diff --git a/packages/opencode/src/tool/registry.ts b/packages/opencode/src/tool/registry.ts
index 9167cb3ea6bc..20c2e98e058e 100644
--- a/packages/opencode/src/tool/registry.ts
+++ b/packages/opencode/src/tool/registry.ts
@@ -151,7 +151,14 @@ const layer = Layer.effect(
                   directory: ctx.directory,
                   worktree: ctx.worktree,
                 }
-                const result = yield* Effect.promise(() => def.execute(args as any, pluginCtx))
+                // Zod-declared plugin tools must parse args like Tool.define()
+                // does: fill .default() and reject constraint violations with
+                // a model-facing error instead of passing raw args through.
+                // Legacy JSON-schema tools keep pass-through (no parser).
+                const input = args ?? {}
+                const parsed = zodParams ? zodParams.safeParse(input) : { success: true as const, data: input }
+                if (parsed.success === false) yield* new Tool.InvalidArgumentsError({ tool: id, detail: parsed.error.message })
+                const result = yield* Effect.promise(() => def.execute(parsed.data as any, pluginCtx))
                 const output = typeof result === "string" ? result : result.output
                 const metadata = typeof result === "string" ? {} : (result.metadata ?? {})
                 const attachments = typeof result === "string" ? undefined : result.attachments
@@ -168,6 +175,7 @@ const layer = Layer.effect(
                   },
                 }
               }).pipe(
+                Effect.orDie,
                 Effect.withSpan("Tool.execute", {
                   attributes: {
                     "tool.name": id,
diff --git a/packages/opencode/test/tool/registry.test.ts b/packages/opencode/test/tool/registry.test.ts
index c8c5fac59559..80e59fe25af8 100644
--- a/packages/opencode/test/tool/registry.test.ts
+++ b/packages/opencode/test/tool/registry.test.ts
@@ -2,7 +2,7 @@ import { afterEach, describe, expect } from "bun:test"
 import path from "path"
 import fs from "fs/promises"
 import { fileURLToPath, pathToFileURL } from "url"
-import { Effect, Layer, Result, Schema } from "effect"
+import { Effect, Layer, Result, Schema, Cause, Exit } from "effect"
 import { LayerNode } from "@opencode-ai/core/effect/layer-node"
 import { ToolRegistry } from "@/tool/registry"
 import { Tool } from "@/tool/tool"
@@ -417,6 +417,57 @@ describe("tool.registry", () => {
     20_000,
   )
 
+  it.instance("parses plugin tool args with Zod: fills defaults and rejects violations (#49279)", () =>
+    Effect.gen(function* () {
+      const test = yield* TestInstance
+      const customTools = path.join(test.directory, ".opencode", "tools")
+      const pluginTool = pathToFileURL(path.resolve(import.meta.dir, "../../../plugin/src/tool.ts")).href
+      yield* Effect.promise(() => fs.mkdir(customTools, { recursive: true }))
+      yield* Effect.promise(() =>
+        Bun.write(
+          path.join(customTools, "argprobe.ts"),
+          [
+            `import { tool } from ${JSON.stringify(pluginTool)}`,
+            "export default tool({",
+            "  description: 'Echoes back what the runtime passed to execute.',",
+            "  args: {",
+            "    required: tool.schema.string(),",
+            "    withdefault: tool.schema.number().default(42),",
+            "    ranged: tool.schema.number().min(10).default(99),",
+            "  },",
+            "  execute: async (args) => 'RECEIVED ' + JSON.stringify(args),",
+            "})",
+            "",
+          ].join("\n"),
+        ),
+      )
+
+      const registry = yield* ToolRegistry.Service
+      const loaded = (yield* registry.all()).find((tool) => tool.id === "argprobe")
+      if (!loaded) throw new Error("argprobe tool was not loaded")
+      const agents = yield* Agent.Service
+      const ctx = {
+        sessionID: SessionID.make("ses_test"),
+        messageID: MessageID.make("msg_test"),
+        agent: (yield* agents.defaultInfo()).name,
+        abort: new AbortController().signal,
+        messages: [],
+        metadata: () => Effect.void,
+        ask: () => Effect.void,
+      } satisfies Tool.Context
+
+      const result = yield* loaded.execute({ required: "a" }, ctx)
+      expect(result.output).toBe('RECEIVED {"required":"a","withdefault":42,"ranged":99}')
+
+      const exit = yield* loaded.execute({ required: "b", ranged: 1 }, ctx).pipe(Effect.exit)
+      expect(Exit.isFailure(exit)).toBe(true)
+      if (!Exit.isFailure(exit)) return
+      const error = Cause.squash(exit.cause)
+      expect(error).toBeInstanceOf(Tool.InvalidArgumentsError)
+      expect((error as Tool.InvalidArgumentsError).message).toContain("argprobe tool was called with invalid arguments")
+    }),
+  )
+
   it.instance("preserves attachments from structured custom tool results", () =>
     Effect.gen(function* () {
       const test = yield* TestInstance