diff --git a/packages/core/src/context/contextManager.golden.test.ts b/packages/core/src/context/contextManager.golden.test.ts
index d5a2f6950c..25529438e5 100644
--- a/packages/core/src/context/contextManager.golden.test.ts
+++ b/packages/core/src/context/contextManager.golden.test.ts
@@ -71,7 +71,7 @@ describe('ContextManager Golden Tests', () => {
     };
 
     const sidecar = SidecarLoader.fromConfig(mockConfig as any);
-    const tracer = new ContextTracer('/tmp', 'test-session');
+    const tracer = new ContextTracer({ targetDir: '/tmp', sessionId: 'test-session' });
     const eventBus = new ContextEventBus();
     const env = new ContextEnvironmentImpl(
       {} as any,
@@ -132,7 +132,7 @@ describe('ContextManager Golden Tests', () => {
     ).IrMapper.toIr(history, new ContextTokenCalculator(4));
     // In Golden Tests, we just want to ensure the logic doesn't throw or alter unprotected history in weird ways.
     // Since we're skipping processors due to being under budget, it should equal history.
-    const tracer2 = new ContextTracer('/tmp', 'test2');
+    const tracer2 = new ContextTracer({ targetDir: '/tmp', sessionId: 'test2' });
     const eventBus2 = new ContextEventBus();
     const env2 = new ContextEnvironmentImpl(
       {} as any,
diff --git a/packages/core/src/context/system/DeterministicIdGenerator.ts b/packages/core/src/context/system/DeterministicIdGenerator.ts
new file mode 100644
index 0000000000..ae3dad6d33
--- /dev/null
+++ b/packages/core/src/context/system/DeterministicIdGenerator.ts
@@ -0,0 +1,18 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type { IIdGenerator } from './IIdGenerator.js';
+
+export class DeterministicIdGenerator implements IIdGenerator {
+  private counter = 0;
+
+  constructor(private prefix: string = 'id-') {}
+
+  generateId(): string {
+    this.counter++;
+    return `${this.prefix}${this.counter}`;
+  }
+}
diff --git a/packages/core/src/context/system/IFileSystem.ts b/packages/core/src/context/system/IFileSystem.ts
new file mode 100644
index 0000000000..ab53eccc40
--- /dev/null
+++ b/packages/core/src/context/system/IFileSystem.ts
@@ -0,0 +1,16 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+export interface IFileSystem {
+  existsSync(path: string): boolean;
+  statSyncSize(path: string): number;
+  readFileSync(path: string, encoding: 'utf8'): string;
+  writeFileSync(path: string, data: string, encoding: 'utf-8'): void;
+  appendFileSync(path: string, data: string, encoding: 'utf-8'): void;
+  mkdirSync(path: string, options?: { recursive?: boolean }): void;
+  join(...paths: string[]): string;
+  dirname(path: string): string;
+}
diff --git a/packages/core/src/context/system/IIdGenerator.ts b/packages/core/src/context/system/IIdGenerator.ts
new file mode 100644
index 0000000000..2f5cb38449
--- /dev/null
+++ b/packages/core/src/context/system/IIdGenerator.ts
@@ -0,0 +1,9 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+export interface IIdGenerator {
+  generateId(): string;
+}
diff --git a/packages/core/src/context/system/InMemoryFileSystem.ts b/packages/core/src/context/system/InMemoryFileSystem.ts
new file mode 100644
index 0000000000..5bd713b525
--- /dev/null
+++ b/packages/core/src/context/system/InMemoryFileSystem.ts
@@ -0,0 +1,71 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type { IFileSystem } from './IFileSystem.js';
+
+export class InMemoryFileSystem implements IFileSystem {
+  private files = new Map<string, string>();
+
+  // Helper for tests
+  getFiles(): ReadonlyMap<string, string> {
+    return this.files;
+  }
+
+  // Helper for tests
+  setFile(path: string, content: string) {
+    this.files.set(this.normalize(path), content);
+  }
+
+  private normalize(p: string): string {
+     // A very naive normalization for testing purposes.
+     // Ensures '/foo/bar' and '/foo//bar' map to the same key.
+     return p.replace(/\/+/g, '/');
+  }
+
+  existsSync(p: string): boolean {
+    return this.files.has(this.normalize(p));
+  }
+  
+  statSyncSize(p: string): number {
+    const content = this.files.get(this.normalize(p));
+    if (content === undefined) {
+       throw new Error(`ENOENT: no such file or directory, stat '${p}'`);
+    }
+    return content.length; // Naive char length = byte size for testing
+  }
+  
+  readFileSync(p: string, encoding: 'utf8'): string {
+    const content = this.files.get(this.normalize(p));
+    if (content === undefined) {
+       throw new Error(`ENOENT: no such file or directory, open '${p}'`);
+    }
+    return content;
+  }
+  
+  writeFileSync(p: string, data: string, encoding: 'utf-8'): void {
+    this.files.set(this.normalize(p), data);
+  }
+  
+  appendFileSync(p: string, data: string, encoding: 'utf-8'): void {
+    const norm = this.normalize(p);
+    const existing = this.files.get(norm) || '';
+    this.files.set(norm, existing + data);
+  }
+  
+  mkdirSync(p: string, options?: { recursive?: boolean }): void {
+    // In-memory fake doesn't track directories separately from files for our simple use cases
+  }
+
+  join(...paths: string[]): string {
+    return this.normalize(paths.join('/'));
+  }
+
+  dirname(p: string): string {
+    const parts = this.normalize(p).split('/');
+    parts.pop();
+    return parts.length === 0 ? '.' : parts.join('/') || '/';
+  }
+}
diff --git a/packages/core/src/context/system/NodeFileSystem.ts b/packages/core/src/context/system/NodeFileSystem.ts
new file mode 100644
index 0000000000..4eae7b1b90
--- /dev/null
+++ b/packages/core/src/context/system/NodeFileSystem.ts
@@ -0,0 +1,43 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import * as fs from 'node:fs';
+import * as path from 'node:path';
+import type { IFileSystem } from './IFileSystem.js';
+
+export class NodeFileSystem implements IFileSystem {
+  existsSync(p: string): boolean {
+    return fs.existsSync(p);
+  }
+  
+  statSyncSize(p: string): number {
+    return fs.statSync(p).size;
+  }
+  
+  readFileSync(p: string, encoding: 'utf8'): string {
+    return fs.readFileSync(p, encoding);
+  }
+  
+  writeFileSync(p: string, data: string, encoding: 'utf-8'): void {
+    fs.writeFileSync(p, data, encoding);
+  }
+  
+  appendFileSync(p: string, data: string, encoding: 'utf-8'): void {
+    fs.appendFileSync(p, data, encoding);
+  }
+  
+  mkdirSync(p: string, options?: { recursive?: boolean }): void {
+    fs.mkdirSync(p, options);
+  }
+
+  join(...paths: string[]): string {
+    return path.join(...paths);
+  }
+
+  dirname(p: string): string {
+    return path.dirname(p);
+  }
+}
diff --git a/packages/core/src/context/system/NodeIdGenerator.ts b/packages/core/src/context/system/NodeIdGenerator.ts
new file mode 100644
index 0000000000..540ec673ba
--- /dev/null
+++ b/packages/core/src/context/system/NodeIdGenerator.ts
@@ -0,0 +1,14 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { randomUUID } from 'node:crypto';
+import type { IIdGenerator } from './IIdGenerator.js';
+
+export class NodeIdGenerator implements IIdGenerator {
+  generateId(): string {
+    return randomUUID();
+  }
+}
diff --git a/packages/core/src/context/testing/contextTestUtils.ts b/packages/core/src/context/testing/contextTestUtils.ts
index 4d5b4811a4..a55c916a20 100644
--- a/packages/core/src/context/testing/contextTestUtils.ts
+++ b/packages/core/src/context/testing/contextTestUtils.ts
@@ -24,7 +24,7 @@ export function createMockEnvironment(): ContextEnvironment {
     traceDir: '/tmp/.gemini/trace',
     projectTempDir: '/tmp/.gemini/tool-outputs',
     eventBus: new ContextEventBus(),
-    tracer: new ContextTracer('/tmp', 'mock-session'),
+    tracer: new ContextTracer({ targetDir: '/tmp', sessionId: 'mock-session' }),
     charsPerToken: 1,
     tokenCalculator: new ContextTokenCalculator(1),
   };
@@ -97,7 +97,7 @@ import type { BaseLlmClient } from 'src/core/baseLlmClient.js';
 export function setupContextComponentTest(config: Config) {
   const chatHistory = new AgentChatHistory();
   const sidecar = SidecarLoader.fromConfig(config);
-  const tracer = new ContextTracer('/tmp', 'test-session');
+  const tracer = new ContextTracer({ targetDir: '/tmp', sessionId: 'test-session' });
   const eventBus = new ContextEventBus();
   const env = new ContextEnvironmentImpl(
     config.getBaseLlmClient(),
diff --git a/packages/core/src/context/tracer.test.ts b/packages/core/src/context/tracer.test.ts
index c6ab819bc5..335b08438f 100644
--- a/packages/core/src/context/tracer.test.ts
+++ b/packages/core/src/context/tracer.test.ts
@@ -1,67 +1,67 @@
-import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
-import * as fs from 'node:fs';
+import { describe, it, expect, beforeEach, vi } from 'vitest';
 import { ContextTracer } from './tracer.js';
+import { InMemoryFileSystem } from './system/InMemoryFileSystem.js';
+import { DeterministicIdGenerator } from './system/DeterministicIdGenerator.js';
 
-vi.mock('node:fs');
-
-describe('ContextTracer', () => {
-  const originalEnv = process.env;
+describe('ContextTracer (Fake FS & ID Gen)', () => {
+  let fileSystem: InMemoryFileSystem;
+  let idGenerator: DeterministicIdGenerator;
 
   beforeEach(() => {
-    vi.resetAllMocks();
-    process.env = { ...originalEnv };
-  });
-
-  afterEach(() => {
-    process.env = originalEnv;
-  });
-
-  it('initializes, logs events, and auto-saves large assets when GEMINI_CONTEXT_TRACE is true', () => {
-    process.env['GEMINI_CONTEXT_TRACE'] = 'true';
-    const mkdirSyncSpy = vi.spyOn(fs, 'mkdirSync');
-    const appendFileSyncSpy = vi.spyOn(fs, 'appendFileSync');
-    const writeFileSyncSpy = vi.spyOn(fs, 'writeFileSync');
-
-    const tracer = new ContextTracer('/fake/target', 'test-session');
-
-    expect(mkdirSyncSpy).toHaveBeenCalled();
+    fileSystem = new InMemoryFileSystem();
+    idGenerator = new DeterministicIdGenerator('mock-uuid-');
     
+    // We must mock Date.now() to ensure asset file names are perfectly deterministic
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date('2026-01-01T12:00:00Z'));
+  });
+
+  it('initializes, logs events, and auto-saves large assets deterministically', () => {
+    const tracer = new ContextTracer(
+      { enabled: true, targetDir: '/fake/target', sessionId: 'test-session' },
+      fileSystem,
+      idGenerator
+    );
+
+    // Verify Initialization
+    const initTraceLog = fileSystem.readFileSync('/fake/target/.gemini/context_trace/test-session/trace.log', 'utf8');
+    expect(initTraceLog).toContain('[SYSTEM] Context Tracer Initialized');
+
     // Small logging: shouldn't trigger saveAsset
     tracer.logEvent('TestComponent', 'TestAction', { key: 'value' });
-
-    expect(appendFileSyncSpy).toHaveBeenCalledTimes(2); // 1 for init, 1 for TestAction
-    expect(writeFileSyncSpy).not.toHaveBeenCalled();
-    const logCall = appendFileSyncSpy.mock.calls[1][1] as string;
     
-    expect(logCall).toContain('[TestComponent] TestAction');
-    expect(logCall).toContain('{"key":"value"}');
+    const smallTraceLog = fileSystem.readFileSync('/fake/target/.gemini/context_trace/test-session/trace.log', 'utf8');
+    expect(smallTraceLog).toContain('[TestComponent] TestAction');
+    expect(smallTraceLog).toContain('{"key":"value"}');
 
     // Large logging: should trigger auto-asset save
     const hugeString = 'a'.repeat(2000);
     tracer.logEvent('TestComponent', 'LargeAction', { largeKey: hugeString });
     
-    expect(writeFileSyncSpy).toHaveBeenCalled(); // asset saved
+    // 1767268800000 is 2026-01-01T12:00:00Z
+    const expectedAssetPath = '/fake/target/.gemini/context_trace/test-session/assets/1767268800000-mock-uuid-1-largeKey.json';
     
-    expect(appendFileSyncSpy).toHaveBeenCalledTimes(4); // init + TestAction + the inner saveAsset log + LargeAction log
-    const largeLogCall = appendFileSyncSpy.mock.calls[3][1] as string;
-    expect(largeLogCall).toContain('LargeAction');
-    expect(largeLogCall).toContain('"$asset":'); // verifies it was extracted
+    // Assert asset was written to FS
+    expect(fileSystem.existsSync(expectedAssetPath)).toBe(true);
+    
+    const largeTraceLog = fileSystem.readFileSync('/fake/target/.gemini/context_trace/test-session/trace.log', 'utf8');
+    expect(largeTraceLog).toContain('[TestComponent] LargeAction');
+    expect(largeTraceLog).toContain(`{"largeKey":{"$asset":"1767268800000-mock-uuid-1-largeKey.json"}}`);
   });
 
-  it('silently ignores logging when GEMINI_CONTEXT_TRACE is false', () => {
-    process.env['GEMINI_CONTEXT_TRACE'] = 'false';
-    const mkdirSyncSpy = vi.spyOn(fs, 'mkdirSync');
-    const appendFileSyncSpy = vi.spyOn(fs, 'appendFileSync');
-    const writeFileSyncSpy = vi.spyOn(fs, 'writeFileSync');
-
-    const tracer = new ContextTracer('/fake/target', 'test-session');
-    expect(mkdirSyncSpy).not.toHaveBeenCalled();
+  it('silently ignores logging when disabled', () => {
+    const tracer = new ContextTracer(
+      { enabled: false, targetDir: '/fake/target', sessionId: 'test-session' },
+      fileSystem,
+      idGenerator
+    );
 
     tracer.logEvent('TestComponent', 'TestAction');
-    expect(appendFileSyncSpy).not.toHaveBeenCalled();
-
+    
     const hugeString = 'a'.repeat(2000);
     tracer.logEvent('TestComponent', 'LargeAction', { largeKey: hugeString });
-    expect(writeFileSyncSpy).not.toHaveBeenCalled();
+    
+    // FS should be completely empty
+    expect(fileSystem.getFiles().size).toBe(0);
   });
 });
diff --git a/packages/core/src/context/tracer.ts b/packages/core/src/context/tracer.ts
index 7d67075660..ac79d2ef9c 100644
--- a/packages/core/src/context/tracer.ts
+++ b/packages/core/src/context/tracer.ts
@@ -4,27 +4,43 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import * as fs from 'node:fs';
-import * as path from 'node:path';
-import { randomUUID } from 'node:crypto';
 import { debugLogger } from '../utils/debugLogger.js';
+import type { IFileSystem } from './system/IFileSystem.js';
+import { NodeFileSystem } from './system/NodeFileSystem.js';
+import type { IIdGenerator } from './system/IIdGenerator.js';
+import { NodeIdGenerator } from './system/NodeIdGenerator.js';
+
+export interface ContextTracerOptions {
+  enabled?: boolean;
+  targetDir: string;
+  sessionId: string;
+}
 
 export class ContextTracer {
   private traceDir: string;
   private assetsDir: string;
   private enabled: boolean;
+  private fileSystem: IFileSystem;
+  private idGenerator: IIdGenerator;
 
   private readonly MAX_INLINE_SIZE = 1000;
 
-  constructor(targetDir: string, sessionId: string) {
-    this.enabled = process.env['GEMINI_CONTEXT_TRACE'] === 'true';
-    this.traceDir = path.join(targetDir, '.gemini', 'context_trace', sessionId);
-    this.assetsDir = path.join(this.traceDir, 'assets');
+  constructor(
+    options: ContextTracerOptions,
+    fileSystem: IFileSystem = new NodeFileSystem(),
+    idGenerator: IIdGenerator = new NodeIdGenerator()
+  ) {
+    this.enabled = options.enabled ?? false;
+    this.fileSystem = fileSystem;
+    this.idGenerator = idGenerator;
+
+    this.traceDir = this.fileSystem.join(options.targetDir, '.gemini', 'context_trace', options.sessionId);
+    this.assetsDir = this.fileSystem.join(this.traceDir, 'assets');
 
     if (this.enabled) {
       try {
-        fs.mkdirSync(this.assetsDir, { recursive: true });
-        this.logEvent('SYSTEM', 'Context Tracer Initialized', { sessionId });
+        this.fileSystem.mkdirSync(this.assetsDir, { recursive: true });
+        this.logEvent('SYSTEM', 'Context Tracer Initialized', { sessionId: options.sessionId });
       } catch (e) {
         debugLogger.error('Failed to initialize ContextTracer', e);
         this.enabled = false;
@@ -59,8 +75,8 @@ export class ContextTracer {
         ? ` | Details: ${JSON.stringify(processedDetails)}`
         : '';
       const logLine = `[${timestamp}] [${component}] ${action}${detailsStr}\n`;
-      fs.appendFileSync(
-        path.join(this.traceDir, 'trace.log'),
+      this.fileSystem.appendFileSync(
+        this.fileSystem.join(this.traceDir, 'trace.log'),
         logLine,
         'utf-8',
       );
@@ -72,10 +88,10 @@ export class ContextTracer {
   private saveAsset(component: string, assetName: string, data: unknown): string {
     if (!this.enabled) return 'asset-recording-disabled';
     try {
-      const assetId = `${Date.now()}-${randomUUID().slice(0, 6)}-${assetName}.json`;
-      const assetPath = path.join(this.assetsDir, assetId);
+      const assetId = `${Date.now()}-${this.idGenerator.generateId()}-${assetName}.json`;
+      const assetPath = this.fileSystem.join(this.assetsDir, assetId);
 
-      fs.writeFileSync(assetPath, JSON.stringify(data, null, 2), 'utf-8');
+      this.fileSystem.writeFileSync(assetPath, JSON.stringify(data, null, 2), 'utf-8');
       this.logEvent(component, `Saved asset: ${assetName}`, { assetId });
       return assetId;
     } catch (e) {