docs: clarify Kotlin 2.3+/API 2.2 and Java 25/target-17 versioning per Spring Boot 4.x guidance

Co-authored-by: afranken <763000+afranken@users.noreply.github.com>

Author: Copilot

.github/CONTRIBUTING.md

@@ -15,7 +15,7 @@ All third-party contributions to this project must be accompanied by a signed co
 ## Development Setup
 
 **Prerequisites:**
-- Java 25 (compile version; targets JVM 17)
+- Java 25 (compile/run; Kotlin and Java API compatibility targets 17, per Spring Boot 4.x guidance)
 - Maven 3.9+ (use the included `./mvnw` wrapper)
 - Docker (for Docker build and integration tests)
 
@@ -56,7 +56,7 @@ S3Mock uses three test levels:
 2. **Spring Boot tests** - component-level coverage with Spring context
 3. **Integration tests** (`*IT.kt`) - end-to-end tests against the Docker container using real AWS SDK v2 clients, in `integration-tests/src/test/`
 
-Please ensure that your code is covered by at least one of these test types.
+Ensure your code has coverage from at least one of these test types.
 
 ## Submitting Changes
 

AGENTS.md

@@ -8,7 +8,7 @@ Lightweight S3 API mock server for local integration testing.
 > no restating of rules from the root.
 
 ## Tech Stack
-- **Kotlin 2.3** (target JVM 17; build/compile requires JDK 25), Spring Boot 4.0.x, Maven 3.9+
+- **Kotlin 2.3+** (language/API compatibility: 2.2; JVM target: 17; compile/run: JDK 25 — per Spring Boot 4.x guidance), Spring Boot 4.0.x, Maven 3.9+
 - **Testing**: JUnit 5, Mockito, AssertJ, Testcontainers
 - **Container**: Docker/Alpine
 

README.md

@@ -480,7 +480,7 @@ graph LR
 
 ## Build & Run
 
-**Requirements**: Java 17+, Maven 3.9+, Docker (for Docker build and integration tests)
+**Requirements**: Java 25, Maven 3.9+, Docker (for Docker build and integration tests)
 
 **Build:**
 ```shell
@@ -503,9 +503,9 @@ make integration-tests
 ```
 
 **Technology:**
-- Built with Kotlin 2.3.0 and Spring Boot 4.0
-- Tests written in Kotlin
-- Target JVM: 17
+- S3Mock uses Kotlin 2.3+ (language/API compatibility: 2.2) and Spring Boot 4.0
+- Tests use Kotlin
+- JVM target: 17 (compile/run requires JDK 25, per Spring Boot 4.x guidance)
 
 ## Contributing
 

docs/KOTLIN.md

@@ -2,6 +2,16 @@
 
 Canonical reference for Kotlin idioms, naming conventions, and code quality standards used across this project.
 
+## Compiler Settings
+
+S3Mock uses Kotlin 2.3+ with language and API compatibility set to **2.2**, following Spring Boot 4.x guidance:
+
+- **Kotlin compiler**: 2.3+
+- **Language/API version**: 2.2 (`-language-version 2.2`, `-api-version 2.2`)
+- **JVM target**: 17 (compile/run requires JDK 25)
+
+This means you can use any syntax and standard library APIs available in Kotlin 2.2. Kotlin 2.3+ compiler improvements (performance, diagnostics) apply automatically.
+
 ## Idioms
 
 ### Null Safety