diff --git a/.changeset/improve-session-initialization.md b/.changeset/improve-session-initialization.md
new file mode 100644
index 00000000..15092e7d
--- /dev/null
+++ b/.changeset/improve-session-initialization.md
@@ -0,0 +1,5 @@
+---
+'@cloudflare/sandbox': patch
+---
+
+Fix session initialization to eliminate noisy error logs during hot reloads
diff --git a/CLAUDE.md b/CLAUDE.md
index e6a66f4b..14e31ebc 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -291,6 +291,10 @@ Turbo handles task orchestration (`turbo.json`) with dependency-aware builds.
 
 **Subject line should stand alone** - don't require reading the body to understand the change. Body is optional and only needed for non-obvious context.
 
+**Focus on the change, not how it was discovered** - never reference "review feedback", "PR comments", or "code review" in commit messages. Describe what the change does and why, not that someone asked for it.
+
+**Avoid bullet points** - write prose, not lists. If you need bullets to explain a change, you're either committing too much at once or over-explaining implementation details. The body should be a brief paragraph, not a changelog.
+
 Good examples:
 
 ```
diff --git a/packages/sandbox-container/src/services/session-manager.ts b/packages/sandbox-container/src/services/session-manager.ts
index 7c509dab..a03eb740 100644
--- a/packages/sandbox-container/src/services/session-manager.ts
+++ b/packages/sandbox-container/src/services/session-manager.ts
@@ -32,11 +32,10 @@ export class SessionManager {
           success: false,
           error: {
             message: `Session '${options.id}' already exists`,
-            code: ErrorCode.INTERNAL_ERROR,
+            code: ErrorCode.SESSION_ALREADY_EXISTS,
             details: {
-              sessionId: options.id,
-              originalError: 'Session already exists'
-            } satisfies InternalErrorContext
+              sessionId: options.id
+            }
           }
         };
       }
diff --git a/packages/sandbox/src/errors/adapter.ts b/packages/sandbox/src/errors/adapter.ts
index 9875da0f..306a46f3 100644
--- a/packages/sandbox/src/errors/adapter.ts
+++ b/packages/sandbox/src/errors/adapter.ts
@@ -26,6 +26,7 @@ import type {
   PortNotExposedContext,
   ProcessErrorContext,
   ProcessNotFoundContext,
+  SessionAlreadyExistsContext,
   ValidationFailedContext
 } from '@repo/shared/errors';
 import { ErrorCode } from '@repo/shared/errors';
@@ -58,6 +59,7 @@ import {
   ProcessNotFoundError,
   SandboxError,
   ServiceNotRespondingError,
+  SessionAlreadyExistsError,
   ValidationFailedError
 } from './classes';
 
@@ -123,6 +125,12 @@ export function createErrorFromResponse(errorResponse: ErrorResponse): Error {
         errorResponse as unknown as ErrorResponse<ProcessErrorContext>
       );
 
+    // Session Errors
+    case ErrorCode.SESSION_ALREADY_EXISTS:
+      return new SessionAlreadyExistsError(
+        errorResponse as unknown as ErrorResponse<SessionAlreadyExistsContext>
+      );
+
     // Port Errors
     case ErrorCode.PORT_ALREADY_EXPOSED:
       return new PortAlreadyExposedError(
diff --git a/packages/sandbox/src/errors/classes.ts b/packages/sandbox/src/errors/classes.ts
index caad715b..ea3c4e11 100644
--- a/packages/sandbox/src/errors/classes.ts
+++ b/packages/sandbox/src/errors/classes.ts
@@ -28,6 +28,7 @@ import type {
   ProcessExitedBeforeReadyContext,
   ProcessNotFoundContext,
   ProcessReadyTimeoutContext,
+  SessionAlreadyExistsContext,
   ValidationFailedContext
 } from '@repo/shared/errors';
 
@@ -236,6 +237,25 @@ export class ProcessError extends SandboxError<ProcessErrorContext> {
   }
 }
 
+// ============================================================================
+// Session Errors
+// ============================================================================
+
+/**
+ * Error thrown when a session already exists
+ */
+export class SessionAlreadyExistsError extends SandboxError<SessionAlreadyExistsContext> {
+  constructor(errorResponse: ErrorResponse<SessionAlreadyExistsContext>) {
+    super(errorResponse);
+    this.name = 'SessionAlreadyExistsError';
+  }
+
+  // Type-safe accessors
+  get sessionId() {
+    return this.context.sessionId;
+  }
+}
+
 // ============================================================================
 // Port Errors
 // ============================================================================
diff --git a/packages/sandbox/src/errors/index.ts b/packages/sandbox/src/errors/index.ts
index 98936962..fc3ffa05 100644
--- a/packages/sandbox/src/errors/index.ts
+++ b/packages/sandbox/src/errors/index.ts
@@ -109,6 +109,8 @@ export {
   ProcessReadyTimeoutError,
   SandboxError,
   ServiceNotRespondingError,
+  // Session Errors
+  SessionAlreadyExistsError,
   // Validation Errors
   ValidationFailedError
 } from './classes';
diff --git a/packages/sandbox/src/sandbox.ts b/packages/sandbox/src/sandbox.ts
index cc74869c..ad4b3932 100644
--- a/packages/sandbox/src/sandbox.ts
+++ b/packages/sandbox/src/sandbox.ts
@@ -37,7 +37,8 @@ import {
   CustomDomainRequiredError,
   ErrorCode,
   ProcessExitedBeforeReadyError,
-  ProcessReadyTimeoutError
+  ProcessReadyTimeoutError,
+  SessionAlreadyExistsError
 } from './errors';
 import { CodeInterpreter } from './interpreter';
 import { isLocalhostPattern } from './request-handler';
@@ -994,39 +995,38 @@ export class Sandbox<Env = unknown> extends Container<Env> implements ISandbox {
    * we reuse it instead of trying to create a new one.
    */
   private async ensureDefaultSession(): Promise<string> {
-    if (!this.defaultSession) {
-      const sessionId = `sandbox-${this.sandboxName || 'default'}`;
+    const sessionId = `sandbox-${this.sandboxName || 'default'}`;
 
-      try {
-        // Try to create session in container
-        await this.client.utils.createSession({
-          id: sessionId,
-          env: this.envVars || {},
-          cwd: '/workspace'
-        });
+    // Fast path: session already initialized in this instance
+    if (this.defaultSession === sessionId) {
+      return this.defaultSession;
+    }
+
+    // Create session in container
+    try {
+      await this.client.utils.createSession({
+        id: sessionId,
+        env: this.envVars || {},
+        cwd: '/workspace'
+      });
 
+      this.defaultSession = sessionId;
+      await this.ctx.storage.put('defaultSession', sessionId);
+      this.logger.debug('Default session initialized', { sessionId });
+    } catch (error: unknown) {
+      // Session may already exist (e.g., after hot reload or concurrent request)
+      if (error instanceof SessionAlreadyExistsError) {
+        this.logger.debug(
+          'Session exists in container but not in DO state, syncing',
+          { sessionId }
+        );
         this.defaultSession = sessionId;
-        // Persist to storage so it survives hot reloads
         await this.ctx.storage.put('defaultSession', sessionId);
-        this.logger.debug('Default session initialized', { sessionId });
-      } catch (error: unknown) {
-        // If session already exists (e.g., after hot reload), reuse it
-        if (
-          error instanceof Error &&
-          error.message.includes('already exists')
-        ) {
-          this.logger.debug('Reusing existing session after reload', {
-            sessionId
-          });
-          this.defaultSession = sessionId;
-          // Persist to storage in case it wasn't saved before
-          await this.ctx.storage.put('defaultSession', sessionId);
-        } else {
-          // Re-throw other errors
-          throw error;
-        }
+      } else {
+        throw error;
       }
     }
+
     return this.defaultSession;
   }
 
diff --git a/packages/shared/src/errors/codes.ts b/packages/shared/src/errors/codes.ts
index d2b2bba3..7569c5bc 100644
--- a/packages/shared/src/errors/codes.ts
+++ b/packages/shared/src/errors/codes.ts
@@ -43,6 +43,9 @@ export const ErrorCode = {
   PROCESS_PERMISSION_DENIED: 'PROCESS_PERMISSION_DENIED',
   PROCESS_ERROR: 'PROCESS_ERROR',
 
+  // Session Errors (409)
+  SESSION_ALREADY_EXISTS: 'SESSION_ALREADY_EXISTS',
+
   // Port Errors (409)
   PORT_ALREADY_EXPOSED: 'PORT_ALREADY_EXPOSED',
   PORT_IN_USE: 'PORT_IN_USE',
diff --git a/packages/shared/src/errors/contexts.ts b/packages/shared/src/errors/contexts.ts
index 354f6e88..f604e1ed 100644
--- a/packages/shared/src/errors/contexts.ts
+++ b/packages/shared/src/errors/contexts.ts
@@ -48,6 +48,10 @@ export interface ProcessErrorContext {
   stderr?: string;
 }
 
+export interface SessionAlreadyExistsContext {
+  sessionId: string;
+}
+
 /**
  * Process readiness error contexts
  */
diff --git a/packages/shared/src/errors/index.ts b/packages/shared/src/errors/index.ts
index b294aff8..b1a8bd9a 100644
--- a/packages/shared/src/errors/index.ts
+++ b/packages/shared/src/errors/index.ts
@@ -57,6 +57,7 @@ export type {
   ProcessExitedBeforeReadyContext,
   ProcessNotFoundContext,
   ProcessReadyTimeoutContext,
+  SessionAlreadyExistsContext,
   ValidationFailedContext
 } from './contexts';
 // Export utility functions
diff --git a/packages/shared/src/errors/status-map.ts b/packages/shared/src/errors/status-map.ts
index d71f1ab6..942172cf 100644
--- a/packages/shared/src/errors/status-map.ts
+++ b/packages/shared/src/errors/status-map.ts
@@ -42,6 +42,7 @@ export const ERROR_STATUS_MAP: Record<ErrorCode, number> = {
   [ErrorCode.PORT_ALREADY_EXPOSED]: 409,
   [ErrorCode.PORT_IN_USE]: 409,
   [ErrorCode.RESOURCE_BUSY]: 409,
+  [ErrorCode.SESSION_ALREADY_EXISTS]: 409,
 
   // 502 Bad Gateway
   [ErrorCode.SERVICE_NOT_RESPONDING]: 502,
diff --git a/packages/shared/src/errors/suggestions.ts b/packages/shared/src/errors/suggestions.ts
index fabd5aa7..a3b484b4 100644
--- a/packages/shared/src/errors/suggestions.ts
+++ b/packages/shared/src/errors/suggestions.ts
@@ -30,6 +30,9 @@ export function getSuggestion(
     case ErrorCode.PORT_IN_USE:
       return `Port ${context.port} is already in use by another service. Choose a different port`;
 
+    case ErrorCode.SESSION_ALREADY_EXISTS:
+      return `Session "${context.sessionId}" already exists. Use a different session ID or reuse the existing session`;
+
     case ErrorCode.INVALID_PORT:
       return `Port must be between 1 and 65535. Port ${context.port} is ${context.reason}`;