diff --git a/bun.lock b/bun.lock
index eb4c6e6fd5f..c4381ac73ba 100644
--- a/bun.lock
+++ b/bun.lock
@@ -46,7 +46,10 @@
         "@tsconfig/bun": "catalog:",
         "@types/bun": "catalog:",
         "@typescript/native": "catalog:",
+        "@valibot/to-json-schema": "1.8.0",
         "typescript": "catalog:",
+        "valibot": "1.5.0",
+        "zod": "catalog:",
       },
     },
     "packages/app": {
@@ -2662,6 +2665,8 @@
 
     "@ungap/structured-clone": ["@ungap/structured-clone@1.3.3", "", {}, "sha512-60YRaenCQcVjYEKOcG824+DRGGIQ3VKErcBoAEDJZz5bKIs2ZG+X/H9Nk+Q6EVkwJk5QNApxbrc5QtBSwtrXAg=="],
 
+    "@valibot/to-json-schema": ["@valibot/to-json-schema@1.8.0", "", { "peerDependencies": { "valibot": "^1.5.0" } }, "sha512-a0M+uwCuQZEPAo65NYkFSJ14O5c213KoSZmPdoCfuFvUhfULi4T2Z6Tpjv6lTVMW9RoLm74RpRAjNEa43KcN+g=="],
+
     "@volar/kit": ["@volar/kit@2.4.28", "", { "dependencies": { "@volar/language-service": "2.4.28", "@volar/typescript": "2.4.28", "typesafe-path": "^0.2.2", "vscode-languageserver-textdocument": "^1.0.11", "vscode-uri": "^3.0.8" }, "peerDependencies": { "typescript": "*" } }, "sha512-cKX4vK9dtZvDRaAzeoUdaAJEew6IdxHNCRrdp5Kvcl6zZOqb6jTOfk3kXkIkG3T7oTFXguEMt5+9ptyqYR84Pg=="],
 
     "@volar/language-core": ["@volar/language-core@2.4.28", "", { "dependencies": { "@volar/source-map": "2.4.28" } }, "sha512-w4qhIJ8ZSitgLAkVay6AbcnC7gP3glYM3fYwKV3srj8m494E3xtrCv6E+bWviiK/8hs6e6t1ij1s2Endql7vzQ=="],
@@ -4656,6 +4661,8 @@
 
     "util-deprecate": ["util-deprecate@1.0.2", "", {}, "sha512-EPD5q1uXyFxJpCrLnCc1nHnq3gOa6DZBocAIiI2TaSCA7VCJ1UJDMagCzIkXNsUYfD1daK//LTEQ8xiIbrHtcw=="],
 
+    "valibot": ["valibot@1.5.0", "", { "peerDependencies": { "typescript": ">=5" }, "optionalPeers": ["typescript"] }, "sha512-nil6AkP2TChWL43Z5uJ6GTxX01CUA+g8LWUM+N/rB9NBbkUMaUsi9PUNzlUPgoKASgmx9f7eGOYpJ04/fSa6FQ=="],
+
     "validate-npm-package-name": ["validate-npm-package-name@7.0.2", "", {}, "sha512-hVDIBwsRruT73PbK7uP5ebUt+ezEtCmzZz3F59BSr2F6OVFnJ/6h8liuvdLrQ88Xmnk6/+xGGuq+pG9WwTuy3A=="],
 
     "vary": ["vary@1.1.2", "", {}, "sha512-BNGbWLfd0eUPabhkXUVm0j8uuvREyTh5ovRa/dyow/BqAbZJyC+5fU+IzQOzmAKzYqYRAISoRhdQr3eIZ/PXqg=="],
diff --git a/packages/ai/README.md b/packages/ai/README.md
index 36c53c3b3e5..4fc3cff0f32 100644
--- a/packages/ai/README.md
+++ b/packages/ai/README.md
@@ -85,6 +85,22 @@ for await (const event of events) {
 }
 ```
 
+`input` and structured-output `schema` also take a [Standard Schema](https://standardschema.dev), such as one from Zod
+4.2+ or ArkType, which validates and types the arguments. The JSON Schema sent to the model comes from the schema's
+[Standard JSON Schema](https://standardschema.dev/json-schema) export; a schema without one (a plain Valibot schema;
+wrap it in `toStandardJsonSchema` from `@valibot/to-json-schema`) throws when the tool is built:
+
+```ts
+import { z } from "zod"
+
+const weather = tool({
+  name: "weather",
+  description: "Get the weather.",
+  input: z.object({ city: z.string() }),
+  execute: ({ city }) => getWeather(city),
+})
+```
+
 ### Freeform tools
 
 A tool with exactly one required string parameter can set `freeform`, so the model writes that parameter as raw text
diff --git a/packages/ai/package.json b/packages/ai/package.json
index 3c89a6c7629..3bb72149a41 100644
--- a/packages/ai/package.json
+++ b/packages/ai/package.json
@@ -149,7 +149,10 @@
     "@tsconfig/bun": "catalog:",
     "@types/bun": "catalog:",
     "@typescript/native": "catalog:",
-    "typescript": "catalog:"
+    "@valibot/to-json-schema": "1.8.0",
+    "typescript": "catalog:",
+    "valibot": "1.5.0",
+    "zod": "catalog:"
   },
   "dependencies": {
     "@aws-sdk/credential-providers": "3.1057.0",
diff --git a/packages/ai/src/llm.ts b/packages/ai/src/llm.ts
index 858f2704d66..af8ea7bb6ba 100644
--- a/packages/ai/src/llm.ts
+++ b/packages/ai/src/llm.ts
@@ -1,4 +1,4 @@
-import { Effect, JsonSchema, Schema, Stream } from "effect"
+import { Effect, Stream } from "effect"
 import { tryRequest } from "./media-model.js"
 import { LLMClient, Service, type StreamOptions } from "./route/client.js"
 import {
@@ -20,7 +20,7 @@ import {
   type ToolEntryInput,
   withPartial,
 } from "./schema/index.js"
-import { tool, type ExecutableTool, type Tool, type ToolSchema } from "./tool.js"
+import { tool, type ExecutableTool, type Tool, type ToolInput, type ToolInputType } from "./tool.js"
 import { ToolRuntime } from "./tool-runtime.js"
 
 /** Input accepted by `LLM.request`, normalized into the canonical `LLMRequest` class. */
@@ -40,10 +40,10 @@ export type RequestInput<SelectedLanguageModel extends LanguageModel = LanguageM
   readonly http?: HttpOptions.Input
 }
 
-/** An Effect Schema decodes and types `object`; a raw JSON Schema leaves it `unknown`. */
-export type OutputSchema = ToolSchema<any> | JsonSchema.JsonSchema
+/** An Effect Schema or a Standard Schema with a JSON Schema export validates and types `object`; a raw JSON Schema leaves it `unknown`. */
+export type OutputSchema = ToolInput
 
-export type OutputType<S extends OutputSchema> = S extends Schema.Top ? Schema.Schema.Type<S> : unknown
+export type OutputType<S extends OutputSchema> = ToolInputType<S>
 
 export type ObjectInput<
   S extends OutputSchema,
diff --git a/packages/ai/src/standard-schema.ts b/packages/ai/src/standard-schema.ts
new file mode 100644
index 00000000000..a667feb5485
--- /dev/null
+++ b/packages/ai/src/standard-schema.ts
@@ -0,0 +1,56 @@
+// The Standard Schema and Standard JSON Schema interfaces (https://standardschema.dev), copied as the spec recommends so
+// accepting them adds no dependency. Trimmed to the members this package reads.
+
+export interface StandardSchemaV1<Input = unknown, Output = Input> {
+  readonly "~standard": StandardSchemaV1.Props<Input, Output>
+}
+
+export declare namespace StandardSchemaV1 {
+  export interface Props<Input = unknown, Output = Input> {
+    readonly version: 1
+    readonly vendor: string
+    readonly validate: (value: unknown) => Result<Output> | Promise<Result<Output>>
+    readonly types?: Types<Input, Output> | undefined
+  }
+
+  export type Result<Output> = { readonly value: Output; readonly issues?: undefined } | FailureResult
+
+  export interface FailureResult {
+    readonly issues: ReadonlyArray<Issue>
+  }
+
+  export interface Issue {
+    readonly message: string
+    readonly path?: ReadonlyArray<PropertyKey | { readonly key: PropertyKey }> | undefined
+  }
+
+  export interface Types<Input = unknown, Output = Input> {
+    readonly input: Input
+    readonly output: Output
+  }
+
+  export type InferOutput<Schema extends StandardSchemaV1> = NonNullable<Schema["~standard"]["types"]>["output"]
+}
+
+export interface StandardJSONSchemaV1<Input = unknown, Output = Input> {
+  readonly "~standard": StandardJSONSchemaV1.Props<Input, Output>
+}
+
+export declare namespace StandardJSONSchemaV1 {
+  export interface Props<Input = unknown, Output = Input> {
+    readonly version: 1
+    readonly vendor: string
+    readonly types?: StandardSchemaV1.Types<Input, Output> | undefined
+    readonly jsonSchema: Converter
+  }
+
+  export interface Converter {
+    readonly input: (options: Options) => Record<string, unknown>
+    readonly output: (options: Options) => Record<string, unknown>
+  }
+
+  export interface Options {
+    readonly target: "draft-2020-12" | "draft-07" | "openapi-3.0" | ({} & string)
+    readonly libraryOptions?: Record<string, unknown> | undefined
+  }
+}
diff --git a/packages/ai/src/tool.ts b/packages/ai/src/tool.ts
index 0f5858e1f58..ef272521fae 100644
--- a/packages/ai/src/tool.ts
+++ b/packages/ai/src/tool.ts
@@ -7,6 +7,7 @@ import type {
   ToolOutput as ToolOutputType,
 } from "./schema/index.js"
 import { ToolDefinition, ToolFailure, ToolOutput, type ToolDefinitionInput } from "./schema/index.js"
+import type { StandardJSONSchemaV1, StandardSchemaV1 } from "./standard-schema.js"
 import { isRecord } from "./utils/record.js"
 
 /**
@@ -16,11 +17,16 @@ import { isRecord } from "./utils/record.js"
  */
 export type ToolSchema<T> = Schema.Codec<T, any, never, never>
 
-/** A tool's `input`: an Effect Schema decodes and types it; a raw JSON Schema leaves it `unknown`. */
-export type ToolInput = ToolSchema<any> | JsonSchema.JsonSchema
+/**
+ * A tool's `input`: an Effect Schema or a Standard Schema (https://standardschema.dev, such as Zod 4, ArkType or Valibot)
+ * validates and types it; a raw JSON Schema leaves it `unknown`. A Standard Schema must also implement Standard JSON
+ * Schema, which gives the JSON Schema sent to the model.
+ */
+export type ToolInput = ToolSchema<any> | StandardSchemaV1 | JsonSchema.JsonSchema
 
 /** The value `execute` receives for a tool `input`. */
-export type ToolInputType<Input extends ToolInput> = Input extends ToolSchema<infer T> ? T : unknown
+export type ToolInputType<Input extends ToolInput> =
+  Input extends ToolSchema<infer T> ? T : Input extends StandardSchemaV1 ? StandardSchemaV1.InferOutput<Input> : unknown
 
 export interface ToolExecuteContext {
   readonly id: ToolCallPart["id"]
@@ -83,7 +89,7 @@ export interface Tool<Input extends ToolInput = any, Success extends ToolSchema<
   readonly toModelOutput?: ToolToModelOutput<Input, Success>
   readonly toStructuredOutput?: (output: Success["Encoded"]) => unknown
   /** @internal */
-  readonly _decode: (input: unknown) => Effect.Effect<ToolInputType<Input>, Schema.SchemaError>
+  readonly _decode: (input: unknown) => Effect.Effect<ToolInputType<Input>, Schema.SchemaError | Error>
   // Method syntax keeps these parameters bivariant, so a concrete tool stays assignable to `ExecutableTool`.
   /** @internal */
   _encode(value: Schema.Schema.Type<Success>): Effect.Effect<unknown, Schema.SchemaError>
@@ -106,8 +112,9 @@ export type ExecutableTool<Input extends ToolInput = any, Success extends ToolSc
 /**
  * Options for `tool`, where `Execute` is the only part that differs between the Effect and Promise clients. Pass an
  * Effect Schema `input` (and optionally a `success` schema that validates the result, otherwise it is sent as is with
- * `undefined` as `null`), or a raw JSON Schema `input` known only at runtime, such as an MCP tool's, with an optional
- * JSON Schema `outputSchema`. Omit `execute` to declare a tool the model can call without running it here.
+ * `undefined` as `null`), a Standard Schema `input` that also exports JSON Schema, or a raw JSON Schema `input` known
+ * only at runtime, such as an MCP tool's. The last two take an optional JSON Schema `outputSchema`. Omit `execute` to
+ * declare a tool the model can call without running it here.
  */
 export type ToolOptions<
   Input extends ToolInput,
@@ -174,6 +181,22 @@ export function build(
       project(config.toModelOutput, config.toStructuredOutput, input, id, output),
   }
   const input = config.input
+  if (isStandardSchema(input))
+    return {
+      ...shared,
+      input,
+      success: Schema.Unknown,
+      _decode: (value: unknown) => validateStandard(input, value),
+      _encode: (value: unknown) => Effect.succeed(value ?? null),
+      _legacyResult: false,
+      _definition: definition({
+        name: config.name,
+        description: config.description,
+        inputSchema: standardJsonSchema(input),
+        outputSchema: config.outputSchema,
+        freeform: config.freeform,
+      }),
+    }
   if (!Schema.isSchema(input))
     return {
       ...shared,
@@ -241,6 +264,39 @@ const toJsonSchema = (schema: Schema.Top): JsonSchema.JsonSchema => {
   return { ...document.schema, $defs: document.definitions }
 }
 
+const isStandardSchema = (input: ToolInput): input is StandardSchemaV1 => "~standard" in input
+
+// Standard JSON Schema has no affordance for validation and Standard Schema none for JSON Schema, so ask the schema for
+// both. Without a JSON Schema export there is nothing truthful to send the model, so fail rather than guess.
+const standardJsonSchema = (schema: StandardSchemaV1): JsonSchema.JsonSchema => {
+  const props: StandardSchemaV1.Props & Partial<StandardJSONSchemaV1.Props> = schema["~standard"]
+  if (props.jsonSchema === undefined)
+    throw new Error(
+      `This ${props.vendor} schema has no JSON Schema export. Use a Standard Schema that also implements Standard JSON Schema (https://standardschema.dev/json-schema), such as a Zod 4.2+ or ArkType schema or a Valibot schema wrapped in toStandardJsonSchema from @valibot/to-json-schema, or pass a raw JSON Schema.`,
+    )
+  // Effect's JSON Schema export omits `$schema` too; providers only read the schema body.
+  const { $schema, ...body } = props.jsonSchema.input({ target: "draft-2020-12" })
+  return body
+}
+
+const validateStandard = (schema: StandardSchemaV1, value: unknown) =>
+  Effect.suspend(() => {
+    const result = schema["~standard"].validate(value)
+    return result instanceof Promise ? Effect.promise(() => result) : Effect.succeed(result)
+  }).pipe(
+    Effect.flatMap((result) =>
+      result.issues === undefined ? Effect.succeed(result.value) : Effect.fail(new Error(formatIssues(result.issues))),
+    ),
+  )
+
+const formatIssues = (issues: ReadonlyArray<StandardSchemaV1.Issue>) =>
+  issues
+    .map((issue) => {
+      const path = (issue.path ?? []).map((segment) => String(typeof segment === "object" ? segment.key : segment))
+      return path.length === 0 ? issue.message : `${path.join(".")}: ${issue.message}`
+    })
+    .join("; ")
+
 const project = (
   toModelOutput: ((input: ToolModelOutputInput<any, any>) => ReadonlyArray<Content>) | undefined,
   toStructuredOutput: ((output: unknown) => unknown) | undefined,
diff --git a/packages/ai/test/standard-schema.test.ts b/packages/ai/test/standard-schema.test.ts
new file mode 100644
index 00000000000..45498b37d03
--- /dev/null
+++ b/packages/ai/test/standard-schema.test.ts
@@ -0,0 +1,113 @@
+import { describe, expect, test } from "bun:test"
+import { toStandardJsonSchema } from "@valibot/to-json-schema"
+import { Effect, Stream } from "effect"
+import * as v from "valibot"
+import { z } from "zod"
+import { ai, ToolCallPart } from "../src/effect.js"
+import { AI } from "../src/index.js"
+import * as OpenAIChat from "../src/protocols/openai-chat.js"
+import { Auth } from "../src/route.js"
+import { TestLLM } from "../src/testing.js"
+import { toDefinitions, tool } from "../src/tool.js"
+import { ToolRuntime } from "../src/tool-runtime.js"
+import { testEffect } from "./lib/effect.js"
+
+const model = OpenAIChat.route
+  .with({ endpoint: { baseURL: "https://api.openai.test/v1/" }, auth: Auth.bearer("test") })
+  .model({ id: "gpt-4o-mini" })
+
+const call = (name: string, input: unknown) => ToolCallPart.make({ id: "call_1", name, input })
+
+describe("Standard Schema tool input", () => {
+  test("sends the schema's own JSON Schema and validates arguments with Zod", async () => {
+    const weather = tool({
+      name: "weather",
+      description: "Get the weather.",
+      input: z.object({ city: z.string(), days: z.string().transform(Number) }),
+      execute: (input) => Effect.succeed(`${input.city}:${input.days + 1}`),
+    })
+
+    expect(toDefinitions([weather])[0]?.inputSchema).toEqual({
+      type: "object",
+      properties: { city: { type: "string" }, days: { type: "string" } },
+      required: ["city", "days"],
+    })
+    expect(
+      (await Effect.runPromise(ToolRuntime.dispatch([weather], call("weather", { city: "Paris", days: "2" })))).result,
+    ).toEqual({
+      type: "text",
+      value: "Paris:3",
+    })
+    expect(
+      (await Effect.runPromise(ToolRuntime.dispatch([weather], call("weather", { city: 1, days: "2" })))).result,
+    ).toEqual({
+      type: "error",
+      value: "Invalid tool input: city: Invalid input: expected string, received number",
+    })
+  })
+
+  test("accepts a Valibot schema with a JSON Schema export in the Promise client", async () => {
+    const weather = AI.tool({
+      name: "weather",
+      description: "Get the weather.",
+      input: toStandardJsonSchema(v.object({ city: v.string() })),
+      execute: (input) => input.city.toUpperCase(),
+    })
+
+    expect(toDefinitions([weather])[0]?.inputSchema).toMatchObject({
+      type: "object",
+      properties: { city: { type: "string" } },
+      required: ["city"],
+    })
+    expect(
+      (await Effect.runPromise(ToolRuntime.dispatch([weather], call("weather", { city: "Paris" })))).result,
+    ).toEqual({
+      type: "text",
+      value: "PARIS",
+    })
+  })
+
+  test("fails clearly when the schema has no JSON Schema export", () => {
+    expect(() =>
+      tool({ name: "weather", description: "Get the weather.", input: v.object({ city: v.string() }) }),
+    ).toThrow("This valibot schema has no JSON Schema export.")
+  })
+})
+
+describe("Standard Schema structured output", () => {
+  const scripted = testEffect(TestLLM.testLayer())
+
+  scripted.effect("validates the answer and sends the schema's JSON Schema", () =>
+    Effect.gen(function* () {
+      const client = yield* TestLLM.Test
+      yield* client.push(TestLLM.toolCall("call-1", "generate_object", { city: "Paris" }))
+
+      const events = Array.from(
+        yield* ai
+          .stream({ model, prompt: "Pick a city.", schema: z.object({ city: z.string() }) })
+          .pipe(Stream.runCollect),
+      )
+
+      const finish = events.at(-1)
+      expect(finish?.type === "finish" && finish.object.city).toBe("Paris")
+      expect((yield* client.requests())[0]?.tools[0]?.inputSchema).toEqual({
+        type: "object",
+        properties: { city: { type: "string" } },
+        required: ["city"],
+      })
+    }),
+  )
+
+  scripted.effect("fails the request when the answer does not match", () =>
+    Effect.gen(function* () {
+      const client = yield* TestLLM.Test
+      yield* client.push(TestLLM.toolCall("call-1", "generate_object", { city: 1 }))
+
+      const error = yield* ai
+        .generate({ model, prompt: "Pick a city.", schema: z.object({ city: z.string() }) })
+        .pipe(Effect.flip)
+
+      expect(error.message).toContain("schema decode failed: city: Invalid input: expected string")
+    }),
+  )
+})
diff --git a/packages/ai/test/standard-schema.types.ts b/packages/ai/test/standard-schema.types.ts
new file mode 100644
index 00000000000..5215fe3c554
--- /dev/null
+++ b/packages/ai/test/standard-schema.types.ts
@@ -0,0 +1,40 @@
+import { toStandardJsonSchema } from "@valibot/to-json-schema"
+import { Effect } from "effect"
+import * as v from "valibot"
+import { z } from "zod"
+import { ai, tool } from "../src/effect.js"
+import { AI } from "../src/index.js"
+import * as OpenAIChat from "../src/protocols/openai-chat.js"
+import { Auth } from "../src/route.js"
+
+const model = OpenAIChat.route.with({ auth: Auth.bearer("fixture") }).model({ id: "gpt-4o-mini" })
+
+tool({
+  name: "weather",
+  description: "Get the weather.",
+  input: z.object({ days: z.string().transform(Number) }),
+  execute: (input) => Effect.succeed(input.days satisfies number),
+})
+
+AI.tool({
+  name: "weather",
+  description: "Get the weather.",
+  input: toStandardJsonSchema(v.object({ city: v.string() })),
+  // @ts-expect-error `city` is a string.
+  execute: (input) => input.city satisfies number,
+})
+
+AI.tool({
+  name: "run_code",
+  description: "Run JavaScript.",
+  input: z.object({ code: z.string() }),
+  freeform: { type: "text" },
+})
+
+ai.generate({ model, prompt: "Pick a city.", schema: z.object({ city: z.string() }) }).pipe(
+  Effect.map((response) => response.object.city satisfies string),
+)
+
+AI.ai
+  .generate({ model, prompt: "Pick a city.", schema: z.object({ city: z.string() }) })
+  .then((response) => response.object.city satisfies string)