feat: multi-turn conversations and Claude fallback for Gemini provider

Major improvements to Gemini provider:

**Shared Conversation History**
- Add ConversationMessage interface for provider-agnostic history
- Both Claude and Gemini agents read/write shared conversationHistory
- Context persists across provider switches via claudeSessionId linkage

**Multi-Turn Gemini API**
- Replace stateless single-query with full conversation context
- queryGeminiMultiTurn() sends entire history for coherent responses
- Maps 'assistant' role to 'model' for Gemini API compatibility

**Automatic Fallback to Claude**
- Detect rate limits (429), server errors (5xx), network failures
- Fall back to Claude SDK when Gemini API fails
- Reset 'processing' messages to 'pending' before fallback

**Mid-Session Provider Switching**
- Track currentProvider on ActiveSession
- Provider changes take effect after current generator finishes
- Avoids race conditions from aborting active generators

Files changed:
- worker-types.ts: Add ConversationMessage, currentProvider tracking
- GeminiAgent.ts: Multi-turn queries, fallback logic
- SDKAgent.ts: Capture messages to shared history
- SessionManager.ts: Initialize new session fields
- SessionRoutes.ts: Provider selection and switching logic
- worker-service.ts: Wire up fallback agent dependency

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/services/worker-service.ts

@@ -85,6 +85,7 @@ export class WorkerService {
     this.sseBroadcaster = new SSEBroadcaster();
     this.sdkAgent = new SDKAgent(this.dbManager, this.sessionManager);
     this.geminiAgent = new GeminiAgent(this.dbManager, this.sessionManager);
+    this.geminiAgent.setFallbackAgent(this.sdkAgent);  // Enable fallback to Claude on Gemini API failure
     this.paginationHelper = new PaginationHelper(this.dbManager);
     this.settingsManager = new SettingsManager(this.dbManager);
     this.sessionEventBroadcaster = new SessionEventBroadcaster(this.sseBroadcaster, this);

src/services/worker-types.ts

@@ -8,6 +8,15 @@ import type { Response } from 'express';
 // Active Session Types
 // ============================================================================
 
+/**
+ * Provider-agnostic conversation message for shared history
+ * Used to maintain context across Claude↔Gemini provider switches
+ */
+export interface ConversationMessage {
+  role: 'user' | 'assistant';
+  content: string;
+}
+
 export interface ActiveSession {
   sessionDbId: number;
   claudeSessionId: string;
@@ -22,6 +31,8 @@ export interface ActiveSession {
   cumulativeInputTokens: number;   // Track input tokens for discovery cost
   cumulativeOutputTokens: number;  // Track output tokens for discovery cost
   pendingProcessingIds: Set<number>;  // Track ALL message IDs yielded but not yet processed
+  conversationHistory: ConversationMessage[];  // Shared conversation history for provider switching
+  currentProvider: 'claude' | 'gemini' | null;  // Track which provider is currently running
 }
 
 export interface PendingMessage {

src/services/worker/GeminiAgent.ts

@@ -19,7 +19,7 @@ import { parseObservations, parseSummary } from '../../sdk/parser.js';
 import { buildInitPrompt, buildObservationPrompt, buildSummaryPrompt, buildContinuationPrompt } from '../../sdk/prompts.js';
 import { SettingsDefaultsManager } from '../../shared/SettingsDefaultsManager.js';
 import { USER_SETTINGS_PATH } from '../../shared/paths.js';
-import type { ActiveSession, PendingMessage } from '../worker-types.js';
+import type { ActiveSession, PendingMessage, ConversationMessage } from '../worker-types.js';
 import { ModeManager } from '../domain/ModeManager.js';
 
 // Gemini API endpoint
@@ -43,17 +43,58 @@ interface GeminiResponse {
   };
 }
 
+/**
+ * Gemini content message format
+ * role: "user" or "model" (Gemini uses "model" not "assistant")
+ */
+interface GeminiContent {
+  role: 'user' | 'model';
+  parts: Array<{ text: string }>;
+}
+
+// Forward declaration for fallback agent type
+type FallbackAgent = {
+  startSession(session: ActiveSession, worker?: any): Promise<void>;
+};
+
 export class GeminiAgent {
   private dbManager: DatabaseManager;
   private sessionManager: SessionManager;
+  private fallbackAgent: FallbackAgent | null = null;
 
   constructor(dbManager: DatabaseManager, sessionManager: SessionManager) {
     this.dbManager = dbManager;
     this.sessionManager = sessionManager;
   }
 
+  /**
+   * Set the fallback agent (Claude SDK) for when Gemini API fails
+   * Must be set after construction to avoid circular dependency
+   */
+  setFallbackAgent(agent: FallbackAgent): void {
+    this.fallbackAgent = agent;
+  }
+
+  /**
+   * Check if an error should trigger fallback to Claude
+   */
+  private shouldFallbackToClaude(error: any): boolean {
+    const message = error?.message || '';
+    // Fall back on rate limit (429), server errors (5xx), or network issues
+    return (
+      message.includes('429') ||
+      message.includes('500') ||
+      message.includes('502') ||
+      message.includes('503') ||
+      message.includes('ECONNREFUSED') ||
+      message.includes('ETIMEDOUT') ||
+      message.includes('fetch failed')
+    );
+  }
+
   /**
    * Start Gemini agent for a session
+   * Uses multi-turn conversation to maintain context across messages
    */
   async startSession(session: ActiveSession, worker?: any): Promise<void> {
     try {
@@ -72,10 +113,14 @@ export class GeminiAgent {
         ? buildInitPrompt(session.project, session.claudeSessionId, session.userPrompt, mode)
         : buildContinuationPrompt(session.userPrompt, session.lastPromptNumber, session.claudeSessionId, mode);
 
-      // Query Gemini with initial prompt
-      const initResponse = await this.queryGemini(initPrompt, apiKey, model);
+      // Add to conversation history and query Gemini with full context
+      session.conversationHistory.push({ role: 'user', content: initPrompt });
+      const initResponse = await this.queryGeminiMultiTurn(session.conversationHistory, apiKey, model);
 
       if (initResponse.content) {
+        // Add response to conversation history
+        session.conversationHistory.push({ role: 'assistant', content: initResponse.content });
+
         // Track token usage
         const tokensUsed = initResponse.tokensUsed || 0;
         session.cumulativeInputTokens += Math.floor(tokensUsed * 0.7);  // Rough estimate
@@ -103,10 +148,14 @@ export class GeminiAgent {
             cwd: message.cwd
           });
 
-          // Query Gemini
-          const obsResponse = await this.queryGemini(obsPrompt, apiKey, model);
+          // Add to conversation history and query Gemini with full context
+          session.conversationHistory.push({ role: 'user', content: obsPrompt });
+          const obsResponse = await this.queryGeminiMultiTurn(session.conversationHistory, apiKey, model);
 
           if (obsResponse.content) {
+            // Add response to conversation history
+            session.conversationHistory.push({ role: 'assistant', content: obsResponse.content });
+
             const tokensUsed = obsResponse.tokensUsed || 0;
             session.cumulativeInputTokens += Math.floor(tokensUsed * 0.7);
             session.cumulativeOutputTokens += Math.floor(tokensUsed * 0.3);
@@ -124,10 +173,14 @@ export class GeminiAgent {
             last_assistant_message: message.last_assistant_message || ''
           }, mode);
 
-          // Query Gemini
-          const summaryResponse = await this.queryGemini(summaryPrompt, apiKey, model);
+          // Add to conversation history and query Gemini with full context
+          session.conversationHistory.push({ role: 'user', content: summaryPrompt });
+          const summaryResponse = await this.queryGeminiMultiTurn(session.conversationHistory, apiKey, model);
 
           if (summaryResponse.content) {
+            // Add response to conversation history
+            session.conversationHistory.push({ role: 'assistant', content: summaryResponse.content });
+
             const tokensUsed = summaryResponse.tokensUsed || 0;
             session.cumulativeInputTokens += Math.floor(tokensUsed * 0.7);
             session.cumulativeOutputTokens += Math.floor(tokensUsed * 0.3);
@@ -140,7 +193,8 @@ export class GeminiAgent {
       const sessionDuration = Date.now() - session.startTime;
       logger.success('SDK', 'Gemini agent completed', {
         sessionId: session.sessionDbId,
-        duration: `${(sessionDuration / 1000).toFixed(1)}s`
+        duration: `${(sessionDuration / 1000).toFixed(1)}s`,
+        historyLength: session.conversationHistory.length
       });
 
       this.dbManager.getSessionStore().markSessionCompleted(session.sessionDbId);
@@ -148,25 +202,65 @@ export class GeminiAgent {
     } catch (error: any) {
       if (error.name === 'AbortError') {
         logger.warn('SDK', 'Gemini agent aborted', { sessionId: session.sessionDbId });
-      } else {
-        logger.failure('SDK', 'Gemini agent error', { sessionDbId: session.sessionDbId }, error);
+        throw error;
       }
+
+      // Check if we should fall back to Claude
+      if (this.shouldFallbackToClaude(error) && this.fallbackAgent) {
+        logger.warn('SDK', 'Gemini API failed, falling back to Claude SDK', {
+          sessionDbId: session.sessionDbId,
+          error: error.message,
+          historyLength: session.conversationHistory.length
+        });
+
+        // Reset any 'processing' messages back to 'pending' so Claude can retry them
+        // This handles the case where Gemini failed mid-processing a message
+        const pendingStore = this.sessionManager.getPendingMessageStore();
+        const resetCount = pendingStore.resetStuckMessages(0);  // 0 = reset ALL processing messages
+        if (resetCount > 0) {
+          logger.info('SDK', 'Reset processing messages for fallback', {
+            sessionDbId: session.sessionDbId,
+            resetCount
+          });
+        }
+
+        // Fall back to Claude - it will use the same session with shared conversationHistory
+        // Note: Claude SDK will continue processing from current state
+        return this.fallbackAgent.startSession(session, worker);
+      }
+
+      logger.failure('SDK', 'Gemini agent error', { sessionDbId: session.sessionDbId }, error);
       throw error;
-    } finally {
-      // Cleanup
-      this.sessionManager.deleteSession(session.sessionDbId).catch(() => {});
     }
   }
 
   /**
-   * Query Gemini via REST API
+   * Convert shared ConversationMessage array to Gemini's contents format
+   * Maps 'assistant' role to 'model' for Gemini API compatibility
    */
-  private async queryGemini(
-    prompt: string,
+  private conversationToGeminiContents(history: ConversationMessage[]): GeminiContent[] {
+    return history.map(msg => ({
+      role: msg.role === 'assistant' ? 'model' : 'user',
+      parts: [{ text: msg.content }]
+    }));
+  }
+
+  /**
+   * Query Gemini via REST API with full conversation history (multi-turn)
+   * Sends the entire conversation context for coherent responses
+   */
+  private async queryGeminiMultiTurn(
+    history: ConversationMessage[],
     apiKey: string,
     model: GeminiModel
   ): Promise<{ content: string; tokensUsed?: number }> {
-    logger.debug('SDK', `Querying Gemini (${model})`, { promptLength: prompt.length });
+    const contents = this.conversationToGeminiContents(history);
+    const totalChars = history.reduce((sum, m) => sum + m.content.length, 0);
+
+    logger.debug('SDK', `Querying Gemini multi-turn (${model})`, {
+      turns: history.length,
+      totalChars
+    });
 
     const url = `${GEMINI_API_URL}/${model}:generateContent?key=${apiKey}`;
 
@@ -176,9 +270,7 @@ export class GeminiAgent {
         'Content-Type': 'application/json',
       },
       body: JSON.stringify({
-        contents: [{
-          parts: [{ text: prompt }]
-        }],
+        contents,
         generationConfig: {
           temperature: 0.3,  // Lower temperature for structured extraction
           maxOutputTokens: 4096,

src/services/worker/SDKAgent.ts

@@ -184,20 +184,31 @@ export class SDKAgent {
    * - SessionManager.initializeSession already fetched this from database
    * - Database row was created by new-hook's createSDKSession call
    * - We just use the session_id we're given - simple and reliable
+   *
+   * SHARED CONVERSATION HISTORY:
+   * - Each user message is added to session.conversationHistory
+   * - This allows provider switching (Claude→Gemini) with full context
+   * - SDK manages its own internal state, but we mirror it for interop
    */
   private async *createMessageGenerator(session: ActiveSession): AsyncIterableIterator<SDKUserMessage> {
     // Load active mode
     const mode = ModeManager.getInstance().getActiveMode();
 
+    // Build initial prompt
+    const initPrompt = session.lastPromptNumber === 1
+      ? buildInitPrompt(session.project, session.claudeSessionId, session.userPrompt, mode)
+      : buildContinuationPrompt(session.userPrompt, session.lastPromptNumber, session.claudeSessionId, mode);
+
+    // Add to shared conversation history for provider interop
+    session.conversationHistory.push({ role: 'user', content: initPrompt });
+
     // Yield initial user prompt with context (or continuation if prompt #2+)
     // CRITICAL: Both paths use session.claudeSessionId from the hook
     yield {
       type: 'user',
       message: {
         role: 'user',
-        content: session.lastPromptNumber === 1
-          ? buildInitPrompt(session.project, session.claudeSessionId, session.userPrompt, mode)
-          : buildContinuationPrompt(session.userPrompt, session.lastPromptNumber, session.claudeSessionId, mode)
+        content: initPrompt
       },
       session_id: session.claudeSessionId,
       parent_tool_use_id: null,
@@ -212,36 +223,46 @@ export class SDKAgent {
           session.lastPromptNumber = message.prompt_number;
         }
 
+        const obsPrompt = buildObservationPrompt({
+          id: 0, // Not used in prompt
+          tool_name: message.tool_name!,
+          tool_input: JSON.stringify(message.tool_input),
+          tool_output: JSON.stringify(message.tool_response),
+          created_at_epoch: Date.now(),
+          cwd: message.cwd
+        });
+
+        // Add to shared conversation history for provider interop
+        session.conversationHistory.push({ role: 'user', content: obsPrompt });
+
         yield {
           type: 'user',
           message: {
             role: 'user',
-            content: buildObservationPrompt({
-              id: 0, // Not used in prompt
-              tool_name: message.tool_name!,
-              tool_input: JSON.stringify(message.tool_input),
-              tool_output: JSON.stringify(message.tool_response),
-              created_at_epoch: Date.now(),
-              cwd: message.cwd
-            })
+            content: obsPrompt
           },
           session_id: session.claudeSessionId,
           parent_tool_use_id: null,
           isSynthetic: true
         };
       } else if (message.type === 'summarize') {
+        const summaryPrompt = buildSummaryPrompt({
+          id: session.sessionDbId,
+          sdk_session_id: session.sdkSessionId,
+          project: session.project,
+          user_prompt: session.userPrompt,
+          last_user_message: message.last_user_message || '',
+          last_assistant_message: message.last_assistant_message || ''
+        }, mode);
+
+        // Add to shared conversation history for provider interop
+        session.conversationHistory.push({ role: 'user', content: summaryPrompt });
+
         yield {
           type: 'user',
           message: {
             role: 'user',
-            content: buildSummaryPrompt({
-              id: session.sessionDbId,
-              sdk_session_id: session.sdkSessionId,
-              project: session.project,
-              user_prompt: session.userPrompt,
-              last_user_message: message.last_user_message || '',
-              last_assistant_message: message.last_assistant_message || ''
-            }, mode)
+            content: summaryPrompt
           },
           session_id: session.claudeSessionId,
           parent_tool_use_id: null,
@@ -254,8 +275,16 @@ export class SDKAgent {
   /**
    * Process SDK response text (parse XML, save to database, sync to Chroma)
    * @param discoveryTokens - Token cost for discovering this response (delta, not cumulative)
+   *
+   * Also captures assistant responses to shared conversation history for provider interop.
+   * This allows Gemini to see full context if provider is switched mid-session.
    */
   private async processSDKResponse(session: ActiveSession, text: string, worker: any | undefined, discoveryTokens: number): Promise<void> {
+    // Add assistant response to shared conversation history for provider interop
+    if (text) {
+      session.conversationHistory.push({ role: 'assistant', content: text });
+    }
+
     // Parse observations
     const observations = parseObservations(text, session.claudeSessionId);
 

src/services/worker/SessionManager.ts

@@ -117,7 +117,9 @@ export class SessionManager {
       startTime: Date.now(),
       cumulativeInputTokens: 0,
       cumulativeOutputTokens: 0,
-      pendingProcessingIds: new Set()
+      pendingProcessingIds: new Set(),
+      conversationHistory: [],  // Initialize empty - will be populated by agents
+      currentProvider: null  // Will be set when generator starts
     };
 
     this.sessions.set(sessionDbId, session);

src/services/worker/http/routes/SessionRoutes.ts

@@ -42,7 +42,10 @@ export class SessionRoutes extends BaseRouteHandler {
 
   /**
    * Get the appropriate agent based on settings
-   * Falls back to Claude SDK if Gemini is selected but not configured
+   * Throws error if Gemini is selected but not configured (no silent fallback)
+   *
+   * Note: Session linking via claudeSessionId allows provider switching mid-session.
+   * The conversationHistory on ActiveSession maintains context across providers.
    */
   private getActiveAgent(): SDKAgent | GeminiAgent {
     if (isGeminiSelected()) {
@@ -50,36 +53,82 @@ export class SessionRoutes extends BaseRouteHandler {
         logger.debug('SESSION', 'Using Gemini agent');
         return this.geminiAgent;
       } else {
-        logger.warn('SESSION', 'Gemini selected but no API key configured, falling back to Claude SDK');
-        return this.sdkAgent;
+        throw new Error('Gemini provider selected but no API key configured. Set CLAUDE_MEM_GEMINI_API_KEY in settings or GEMINI_API_KEY environment variable.');
       }
     }
     return this.sdkAgent;
   }
 
+  /**
+   * Get the currently selected provider name
+   */
+  private getSelectedProvider(): 'claude' | 'gemini' {
+    return (isGeminiSelected() && isGeminiAvailable()) ? 'gemini' : 'claude';
+  }
+
   /**
    * Ensures agent generator is running for a session
    * Auto-starts if not already running to process pending queue
    * Uses either Claude SDK or Gemini based on settings
+   *
+   * Provider switching: If provider setting changed while generator is running,
+   * we let the current generator finish naturally (max 5s linger timeout).
+   * The next generator will use the new provider with shared conversationHistory.
    */
   private ensureGeneratorRunning(sessionDbId: number, source: string): void {
     const session = this.sessionManager.getSession(sessionDbId);
-    if (session && !session.generatorPromise) {
-      const agent = this.getActiveAgent();
-      const agentName = (isGeminiSelected() && isGeminiAvailable()) ? 'Gemini' : 'Claude SDK';
+    if (!session) return;
 
-      logger.info('SESSION', `Generator auto-starting (${source}) using ${agentName}`, {
-        sessionId: sessionDbId,
-        queueDepth: session.pendingMessages.length
-      });
+    const selectedProvider = this.getSelectedProvider();
 
-      session.generatorPromise = agent.startSession(session, this.workerService)
-        .finally(() => {
-          logger.info('SESSION', `Generator finished`, { sessionId: sessionDbId });
-          session.generatorPromise = null;
-          this.workerService.broadcastProcessingStatus();
-        });
+    // Start generator if not running
+    if (!session.generatorPromise) {
+      this.startGeneratorWithProvider(session, selectedProvider, source);
+      return;
     }
+
+    // Generator is running - check if provider changed
+    if (session.currentProvider && session.currentProvider !== selectedProvider) {
+      logger.info('SESSION', `Provider changed, will switch after current generator finishes`, {
+        sessionId: sessionDbId,
+        currentProvider: session.currentProvider,
+        selectedProvider,
+        historyLength: session.conversationHistory.length
+      });
+      // Let current generator finish naturally, next one will use new provider
+      // The shared conversationHistory ensures context is preserved
+    }
+  }
+
+  /**
+   * Start a generator with the specified provider
+   */
+  private startGeneratorWithProvider(
+    session: ReturnType<typeof this.sessionManager.getSession>,
+    provider: 'claude' | 'gemini',
+    source: string
+  ): void {
+    if (!session) return;
+
+    const agent = provider === 'gemini' ? this.geminiAgent : this.sdkAgent;
+    const agentName = provider === 'gemini' ? 'Gemini' : 'Claude SDK';
+
+    logger.info('SESSION', `Generator auto-starting (${source}) using ${agentName}`, {
+      sessionId: session.sessionDbId,
+      queueDepth: session.pendingMessages.length,
+      historyLength: session.conversationHistory.length
+    });
+
+    // Track which provider is running
+    session.currentProvider = provider;
+
+    session.generatorPromise = agent.startSession(session, this.workerService)
+      .finally(() => {
+        logger.info('SESSION', `Generator finished`, { sessionId: session.sessionDbId });
+        session.generatorPromise = null;
+        session.currentProvider = null;
+        this.workerService.broadcastProcessingStatus();
+      });
   }
 
   setupRoutes(app: express.Application): void {
@@ -150,24 +199,8 @@ export class SessionRoutes extends BaseRouteHandler {
       });
     }
 
-    // Start agent in background (pass worker ref for spinner control)
-    const agent = this.getActiveAgent();
-    const agentName = isGeminiSelected() ? 'Gemini' : 'Claude SDK';
-
-    logger.info('SESSION', `Generator starting using ${agentName}`, {
-      sessionId: sessionDbId,
-      project: session.project,
-      promptNum: session.lastPromptNumber
-    });
-
-    session.generatorPromise = agent.startSession(session, this.workerService)
-      .finally(() => {
-        // Clear generator reference when completed
-        logger.info('SESSION', `Generator finished`, { sessionId: sessionDbId });
-        session.generatorPromise = null;
-        // Broadcast status change (generator finished, may stop spinner)
-        this.workerService.broadcastProcessingStatus();
-      });
+    // Start agent in background using the helper method
+    this.startGeneratorWithProvider(session, this.getSelectedProvider(), 'init');
 
     // Broadcast session started event
     this.eventBroadcaster.broadcastSessionStarted(sessionDbId, session.project);