diff --git a/config/checkstyle/checkstyle-suppressions.xml b/config/checkstyle/checkstyle-suppressions.xml
new file mode 100644
index 0000000..c598199
--- /dev/null
+++ b/config/checkstyle/checkstyle-suppressions.xml
@@ -0,0 +1,22 @@
+<?xml version="1.0"?>
+<!DOCTYPE suppressions PUBLIC
+    "-//Checkstyle//DTD SuppressionFilter Configuration 1.2//EN"
+    "https://checkstyle.org/dtds/suppressions_1_2.dtd">
+
+<!-- Existing-code suppressions; new code must pass. Clean these up over time. -->
+<suppressions>
+    <!-- WireLogFormatter: deeply nested if-chains from protocol formatting -->
+    <suppress checks="NestedIfDepth"
+              files="WireLogFormatter\.java"/>
+
+    <!-- ExecutePsop builder-facing constructor carries the full option set -->
+    <suppress checks="ParameterNumber"
+              files="ExecutePsop\.java"/>
+
+    <!-- Utility classes with intentional public constructors (builder/entry-point API surface) -->
+    <suppress checks="HideUtilityClassConstructor"
+              files="FailureMapping\.java|LoadPsop\.java|ExecutePsop\.java"/>
+    <!-- Spring Boot entry-point class: implicit constructor is the standard pattern -->
+    <suppress checks="HideUtilityClassConstructor"
+              files="SpringWorkbenchApplication\.java"/>
+</suppressions>
diff --git a/config/checkstyle/checkstyle.xml b/config/checkstyle/checkstyle.xml
new file mode 100644
index 0000000..6efaaf0
--- /dev/null
+++ b/config/checkstyle/checkstyle.xml
@@ -0,0 +1,82 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC
+    "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
+    "https://checkstyle.org/dtds/configuration_1_3.dtd">
+
+<!--
+  Pragmatic Checkstyle rules for the workflow engine.
+  Formatting (indent, line length, import order) is handled by Spotless;
+  this config focuses on naming, complexity, and structural checks.
+-->
+<module name="Checker">
+    <property name="charset" value="UTF-8"/>
+
+    <property name="severity" value="error"/>
+
+    <module name="SuppressionFilter">
+        <property name="file" value="${config_loc}/checkstyle-suppressions.xml"/>
+        <property name="optional" value="false"/>
+    </module>
+
+    <!-- File-level checks -->
+    <module name="FileTabCharacter">
+        <property name="eachLine" value="true"/>
+    </module>
+    <module name="NewlineAtEndOfFile"/>
+
+    <!-- TreeWalker: AST-based checks -->
+    <module name="TreeWalker">
+
+        <!-- Naming conventions (underscores allowed in method names: spec-section test cases) -->
+        <module name="TypeName"/>
+        <module name="MethodName">
+            <property name="format" value="^[a-z][a-zA-Z0-9_]*$"/>
+        </module>
+        <module name="MemberName"/>
+        <module name="ParameterName"/>
+        <module name="LocalVariableName"/>
+        <!-- Allow conventional lowercase names: log (SLF4J), mapper (Jackson) -->
+        <module name="ConstantName">
+            <property name="format" value="^log$|^mapper$|^cardMapper$|^[A-Z][A-Z0-9]*(_[A-Z0-9]+)*$"/>
+        </module>
+        <module name="PackageName">
+            <property name="format" value="^[a-z]+(\.[a-z][a-z0-9]*)*$"/>
+        </module>
+
+        <!-- Imports: unused/redundant are errors; star imports warned (cleanup ongoing) -->
+        <module name="UnusedImports"/>
+        <module name="RedundantImport"/>
+
+        <!-- Complexity (thresholds calibrated for existing codebase; tighten over time) -->
+        <module name="CyclomaticComplexity">
+            <property name="max" value="35"/>
+        </module>
+        <module name="MethodLength">
+            <property name="max" value="150"/>
+        </module>
+        <module name="ParameterNumber">
+            <property name="max" value="12"/>
+        </module>
+        <module name="NestedIfDepth">
+            <property name="max" value="3"/>
+        </module>
+        <module name="NestedTryDepth">
+            <property name="max" value="2"/>
+        </module>
+
+        <!-- Common bug patterns -->
+        <module name="EmptyCatchBlock">
+            <property name="exceptionVariableName" value="expected|ignore|ignored"/>
+        </module>
+        <module name="EqualsHashCode"/>
+        <module name="StringLiteralEquality"/>
+        <module name="ArrayTypeStyle"/>
+        <module name="OneStatementPerLine"/>
+
+        <!-- Design -->
+        <module name="HideUtilityClassConstructor"/>
+        <module name="InterfaceIsType"/>
+        <module name="MutableException"/>
+
+    </module>
+</module>
diff --git a/config/spotbugs/spotbugs-exclude.xml b/config/spotbugs/spotbugs-exclude.xml
new file mode 100644
index 0000000..e49f56a
--- /dev/null
+++ b/config/spotbugs/spotbugs-exclude.xml
@@ -0,0 +1,91 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  SpotBugs exclusion filter for the workflow engine.
+  These patterns are either intentional design choices or false positives
+  for this codebase's patterns. Clean up over time; new findings still block verify.
+-->
+<FindBugsFilter>
+    <!-- Returning/storing mutable collections in builders, records and config accessors is
+         the intended API surface; defensive copies are made at trust boundaries. -->
+    <Match>
+        <Or>
+            <Bug pattern="EI_EXPOSE_REP"/>
+            <Bug pattern="EI_EXPOSE_REP2"/>
+            <Bug pattern="EI_EXPOSE_REP2_EI_EXPOSE_REP2"/>
+        </Or>
+    </Match>
+
+    <!-- Java serialization is not used by this project. -->
+    <Match>
+        <Or>
+            <Bug pattern="SE_BAD_FIELD"/>
+            <Bug pattern="SE_NO_SERIALVERSIONID"/>
+        </Or>
+    </Match>
+
+    <!-- Throwing from constructors: no finalizers in this codebase, finalizer attack N/A. -->
+    <Match>
+        <Bug pattern="CT_CONSTRUCTOR_THROW"/>
+    </Match>
+
+    <!-- Single-use SecureRandom: fresh instance per AES-GCM IV is intentional and safe. -->
+    <Match>
+        <Bug pattern="DMI_RANDOM_USED_ONLY_ONCE"/>
+    </Match>
+
+    <!-- Duplicated branch bodies in normalization (kept for symmetry with sibling branches). -->
+    <Match>
+        <Bug pattern="DB_DUPLICATE_BRANCHES"/>
+    </Match>
+
+    <!-- CompletableFuture exception paths in WorkflowExecutor intentionally ignore
+         secondary failures when aggregating step results. -->
+    <Match>
+        <Bug pattern="DM_DEFAULT_ENCODING,DE_MIGHT_IGNORE"/>
+    </Match>
+
+    <!-- Generated fixtures and test code -->
+    <Match>
+        <Or>
+            <Class name="~.*Test.*"/>
+            <Class name="~.*Mock.*"/>
+        </Or>
+    </Match>
+
+    <!-- Vendor SDK adapter classes: the vendor API shape is out of our control. -->
+    <Match>
+        <Or>
+            <Class name="~.*OrderHttpClientAdapter.*"/>
+            <Class name="~.*OrderGatewayClientRuntime.*"/>
+            <Class name="~.*EastcomOrderByteBufWorkaround.*"/>
+            <Class name="~.*MockGatewayServer.*"/>
+            <Class name="~.*MockOrderHttpSessionClient.*"/>
+        </Or>
+    </Match>
+
+    <!-- Samples/demo code: accepted demo-quality items; review on next touch.
+         - Dead store in parseNegotiationContext (leftover context map)
+         - Vacuous instanceof in NotificationPolicy (defensive String check)
+         - NegotiationStrategy intentionally shadows the control interface name (sample mirror)
+         - Boolean-returning booleanParam helper may return null to signal "unset"
+         - handleLogin catches Exception broadly as the demo login boundary
+         - doPrivileged warning: SecurityManager is not used by this project -->
+    <Match>
+        <Or>
+            <Class name="~.*NegotiationBaseAgentExecutor.*"/>
+            <Class name="~.*NotificationPolicy.*"/>
+            <Class name="~.*examples.negotiation.NegotiationStrategy"/>
+            <Class name="~.*JdkHttpA2AServer.*"/>
+            <Class name="~.*A2ATInitialization.*"/>
+        </Or>
+        <Bug pattern="DP_CREATE_CLASSLOADER_INSIDE_DO_PRIVILEGED"/>
+    </Match>
+    <Match>
+        <Or>
+            <Class name="~.*NegotiationBaseAgentExecutor.*"/>
+            <Class name="~.*NotificationPolicy.*"/>
+            <Class name="~.*examples.negotiation.NegotiationStrategy"/>
+            <Class name="~.*JdkHttpA2AServer.*"/>
+        </Or>
+    </Match>
+</FindBugsFilter>
diff --git a/pom.xml b/pom.xml
index 3234d06..f2e8188 100644
--- a/pom.xml
+++ b/pom.xml
@@ -174,8 +174,12 @@
                 <version>2.46.1</version>
                 <configuration>
                     <java>
+                        <excludes>
+                            <!-- google-java-format cannot parse package-info.java -->
+                            <exclude>**/package-info.java</exclude>
+                        </excludes>
                         <googleJavaFormat>
-                            <version>1.22.0</version>
+                            <version>1.24.0</version>
                             <style>GOOGLE</style>
                         </googleJavaFormat>
                         <removeUnusedImports/>
@@ -191,6 +195,81 @@
                     </execution>
                 </executions>
             </plugin>
+
+            <!-- Checkstyle: naming, complexity, structural checks (formatting is Spotless) -->
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-checkstyle-plugin</artifactId>
+                <version>3.6.0</version>
+                <dependencies>
+                    <dependency>
+                        <groupId>com.puppycrawl.tools</groupId>
+                        <artifactId>checkstyle</artifactId>
+                        <version>10.20.0</version>
+                    </dependency>
+                </dependencies>
+                <configuration>
+                    <configLocation>${maven.multiModuleProjectDirectory}/config/checkstyle/checkstyle.xml</configLocation>
+                    <propertyExpansion>config_loc=${maven.multiModuleProjectDirectory}/config/checkstyle</propertyExpansion>
+                    <includeTestSourceDirectory>false</includeTestSourceDirectory>
+                    <consoleOutput>true</consoleOutput>
+                    <failOnViolation>true</failOnViolation>
+                </configuration>
+                <executions>
+                    <execution>
+                        <id>checkstyle-check</id>
+                        <phase>validate</phase>
+                        <goals>
+                            <goal>check</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
+
+            <!-- SpotBugs: bug pattern detection (null dereference, resource leaks, concurrency) -->
+            <plugin>
+                <groupId>com.github.spotbugs</groupId>
+                <artifactId>spotbugs-maven-plugin</artifactId>
+                <version>4.8.6.4</version>
+                <configuration>
+                    <effort>Max</effort>
+                    <threshold>Medium</threshold>
+                    <excludeFilterFile>${maven.multiModuleProjectDirectory}/config/spotbugs/spotbugs-exclude.xml</excludeFilterFile>
+                    <xmlOutput>true</xmlOutput>
+                </configuration>
+                <executions>
+                    <execution>
+                        <id>spotbugs-check</id>
+                        <phase>verify</phase>
+                        <goals>
+                            <goal>check</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
+
+            <!-- JaCoCo: test coverage reporting (no threshold enforcement) -->
+            <plugin>
+                <groupId>org.jacoco</groupId>
+                <artifactId>jacoco-maven-plugin</artifactId>
+                <version>0.8.12</version>
+                <executions>
+                    <execution>
+                        <id>jacoco-prepare-agent</id>
+                        <goals>
+                            <goal>prepare-agent</goal>
+                        </goals>
+                    </execution>
+                    <execution>
+                        <id>jacoco-report</id>
+                        <phase>test</phase>
+                        <goals>
+                            <goal>report</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
+
             <plugin>
                 <groupId>org.codehaus.mojo</groupId>
                 <artifactId>flatten-maven-plugin</artifactId>
diff --git a/spring-boot-starter/pom.xml b/spring-boot-starter/pom.xml
index 4340889..f67bdb8 100644
--- a/spring-boot-starter/pom.xml
+++ b/spring-boot-starter/pom.xml
@@ -75,6 +75,12 @@
             <version>${spring-boot.version}</version>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.jspecify</groupId>
+            <artifactId>jspecify</artifactId>
+            <version>1.0.0</version>
+            <scope>provided</scope>
+        </dependency>
     </dependencies>
 
     <build>
diff --git a/spring-boot-starter/src/main/java/dev/openan/workflow/engine/spring/package-info.java b/spring-boot-starter/src/main/java/dev/openan/workflow/engine/spring/package-info.java
new file mode 100644
index 0000000..e608b77
--- /dev/null
+++ b/spring-boot-starter/src/main/java/dev/openan/workflow/engine/spring/package-info.java
@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2026 Huawei Technologies Co., Ltd.
+ * All Rights Reserved.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *    not use this file except in compliance with the License. You may obtain
+ *    a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an AS IS BASIS, WITHOUT
+ *    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ *    License for the specific language governing permissions and limitations
+ *    under the License.
+ */
+
+/**
+ * Spring Boot auto-configuration for the A2A server side.
+ */
+@org.jspecify.annotations.NullMarked
+package dev.openan.workflow.engine.spring;
diff --git a/workflow-engine/pom.xml b/workflow-engine/pom.xml
index d70d6ab..fa58fbe 100644
--- a/workflow-engine/pom.xml
+++ b/workflow-engine/pom.xml
@@ -140,6 +140,13 @@
             <version>${slf4j.version}</version>
             <scope>test</scope>
         </dependency>
+        <!-- Null-safety annotations matching the A2A SDK's JSpecify contract -->
+        <dependency>
+            <groupId>org.jspecify</groupId>
+            <artifactId>jspecify</artifactId>
+            <version>1.0.0</version>
+            <scope>provided</scope>
+        </dependency>
     </dependencies>
 
     <build>
diff --git a/workflow-engine/src/main/java/dev/openan/workflow/engine/client/DefaultWorkflowEngineClient.java b/workflow-engine/src/main/java/dev/openan/workflow/engine/client/DefaultWorkflowEngineClient.java
index 0eb90de..525f125 100644
--- a/workflow-engine/src/main/java/dev/openan/workflow/engine/client/DefaultWorkflowEngineClient.java
+++ b/workflow-engine/src/main/java/dev/openan/workflow/engine/client/DefaultWorkflowEngineClient.java
@@ -23,9 +23,22 @@
 import dev.openan.workflow.engine.control.EventCallback;
 import dev.openan.workflow.engine.control.EventType;
 import dev.openan.workflow.engine.control.NegotiationStrategy;
-import dev.openan.workflow.engine.model.*;
+import dev.openan.workflow.engine.model.BusinessFailure;
+import dev.openan.workflow.engine.model.MessageContent;
+import dev.openan.workflow.engine.model.NegotiationReply;
+import dev.openan.workflow.engine.model.NegotiationRequest;
+import dev.openan.workflow.engine.model.ReceivedMessage;
+import dev.openan.workflow.engine.model.SendMessageResult;
+import dev.openan.workflow.engine.model.TaskRequest;
 import java.time.Duration;
-import java.util.*;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+import java.util.UUID;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CompletionException;
 import java.util.concurrent.ExecutionException;
diff --git a/workflow-engine/src/main/java/dev/openan/workflow/engine/client/package-info.java b/workflow-engine/src/main/java/dev/openan/workflow/engine/client/package-info.java
index 20bb65d..60380af 100644
--- a/workflow-engine/src/main/java/dev/openan/workflow/engine/client/package-info.java
+++ b/workflow-engine/src/main/java/dev/openan/workflow/engine/client/package-info.java
@@ -11,7 +11,7 @@
  *         http://www.apache.org/licenses/LICENSE-2.0
  *
  *    Unless required by applicable law or agreed to in writing, software
- *    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *    distributed under the License is distributed on an AS IS BASIS, WITHOUT
  *    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
  *    License for the specific language governing permissions and limitations
  *    under the License.
@@ -21,4 +21,5 @@
  * Protocol adaptation: transport runtimes, authentication, negotiation correlation, independent
  * subscriptions, and wire observability. Content generation stays with the host.
  */
+@org.jspecify.annotations.NullMarked
 package dev.openan.workflow.engine.client;