agentHost: lean tools + CLI-parity prompt for HydraFusion

src/vs/platform/agentHost/common/copilotCliConfig.ts

@@ -121,6 +121,33 @@ export interface ICopilotCliModelCapabilityOverride {
 /** Map of model id → capability override. */
 export type CopilotCliModelCapabilityOverrides = Record<string, ICopilotCliModelCapabilityOverride>;
 
+/**
+ * Tools excluded by default for {@link COPILOT_HYDRA_FUSION_MODEL_ID}, matching
+ * the Copilot CLI's lean tool set. Their definitions add ~13K tokens to every
+ * HydraFusion request. `ask_user` and `web_fetch` stay available because the
+ * interactive CLI ships them. A configured `availableTools` or `excludedTools`
+ * capability override replaces this default.
+ */
+export const HYDRAFUSION_DEFAULT_EXCLUDED_TOOLS: readonly string[] = [
+	'usages', 'problems',
+	'createJupyterNotebook', 'editNotebook', 'runNotebookCell', 'getNotebookSummary', 'readNotebookCellOutput',
+	'runTask', 'getTaskOutput', 'createAndRunTask',
+	'openBrowserPage', 'readPage', 'screenshotPage', 'navigatePage', 'clickElement', 'dragElement', 'hoverElement', 'typeInPage', 'runPlaywrightCode', 'handleDialog',
+	'runTests', 'testFailure',
+	'addComment', 'listComments', 'replyToComment', 'deleteComments', 'resolveComments', 'viewUnreviewedComments',
+	'list_sessions', 'get_current_session', 'create_session', 'rename_chat', 'send_message', 'get_session_context', 'delete_session',
+	'setAgentMergeEnabled', 'readAgentMergeCI', 'replyToAgentMergeReviewThread', 'rerunAgentMergeWorkflow',
+	'add_artifact_or_reference', 'remove_artifact_or_reference', 'list_artifacts_and_references',
+];
+
+/**
+ * Built-in `excludedTools` default for a model (after `family` aliasing), used
+ * only when no `availableTools` or `excludedTools` capability override applies.
+ */
+export function getDefaultExcludedTools(modelId: string | undefined): readonly string[] | undefined {
+	return modelId === COPILOT_HYDRA_FUSION_MODEL_ID ? HYDRAFUSION_DEFAULT_EXCLUDED_TOOLS : undefined;
+}
+
 /** Wildcard entry key matching every model id; a specific model-id entry wins field-by-field. */
 export const MODEL_CAPABILITY_OVERRIDE_WILDCARD = '*';
 

src/vs/platform/agentHost/node/copilot/copilotSessionLauncher.ts

@@ -17,7 +17,7 @@ import { AgentSession } from '../../common/agent.js';
 import type { IAgentProviderSendStageRecorder } from '../../common/agentHostTelemetry.js';
 import { getByokLmSelectionModelId, resolveByokLmEnablement, type IByokLmModelInfo } from '../../common/agentHostByokLm.js';
 import { AgentHostByokModelsEnabledConfigKey, AgentHostMcpConnectorsEnabledConfigKey, AgentHostSessionSyncEnabledConfigKey, platformRootSchema, type AgentHostMcpServers } from '../../common/agentHostSchema.js';
-import { CopilotCliConfigKey, copilotCliConfigSchema, normalizeModelFamilyAlias, normalizeToolSearchDeferThreshold, resolveModelCapabilityOverrideField } from '../../common/copilotCliConfig.js';
+import { CopilotCliConfigKey, copilotCliConfigSchema, getDefaultExcludedTools, normalizeModelFamilyAlias, normalizeToolSearchDeferThreshold, resolveModelCapabilityOverrideField } from '../../common/copilotCliConfig.js';
 import { IAgentHostOTelService } from '../../common/otel/agentHostOTelService.js';
 import { reasoningEffortLevels, type ReasoningEffortLevel } from '../../common/reasoningEffort.js';
 import { getSessionSandboxConfig } from '../sessionSandbox.js';
@@ -1010,7 +1010,16 @@ export class CopilotSessionLauncher implements ICopilotSessionLauncher {
 			this._logService.warn(`[Copilot:${plan.sessionId}] Ignoring unusable 'excludedTools' capability override for '${modelId}'; expected an array of tool patterns`);
 		});
 		const availableTools = getToolFilterOverride(availableToolsOverride, 'availableTools', modelId, this._logService, plan.sessionId);
-		const excludedTools = getToolFilterOverride(excludedToolsOverride, 'excludedTools', modelId, this._logService, plan.sessionId);
+		const configuredExcludedTools = getToolFilterOverride(excludedToolsOverride, 'excludedTools', modelId, this._logService, plan.sessionId);
+		// A configured filter on either field replaces the built-in default, so an
+		// `availableTools` override can re-enable a default-excluded tool.
+		const defaultExcludedTools = availableTools === undefined && configuredExcludedTools === undefined
+			? getDefaultExcludedTools(modelFamily ?? model?.id)
+			: undefined;
+		if (defaultExcludedTools) {
+			this._logService.info(`[Copilot:${plan.sessionId}] Applying default 'excludedTools' for '${modelFamily ?? modelId}' (${defaultExcludedTools.length} tools)`);
+		}
+		const excludedTools = configuredExcludedTools ?? defaultExcludedTools;
 		const sdkAvailableTools = toSdkToolFilterPatterns(availableTools);
 		const configuredSdkExcludedTools = plan.isEphemeral
 			? [...(toSdkToolFilterPatterns(excludedTools) ?? []), ...EPHEMERAL_DISABLED_COPILOT_TOOLS]

src/vs/platform/agentHost/node/copilot/prompts/AGENTS.md

@@ -9,6 +9,7 @@ This directory customizes the system prompt for Copilot CLI **agent host** (ahp+
 - `toolInstructions.ts` — the model-agnostic `tool_instructions` layer: gated or unconditional nudges (`TOOL_INSTRUCTION_LINES`) composed into the SDK's `tool_instructions` section, including the default-model guidance for subagents.
 - `anthropicPrompt.ts` — example per-model contributor (Claude Opus 4.8).
 - `openaiPrompt.ts` — OpenAI targeted post-edit inspection guidance, appended to `code_change_rules` without replacing the SDK foundation prompt.
+- `hydraFusionPrompt.ts` — HydraFusion (`hydrafusion`): replaces `tone` and `tool_efficiency` with the Copilot CLI's brevity, search/delegation, and tool-efficiency guidance. Pairs with the launcher's default `excludedTools` for the model (`HYDRAFUSION_DEFAULT_EXCLUDED_TOOLS`: 42 VS Code-only tools; `ask_user` and `web_fetch` stay, as in the interactive CLI).
 - `allPrompts.ts` — side-effect import hub; importing it registers every contributor into the shared `agentHostPromptRegistry`.
 
 ## How the system message is built
@@ -84,7 +85,7 @@ This branch's OpenAI contributor is unconditional for GPT families, legacy `o1`/
 
 ## Related — per-model experimentation knobs (`copilotCliConfig.ts`)
 
-`chat.agentHost.copilot.modelCapabilityOverrides` entries (keyed by model id; `'*'` matches every model, a specific entry wins field-by-field) carry the experimentation knobs the launcher applies: `family` (prompt and tool-profile alias, so a preview model resolves through another family's contributor), `reasoningEffort` (wins over the model picker's thinking level; set it on the `'*'` entry to pin every model, re-applied on session resume and mid-session model change), `availableTools`/`excludedTools` (SDK tool filters; applied on launch and resume, but not on a mid-session model change — and enforced against every SDK-registered tool, including the host's shell and server tools, not just the forwarded client tools), `modelCapabilities` (per-property overrides passed through to the SDK's `modelCapabilities` field — e.g. vision support, token limits — applied on every launch and resume), and `promptOverrideString`/`promptOverrideFile` (YAML system-prompt and tool-description overrides, applied on launch and resume).
+`chat.agentHost.copilot.modelCapabilityOverrides` entries (keyed by model id; `'*'` matches every model, a specific entry wins field-by-field) carry the experimentation knobs the launcher applies: `family` (prompt and tool-profile alias, so a preview model resolves through another family's contributor), `reasoningEffort` (wins over the model picker's thinking level; set it on the `'*'` entry to pin every model, re-applied on session resume and mid-session model change), `availableTools`/`excludedTools` (SDK tool filters; applied on launch and resume, but not on a mid-session model change — and enforced against every SDK-registered tool, including the host's shell and server tools, not just the forwarded client tools; `hydrafusion` defaults `excludedTools` to `HYDRAFUSION_DEFAULT_EXCLUDED_TOOLS` when neither filter is configured for it, and any configured filter — including `excludedTools: []` to restore every tool — replaces that default), `modelCapabilities` (per-property overrides passed through to the SDK's `modelCapabilities` field — e.g. vision support, token limits — applied on every launch and resume), and `promptOverrideString`/`promptOverrideFile` (YAML system-prompt and tool-description overrides, applied on launch and resume).
 
 `family` is host-side only: it selects the prompt contributor and the tool-search capability gate, and the model id sent to the runtime is unchanged, so the session still runs on the selected model. That is the point — a preview model can be evaluated against a known family's prompt and tool profile while still hitting its own endpoint.
 

src/vs/platform/agentHost/node/copilot/prompts/allPrompts.ts

@@ -13,4 +13,5 @@
 //   import './geminiPrompt.js';
 
 import './anthropicPrompt.js';
+import './hydraFusionPrompt.js';
 import './openaiPrompt.js';

src/vs/platform/agentHost/node/copilot/prompts/hydraFusionPrompt.ts

@@ -0,0 +1,55 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import type { SectionOverride, SystemMessageSection } from '@github/copilot-sdk';
+import { COPILOT_HYDRA_FUSION_MODEL_ID } from '../../../common/copilotCliConfig.js';
+import type { ModelSelection } from '../../../common/state/protocol/state.js';
+import { agentHostPromptRegistry, type IAgentHostPrompt } from './promptRegistry.js';
+
+const HYDRAFUSION_TONE = [
+	'# Tone and style',
+	'* When providing output or explanation to the user, limit your response to 100 words or less.',
+	'* Be concise in routine responses. For complex tasks, briefly explain your approach before implementing.',
+	'* Prioritize brevity. Default to the shortest possible response that satisfies the request. Cut filler, recap, and process narration.',
+].join('\n');
+
+const HYDRAFUSION_TOOL_EFFICIENCY = [
+	'# Search and delegation',
+	'* Give sub-agents comprehensive context; response-brevity rules do not apply to their prompts.',
+	'* Search files/text only in the cwd or its descendants unless absolutely necessary. For code, prefer: available code intelligence > available LSP > glob > grep with a glob > bash.',
+	'* For broad unfamiliar-code exploration, prefer `search_code_subagent`; use direct search tools for narrow lookups you can resolve in a couple of calls.',
+	'',
+	'# Tool usage efficiency',
+	'CRITICAL: Maximize tool efficiency:',
+	'* For simple searches, reads, or edits requiring only 2-5 direct calls, use grep, glob, view, edit yourself; delegate only complex/long work that benefits from separate context, since sub-agents add latency.',
+	'* **USE PARALLEL TOOL CALLING** - when you need to perform multiple independent operations, make ALL tool calls in a SINGLE response. For example, if you need to read 3 files, make 3 view tool calls in one response, NOT 3 sequential responses.',
+	'* Chain related bash commands with && instead of separate calls',
+	'* Suppress verbose output (use --quiet, --no-pager, pipe to grep/head when appropriate)',
+	'* Batching does not replace investigation; take as many turns as needed to understand before acting.',
+	'* Default task agents to sync; use background only while doing independent work, not to poll while idle.',
+].join('\n');
+
+/**
+ * HydraFusion prompt: replaces the `tone` and `tool_efficiency` sections with
+ * the Copilot CLI's guidance so HydraFusion makes as few, as concise requests
+ * in the agent host as it does in the standalone CLI. Pairs with the lean
+ * default tool set (`HYDRAFUSION_DEFAULT_EXCLUDED_TOOLS`).
+ */
+class HydraFusionPromptResolver implements IAgentHostPrompt {
+	static readonly familyPrefixes: readonly string[] = [];
+
+	static matchesModel(model: ModelSelection): boolean {
+		return model.id === COPILOT_HYDRA_FUSION_MODEL_ID;
+	}
+
+	resolveSectionOverrides(): Partial<Record<SystemMessageSection, SectionOverride>> {
+		return {
+			tone: { action: 'replace', content: HYDRAFUSION_TONE },
+			tool_efficiency: { action: 'replace', content: HYDRAFUSION_TOOL_EFFICIENCY },
+		};
+	}
+}
+
+agentHostPromptRegistry.registerPrompt(HydraFusionPromptResolver);

src/vs/platform/agentHost/test/node/agentHostPromptRegistry.test.ts

@@ -264,6 +264,34 @@ suite('AgentHostPromptRegistry', () => {
 		});
 	});
 
+	suite('HydraFusion contributor (registered via allPrompts)', () => {
+		// Section text is pinned by its headings so copy edits don't churn the test.
+		const headings = (override: SectionOverride | undefined) => override && {
+			action: override.action,
+			headings: override.content?.split('\n').filter(line => line.startsWith('#')),
+		};
+
+		test('replaces tone and tool_efficiency with the CLI guidance without a setting', () => {
+			const result = agentHostPromptRegistry.resolveSystemMessageConfig({ id: 'hydrafusion' }, context());
+			assert.ok(result.mode === 'customize');
+			const { tone, tool_efficiency, ...sections } = result.sections ?? {};
+			assert.deepStrictEqual({ ...result, sections, tone: headings(tone), tool_efficiency: headings(tool_efficiency) }, {
+				...withUniversalAgentHostInstructions(COPILOT_AGENT_HOST_SYSTEM_MESSAGE),
+				tone: { action: 'replace', headings: ['# Tone and style'] },
+				tool_efficiency: { action: 'replace', headings: ['# Search and delegation', '# Tool usage efficiency'] },
+			});
+		});
+
+		test('does not match other models', () => {
+			for (const id of ['hydrafusion-preview', 'claude-sonnet-5', 'unknown']) {
+				assert.deepStrictEqual(
+					agentHostPromptRegistry.resolveSystemMessageConfig({ id }, context()),
+					withUniversalAgentHostInstructions(COPILOT_AGENT_HOST_SYSTEM_MESSAGE)
+				);
+			}
+		});
+	});
+
 	suite('model capability overrides (family alias)', () => {
 		// Mirrors the launcher's composition in `_buildSessionConfig`: the
 		// resolved family becomes the effective model id handed to the registry.

src/vs/platform/agentHost/test/node/copilotSessionLauncher.test.ts

@@ -22,7 +22,7 @@ import { AgentHostByokModelsEnabledConfigKey, AgentHostMcpConnectorsEnabledConfi
 import type { IAgentHostManagedSettingsPermissions } from '../../common/agentHostManagedSettings.js';
 import { toClientPluginMcpDefaultCwdsMeta } from '../../common/meta/clientPluginCustomizationMeta.js';
 import { readSessionSandboxState } from '../../common/meta/agentSandboxStateMeta.js';
-import { CopilotCliConfigKey, copilotCliConfigSchema } from '../../common/copilotCliConfig.js';
+import { COPILOT_HYDRA_FUSION_MODEL_ID, CopilotCliConfigKey, copilotCliConfigSchema, HYDRAFUSION_DEFAULT_EXCLUDED_TOOLS } from '../../common/copilotCliConfig.js';
 import type { IAgentHostOTelService } from '../../common/otel/agentHostOTelService.js';
 import { reasoningEffortLevels } from '../../common/reasoningEffort.js';
 import { autoModeTiers } from '../../common/autoModeTiers.js';
@@ -1886,6 +1886,35 @@ suite('CopilotSessionLauncher resume config', () => {
 		store.dispose();
 	});
 
+	test('hydrafusion defaults to the lean excluded tool set unless a tool filter is configured', async () => {
+		const store = new DisposableStore();
+		const hydraFusion: ModelSelection = { id: COPILOT_HYDRA_FUSION_MODEL_ID };
+		const semanticSearch = `builtin:${SEMANTIC_SEARCH_TOOL_NAME}`;
+		const defaulted = await buildResumeConfig(createLauncher(store, {}), hydraFusion);
+		const otherModel = await buildResumeConfig(createLauncher(store, {}), { id: 'gpt-5' });
+		const aliased = await buildResumeConfig(createLauncher(store, { modelCapabilityOverrides: { 'preview-model': { family: COPILOT_HYDRA_FUSION_MODEL_ID } } }), { id: 'preview-model' });
+		const userExcluded = await buildResumeConfig(createLauncher(store, { modelCapabilityOverrides: { [COPILOT_HYDRA_FUSION_MODEL_ID]: { excludedTools: ['mcp:*'] } } }), hydraFusion);
+		const userAvailable = await buildResumeConfig(createLauncher(store, { modelCapabilityOverrides: { [COPILOT_HYDRA_FUSION_MODEL_ID]: { availableTools: ['web_fetch'] } } }), hydraFusion);
+		const wildcard = await buildResumeConfig(createLauncher(store, { modelCapabilityOverrides: { '*': { excludedTools: [] } } }), hydraFusion);
+
+		assert.deepStrictEqual(
+			[defaulted, otherModel, aliased, userExcluded, userAvailable, wildcard].map(config => [config.availableTools, config.excludedTools]),
+			[
+				[undefined, [...HYDRAFUSION_DEFAULT_EXCLUDED_TOOLS, semanticSearch]],
+				[undefined, [semanticSearch]],
+				[undefined, [...HYDRAFUSION_DEFAULT_EXCLUDED_TOOLS, semanticSearch]],
+				[undefined, ['mcp:*', semanticSearch]],
+				[['web_fetch'], [semanticSearch]],
+				[undefined, [semanticSearch]],
+			]
+		);
+		assert.deepStrictEqual(
+			{ count: HYDRAFUSION_DEFAULT_EXCLUDED_TOOLS.length, keepsCliTools: ['ask_user', 'web_fetch'].filter(tool => HYDRAFUSION_DEFAULT_EXCLUDED_TOOLS.includes(tool)) },
+			{ count: 42, keepsCliTools: [] }
+		);
+		store.dispose();
+	});
+
 	test('forwards a configured modelCapabilities override and ignores a non-object one', async () => {
 		const store = new DisposableStore();
 		const model: ModelSelection = { id: 'gpt-5', config: { thinkingLevel: 'medium' } };