing - opt-in for context editing

Centralized model support checks

• Added modelSupportsToolSearch() and modelSupportsContextEditing() to chatModelCapabilities.ts
• Added supportsToolSearch/supportsContextEditing properties to IChatEndpoint
• Extended IChatModelCapabilities.supports with tool_search and context_editing

Request body gating

• messagesApi.ts reads options.modelCapabilities instead of location-based checks
• Removed isAllowedConversationAgent and isSubagent computations

Test Rubric

Capability	Agent mode	Ask agent	Subagent (runSubagent)	Search subagent	Execution subagent	Claude agent
enableThinking	✅	✅	✅	✅	✅	✅
reasoningEffort	✅	✅	✅	❌	❌	❌
enableToolSearch	✅ (config + model)	✅ (config + model)	❌	❌	❌	❌
enableContextEditing	✅ (config + model)	✅ (config + model)	❌	❌	❌	❌

[Copilot]

Pull request overview

This PR refactors per-request model capability controls by replacing the flat enableThinking / reasoningEffort fields on IMakeChatRequestOptions with a consolidated modelCapabilities?: IModelCapabilityOptions object, and centralizes model capability checks (tool search/context editing) so endpoints can use server-provided metadata.

Changes:

• Introduces IModelCapabilityOptions and plumbs modelCapabilities through request creation paths (Messages API + Responses API).
• Moves tool-search/context-editing model support helpers into chatModelCapabilities.ts and exposes supportsToolSearch / supportsContextEditing on IChatEndpoint.
• Prevents reasoningEffort from flowing into search/execution subagent loops.

Show a summary per file

File	Description
extensions/copilot/src/platform/networking/common/networking.ts	Adds IModelCapabilityOptions and replaces request-level thinking fields with modelCapabilities.
extensions/copilot/src/platform/networking/common/anthropic.ts	Switches capability checks to use endpoint metadata + shared helpers.
extensions/copilot/src/platform/endpoint/test/node/messagesApi.spec.ts	Updates reasoning-effort tests to use modelCapabilities.
extensions/copilot/src/platform/endpoint/node/responsesApi.ts	Reads reasoning effort from options.modelCapabilities.
extensions/copilot/src/platform/endpoint/node/messagesApi.ts	Gates tool search/context editing/thinking off modelCapabilities instead of flat options.
extensions/copilot/src/platform/endpoint/node/chatEndpoint.ts	Populates endpoint capability booleans from server metadata; adjusts Anthropic beta header construction.
extensions/copilot/src/platform/endpoint/common/endpointProvider.ts	Extends metadata schema with tool_search / context_editing capability booleans.
extensions/copilot/src/platform/endpoint/common/chatModelCapabilities.ts	Becomes the central home for tool-search/context-editing model support checks.
extensions/copilot/src/extension/tools/node/toolSearchTool.ts	Imports tool-search supported model prefixes from chatModelCapabilities.
extensions/copilot/src/extension/prompt/node/searchSubagentToolCallingLoop.ts	Strips reasoningEffort from subagent requests via modelCapabilities.
extensions/copilot/src/extension/prompt/node/executionSubagentToolCallingLoop.ts	Strips reasoningEffort from subagent requests via modelCapabilities.
extensions/copilot/src/extension/prompt/node/defaultIntentRequestHandler.ts	Sets per-request capability opt-ins (thinking/tool search/context editing) on outbound requests.
extensions/copilot/src/extension/intents/node/toolCallingLoop.ts	Updates loop fetch options shape to pass modelCapabilities.
extensions/copilot/src/extension/externalAgents/node/oaiLanguageModelServer.ts	Updates internal proxy requests to use modelCapabilities.enableThinking.
extensions/copilot/src/extension/conversation/vscode-node/languageModelAccess.ts	Plumbs reasoning effort into modelCapabilities for extension-language-model requests.
extensions/copilot/src/extension/chatSessions/claude/node/claudeLanguageModelServer.ts	Updates internal proxy requests to use modelCapabilities.enableThinking.

Copilot's findings

• Files reviewed: 16/16 changed files
• Comments generated: 4

[github-actions]

Screenshot Changes

Base: 0ee18249 Current: 962c95e1

Added (2)

chat/aiCustomizations/aiCustomizationManagementEditor/WelcomePage/Dark

chat/aiCustomizations/aiCustomizationManagementEditor/WelcomePage/Light