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Use when DTOs fail to deserialize, default parameters or nullability behave unexpectedly, date-time or enum formats drift, polymorphic payloads are involved, PATCH semantics must distinguish null from absent, or ObjectMapper changes risk breaking existing API or message contracts.\nmetadata:\n  short-description: \"Fix Kotlin JSON edge cases safely\"\n  author: Kotlin\n  source: https:\u002F\u002Fgithub.com\u002FKotlin\u002Fkotlin-backend-agent-skills\u002Ftree\u002Fmain\u002F.agents\u002Fskills\u002Fjackson-kotlin-serialization-specialist\n---\n\n# Jackson Kotlin Serialization Specialist\n\nSource mapping: Tier 2 high-value skill derived from `Kotlin_Spring_Developer_Pipeline.md` (`SK-08`).\n\n## Mission\n\nMake Kotlin plus Jackson behavior explicit, compatible, and testable.\nTreat wire-format correctness as a contract problem, not only a mapper-configuration problem.\n\n## Read First\n\n- The actual DTO or event model classes.\n- The exact failing JSON payload or expected payload examples.\n- `ObjectMapper` customizers, Spring Boot Jackson properties, and any per-client mapper overrides.\n- Build files to verify Jackson module alignment with Spring Boot and Kotlin versions.\n- The boundary where serialization matters: MVC, WebFlux, Kafka, Redis, persistence JSON column, or external HTTP client.\n\n## Diagnose In This Order\n\n1. Verify module presence and alignment:\n   - `jackson-module-kotlin`\n   - `JavaTimeModule`\n   - other custom modules or serializers\n2. Verify constructor semantics:\n   - default parameters\n   - required parameters\n   - nullable versus non-null\n3. Verify field presence semantics:\n   - absent\n   - present with `null`\n   - present with value\n4. Verify naming, inclusion, and date-time strategy.\n5. Verify polymorphism or custom serializer behavior.\n6. Verify whether the real bug comes from a local mapper override rather than the global mapper.\n\n## Core Kotlin Rules\n\n- Keep DTOs immutable unless the project already has a strong alternative convention.\n- Do not switch `val` to `var` just to appease Jackson.\n- Do not add empty constructors to Kotlin DTOs as a workaround if the Kotlin module can model the contract correctly.\n- Treat nullable as a wire-contract decision, not a convenient escape hatch.\n- Be explicit about value classes, sealed hierarchies, and default parameter behavior.\n\n## Advanced Serialization Traps\n\n- Missing field and explicit `null` are not the same. For PATCH-like contracts, model tri-state semantics deliberately.\n- Default constructor values can silently hide client mistakes if the field should have been required.\n- `@JsonInclude` may improve payload size but can also erase signal that clients rely on.\n- Non-null primitives, `FAIL_ON_NULL_FOR_PRIMITIVES`, and Kotlin non-null types interact differently across payload shapes.\n- Sealed classes need stable, versionable type discriminators. Do not treat polymorphic type ids as an internal detail once they are on the wire.\n- Enum serialization by name, code, or custom object form is a public compatibility choice.\n- Date-time serialization must make timezone assumptions explicit. `Instant`, `OffsetDateTime`, and `LocalDateTime` are not interchangeable.\n- `@JvmInline value class` support may differ by Jackson version and serializer context. Verify scalar form and map-key behavior explicitly.\n- Global `ObjectMapper` changes can break unrelated endpoints or message consumers. Prefer narrow fixes when the issue is boundary-specific.\n\n## Boundary-Specific Nuances\n\n- MVC request and response mapping, Kafka message mapping, Redis payload mapping, and JSON-column mapping often use different mapper lifecycles even inside one codebase.\n- `ObjectMapper.copy()` can preserve most configuration while still drifting from future global changes. If a subsystem owns a private mapper, document that divergence.\n- Kotlin default parameters interact differently with creator annotations, mix-ins, and custom deserializers. If custom deserialization exists, verify constructor invocation explicitly.\n- Unknown enum handling, unknown-property handling, and coercion rules are compatibility decisions. A permissive setting may preserve old clients or may quietly accept garbage.\n- If the API is documented through OpenAPI or consumer contracts, make sure the documented nullability and actual wire behavior match. Kotlin type hints alone are not enough.\n\n## Expert Heuristics\n\n- If the same model is used on both inbound and outbound boundaries, check whether the optimal serializer settings are actually symmetrical. Often they are not.\n- If clients rely on partial update semantics, prefer an explicit patch model rather than trying to infer intent from ordinary DTO nullability.\n- If a serializer bug appears after a dependency upgrade, inspect feature defaults and module registration order before rewriting DTOs.\n- If compatibility matters, prove the fix with golden JSON examples or snapshot-style serialization tests, not only with one happy-path request.\n\n## Design Rules\n\n- Choose one naming strategy and document it.\n- Keep transport DTOs separate from persistence and domain objects when contract stability matters.\n- If payload evolution matters, favor additive fields and backward-compatible defaults over silent semantic changes.\n- If multiple serialization contexts exist, decide which behavior is global and which is boundary-specific.\n\n## Output Contract\n\nReturn these sections:\n\n- `Observed behavior`: what the current mapper does.\n- `Contract expectation`: what the wire format 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Use this when users request creating art using code, generative art, algorithmic art, flow fields, or particle systems. Create original algorithmic art rather than copying existing artists' work to avoid copyright violations.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[641,644,647,650],{"name":642,"slug":643,"type":15},"Creative","creative",{"name":645,"slug":646,"type":15},"Generative Art","generative-art",{"name":648,"slug":649,"type":15},"Graphics","graphics",{"name":651,"slug":652,"type":15},"JavaScript","javascript","2026-07-13T06:41:35.540127",{"slug":655,"name":655,"fn":656,"description":657,"org":658,"tags":659,"stars":25,"repoUrl":26,"updatedAt":670},"antfu","configure JavaScript projects with Anthony Fu's tools","Anthony Fu's opinionated tooling and conventions for JavaScript\u002FTypeScript projects. Use when setting up new projects, configuring ESLint\u002FPrettier alternatives, monorepos, library publishing, or when the user mentions Anthony Fu's preferences.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[660,663,666,667],{"name":661,"slug":662,"type":15},"Best Practices","best-practices",{"name":664,"slug":665,"type":15},"Engineering","engineering",{"name":651,"slug":652,"type":15},{"name":668,"slug":669,"type":15},"TypeScript","typescript","2026-07-13T06:43:13.153309",{"slug":672,"name":672,"fn":673,"description":674,"org":675,"tags":676,"stars":25,"repoUrl":26,"updatedAt":686},"brand-guidelines","apply Anthropic brand guidelines","Applies Anthropic's official brand colors and typography to any sort of artifact that may benefit from having Anthropic's look-and-feel. Use it when brand colors or style guidelines, visual formatting, or company design standards apply.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[677,680,683],{"name":678,"slug":679,"type":15},"Branding","branding",{"name":681,"slug":682,"type":15},"Design","design",{"name":684,"slug":685,"type":15},"Typography","typography","2026-07-13T06:43:06.077629",{"slug":688,"name":688,"fn":689,"description":690,"org":691,"tags":692,"stars":25,"repoUrl":26,"updatedAt":701},"canvas-design","create visual art and design assets","Create beautiful visual art in .png and .pdf documents using design philosophy. You should use this skill when the user asks to create a poster, piece of art, design, or other static piece. Create original visual designs, never copying existing artists' work to avoid copyright violations.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[693,694,695,698],{"name":642,"slug":643,"type":15},{"name":681,"slug":682,"type":15},{"name":696,"slug":697,"type":15},"Images","images",{"name":699,"slug":700,"type":15},"PDF","pdf","2026-07-13T06:39:58.803113",{"slug":703,"name":703,"fn":704,"description":705,"org":706,"tags":707,"stars":25,"repoUrl":26,"updatedAt":720},"ci-cd-containerization-advisor","design CI\u002FCD pipelines for Kotlin applications","Design reproducible build, image, and deployment pipelines for Kotlin plus Spring applications, including CI verification, layered containers, rollout safety, and deployment-time migration coordination. Use when creating or improving Dockerfiles, CI workflows, image hardening, Kubernetes manifests, release gates, or deployment strategies for Spring Boot services, especially where build reproducibility and operational safety matter.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[708,711,714,717,718,719],{"name":709,"slug":710,"type":15},"CI\u002FCD","ci-cd",{"name":712,"slug":713,"type":15},"Containers","containers",{"name":715,"slug":716,"type":15},"Deployment","deployment",{"name":664,"slug":665,"type":15},{"name":13,"slug":14,"type":15},{"name":17,"slug":18,"type":15},"2026-07-13T06:41:47.83899",{"slug":722,"name":722,"fn":723,"description":724,"org":725,"tags":726,"stars":25,"repoUrl":26,"updatedAt":737},"cloudflare-deploy","deploy applications to Cloudflare","Deploy applications and infrastructure to Cloudflare using Workers, Pages, and related platform services. Use when the user asks to deploy, host, publish, or set up a project on Cloudflare.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[727,730,733,736],{"name":728,"slug":729,"type":15},"Cloudflare","cloudflare",{"name":731,"slug":732,"type":15},"Cloudflare Pages","cloudflare-pages",{"name":734,"slug":735,"type":15},"Cloudflare Workers","cloudflare-workers",{"name":715,"slug":716,"type":15},"2026-07-17T06:04:42.853896",{"slug":739,"name":739,"fn":740,"description":741,"org":742,"tags":743,"stars":25,"repoUrl":26,"updatedAt":753},"compose-ui-control","interact with Compose Desktop applications","Control a running Compose Desktop application via HTTP. Use when you need to interact with UI elements, click buttons, enter text, wait for elements to appear, or capture screenshots in a Compose Desktop app that has compose-ui-test-server enabled.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[744,747,750],{"name":745,"slug":746,"type":15},"Automation","automation",{"name":748,"slug":749,"type":15},"Desktop","desktop",{"name":751,"slug":752,"type":15},"UI Components","ui-components","2026-07-13T06:40:38.798626",128,{"items":756,"total":877},[757,773,782,791,802,812,821,830,839,849,858,869],{"slug":758,"name":758,"fn":759,"description":760,"org":761,"tags":762,"stars":770,"repoUrl":771,"updatedAt":772},"mps-aspect-accessories","configure JetBrains MPS module dependencies","Wire MPS module and model dependencies, used languages, used devkits, extended languages, runtime solutions, accessory models, and language\u002Fdependency versions. Use when adding\u002Fremoving module dependencies, importing languages or devkits into a model, declaring runtime solutions, or shipping accessory content visible to consumers without explicit import.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[763,766,769],{"name":764,"slug":765,"type":15},"Architecture","architecture",{"name":767,"slug":768,"type":15},"Configuration","configuration",{"name":664,"slug":665,"type":15},1650,"https:\u002F\u002Fgithub.com\u002FJetBrains\u002FMPS","2026-07-17T06:06:57.311661",{"slug":774,"name":774,"fn":775,"description":776,"org":777,"tags":778,"stars":770,"repoUrl":771,"updatedAt":781},"mps-aspect-actions","define and edit MPS node factories","Use when defining or editing MPS node factories (the \"actions\" aspect) — `NodeFactories` roots, per-concept `NodeFactory` setup functions that initialize a freshly created node and optionally copy data from a replaced `sampleNode`, plus the actions aspect's `CopyPasteHandlers` and `PasteWrappers` roots. Reach for this skill when a substitution, side transform, completion replacement, or `add new initialized(...)` should preserve fields from the node it is replacing, or when defaults set in a constructor are not enough.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[779,780],{"name":764,"slug":765,"type":15},{"name":664,"slug":665,"type":15},"2026-07-17T06:04:48.066901",{"slug":783,"name":783,"fn":784,"description":785,"org":786,"tags":787,"stars":770,"repoUrl":771,"updatedAt":790},"mps-aspect-behavior","define and edit MPS concept behavior","Use when defining or editing MPS `ConceptBehavior` — per-concept methods (non-virtual \u002F virtual \u002F abstract \u002F static \u002F virtual static), constructors, virtual dispatch (MRO), super and interface-default calls (`super\u003CInterface>.method`), overriding methods from `lang.core.behavior` interfaces such as `ScopeProvider.getScope` \u002F `INamedConcept.getName` \u002F `BaseConcept.getPresentation`, calling sibling methods (`LocalBehaviorMethodCall`) and behavior methods from other aspects via `node.method(...)`. Reach for this skill whenever the task involves authoring or modifying `\u003Clang>\u002FlanguageModels\u002Fbehavior.mps`.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[788,789],{"name":764,"slug":765,"type":15},{"name":664,"slug":665,"type":15},"2026-07-13T06:45:21.757084",{"slug":792,"name":792,"fn":793,"description":794,"org":795,"tags":796,"stars":770,"repoUrl":771,"updatedAt":801},"mps-aspect-constraints","define JetBrains MPS language constraints","Use when defining or editing MPS language constraints — property validators \u002F setters \u002F getters, referent search scopes (imperative or inherited via `ScopeProvider.getScope`), `referentSetHandler` side effects, default-scope blocks, `canBeChild` \u002F `canBeParent` \u002F `canBeAncestor` \u002F `canBeRoot` placement rules, `defaultConcreteConcept` for abstract concepts, `set \u003Cread-only>` and `{name}` aliasing, and scope helpers (`SimpleRoleScope`, `ListScope`, `CompositeScope`, `HidingByNameScope`). Reach for this skill whenever the task involves authoring or modifying `\u003Clang>\u002FlanguageModels\u002Fconstraints.mps`.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[797,798],{"name":764,"slug":765,"type":15},{"name":799,"slug":800,"type":15},"Code Analysis","code-analysis","2026-07-23T05:41:33.639365",{"slug":803,"name":803,"fn":804,"description":805,"org":806,"tags":807,"stars":770,"repoUrl":771,"updatedAt":811},"mps-aspect-dataflow","define and debug MPS dataflow builders","Use when defining or debugging MPS dataflow builders for a concept — control\u002Fdata flow declarations that drive reachability analysis and variable-use checking. Covers DataFlowBuilderDeclaration, BuilderBlock, emit instructions (code for, jump, ifjump, label, read, write, ret, mayBeUnreachable), positions (AfterPosition, BeforePosition, LabelPosition), the jetbrains.mps.lang.dataFlow language, the NodeParameter implicit, BL+smodel usage inside builder bodies, and IBuilderMode for advanced analyses such as nullable\u002Fnon-null tracking.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[808],{"name":809,"slug":810,"type":15},"Data Analysis","data-analysis","2026-07-13T06:45:19.114674",{"slug":813,"name":813,"fn":814,"description":815,"org":816,"tags":817,"stars":770,"repoUrl":771,"updatedAt":820},"mps-aspect-editor","define MPS editor layouts","Use when creating or changing MPS editor definitions — the overall workflow from scaffolding a `ConceptEditorDeclaration` through componentizing reusable `EditorComponentDeclaration`s, refining cell models and cell layouts, applying style sheets and indent-layout style items, wiring smart references, leveraging inheritance via super-concepts and interfaces, inspecting (`print_node_json`, `show_node_representation`) and validating (`check_root_node_problems`). Covers `jetbrains.mps.lang.editor` cell models (`CellModel_RefNode`\u002F`CellModel_RefNodeList`\u002F`CellModel_RefCell`\u002F`CellModel_Property`\u002F`CellModel_Constant`), layout choices, and JSON blueprints for common editor shapes. For the non-layout side (action maps, keymaps, transformation\u002Fsubstitute menus) use `mps-aspect-editor-menus-and-keymaps`.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[818,819],{"name":681,"slug":682,"type":15},{"name":751,"slug":752,"type":15},"2026-07-23T05:41:56.638151",{"slug":822,"name":822,"fn":823,"description":824,"org":825,"tags":826,"stars":770,"repoUrl":771,"updatedAt":829},"mps-aspect-editor-menus-and-keymaps","author MPS editor menus and keymaps","Use when authoring the **non-layout** parts of the MPS editor aspect — what happens when the user types, presses a key, triggers completion, pastes, or invokes a context action. Covers action maps (`CellActionMapDeclaration`), cell keymaps (`CellKeyMapDeclaration`), transformation menus (`TransformationMenu_Default` \u002F `_Named` \u002F `_Contribution`), substitute menus (`SubstituteMenu_Default` \u002F `SubstituteMenu` \u002F contributions), side transforms (LEFT\u002FRIGHT), legacy cell menus, paste wrappers and copy-paste handlers (in the actions language), completion styling, reference presentation, two-step deletion, and the editor selection API. Trigger terms: `actionMap`, `keyMap`, `delete_action_id`, `transformationMenu`, `substituteMenu`, `Ctrl+Space`, `Ctrl+Alt+B`, side transform, paste wrapper, completion styling, `PasteWrappers`, `CopyPasteHandlers`. For the **layout** side (cells, layouts, style sheets) use `mps-aspect-editor` instead.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[827,828],{"name":664,"slug":665,"type":15},{"name":751,"slug":752,"type":15},"2026-07-23T05:41:49.666535",{"slug":831,"name":831,"fn":832,"description":833,"org":834,"tags":835,"stars":770,"repoUrl":771,"updatedAt":838},"mps-aspect-generation-plan","modify MPS generation plans","Use when defining or modifying an MPS generation plan — explicit ordering of generators, checkpoints for cross-model reference resolution, forks for parallel branches, IncludePlan composition, conditional PlanContribution activation, ParameterEquals\u002FConceptListSelector fork selectors, and InitModelAttributes for targetFacet routing. Apply when working with @genplan models, the jetbrains.mps.lang.generator.plan language, attaching plans via DevKits or the Custom generation facet, or debugging cross-model mapping label resolution.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[836,837],{"name":764,"slug":765,"type":15},{"name":664,"slug":665,"type":15},"2026-07-13T06:44:59.507855",{"slug":840,"name":840,"fn":841,"description":842,"org":843,"tags":844,"stars":770,"repoUrl":771,"updatedAt":848},"mps-aspect-generator","define JetBrains MPS generator rules","Use when defining or modifying MPS generators — author a generator module, add or edit root\u002Freduction\u002Fweaving\u002Fpattern mapping rules, attach template macros ($COPY_SRC, $LOOP, $IF, $PROPERTY, $REF, $SWITCH, $MAP_SRC, $WEAVE, $INSERT, $LABEL, $TRACE, $VAR), wire mapping labels, build template switches, write pre\u002Fpost mapping scripts, navigate `genContext`, or debug \"rule didn't fire\", missing references, empty output, infinite reduction loops, and generated-Java compile failures.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[845,846,847],{"name":764,"slug":765,"type":15},{"name":799,"slug":800,"type":15},{"name":664,"slug":665,"type":15},"2026-07-17T06:06:58.042999",{"slug":850,"name":850,"fn":851,"description":852,"org":853,"tags":854,"stars":770,"repoUrl":771,"updatedAt":857},"mps-aspect-intentions","define and edit MPS intentions","Use when defining or editing MPS intentions (the Alt+Enter context-action aspect) — adding `IntentionDeclaration` roots, parameterized or surround-with variants, description\u002FisApplicable\u002Fexecute blocks, child-filter functions, factory-initialized AST splicing, or debugging why an intention is not offered. Lives in the language's `intentions` model and uses `jetbrains.mps.lang.intentions`.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[855,856],{"name":764,"slug":765,"type":15},{"name":664,"slug":665,"type":15},"2026-07-23T05:41:48.692899",{"slug":859,"name":859,"fn":860,"description":861,"org":862,"tags":863,"stars":770,"repoUrl":771,"updatedAt":868},"mps-aspect-migrations","author and debug MPS migration scripts","Use when authoring or debugging MPS migration scripts that upgrade user models after a language definition changes — covers jetbrains.mps.lang.migration (MigrationScript class-based, PureMigrationScript declarative, MoveConcept\u002FMoveContainmentLink\u002FMoveReferenceLink\u002FMoveProperty, ordering via OrderDependency, data exchange via putData\u002FgetData, RefactoringLog, ConceptMigrationReference) and jetbrains.mps.lang.script Enhancement Scripts (MigrationScript with MigrationScriptPart_Instance, ExtractInterfaceMigration, FactoryMigrationScriptPart, CommentMigrationScriptPart) — when a model needs version-gated upgrade, concept rename or removal, link or property rename, instance-level transformation, or composition of migration steps.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[864,865],{"name":23,"slug":24,"type":15},{"name":866,"slug":867,"type":15},"Migration","migration","2026-07-13T06:45:20.372122",{"slug":870,"name":870,"fn":871,"description":872,"org":873,"tags":874,"stars":770,"repoUrl":771,"updatedAt":876},"mps-aspect-structure-concepts","define concepts in MPS structure aspect","Define concepts, interface concepts, enumerations, and constrained data types in an MPS language's `structure` aspect. Covers smart-reference detection, alias rules, cardinality, INamedConcept usage, bulk creation, and the full `mps_mcp_alter_structure` \u002F `mps_mcp_query_structure` reference. Use when authoring or modifying a language's structure model.",{"slug":8,"name":9,"logoUrl":10,"githubOrg":9},[875],{"name":20,"slug":21,"type":15},"2026-07-23T05:41:30.705975",188]