feat: enable thin harness by default on this branch

packages/core/src/agents/registry.ts

@@ -104,6 +104,11 @@ export class AgentRegistry {
   private async loadAgents(): Promise<void> {
     this.agents.clear();
     this.allDefinitions.clear();
+
+    if (this.config.isThinHarness()) {
+      return;
+    }
+
     this.loadBuiltInAgents();
 
     if (!this.config.isAgentsEnabled()) {
@@ -240,6 +245,10 @@ export class AgentRegistry {
   }
 
   private loadBuiltInAgents(): void {
+    if (this.config.isThinHarness()) {
+      return;
+    }
+
     this.registerLocalAgent(CodebaseInvestigatorAgent(this.config));
     this.registerLocalAgent(CliHelpAgent(this.config));
     this.registerLocalAgent(GeneralistAgent(this.config));

packages/core/src/config/config.ts

@@ -633,6 +633,7 @@ export interface ConfigParameters {
   tracker?: boolean;
   planSettings?: PlanSettings;
   modelSteering?: boolean;
+  thinHarness?: boolean;
   onModelChange?: (model: string) => void;
   mcpEnabled?: boolean;
   extensionsEnabled?: boolean;
@@ -853,6 +854,7 @@ export class Config implements McpContext, AgentLoopContext {
   private readonly planEnabled: boolean;
   private readonly trackerEnabled: boolean;
   private readonly planModeRoutingEnabled: boolean;
+  private readonly thinHarness: boolean;
   private readonly modelSteering: boolean;
   private contextManager?: ContextManager;
   private terminalBackground: string | undefined = undefined;
@@ -996,6 +998,8 @@ export class Config implements McpContext, AgentLoopContext {
       modelConfigServiceConfig ?? DEFAULT_MODEL_CONFIGS,
     );
 
+    this.thinHarness =
+      params.thinHarness ?? process.env['GEMINI_THIN_HARNESS'] !== 'false';
     this.experimentalJitContext = params.experimentalJitContext ?? true;
     this.topicUpdateNarration = params.topicUpdateNarration ?? false;
     this.modelSteering = params.modelSteering ?? false;
@@ -1236,30 +1240,35 @@ export class Config implements McpContext, AgentLoopContext {
 
     this._toolRegistry = await this.createToolRegistry();
     discoverToolsHandle?.end();
-    this.mcpClientManager = new McpClientManager(
-      this.clientVersion,
-      this._toolRegistry,
-      this,
-      this.eventEmitter,
-    );
-    // We do not await this promise so that the CLI can start up even if
-    // MCP servers are slow to connect.
-    this.mcpInitializationPromise = Promise.allSettled([
-      this.mcpClientManager.startConfiguredMcpServers(),
-      this.getExtensionLoader().start(this),
-    ]).then((results) => {
-      for (const result of results) {
-        if (result.status === 'rejected') {
-          debugLogger.error('Error initializing MCP clients:', result.reason);
+
+    if (this.thinHarness) {
+      this.mcpInitializationPromise = Promise.resolve();
+    } else {
+      this.mcpClientManager = new McpClientManager(
+        this.clientVersion,
+        this._toolRegistry,
+        this,
+        this.eventEmitter,
+      );
+      // We do not await this promise so that the CLI can start up even if
+      // MCP servers are slow to connect.
+      this.mcpInitializationPromise = Promise.allSettled([
+        this.mcpClientManager.startConfiguredMcpServers(),
+        this.getExtensionLoader().start(this),
+      ]).then((results) => {
+        for (const result of results) {
+          if (result.status === 'rejected') {
+            debugLogger.error('Error initializing MCP clients:', result.reason);
+          }
         }
-      }
-    });
+      });
+    }
 
     if (!this.interactive || this.acpMode) {
       await this.mcpInitializationPromise;
     }
 
-    if (this.skillsSupport) {
+    if (this.skillsSupport && !this.thinHarness) {
       this.getSkillManager().setAdminSettings(this.adminSkillsEnabled);
       if (this.adminSkillsEnabled) {
         await this.getSkillManager().discoverSkills(
@@ -2024,6 +2033,10 @@ export class Config implements McpContext, AgentLoopContext {
   }
 
   getUserMemory(): string | HierarchicalMemory {
+    if (this.thinHarness) {
+      return { global: '', extension: '', project: '' };
+    }
+
     if (this.experimentalJitContext && this.contextManager) {
       return {
         global: this.contextManager.getGlobalMemory(),
@@ -2120,6 +2133,9 @@ export class Config implements McpContext, AgentLoopContext {
   }
 
   getGeminiMdFileCount(): number {
+    if (this.thinHarness) {
+      return 0;
+    }
     if (this.experimentalJitContext && this.contextManager) {
       return this.contextManager.getLoadedPaths().size;
     }
@@ -2131,6 +2147,9 @@ export class Config implements McpContext, AgentLoopContext {
   }
 
   getGeminiMdFilePaths(): string[] {
+    if (this.thinHarness) {
+      return [];
+    }
     if (this.experimentalJitContext && this.contextManager) {
       return Array.from(this.contextManager.getLoadedPaths());
     }
@@ -2493,7 +2512,7 @@ export class Config implements McpContext, AgentLoopContext {
   }
 
   isAgentsEnabled(): boolean {
-    return this.enableAgents;
+    return this.enableAgents && !this.thinHarness;
   }
 
   isEventDrivenSchedulerEnabled(): boolean {
@@ -2786,14 +2805,14 @@ export class Config implements McpContext, AgentLoopContext {
   }
 
   isSkillsSupportEnabled(): boolean {
-    return this.skillsSupport;
+    return this.skillsSupport && !this.thinHarness;
   }
 
   /**
    * Reloads skills by re-discovering them from extensions and local directories.
    */
   async reloadSkills(): Promise<void> {
-    if (!this.skillsSupport) {
+    if (!this.skillsSupport || this.thinHarness) {
       return;
     }
 
@@ -2843,6 +2862,10 @@ export class Config implements McpContext, AgentLoopContext {
     }
   }
 
+  isThinHarness(): boolean {
+    return this.thinHarness;
+  }
+
   isInteractive(): boolean {
     return this.interactive;
   }
@@ -3041,6 +3064,25 @@ export class Config implements McpContext, AgentLoopContext {
         );
       }
 
+      if (this.thinHarness) {
+        const allowedTools = [
+          'LSTool',
+          'ReadFileTool',
+          'RipGrepTool',
+          'GrepTool',
+          'GlobTool',
+          'EditTool',
+          'WriteFileTool',
+          'WebFetchTool',
+          'ShellTool',
+          'WebSearchTool',
+          'AskUserTool',
+        ];
+        if (!allowedTools.includes(normalizedClassName)) {
+          isEnabled = false;
+        }
+      }
+
       if (isEnabled) {
         registerFn();
       }
@@ -3144,7 +3186,9 @@ export class Config implements McpContext, AgentLoopContext {
     // Register Subagents as Tools
     this.registerSubAgentTools(registry);
 
-    await registry.discoverAllTools();
+    if (!this.thinHarness) {
+      await registry.discoverAllTools();
+    }
     registry.sortTools();
     return registry;
   }
@@ -3153,6 +3197,10 @@ export class Config implements McpContext, AgentLoopContext {
    * Registers SubAgentTools for all available agents.
    */
   private registerSubAgentTools(registry: ToolRegistry): void {
+    if (this.thinHarness) {
+      return;
+    }
+
     const agentsOverrides = this.getAgentsSettings().overrides ?? {};
     const definitions = this.agentRegistry.getAllDefinitions();
 

packages/core/src/prompts/promptProvider.ts

@@ -44,6 +44,11 @@ export class PromptProvider {
     userMemory?: string | HierarchicalMemory,
     interactiveOverride?: boolean,
   ): string {
+    const config = context.config;
+    if (config.isThinHarness()) {
+      return 'You are Gemini CLI, a helpful assistant specializing in software engineering tasks.';
+    }
+
     const systemMdResolution = resolvePathFromEnv(
       process.env['GEMINI_SYSTEM_MD'],
     );
@@ -216,11 +221,13 @@ export class PromptProvider {
     }
 
     // --- Finalization (Shell) ---
-    const finalPrompt = activeSnippets.renderFinalShell(
-      basePrompt,
-      userMemory,
-      contextFilenames,
-    );
+    const finalPrompt = context.config.isThinHarness()
+      ? basePrompt
+      : activeSnippets.renderFinalShell(
+          basePrompt,
+          userMemory,
+          contextFilenames,
+        );
 
     // Sanitize erratic newlines from composition
     const sanitizedPrompt = finalPrompt.replace(/\n{3,}/g, '\n\n');

packages/core/src/utils/memoryDiscovery.ts

@@ -699,6 +699,18 @@ export async function loadServerHierarchicalMemory(
  * Returns the result of the call to `loadHierarchicalGeminiMemory`.
  */
 export async function refreshServerHierarchicalMemory(config: Config) {
+  if (config.isThinHarness()) {
+    const emptyResult: LoadServerHierarchicalMemoryResponse = {
+      memoryContent: { global: '', extension: '', project: '' },
+      fileCount: 0,
+      filePaths: [],
+    };
+    config.setUserMemory(emptyResult.memoryContent);
+    config.setGeminiMdFileCount(0);
+    config.setGeminiMdFilePaths([]);
+    return emptyResult;
+  }
+
   const result = await loadServerHierarchicalMemory(
     config.getWorkingDir(),
     config.shouldLoadMemoryFromIncludeDirectories()