diff --git a/packages/opencode/src/tool/registry.ts b/packages/opencode/src/tool/registry.ts
index 6ef6d39a65..67df37c34a 100644
--- a/packages/opencode/src/tool/registry.ts
+++ b/packages/opencode/src/tool/registry.ts
@@ -55,6 +55,13 @@ import { Reference } from "@/reference/reference"
 import { BackgroundJob } from "@/background/job"
 import { SessionStatus } from "@/session/status"
 import { RuntimeFlags } from "@/effect/runtime-flags"
+import {
+  objectFromShape,
+  safeParse,
+  type AnyObjectSchema,
+  type AnySchema,
+} from "@modelcontextprotocol/sdk/server/zod-compat.js"
+import { toJsonSchemaCompat } from "@modelcontextprotocol/sdk/server/zod-json-schema-compat.js"
 
 const log = Log.create({ service: "tool.registry" })
 
@@ -150,10 +157,10 @@ export const layer: Layer.Layer<
           const args = def.args ?? {}
           const entries = Object.entries(args)
           const allZod = entries.every((entry) => isZodType(entry[1]))
-          const zodParams = allZod ? z.object(args) : undefined
+          const zodParams = allZod ? objectFromShape(args as Record<string, AnySchema>) : undefined
           const jsonSchema = zodParams ? zodJsonSchema(zodParams) : legacyJsonSchema(entries)
           const parameters = zodParams
-            ? Schema.declare<unknown>((u): u is unknown => zodParams.safeParse(u).success)
+            ? Schema.declare<unknown>((u): u is unknown => safeParse(zodParams, u).success)
             : Schema.Unknown
           return {
             id,
@@ -402,7 +409,11 @@ export const defaultLayer = Layer.suspend(() =>
     .pipe(Layer.provide(RuntimeFlags.defaultLayer)),
 )
 
-function isZodType(value: unknown): value is z.ZodType {
+function isZodType(value: unknown): value is AnySchema {
+  return typeof value === "object" && value !== null && ("_zod" in value || "_def" in value)
+}
+
+function isZod4Type(value: unknown): value is z.ZodType {
   return typeof value === "object" && value !== null && "_zod" in value
 }
 
@@ -425,8 +436,12 @@ function legacyJsonSchema(entries: [string, unknown][]): JSONSchema7 {
   }
 }
 
-function zodJsonSchema(schema: z.ZodType): JSONSchema7 {
-  const result = normalizeZodJsonSchema(z.toJSONSchema(schema, { io: "input", metadata: zodMetadataRegistry(schema) }))
+function zodJsonSchema(schema: AnyObjectSchema): JSONSchema7 {
+  const result = normalizeZodJsonSchema(
+    isZod4Type(schema)
+      ? z.toJSONSchema(schema, { io: "input", metadata: zodMetadataRegistry(schema) })
+      : toJsonSchemaCompat(schema, { pipeStrategy: "input" }),
+  )
   if (!isJsonSchemaObject(result)) throw new Error("plugin tool Zod schema produced a non-object JSON Schema")
   const { $defs, ...rest } = result
   return (
@@ -434,7 +449,7 @@ function zodJsonSchema(schema: z.ZodType): JSONSchema7 {
   ) as JSONSchema7
 }
 
-function zodMetadataRegistry(schema: z.ZodType) {
+function zodMetadataRegistry(schema: AnyObjectSchema) {
   const registry = z.registry<Record<string, unknown>>()
   const seen = new WeakSet<object>()
   const collect = (value: unknown) => {
@@ -442,7 +457,7 @@ function zodMetadataRegistry(schema: z.ZodType) {
     if (seen.has(value)) return
     seen.add(value)
 
-    if (isZodType(value)) {
+    if (isZod4Type(value)) {
       const metadata = typeof value.meta === "function" ? value.meta() : undefined
       const description = typeof value.description === "string" ? value.description : undefined
       const merged = {
diff --git a/packages/opencode/test/tool/registry.test.ts b/packages/opencode/test/tool/registry.test.ts
index d3549e66f3..aef5d2c49a 100644
--- a/packages/opencode/test/tool/registry.test.ts
+++ b/packages/opencode/test/tool/registry.test.ts
@@ -34,6 +34,7 @@ import { ProviderID, ModelID } from "@/provider/schema"
 import { ToolJsonSchema } from "@/tool/json-schema"
 import { MessageID, SessionID } from "@/session/schema"
 import { RuntimeFlags } from "@/effect/runtime-flags"
+import z3 from "zod/v3"
 
 const node = CrossSpawnSpawner.defaultLayer
 const configLayer = TestConfig.layer({
@@ -80,7 +81,7 @@ const brokenPluginLayer = Layer.succeed(
     init: () => Effect.void,
     trigger: ((_name: unknown, _input: unknown, output: unknown) =>
       Effect.succeed(output)) as Plugin.Interface["trigger"],
-    list: () =>
+    list: (() =>
       Effect.succeed([
         {
           tool: {
@@ -91,7 +92,41 @@ const brokenPluginLayer = Layer.succeed(
             },
           },
         },
-      ]),
+      ])) as unknown as Plugin.Interface["list"],
+  }),
+)
+
+const zod3PluginLayer = Layer.succeed(
+  Plugin.Service,
+  Plugin.Service.of({
+    init: () => Effect.void,
+    trigger: ((_name: unknown, _input: unknown, output: unknown) =>
+      Effect.succeed(output)) as Plugin.Interface["trigger"],
+    list: (() =>
+      Effect.succeed([
+        {
+          tool: {
+            ctx_batch_execute: {
+              description: "context-mode batch executor",
+              args: {
+                batch: z3
+                  .preprocess(
+                    (value) => (typeof value === "string" ? JSON.parse(value) : value),
+                    z3
+                      .array(
+                        z3.object({
+                          command: z3.string().describe("Command to execute"),
+                        }),
+                      )
+                      .min(1),
+                  )
+                  .describe("Commands to execute as a batch"),
+              },
+              execute: async () => "ok",
+            },
+          },
+        },
+      ])) as unknown as Plugin.Interface["list"],
   }),
 )
 
@@ -105,6 +140,7 @@ const background = testEffect(
 const withBrokenPlugin = testEffect(
   Layer.mergeAll(registryLayer({ plugin: brokenPluginLayer }), node, Agent.defaultLayer),
 )
+const withZod3Plugin = testEffect(Layer.mergeAll(registryLayer({ plugin: zod3PluginLayer }), node, Agent.defaultLayer))
 
 afterEach(async () => {
   await disposeAllInstances()
@@ -349,6 +385,52 @@ describe("tool.registry", () => {
     }),
   )
 
+  withZod3Plugin.instance("loads plugin tools with Zod 3 args as JSON Schema", () =>
+    Effect.gen(function* () {
+      const registry = yield* ToolRegistry.Service
+      const loaded = (yield* registry.all()).find((tool) => tool.id === "ctx_batch_execute")
+      if (!loaded) throw new Error("ctx_batch_execute tool was not loaded")
+
+      expect(loaded.jsonSchema).toMatchObject({
+        type: "object",
+        properties: {
+          batch: {
+            type: "array",
+            description: "Commands to execute as a batch",
+            items: {
+              type: "object",
+              properties: {
+                command: { type: "string", description: "Command to execute" },
+              },
+            },
+            minItems: 1,
+          },
+        },
+        required: ["batch"],
+      })
+      expect(JSON.stringify(loaded.jsonSchema)).not.toContain("_def")
+      expect(JSON.stringify(loaded.jsonSchema)).not.toContain("_zod")
+      expect(Result.isSuccess(Schema.decodeUnknownResult(loaded.parameters)({ batch: [{ command: "pwd" }] }))).toBe(
+        true,
+      )
+      expect(Result.isSuccess(Schema.decodeUnknownResult(loaded.parameters)({}))).toBe(false)
+    }),
+  )
+
+  withZod3Plugin.instance("validates plugin tools with Zod 3 preprocessors", () =>
+    Effect.gen(function* () {
+      const registry = yield* ToolRegistry.Service
+      const loaded = (yield* registry.all()).find((tool) => tool.id === "ctx_batch_execute")
+      if (!loaded) throw new Error("ctx_batch_execute tool was not loaded")
+
+      expect(
+        Result.isSuccess(
+          Schema.decodeUnknownResult(loaded.parameters)({ batch: JSON.stringify([{ command: "pwd" }]) }),
+        ),
+      ).toBe(true)
+    }),
+  )
+
   it.instance(
     "preserves Zod arg descriptions from older config-scoped plugin packages",
     () =>