diff --git a/js/ai/src/formats/json.ts b/js/ai/src/formats/json.ts
index cc4beecd0a..5d7465546a 100644
--- a/js/ai/src/formats/json.ts
+++ b/js/ai/src/formats/json.ts
@@ -23,8 +23,19 @@ export const jsonFormatter: Formatter<unknown, unknown> = {
     format: 'json',
     contentType: 'application/json',
     constrained: true,
+    defaultInstructions: false,
   },
-  handler: () => {
+  handler: (schema) => {
+    let instructions: string | undefined;
+
+    if (schema) {
+      instructions = `Output should be in JSON format and conform to the following schema:
+
+\`\`\`
+${JSON.stringify(schema)}
+\`\`\`
+`;
+    }
     return {
       parseChunk: (chunk) => {
         return extractJson(chunk.accumulatedText);
@@ -33,6 +44,8 @@ export const jsonFormatter: Formatter<unknown, unknown> = {
       parseMessage: (message) => {
         return extractJson(message.text);
       },
+
+      instructions,
     };
   },
 };
diff --git a/js/ai/src/formats/types.ts b/js/ai/src/formats/types.ts
index 7f0c9fbd5a..3cd17e3d3a 100644
--- a/js/ai/src/formats/types.ts
+++ b/js/ai/src/formats/types.ts
@@ -23,7 +23,9 @@ export type OutputContentTypes = 'application/json' | 'text/plain';
 
 export interface Formatter<O = unknown, CO = unknown> {
   name: string;
-  config: ModelRequest['output'];
+  config: ModelRequest['output'] & {
+    defaultInstructions?: false;
+  };
   handler: (schema?: JSONSchema) => {
     parseMessage(message: Message): O;
     parseChunk?: (chunk: GenerateResponseChunk) => CO;
diff --git a/js/ai/src/generate.ts b/js/ai/src/generate.ts
index d0b3ebef14..8d9a1766f6 100755
--- a/js/ai/src/generate.ts
+++ b/js/ai/src/generate.ts
@@ -33,7 +33,10 @@ import {
   resolveFormat,
   resolveInstructions,
 } from './formats/index.js';
-import { generateHelper } from './generate/action.js';
+import {
+  generateHelper,
+  shouldInjectFormatInstructions,
+} from './generate/action.js';
 import { GenerateResponseChunk } from './generate/chunk.js';
 import { GenerateResponse } from './generate/response.js';
 import { Message } from './message.js';
@@ -211,14 +214,19 @@ export async function toGenerateRequest(
   );
 
   const out = {
-    messages: injectInstructions(messages, instructions),
+    messages: shouldInjectFormatInstructions(
+      resolvedFormat?.config,
+      options.output
+    )
+      ? injectInstructions(messages, instructions)
+      : messages,
     config: options.config,
     docs: options.docs,
     tools: tools?.map(toToolDefinition) || [],
     output: {
       ...(resolvedFormat?.config || {}),
-      schema: resolvedSchema,
       ...options.output,
+      schema: resolvedSchema,
     },
   } as GenerateRequest;
   if (!out?.output?.schema) delete out?.output?.schema;
@@ -343,16 +351,11 @@ export async function generate<
     resolvedOptions.output.format = 'json';
   }
   const resolvedFormat = await resolveFormat(registry, resolvedOptions.output);
-  const instructions = resolveInstructions(
-    resolvedFormat,
-    resolvedSchema,
-    resolvedOptions?.output?.instructions
-  );
 
   const params: GenerateActionOptions = {
     model: resolvedModel.modelAction.__action.name,
     docs: resolvedOptions.docs,
-    messages: injectInstructions(messages, instructions),
+    messages: messages,
     tools,
     toolChoice: resolvedOptions.toolChoice,
     config: {
diff --git a/js/ai/src/generate/action.ts b/js/ai/src/generate/action.ts
index 74f69ad072..2dc71bf8ad 100644
--- a/js/ai/src/generate/action.ts
+++ b/js/ai/src/generate/action.ts
@@ -41,6 +41,7 @@ import {
 import {
   GenerateActionOptions,
   GenerateActionOptionsSchema,
+  GenerateActionOutputConfig,
   GenerateRequest,
   GenerateRequestSchema,
   GenerateResponseChunkData,
@@ -172,7 +173,14 @@ function applyFormat(
   );
 
   if (resolvedFormat) {
-    outRequest.messages = injectInstructions(outRequest.messages, instructions);
+    if (
+      shouldInjectFormatInstructions(resolvedFormat.config, rawRequest?.output)
+    ) {
+      outRequest.messages = injectInstructions(
+        outRequest.messages,
+        instructions
+      );
+    }
     outRequest.output = {
       // use output config from the format
       ...resolvedFormat.config,
@@ -184,6 +192,16 @@ function applyFormat(
   return outRequest;
 }
 
+export function shouldInjectFormatInstructions(
+  formatConfig?: Formatter['config'],
+  rawRequestConfig?: z.infer<typeof GenerateActionOutputConfig>
+) {
+  return (
+    formatConfig?.defaultInstructions !== false ||
+    rawRequestConfig?.instructions
+  );
+}
+
 function applyTransferPreamble(
   rawRequest: GenerateActionOptions,
   transferPreamble?: GenerateActionOptions
diff --git a/js/ai/src/model.ts b/js/ai/src/model.ts
index 3f7e2ca576..368d48a24e 100644
--- a/js/ai/src/model.ts
+++ b/js/ai/src/model.ts
@@ -699,6 +699,14 @@ export async function resolveModel<C extends z.ZodTypeAny = z.ZodTypeAny>(
   return out;
 }
 
+export const GenerateActionOutputConfig = z.object({
+  format: z.string().optional(),
+  contentType: z.string().optional(),
+  instructions: z.union([z.boolean(), z.string()]).optional(),
+  jsonSchema: z.any().optional(),
+  constrained: z.boolean().optional(),
+});
+
 export const GenerateActionOptionsSchema = z.object({
   /** A model name (e.g. `vertexai/gemini-1.0-pro`). */
   model: z.string(),
@@ -713,15 +721,7 @@ export const GenerateActionOptionsSchema = z.object({
   /** Configuration for the generation request. */
   config: z.any().optional(),
   /** Configuration for the desired output of the request. Defaults to the model's default output if unspecified. */
-  output: z
-    .object({
-      format: z.string().optional(),
-      contentType: z.string().optional(),
-      instructions: z.union([z.boolean(), z.string()]).optional(),
-      jsonSchema: z.any().optional(),
-      constrained: z.boolean().optional(),
-    })
-    .optional(),
+  output: GenerateActionOutputConfig.optional(),
   /** Options for resuming an interrupted generation. */
   resume: z
     .object({
diff --git a/js/ai/tests/model/middleware_test.ts b/js/ai/tests/model/middleware_test.ts
index 49cd624fd3..cdd70d1caa 100644
--- a/js/ai/tests/model/middleware_test.ts
+++ b/js/ai/tests/model/middleware_test.ts
@@ -396,7 +396,7 @@ describe('augmentWithContext', () => {
   });
 });
 
-describe('simulateConstrainedGeneration', () => {
+describe.only('simulateConstrainedGeneration', () => {
   let registry: Registry;
 
   beforeEach(() => {
@@ -555,4 +555,70 @@ describe('simulateConstrainedGeneration', () => {
       tools: [],
     });
   });
+
+  it('uses format instructions when instructions is explicitly set to true', async () => {
+    let pm = defineProgrammableModel(registry, {
+      supports: { constrained: 'all' },
+    });
+    pm.handleResponse = async (req, sc) => {
+      return {
+        message: {
+          role: 'model',
+          content: [{ text: '```\n{"foo": "bar"}\n```' }],
+        },
+      };
+    };
+
+    const { output } = await generate(registry, {
+      model: 'programmableModel',
+      prompt: 'generate json',
+      output: {
+        instructions: true,
+        constrained: false,
+        schema: z.object({
+          foo: z.string(),
+        }),
+      },
+    });
+    assert.deepEqual(output, { foo: 'bar' });
+    assert.deepStrictEqual(pm.lastRequest, {
+      config: {},
+      messages: [
+        {
+          role: 'user',
+          content: [
+            { text: 'generate json' },
+            {
+              metadata: {
+                purpose: 'output',
+              },
+              text:
+                'Output should be in JSON format and conform to the following schema:\n' +
+                '\n' +
+                '```\n' +
+                '{"type":"object","properties":{"foo":{"type":"string"}},"required":["foo"],"additionalProperties":true,"$schema":"http://json-schema.org/draft-07/schema#"}\n' +
+                '```\n',
+            },
+          ],
+        },
+      ],
+      output: {
+        constrained: false,
+        contentType: 'application/json',
+        format: 'json',
+        schema: {
+          $schema: 'http://json-schema.org/draft-07/schema#',
+          additionalProperties: true,
+          properties: {
+            foo: {
+              type: 'string',
+            },
+          },
+          required: ['foo'],
+          type: 'object',
+        },
+      },
+      tools: [],
+    });
+  });
 });
diff --git a/js/core/src/action.ts b/js/core/src/action.ts
index 4fee1628cd..4b9441f19b 100644
--- a/js/core/src/action.ts
+++ b/js/core/src/action.ts
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import { JSONSchema7 } from 'json-schema';
+import { type JSONSchema7 } from 'json-schema';
 import * as z from 'zod';
 import { lazy } from './async.js';
 import { ActionContext, getContext, runWithContext } from './context.js';
@@ -26,7 +26,7 @@ import {
   setCustomMetadataAttributes,
 } from './tracing.js';
 
-export { Status, StatusCodes, StatusSchema } from './statusTypes.js';
+export { StatusCodes, StatusSchema, type Status } from './statusTypes.js';
 export { JSONSchema7 };
 
 /**
diff --git a/js/core/src/error.ts b/js/core/src/error.ts
index bda27d595f..305220a48e 100644
--- a/js/core/src/error.ts
+++ b/js/core/src/error.ts
@@ -15,7 +15,7 @@
  */
 
 import { Registry } from './registry.js';
-import { httpStatusCode, StatusName } from './statusTypes.js';
+import { httpStatusCode, type StatusName } from './statusTypes.js';
 
 export { StatusName };
 
diff --git a/js/genkit/src/registry.ts b/js/genkit/src/registry.ts
index 8c45c10d46..367d697b5d 100644
--- a/js/genkit/src/registry.ts
+++ b/js/genkit/src/registry.ts
@@ -15,8 +15,8 @@
  */
 
 export {
-  ActionType,
-  AsyncProvider,
   Registry,
-  Schema,
+  type ActionType,
+  type AsyncProvider,
+  type Schema,
 } from '@genkit-ai/core/registry';
diff --git a/js/genkit/tests/formats_test.ts b/js/genkit/tests/formats_test.ts
index 774bbd47e4..b23ef49678 100644
--- a/js/genkit/tests/formats_test.ts
+++ b/js/genkit/tests/formats_test.ts
@@ -93,7 +93,7 @@ describe('formats', () => {
   });
 
   it('lets you define and use a custom output format with simulated constrained generation', async () => {
-    defineEchoModel(ai, { supports: { constrained: false } });
+    defineEchoModel(ai, { supports: { constrained: 'none' } });
 
     const { output } = await ai.generate({
       model: 'echoModel',
diff --git a/js/genkit/tests/prompts_test.ts b/js/genkit/tests/prompts_test.ts
index 14d547c814..3f4ea276e2 100644
--- a/js/genkit/tests/prompts_test.ts
+++ b/js/genkit/tests/prompts_test.ts
@@ -173,7 +173,7 @@ describe('definePrompt', () => {
   });
 });
 
-describe.only('definePrompt', () => {
+describe('definePrompt', () => {
   describe('default model', () => {
     let ai: GenkitBeta;
 
@@ -310,7 +310,7 @@ describe.only('definePrompt', () => {
     });
   });
 
-  describe.only('default model ref', () => {
+  describe('default model ref', () => {
     let ai: GenkitBeta;
 
     beforeEach(() => {
diff --git a/js/testapps/flow-simple-ai/src/index.ts b/js/testapps/flow-simple-ai/src/index.ts
index 79da26e118..fea7b141f5 100644
--- a/js/testapps/flow-simple-ai/src/index.ts
+++ b/js/testapps/flow-simple-ai/src/index.ts
@@ -654,3 +654,85 @@ ai.defineFlow('blockingMiddleware', async () => {
   });
   return text;
 });
+
+ai.defineFlow('formatJson', async (input, { sendChunk }) => {
+  const { output, text } = await ai.generate({
+    prompt: `generate an RPG game character of type ${input || 'archer'}`,
+    output: {
+      constrained: false,
+      instructions: true,
+      schema: z
+        .object({
+          name: z.string(),
+          weapon: z.string(),
+        })
+        .strict(),
+    },
+    onChunk: (c) => sendChunk(c.output),
+  });
+  return { output, text };
+});
+
+ai.defineFlow('formatJsonManualSchema', async (input, { sendChunk }) => {
+  const { output, text } = await ai.generate({
+    prompt: `generate one RPG game character of type ${input || 'archer'} and generated JSON must match this interface
+    
+    \`\`\`typescript
+    interface Character {
+      name: string;
+      weapon: string;
+    }
+    \`\`\`
+    `,
+    output: {
+      constrained: true,
+      instructions: false,
+      schema: z
+        .object({
+          name: z.string(),
+          weapon: z.string(),
+        })
+        .strict(),
+    },
+    onChunk: (c) => sendChunk(c.output),
+  });
+  return { output, text };
+});
+
+ai.defineFlow('testArray', async (input, { sendChunk }) => {
+  const { output } = await ai.generate({
+    prompt: `10 different weapons for ${input}`,
+    output: {
+      format: 'array',
+      schema: z.array(z.string()),
+    },
+    onChunk: (c) => sendChunk(c.output),
+  });
+  return output;
+});
+
+ai.defineFlow('formatEnum', async (input, { sendChunk }) => {
+  const { output } = await ai.generate({
+    prompt: `classify the denger level of sky diving`,
+    output: {
+      format: 'enum',
+      schema: z.enum(['safe', 'dangerous', 'medium']),
+    },
+    onChunk: (c) => sendChunk(c.output),
+  });
+  return output;
+});
+
+ai.defineFlow('formatJsonl', async (input, { sendChunk }) => {
+  const { output } = await ai.generate({
+    prompt: `generate 5 randon persons`,
+    output: {
+      format: 'jsonl',
+      schema: z.array(
+        z.object({ name: z.string(), surname: z.string() }).strict()
+      ),
+    },
+    onChunk: (c) => sendChunk(c.output),
+  });
+  return output;
+});
diff --git a/js/testapps/format-tester/src/index.ts b/js/testapps/format-tester/src/index.ts
index 84dcfb5905..59716c1629 100644
--- a/js/testapps/format-tester/src/index.ts
+++ b/js/testapps/format-tester/src/index.ts
@@ -154,6 +154,7 @@ if (!models.length) {
     'vertexai/gemini-1.5-flash',
     'googleai/gemini-1.5-pro',
     'googleai/gemini-1.5-flash',
+    'googleai/gemini-2.0-flash',
   ];
 }