Genkit Dart 1.0: GenkitRouter Replaces startFlowServer, and Old Flutter Clients Need a Flag

Google tagged Genkit Dart 1.0.0 on October 7, 2026, and the Flutter blog announcement followed on October 8. The core genkit package, schemantic, and the Google GenAI, Vertex AI, Anthropic, OpenAI, MCP, middleware, and shelf plugins are now 1.0.0 and covered by semver. genkit_otel, genkit_chrome, genkit_firebase_ai, and genkit_a2ui stay on 0.x.

The release notes are upfront that freezing the API meant “a lot of breaking changes”. Most are renames. The one that can break an app already in your users’ hands is in HTTP serving.

startFlowServer is gone, GenkitRouter lives in core

Up to 0.17, serving flows meant genkit_shelf and startFlowServer. In 1.0, HTTP serving moved into package:genkit/io.dart, runs on plain dart:io, and genkit_shelf became a thin adapter. startFlowServer and FlowWithContextProvider were removed (#533):

// before (0.17)
await startFlowServer(
  flows: [helloFlow, FlowWithContextProvider(flow: secureFlow, context: bearerAuth)],
  cors: {'origin': '*'},
);

// after (1.0.0)
final genkit = GenkitRouter()
  ..addAction(helloFlow)
  ..addAction(secureFlow, contextProvider: bearerAuth);
await genkit.serve(cors: const CorsOptions());

Context providers now receive a framework-neutral RequestData instead of a shelf Request, with lowercased header names, so request.headers['Authorization'] lookups need to become 'authorization'. You can also mount the router inside your own server with handleHttpRequest(request, basePath: '/api').

The streamed error frame changed

Streamed responses now use Content-Type: text/event-stream, and a failed stream ends with a data: frame carrying the error, matching the Go and Python servers. I ran a two-flow server on Dart 3.12.2 with genkit 1.0.0, where the streaming flow sends one chunk and then throws:

final boom = ai.defineFlow(
  name: 'boom',
  inputSchema: .string(),
  outputSchema: .string(),
  streamSchema: .string(),
  fn: (String name, ctx) async {
    ctx.sendChunk('partial');
    throw GenkitException('nope', status: StatusCode.internal);
  },
);

POST /boom?stream=true with the 1.0 defaults:

content-type: text/event-stream

data: {"message":"partial"}

data: {"error":{"code":500,"status":"INTERNAL","message":"nope"}}

Per the release notes, clients from genkit 0.17 and earlier only understand the old error: frame. If a Flutter app built against 0.17 is calling your backend, upgrading the server alone means that client stops seeing errors the way it used to. That is the case for any version still installed on phones that have not updated.

The fix is one constructor argument:

final genkit = GenkitRouter(sendLegacyErrorFrame: true);

With it set, the same request ends with the legacy frame:

data: {"message":"partial"}

error: {"error":{"code":500,"status":"INTERNAL","message":"nope"}}

Non-streaming calls are unaffected: POST /hello returned {"result":"Hello, Marius"} in both modes.

Other renames worth grepping for

• ExecutablePrompt is now Prompt<Input, Output>, and prompts return typed output like generate does.
• Agents, sessions, snapshots, and bidi streaming moved behind package:genkit/experimental.dart, which triggers the experimental_member_use analyzer warning on import.
• agents() is no longer re-exported from genkit_middleware; import package:genkit_middleware/agents.dart.
• Plugin authors: defineMiddleware is now generateMiddleware, and app code gets ai.defineGenerateMiddleware.

If you serve Genkit flows to a mobile client, the safe upgrade order is: ship the server on 1.0.0 with sendLegacyErrorFrame: true, release the Flutter app on the 1.0 client, and drop the flag once your minimum supported app version is past 0.17.