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Building the Claude API for the Frontier

Every Claude launch puts the spotlight on the model, but just as much work goes into making sure the API evolves with it.

Lucca SiaudzionisguestJake Dejnohost
Nov 12, 202618mWatch on YouTube

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At a glance

WHAT IT’S REALLY ABOUT

Inside Claude’s Messages API: knobs, caching, versioning, and safety tradeoffs

  1. Lucca explains the Messages API as the cornerstone of the Claude Platform, combining a familiar chat-and-tools structure with built-in controls that enable deeper behavior tuning than prompting alone.
  2. Feature decisions are described as a two-way process between research learnings and customer-driven needs, with internal experimentation turning into shipped API tools.
  3. Both speakers emphasize evaluation as the main way to ensure new knobs, models, and safeguards improve outcomes without breaking user workflows, and they recommend customers run their own use-case-specific evals.
  4. Model versioning is framed as faster-moving than traditional APIs, using model identifiers plus rollout and rollback mechanisms (including beta headers) while managing additive changes and more difficult deprecations.
  5. Cost and safety are treated as parallel priorities: prompt caching is presented as a primary cost lever, while Mythos/Fable access differences, classifiers, and distillation-related protections aim to reduce misuse without sacrificing developer experience.

IDEAS WORTH REMEMBERING

5 ideas

The Messages API is positioned as Claude’s foundational, highly customizable building block.

Lucca describes the Messages API as the core interface for building on Claude, organized around user turns, assistant turns, and tool calls, with built-in controls that go beyond what prompting alone can achieve.

API capabilities are shaped by continuous feedback between research and customer demand.

Lucca says features and “knobs” emerge from a two-way loop: research discoveries (like noticing new strengths) inform productization, while customer needs feed back into research goals for future models.

Mid-conversation controls matter most when they don’t break prompt caching.

Lucca highlights changing effort and changing system prompts mid-conversation as powerful because they preserve prompt caching, avoiding a cost/performance tradeoff developers would otherwise face when altering behavior during an agentic flow.

Evals are treated as the core mechanism for validating changes and upgrades.

Both speakers emphasize frequent evaluation: the team runs many internal evals before launch, and Lucca recommends customers run their own evals because the API is generic while each use case is specific.

Model evolution requires migration paths, rollback mechanisms, and eventual deprecation deadlines.

The discussion frames model strings, beta headers, and toggles as practical ways to roll forward while still enabling rollback time, with Lucca noting deprecations are hardest when users rely on removed capabilities.

WORDS WORTH SAVING

5 quotes

The easiest way to see it is as the cornerstone of our API. So it, it allows you to customize it and build almost whatever you can imagine with it.

— Lucca Siaudzionis

You take the same model in high effort and low effort, feels like two separate models.

— Lucca Siaudzionis

So being able to change effort and system prompts mid-conversation are two things for me that really show the power of being able to have a knob later on.

— Lucca Siaudzionis

Whenever we get a new model, we start seeing how smart it is. We get the, the first feeling is the, this excitement like, wow, this model can do things that it couldn't do before.

— Lucca Siaudzionis

So we run an absurd amount of evals.

— Lucca Siaudzionis

Read the available transcript
Messages API structure (turns and tool calls)Behavior controls and “knobs” (effort, system prompts, thinking)Research-to-product and customer feedback loopEvals and customer-run validationModel strings, rollouts, and deprecationsPrompt caching and cost-per-task vs cost-per-tokenSafety, classifiers, Mythos vs Fable, and distillation protections

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SPEAKERS

  • Lucca Siaudzionis

    guest

    Anthropic/Claude Platform team member focused on building and shipping Claude API capabilities (e.g., Messages API, tooling, rollouts, and safety mechanisms).

  • Jake Dejno

    host

    Anthropic/Claude Platform interviewer/host who leads product and engineering discussions about the Claude API and developer experience.

High quality AI-generated summary created from speaker-labeled transcript.

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