Claude 4 API: Tool Use Streaming & Prompt Caching vs Together AI MCP Server Registry

“The primitive here is clean: incremental tool-call deltas over SSE, and a cache-control header you attach to prompt segments to pin them server-side. The DX bet is that complexity lives in the HTTP layer, not in a new SDK abstraction — you opt in per-request, no new mental model required. The moment of truth is calling `stream=true` on a tool-use request and watching partial JSON arguments arrive before the model finishes thinking, which actually matters for agent loops where you want to dispatch work early. This is not a weekend-script replacement — implementing correct incremental JSON parsing for partial tool arguments plus a reliable distributed cache with 2M token capacity is a real engineering problem Anthropic has solved for you. The specific decision that earns the ship: cache invalidation is explicit and cache hits are reflected in the usage object, so you can actually measure what you're saving instead of guessing.”

“The primitive here is clean: a versioned, typed registry of MCP server definitions that a CLI can resolve and deploy without the usual copy-paste-from-docs ritual. The DX bet is that discoverability is the actual bottleneck — not building an MCP server from scratch, but finding one that already works against your Postgres or Salesforce instance. That bet is correct; I've wasted more hours than I'd like to admit hunting for a working MCP config. The moment of truth is `mcp install` resolving to a running server with zero env-var archaeology — if that actually works on the 300th integration the same as the first, this is infrastructure. The skip risk is that 'production-ready' in a community registry means 'worked once on someone's laptop,' so trust but verify before pointing this at anything sensitive.”

“Direct competitors are OpenAI's cached completions and Google's context caching in Gemini 1.5 — both shipping for months — so Anthropic is catching up, not leading. The specific scenario where this breaks: cache hit rates depend entirely on prompt structure, and developers who dynamically compose system prompts (inserting user-specific context at the top) will see near-zero cache utilization and pay full price while assuming they're saving money. The prediction: this feature doesn't get killed — it becomes table stakes infrastructure and Anthropic wins by having the largest cache window (2M vs. competitors' current limits). What would have to be true for me to be wrong: OpenAI ships a 10M token cache window before Anthropic's ecosystem matures, commoditizing the advantage. Still a ship because the streaming tool-use delta is genuinely differentiated — no competitor has clean partial-argument streaming for tool calls yet, and that changes agent loop architecture in ways that matter.”

“Direct competitors are Smithery, mcp.run, and the increasingly crowded roster of MCP marketplaces — Together AI is not first here. The specific scenario where this breaks is enterprise brownfield: the moment a team needs an MCP server for an internal API that isn't in the catalog, they're back to writing one from scratch, and now they also have to figure out how to publish it back. The '300+ integrations' number needs scrutiny — quantity in a registry means nothing if 250 of them are unmaintained forks of the same Postgres connector. What keeps this alive is Together AI's model inference business: the registry is a distribution play to keep developers in their ecosystem, not a standalone product, which paradoxically makes the registry more likely to survive than a pure-play alternative. What kills it in 12 months is Anthropic or OpenAI shipping a first-party registry with the same integrations and better model-side tooling.”

“The thesis this bets on: by 2027, the dominant AI application architecture is a persistent agent with a large, stable context (tools, memory, instructions) that gets reused across thousands of user interactions — making context I/O cost the primary unit economics lever, not generation cost. The dependency that has to hold: agents don't collapse back to stateless chatbots, and context windows keep growing faster than per-token prices fall. The second-order effect nobody's talking about: prompt caching at 2M tokens makes it economically viable to give every enterprise user a fully-loaded, role-specific agent context at request time — which shifts competitive differentiation from 'who has the best model' to 'who has the best cached context corpus,' effectively making knowledge curation the new moat. This tool is riding the trend of context-window expansion-as-infrastructure, and it's on-time, not early — but the streaming tool-use primitive is ahead of the curve on agent loop efficiency. The future state where this is infrastructure: every production agentic system has a cache manifest the same way it has a CDN config.”

“The thesis here is falsifiable: within 2-3 years, agentic applications will require composable, pre-vetted tool integrations the same way web apps required npm packages, and whoever owns the canonical registry owns a layer of the stack. The dependency is that MCP actually becomes the dominant protocol for tool-calling — if OpenAI's or Google's tool-use format wins instead, this registry is stranded. The second-order effect that matters isn't developer productivity; it's that a registry with adoption creates data on which integrations are actually used at scale, which is a defensible moat Together AI can exploit to tune models against real-world tool-use patterns. Together AI is riding the MCP standardization wave and is approximately on-time — not early enough to define the protocol, but early enough to own the registry layer before the obvious players consolidate it. The future state where this is infrastructure: every new agentic framework defaults to this registry the way new Node projects default to npm.”

“The buyer is the engineering team at any company running Claude in production with long system prompts or multi-step agents — this comes out of the AI infrastructure budget, not a new budget line, which means no procurement friction. The pricing architecture is sound: cache reads at ~90% discount means the savings are real and measurable in the first billing cycle, which creates immediate retention — developers who restructure prompts to maximize cache hits are now architecturally coupled to Anthropic's caching implementation. The moat question is the honest one: this is infrastructure that OpenAI and Google will match, so the defensible position isn't the feature itself but the ecosystem of developers who've restructured their codebases around it. What survives a 10x model price drop: the streaming tool-use architecture, because that's about latency, not cost. The specific business decision that makes this viable is pricing cache reads as a separate SKU — it lets Anthropic capture value from high-volume production workloads without losing price-sensitive experimenters.”

“The buyer here isn't paying for the registry — it's free — which means the actual business logic is that the registry accelerates adoption of Together AI's inference API, and the registry's success is measured in GPU-hours sold, not in registry installs. That's a coherent distribution strategy, but it means the registry itself has no independent unit economics and will be deprioritized the moment it stops converting to inference revenue. The moat is weak: the registry format is open, the servers are community-contributed, and any better-capitalized competitor can clone the catalog in 90 days. What would make this a ship as a standalone business is if Together AI starts charging for hosted MCP server execution or adds proprietary connectors that require their inference stack — right now it's a marketing asset dressed up as infrastructure, and marketing assets don't compound.”