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Custom Business Logic Extensibility in CAP (CDS-Oyster)
A Devtoberfest 2026 session walking @sap/cds-oyster@sap/cds-oysterRuns tenant-specific extension code inside a WebAssembly sandbox, so a SaaS application can accept custom logic from a customer without running it unguarded.sap official, cap — at version 0.55, with 1.0 stated for Q4 — end to end on a multitenant xtravelsxtravelsA travel-booking application built on the master data that xflights publishes, composing another package's schema into its own rather than owning master data itself.sap official, cap, from the application developer's side and the extension developer's side. The application developer adds a submit-for-review action to the status flow and opens exactly one surface for custom code: a separate extension service marked as extensible code, holding a no-op validate action and two read-only helper entities, which the host application calls before the status change; nothing else is extensible, because the annotation defaults to false. The extension developer works in a project whose base model came from cds pull, with test data, a server.js and types from @cap-js/cds-typer@cap-js/cds-typerGenerates TypeScript types from a CDS model.sap official, cap added so the UI and code completion work, debugs the handler locally and pushes it as an @sap/cds-mtxs@sap/cds-mtxsMultitenancy, tenant provisioning and model extensibility for CAP.sap official, cap extension, after which the next request rejects a travel over the policy limit without a restart — the code having been stored as an annotation in the tenant's model and run in a Rust WebAssembly sandbox with no database connection, whose queries go back to the CAP runtime as CQL strings, which is tenant extensibilityTenant extensibilityLetting a customer extend the model of a running SaaS application with extra fields and entities per tenant, without forking the application, as distinct from extending the application's code for every customer at once.SAP and community, unassessed extended from fields to logic.
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Explains
@sap/cds-oysterCAP@sap/cds-oysterRuns tenant-specific extension code inside a WebAssembly sandbox, so a SaaS application can accept custom logic from a customer without running it unguarded.@sap/cds-mtxsCAP@sap/cds-mtxsMultitenancy, tenant provisioning and model extensibility for CAP.Tenant extensibilityCAPTenant extensibilityLetting a customer extend the model of a running SaaS application with extra fields and entities per tenant, without forking the application, as distinct from extending the application's code for every customer at once.
Since then
No major release of @sap/cds-oyster since this was published
Who is behind it
Nick JosipovicNick JosipovicWrote this
In the atlas: contributes to 4, wrote 1@nkaputnikDJ AdamsDJ AdamsWrote this
Developer Advocate
In the atlas: contributes to 20, wrote 22@qmacroauthors
In the atlas: contributes to 4, wrote 1@nkaputnikDJ AdamsDJ AdamsWrote this
Developer Advocate
In the atlas: contributes to 20, wrote 22@qmacroauthors
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Multitenant SaaS Application using CAPMultitenant SaaS Application using CAPSAP's reference architecture for a multitenant SaaS application on BTP built with CAP: one deployment serving many tenants with logically separated data, the subscription and onboarding flow, and the BTP services the design leans on. Written for partners and line-of-business teams building on BTP, and it is SAP's own side of the shape multitenancy takes on BTP.published 2025-01-31
Developing CAP java/node.js mixed mono Multitenant SaaS Applications with Shared DatabaseDeveloping CAP java/node.js mixed mono Multitenant SaaS Applications with Shared DatabaseTwo CAP projects, one Java and one Node.js, in one npm workspace sharing a single tenant database. The interesting part is which sidecar wins: both sub-projects get an MTX sidecar, both have cds.xt.SaasProvisioningService switched off, and a third onboarding project keeps provisioning to itself. Configuration listings rather than prose, and it assumes you already know why you would want this.xuelong, published 2025-01-08
Developing CAP java/node.js mixed mono Multitenant SaaS Applications with Shared Database
@sap/cds-mtxs has been through 2 major lines since this was published: 3 (2025-06), 4 (2026-06). Affects A recipe, Runnable code.
Everything new: Try out the latest features in CAPEverything new: Try out the latest features in CAPCAP (@sap/cds) gains the features this hands-on session works through: declarative constraints, which is the input validation concern, status flows as status transition flows, a cds REPL, extensibility through @sap/cds-oyster, and HCQL, which SAP's abstract calls CAP's native query-over-HTTP protocol, after an opening stretch of AI-driven app development. The abstract names the features and stops there; what each one does is not described. A TechEd 2026 hands-on session (Wednesday 28 October, Berlin), presented by Daniel Hutzel, Daniel Schlachter, Marc Huber and Tim Back, written here from SAP's abstract; the recording is not yet published.
CAP in the age of AI #7 - vector embeddings wrap-upCAP in the age of AI #7 - vector embeddings wrap-upThe same embeddings model taken from SQLite to SAP HANA Cloud, then handed to an agent. Locally it is @cap-js/ai that adds the vector functions SQLite lacks and downloads an all-MiniLM L6v2 ONNX model on first start; on HANA natural language processing has to be switched on, the model name is a mandatory argument, the vectors it returns are 768 dimensions, and dimensions must match on both sides of a comparison. HANA itself can be given SAP AI Core as a remote source for other embedding models, which a CDS.ql expression has to inject because the postfix notation only takes what the CDS model holds, and the hour ends with a search function over cosine similarity exposed over @cap-js/mcp, so an agent asked about low output from solar panels calls semantic search instead of writing the query itself.published 2026-10-16
CAP in the age of AI #6 - vector embeddingsCAP in the age of AI #6 - vector embeddingsVector embeddings put to work in the incident management sample, answering whether this problem has been seen before. An embedding element of type Vector is calculated and stored from the vector_embedding function, so the database refreshes it whenever the text behind it changes, and cosine_similarity compares it with a freshly embedded string; because a title says little, a scheduled action first summarises each incident's conversation through a model on SAP AI Core and the embedding is taken from that summary. The similar-incidents list is a view joining incidents to themselves, with order and limit set in a before handler on the expanded query, and the vector element excluded from the service, since OData knows nothing about vectors — on SAP HANA Cloud the function is native, while SQLite needs packages added to the project.published 2026-10-09
CAP in the age of AI #5 - agents wrap-upCAP in the age of AI #5 - agents wrap-upThe agents topic closed out on what sits around the agent rather than inside it, now that the documentation has moved into capire. The chat preview's session is persisted in the database rather than kept in memory, traces go out through @cap-js/telemetry to a self-hosted MLflow under the OpenTelemetry conventions for generative AI, and the panel is explicit that @agent reuses the MCP adapter's code programmatically without serving an MCP endpoint. It ends in the xtravels sample, where a travel agent delegates to a hotel-service sub-agent and reaches a separately started flight service over MCP through the service binding CAP resolves anyway. Earlier in the hour the same agent answers from a model running on the presenter's laptop, reached as an OpenAI-compatible endpoint.published 2026-10-02
What's in Daniel Hutzel's brain?What's in Daniel Hutzel's brain?A Devtoberfest 2026 session in the building-apps track, in which Daniel Hutzel and DJ Adams work through what the CAP team is building for agents. A bookshop gets an MCP endpoint beside its OData one from a single annotation, with the plugin writing the coding agent's configuration itself; in the xt-travels sample five services that know nothing about each other are served over MCP and an agent does the orchestration the hand-built UI and its value helps used to do. Hutzel then annotates a service as an agent so the ReAct loop and the choice of model move into the CAP service, and finally drops MCP for a skill that teaches the agent CQL instead, which is where the edges to Serve your application over MCP, Agent tooling and Agent guidance come from. The mechanism under all of it, a plugin file in the package root and event handlers that can be added to any CAP service, is the edge to Plugin authoring.published 2026-10-02
CAP in the age of AI #4 - more on agentsCAP in the age of AI #4 - more on agentsAsking a human first, and telling the agent how to behave. Ordering a book through @cap-js/agents happens without a question being asked; the human-in-the-loop annotation on the action turns the same request into an approve-or-reject step, which the episode follows down to the A2A status update that stores the task in an input-required state in the database — A2A itself being JSON-RPC 2.0 over HTTP with server-sent events for the stream. The rest is instructions on disk: an agents.md in the service's own folder removes the question about how many copies to order, a skill under a skills folder is advertised by its description and read only when the agent decides the case applies, and either file makes the plugin build a deep agent instead of the plain one.published 2026-09-11
CAP in the age of AI #3 - from MCP to A2ACAP in the age of AI #3 - from MCP to A2AThe episode that finishes @cap-js/mcp with authorisation and then crosses to agents. @requires and @restrict are the same annotations CAP already has, and the plugin reads them to filter the tool list and the describe output per user and per request, so a model is not told about entities the caller cannot reach — where the OData metadata of the same service lists every exposed entity regardless. The second half annotates a service with @agent from @cap-js/agents, which serves it over A2A with an agent card and a chat preview, runs first against a mocked response and then against SAP AI Core bound with cds bind and a hybrid profile, and reuses the MCP adapter's tools rather than defining new ones.published 2026-08-21
CAP in the age of AI #2 - more on the MCP adapterCAP in the age of AI #2 - more on the MCP adapterWhat the query tool costs in tokens, and the third tool @cap-js/mcp registers. Counted against a model running locally under llama.cpp, the same query is 26 tokens as CQL and 173 as CQN, and the tool definition sent with every system prompt drops from around 5,500 tokens to around 600 — the reason CQL became the default format. The call tool then exposes a service's unbound actions and functions, as one generic tool or, behind a flag, one tool per action with its own description and input schema; a question from the chat about extension points gets the answer that the adapter does the tools, which the panel calls a small subset of the protocol.published 2026-08-14
Add MCP capabilities to a CAP serviceAdd MCP capabilities to a CAP serviceA fifteen-minute walkthrough of turning a plain CAP service into an MCP server: annotate the service with @mcp, install @cap-js/mcp, and it is served alongside OData at /mcp/<path>. The part worth reading it for is what happens next — cds watch writes the endpoint into the local agent's own configuration file, so OpenCode or Claude Code can call the service without anyone editing JSON by hand. It is built on the tiny-sample facet, so there is nothing to prepare, and it shows the protocol traffic rather than only the result. Written while the adapter was in beta, and it says so.published 2026-08-13
CAP in the age of AI #1 - the MCP adapterCAP in the age of AI #1 - the MCP adapterThe episode that opens the series and the first pass over @cap-js/mcp, with four of the people building it on the call. Two of the adapter's tools are worked through: describe, the one an MCP client reaches for first to get the overview of what it may do, and query, which the panel establishes as read-only. It also writes the server entry into a local agent's own configuration file, which episode three comes back to as auto-wiring.published 2026-08-07
CAP in the age of AICAP in the age of AISeven live episodes of Hands-on SAP Dev in which DJ Adams works through CAP's AI features with the people who build them, each one a screen-share against a sample project. The first three are @cap-js/mcp: the @mcp annotation on a service, the describe, query and call tools, CQL as the query format, and what authorisation takes out of the tool list. From episode three the run moves to A2A and @cap-js/agents — human approval of an action, agents.md and skills, session persistence, telemetry and sub-agents — and the last two episodes are vector embeddings in CDS, local on SQLite and native on SAP HANA Cloud.published 2026-08-07
Avoid design time CAP server restarts when maintaining local data filesAvoid design time CAP server restarts when maintaining local data filescds watch restarting on every file change is the feature; this is the case where it is not. Editing a CSV seed file mid-edit restarts the server against half-finished data, and the post is about suppressing that without giving up the watch loop. A small friction most people work around silently rather than fix.published 2024-04-26
Avoid design time CAP server restarts when maintaining local data files
Local development changed in @sap/cds-dk 7.9.1, 2024-05-02. Affects Runnable code.
CAP, CDS, CDL, CSN and jq - finding associationsCAP, CDS, CDL, CSN and jq - finding associationsHow associations are actually represented once CDL is compiled into CSN, worked out by parsing the machine-readable form with jq. Useful less as a recipe than as a way in: anything that wants to reason about a CDS model without the compiler has to read CSN, and this is what that looks like from the outside.published 2024-04-16
CAP, CORS and custom headersCAP, CORS and custom headersWhat CAP itself does about CORS, and where it stops. The starting point is a colleague's request and a deliberate constraint: @sap/approuter has full CORS support, but the author wanted to know what the CAP server offers on its own and how to extend it. Ends up covering preflight requests and custom servers, which is the mechanism a frontend hosted outside BTP actually runs into.published 2024-04-11
ISO content for common CAP typesISO content for common CAP typesThe npm package that supplies ISO-standard content — countries, languages, currencies, timezones — for CAP's common types, and how it works underneath. The author flags the length and the reason for it up front, having followed the mechanism further than the question required, which is what makes it worth reading rather than a package announcement.published 2024-03-14
ISO content for common CAP types
Seed data changed in @sap/cds-dk 7.9.1, 2024-05-02. Affects Runnable code.
CAP is important because it's not importantCAP is important because it's not importantAn essay rather than a technique: what CAP actually is, what it gives you, and the case for treating it as foundational precisely because it is not the interesting part of an application. Written in 2019, when that argument still had to be made, and the title is the argument. Also recorded as a Tech Aloud podcast episode and a CodeTalk, both linked from the post.published 2019-11-06
Computed field example in CAPComputed field example in CAPAn answer to a Community question — how to count the entities on the far side of an association — written up as a short post rather than left in a thread. The answer combines CDS with Node.js service events, which is the seam worth noticing: some of what looks like modelling is expressed in the model and some has to be computed when the request arrives, and the boundary between them is not obvious from either side.published 2019-08-21
Computed field example in CAP
Data modelling has changed 3 times since this was published, most recently in @sap/cds 9.9.0, 2026-04-23. Affects A recipe, Runnable code.
Set up VS Code on Chrome OS for local Application Programming Model developmentSet up VS Code on Chrome OS for local Application Programming Model developmentA CAP development environment on a Chromebook, written from preference rather than constraint — the author sets out why Chrome OS is his primary machine before showing the setup. Carries its own updates section noting what has changed since, which for a 2018 post about an operating system is most of it. The interest now is that local CAP development was portable this early.published 2018-10-16
Set up VS Code on Chrome OS for local Application Programming Model development
Local development has changed twice since this was published, most recently in @sap/cds-dk 7.9.1, 2024-05-02. Affects Runnable code.
SAP Cloud Application Programming Model (CAP) - start hereSAP Cloud Application Programming Model (CAP) - start hereThe orientation post for CAP, published in 2018 and revised in 2020 and again in 2021 with help credited to Iwona Hahn — which is the unusual part: a starting-point index that was maintained rather than left to rot, and says so at the top. Read as the shape of what a newcomer was pointed at, and as a marker of how much of that changed inside three years.published 2018-10-10
CAP Node.js Expert Session #6 - The MCP AdapterCAP Node.js Expert Session #6 - The MCP AdapterDaniel Schlachter and DJ Adams add @cap-js/mcp to a bookshop service, inspect its describe, query and action tools, and query the catalogue through an AI agent. The session distinguishes this runtime adapter from the development-time MCP server and demonstrates automatic registration of the running service with a local agent. The recording describes the adapter as beta in July 2026, with read-only queries and writes through unbound actions.
Examine the details of the Model Context Protocol on startupExamine the details of the Model Context Protocol on startupWhat the MCP adapter actually does when a CAP server boots, watched rather than described — the companion to the tutorial that switches it on.
Define, generate, deploy: AI-driven spec-first TerraformDefine, generate, deploy: AI-driven spec-first TerraformAccount provisioning written as a spec first: the abstract has participants write a structured description of a BTP account, its subaccounts, entitlements, governance, best practices and more, and an AI turns it into Terraform scripts that build it. The abstract does not name the Terraform provider the scripts use, the spec format or the AI behind the generation. A hands-on TechEd 2026 session (Wednesday 28 October, Berlin), presented by Christian Lechner, DJ Adams, Piotr Tesny and Vipin VijayKumar, written here from SAP's abstract; the recording is not yet published.
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@sap/cds-oysterartifact
@sap/cds-mtxsartifact
Tenant extensibilitycapability, core and community
YouTube — SAP Developers playlistspublishing source
YouTube — SAP Developers playlistsdevtoberfest-2026
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- Entered the atlasDiscovered via YouTube — SAP Developers playlists · devtoberfest-2026.
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