feat(appkit): add DatabricksAdapter.fromAiGateway for gateway chat completions

[MarioCadenas]

What

Adds DatabricksAdapter.fromAiGateway({ model }) so agents can consume models through the Databricks AI Gateway Chat Completions endpoint (/ai-gateway/mlflow/v1/chat/completions), naming the model in the request body (e.g. system.ai.claude-opus-5-5) instead of a per-endpoint URL.

This is the migration path off the legacy databricks-* serving-endpoint names (/serving-endpoints/{name}/invocations) toward the gateway's UC system.ai.* models.

import { agents, DatabricksAdapter } from "@databricks/appkit/beta";

agents({
  agents: {
    assistant: createAgent({
      instructions: "You are a helpful assistant.",
      model: await DatabricksAdapter.fromAiGateway({
        model: "system.ai.claude-opus-5-5",
      }),
    }),
  },
});

How

The gateway speaks the same OpenAI Chat Completions wire format the adapter already used, so the change is deliberately tiny:

• connectors/serving/client.ts — new streamAiGateway(): hard-codes the gateway path and forces stream: true (mirrors the existing stream() serving helper; keeps the @internal "hard-coded path only" contract of streamPath).
• agents/databricks.ts — optional model on the adapter options, injected into the request body only when set; new fromAiGateway factory that auto-resolves a WorkspaceClient (like fromModelServing).

Everything else is reused unchanged: the multi-step tool loop, SSE parsing, dot→__ tool-name mapping, thought-signature pass-through, DoS caps, and generation params.

Non-breaking

Purely additive (+186 / -0). model is set only on the gateway path. fromModelServing, fromServingEndpoint, and bare-string model: never set a body model, so their requests are byte-identical to before — a test pins this.

Verification

• Live against the dogfood gateway: confirmed the path /ai-gateway/mlflow/v1/chat/completions 200s, model-in-body works (system.ai.claude-opus-5-5 → global.anthropic.claude-opus-5-5), and streaming SSE + tool-calling return in the exact choices[0].delta / tool_calls[] shape the adapter parses.
• Tests: 39/39 pass in databricks.test.ts (2 new — gateway path + model in body; serving path leaves model out).
• Typecheck: no new errors introduced (pre-existing baseline red in unrelated genie/sql-warehouse/mock-workspace-client files only).

Follow-ups (not in scope)

• Markdown frontmatter model: still routes to serving endpoints; wiring gateway selection into frontmatter can be a separate change if wanted.

This pull request and its description were written by Isaac.

[github-actions]

📦 Bundle size report

Compared against bundle-size-baseline.json (main).

@databricks/appkit

npm tarball (packed): 1.2 MB (+2.5 KB) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	1.2 MB (+2.5 KB)	426 KB (+648 B)
Type declarations	446 KB (+2.4 KB)	162 KB (+674 B)
Source maps	2.3 MB (+5.3 KB)	798 KB (+1.2 KB)
Other	11 KB	3.7 KB
Total	4.0 MB (+10 KB)	1.4 MB (+2.5 KB)

Per-entry composition (own code — deps external (as shipped))

Entry	Initial (gz)	Lazy (gz)	Total (gz)	node_modules (min)	Own code (min)
.	96 KB	2.5 KB	98 KB	external	314 KB
./beta	93 KB (+130 B)	457 B	94 KB (+130 B)	external	281 KB (+529 B)
./testing	38 KB	30 KB	69 KB	external	201 KB
./tsdown	520 B	0 B	520 B	external	813 B
./type-generator	23 KB	0 B	23 KB	external	65 KB

Chunks:

Entry	Chunk	Load	Size (gz)
.	index.js	initial	92 KB
.	utils.js	initial	4.0 KB
.	remote-tunnel-manager.js	lazy	2.5 KB
./beta	beta.js	initial	77 KB
./beta	stream-manager.js	initial	5.8 KB
./beta	databricks.js	initial	3.2 KB
./beta	wide-event-emitter.js	initial	3.2 KB
./beta	configuration.js	initial	2.3 KB
./beta	service-context.js	initial	1.3 KB
./beta	client.js	initial	434 B
./beta	client-options.js	initial	219 B
./beta	supervisor-api.js	lazy	192 B
./beta	databricks.js	lazy	142 B
./beta	index.js	lazy	123 B
./testing	manifest.js	initial	26 KB
./testing	index.js	initial	10.0 KB
./testing	wide-event-emitter.js	initial	2.9 KB
./testing	index.js	lazy	26 KB
./testing	remote-tunnel-manager.js	lazy	2.5 KB
./testing	utils.js	lazy	1.2 KB
./tsdown	index.js	initial	520 B
./type-generator	index.js	initial	23 KB

@databricks/appkit-ui

npm tarball (packed): 350 KB (-4 B) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	395 KB	132 KB
Type declarations	229 KB	84 KB (-1 B)
Source maps	766 KB	253 KB
CSS	16 KB	3.2 KB
Total	1.4 MB	472 KB (-1 B)

Per-entry composition (consumer bundle — deps bundled, peerDeps external)

Entry	Initial (gz)	Lazy (gz)	Total (gz)	node_modules (min)	Own code (min)
./js	5.3 KB	49 KB	55 KB	208 KB	14 KB
./js/beta	20 B	0 B	20 B	0 B	0 B
./react	432 KB	49 KB	481 KB	1.3 MB	177 KB
./react/beta	1.0 KB	0 B	1.0 KB	0 B	1.9 KB

Chunks:

Entry	Chunk	Load	Size (gz)
./js	index.js	initial	5.2 KB
./js	chunk	initial	120 B
./js	apache-arrow	lazy	49 KB
./js/beta	beta.js	initial	20 B
./react	index.js	initial	430 KB
./react	tslib	initial	2.1 KB
./react	apache-arrow	lazy	49 KB
./react/beta	beta.js	initial	1.0 KB

[github-actions]

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Scaffolds a new app from this PR's SDK build. Run it in any folder (requires the GitHub CLI — gh auth login — and the Databricks CLI):

gh run download 36149675675 -R databricks/appkit -n appkit-template-0.78.0-pr.1ed8e38-feat-agents-ai-gateway-605 -D appkit-pr-605 \
  && unzip -o "appkit-pr-605/appkit-template-0.78.0-pr.1ed8e38-feat-agents-ai-gateway-605.zip" -d "appkit-pr-605" \
  && databricks apps init --template "appkit-pr-605"

The template pins @databricks/appkit and @databricks/appkit-ui to tarballs built from this branch, so the scaffolded app runs against this PR's code.