robustness(core): static checks to validate history is immutable

packages/cli/src/ui/utils/historyExportUtils.ts

@@ -11,7 +11,9 @@ import type { Content } from '@google/genai';
 /**
  * Serializes chat history to a Markdown string.
  */
-export function serializeHistoryToMarkdown(history: Content[]): string {
+export function serializeHistoryToMarkdown(
+  history: readonly Content[],
+): string {
   return history
     .map((item) => {
       const text =
@@ -49,7 +51,7 @@ export function serializeHistoryToMarkdown(history: Content[]): string {
  * Options for exporting chat history.
  */
 export interface ExportHistoryOptions {
-  history: Content[];
+  history: readonly Content[];
   filePath: string;
 }
 

packages/core/src/commands/types.ts

@@ -31,7 +31,7 @@ export interface MessageActionReturn {
 export interface LoadHistoryActionReturn<HistoryType = unknown> {
   type: 'load_history';
   history: HistoryType;
-  clientHistory: Content[]; // The history for the generative client
+  clientHistory: readonly Content[]; // The history for the generative client
 }
 
 /**

packages/core/src/core/client.ts

@@ -255,15 +255,15 @@ export class GeminiClient {
     return this.chat !== undefined;
   }
 
-  getHistory(): Content[] {
+  getHistory(): readonly Content[] {
     return this.getChat().getHistory();
   }
 
   stripThoughtsFromHistory() {
     this.getChat().stripThoughtsFromHistory();
   }
 
-  setHistory(history: Content[]) {
+  setHistory(history: readonly Content[]) {
     this.getChat().setHistory(history);
     this.updateTelemetryTokenCount();
     this.forceFullIdeContext = true;
@@ -1171,7 +1171,7 @@ export class GeminiClient {
   /**
    * Masks bulky tool outputs to save context window space.
    */
-  private async tryMaskToolOutputs(history: Content[]): Promise<void> {
+  private async tryMaskToolOutputs(history: readonly Content[]): Promise<void> {
     if (!this.config.getToolOutputMaskingEnabled()) {
       return;
     }

packages/core/src/core/geminiChat.ts

@@ -470,7 +470,7 @@ export class GeminiChat {
 
   private async makeApiCallAndProcessStream(
     modelConfigKey: ModelConfigKey,
-    requestContents: Content[],
+    requestContents: readonly Content[],
     prompt_id: string,
     abortSignal: AbortSignal,
     role: LlmRole,
@@ -489,7 +489,7 @@ export class GeminiChat {
     let currentGenerateContentConfig: GenerateContentConfig =
       newAvailabilityConfig;
     let lastConfig: GenerateContentConfig = currentGenerateContentConfig;
-    let lastContentsToUse: Content[] = requestContents;
+    let lastContentsToUse: Content[] = [...requestContents];
 
     const getAvailabilityContext = createAvailabilityContextProvider(
       this.config,
@@ -528,9 +528,9 @@ export class GeminiChat {
         abortSignal,
       };
 
-      let contentsToUse = supportsModernFeatures(modelToUse)
-        ? contentsForPreviewModel
-        : requestContents;
+      let contentsToUse: Content[] = supportsModernFeatures(modelToUse)
+        ? [...contentsForPreviewModel]
+        : [...requestContents];
 
       const hookSystem = this.config.getHookSystem();
       if (hookSystem) {
@@ -687,16 +687,10 @@ export class GeminiChat {
    * @return History contents alternating between user and model for the entire
    * chat session.
    */
-  getHistory(curated: boolean = false): Content[] {
+  getHistory(curated: boolean = false): readonly Content[] {
     const history = curated
       ? extractCuratedHistory(this.history)
       : this.history;
-    // Return a shallow copy of the array to prevent callers from mutating
-    // the internal history array (push/pop/splice). Content objects are
-    // shared references — callers MUST NOT mutate them in place.
-    // This replaces a prior structuredClone() which deep-copied the entire
-    // conversation on every call, causing O(n) memory pressure per turn
-    // that compounded into OOM crashes in long-running sessions.
     return [...history];
   }
 
@@ -714,8 +708,8 @@ export class GeminiChat {
     this.history.push(content);
   }
 
-  setHistory(history: Content[]): void {
-    this.history = history;
+  setHistory(history: readonly Content[]): void {
+    this.history = [...history];
     this.lastPromptTokenCount = estimateTokenCountSync(
       this.history.flatMap((c) => c.parts || []),
     );
@@ -742,7 +736,9 @@ export class GeminiChat {
   // To ensure our requests validate, the first function call in every model
   // turn within the active loop must have a `thoughtSignature` property.
   // If we do not do this, we will get back 400 errors from the API.
-  ensureActiveLoopHasThoughtSignatures(requestContents: Content[]): Content[] {
+  ensureActiveLoopHasThoughtSignatures(
+    requestContents: readonly Content[],
+  ): readonly Content[] {
     // First, find the start of the active loop by finding the last user turn
     // with a text message, i.e. that is not a function response.
     let activeLoopStartIndex = -1;

packages/core/src/core/logger.ts

@@ -27,7 +27,7 @@ export interface LogEntry {
 }
 
 export interface Checkpoint {
-  history: Content[];
+  history: readonly Content[];
   authType?: AuthType;
 }
 

packages/core/src/routing/routingStrategy.ts

@@ -31,7 +31,7 @@ export interface RoutingDecision {
  */
 export interface RoutingContext {
   /** The full history of the conversation. */
-  history: Content[];
+  history: readonly Content[];
   /** The immediate request parts to be processed. */
   request: PartListUnion;
   /** An abort signal to cancel an LLM call during routing. */

packages/core/src/safety/context-builder.ts

@@ -74,7 +74,9 @@ export class ContextBuilder {
   }
 
   // Helper to convert Google GenAI Content[] to Safety Protocol ConversationTurn[]
-  private convertHistoryToTurns(history: Content[]): ConversationTurn[] {
+  private convertHistoryToTurns(
+    history: readonly Content[],
+  ): ConversationTurn[] {
     const turns: ConversationTurn[] = [];
     let currentUserRequest: { text: string } | undefined;
 

packages/core/src/services/chatCompressionService.ts

@@ -130,7 +130,7 @@ export function modelStringToModelConfigAlias(model: string): string {
  * contain massive tool outputs (like large grep results or logs).
  */
 async function truncateHistoryToBudget(
-  history: Content[],
+  history: readonly Content[],
   config: Config,
 ): Promise<Content[]> {
   let functionResponseTokenCounter = 0;

packages/core/src/services/chatRecordingService.ts

@@ -664,7 +664,7 @@ export class ChatRecordingService {
    * Updates the conversation history based on the provided API Content array.
    * This is used to persist changes made to the history (like masking) back to disk.
    */
-  updateMessagesFromHistory(history: Content[]): void {
+  updateMessagesFromHistory(history: readonly Content[]): void {
     if (!this.conversationFile) return;
 
     try {

packages/core/src/services/toolOutputMaskingService.ts

@@ -43,7 +43,7 @@ const EXEMPT_TOOLS = new Set([
 ]);
 
 export interface MaskingResult {
-  newHistory: Content[];
+  newHistory: readonly Content[];
   maskedCount: number;
   tokensSaved: number;
 }
@@ -67,7 +67,10 @@ export interface MaskingResult {
  * are preserved until they collectively reach the threshold.
  */
 export class ToolOutputMaskingService {
-  async mask(history: Content[], config: Config): Promise<MaskingResult> {
+  async mask(
+    history: readonly Content[],
+    config: Config,
+  ): Promise<MaskingResult> {
     const maskingConfig = await config.getToolOutputMaskingConfig();
     if (!maskingConfig.enabled || history.length === 0) {
       return { newHistory: history, maskedCount: 0, tokensSaved: 0 };

packages/core/src/utils/checkpointUtils.ts

@@ -14,7 +14,7 @@ import type { ToolCallRequestInfo } from '../scheduler/types.js';
 
 export interface ToolCallData<HistoryType = unknown, ArgsType = unknown> {
   history?: HistoryType;
-  clientHistory?: Content[];
+  clientHistory?: readonly Content[];
   commitHash?: string;
   toolCall: {
     name: string;

packages/sdk/src/session.ts

@@ -226,7 +226,7 @@ export class GeminiCliSession {
         break;
       }
 
-      const transcript: Content[] = client.getHistory();
+      const transcript: readonly Content[] = client.getHistory();
       const context: SessionContext = {
         sessionId,
         transcript,

packages/sdk/src/types.ts

@@ -51,7 +51,7 @@ export interface AgentShell {
 
 export interface SessionContext {
   sessionId: string;
-  transcript: Content[];
+  transcript: readonly Content[];
   cwd: string;
   timestamp: string;
   fs: AgentFilesystem;