diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1d621ad2..605f5bee 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,9 @@ This file is the canonical source for unreleased changes and published release n
 
 ## [Unreleased]
 
+### Fixed
+
+- `@codeworksh/plugin`: Fixed tools whose parameters are `Schema.Struct({})` being rejected by OpenAI and Anthropic. Their JSON Schema is now `{ "type": "object", "properties": {} }` instead of `{ "not": { "type": "null" } }`.
 ### Internal
 
 - The `codework tui` E2E artifact now records the CLI version as `<version>`, so it no longer goes stale on every version bump.
diff --git a/packages/harness/test/__artifacts__/tools.empty.live.anthropic.json b/packages/harness/test/__artifacts__/tools.empty.live.anthropic.json
new file mode 100644
index 00000000..b2bfb81f
--- /dev/null
+++ b/packages/harness/test/__artifacts__/tools.empty.live.anthropic.json
@@ -0,0 +1,4 @@
+{
+	"finite": "completed",
+	"reply": "finite-7f3a"
+}
diff --git a/packages/harness/test/__artifacts__/tools.empty.live.openai.json b/packages/harness/test/__artifacts__/tools.empty.live.openai.json
new file mode 100644
index 00000000..b2bfb81f
--- /dev/null
+++ b/packages/harness/test/__artifacts__/tools.empty.live.openai.json
@@ -0,0 +1,4 @@
+{
+	"finite": "completed",
+	"reply": "finite-7f3a"
+}
diff --git a/packages/harness/test/tools.empty.live.test.ts b/packages/harness/test/tools.empty.live.test.ts
new file mode 100644
index 00000000..eb19068f
--- /dev/null
+++ b/packages/harness/test/tools.empty.live.test.ts
@@ -0,0 +1,88 @@
+import { Effect, Schema } from "effect";
+import fs from "node:fs/promises";
+import { join } from "node:path";
+import { describe, expect, it } from "vite-plus/test";
+import { ContextCodec } from "../src/context/codec.ts";
+import { Harness } from "../src/effect/harness.ts";
+import { Session } from "../src/effect/session.ts";
+import { define } from "../src/plugin/plugin.ts";
+import * as Tool from "../src/tool/tool.ts";
+import { tmpdir } from "./fixtures/tempdir.ts";
+import { available, LIVE } from "./utils/live.ts";
+
+/**
+ * A tool whose parameters are `Schema.Struct({})` must reach the provider as an object
+ * schema. The token is only known to the tool, so a right reply means the call went through.
+ */
+
+const TOKEN = "finite-7f3a";
+
+const finitePlugin = define({
+	id: "acme.tool.finite",
+	kind: "tool",
+	setup(ctx) {
+		ctx.plugin.tools.add(
+			Tool.register(
+				Tool.make({
+					name: "finite",
+					description: "Return the secret token. Takes no arguments.",
+					promptSnippet: "Return the secret token",
+					parameters: Schema.Struct({}),
+					success: Schema.String,
+					encodeContent: (value) => [{ type: "text", text: value }],
+					handler: () => Effect.succeed(TOKEN),
+				}),
+			),
+		);
+	},
+});
+
+// Direct provider APIs only.
+const direct = LIVE.filter((live) => live.provider === "openai" || live.provider === "anthropic");
+
+describe.each(direct)("parameterless tool against live $name", (live) => {
+	it.skipIf(!available(live))(
+		"calls a tool whose parameters are an empty struct",
+		async () => {
+			await using temp = await tmpdir();
+			const root = await fs.realpath(temp.path);
+			const work = join(root, "work");
+			await fs.mkdir(work, { recursive: true });
+
+			const { calls, reply } = await Effect.gen(function* () {
+				const session = yield* Session.create({ directory: work, model: { provider: live.provider, id: live.id } });
+				yield* session.run("Call the finite tool, then reply with exactly the token it returned and nothing else.");
+				const path = yield* session.path();
+				const messages = yield* Effect.forEach(path, ContextCodec.decodeMessage);
+				const parts = messages.flatMap((message) => message.parts);
+				const last = messages.findLast((message) => message.parts.some((part) => part.type === "text"));
+				return {
+					calls: parts.flatMap((part) => (part.type === "toolCall" ? [part] : [])),
+					reply: (last?.parts ?? []).flatMap((part) => (part.type === "text" ? [part.text] : [])).join(""),
+				};
+			}).pipe(
+				Effect.provide(
+					Harness.layer({
+						home: join(root, "home"),
+						hostCwd: root,
+						database: ":memory:",
+						plugins: [finitePlugin],
+					}),
+				),
+				Effect.scoped,
+				Effect.timeout("120 seconds"),
+				Effect.runPromise,
+			);
+
+			const finite = calls.filter((call) => call.name === "finite").map((call) => call.status);
+			expect(finite).toContain("completed");
+			const result = { finite: "completed", reply: reply.trim().replace(/^`|`$/g, "") };
+			expect(result.reply).toBe(TOKEN);
+
+			await expect(`${JSON.stringify(result, null, "\t")}\n`).toMatchFileSnapshot(
+				`./__artifacts__/tools.empty.live.${live.name}.json`,
+			);
+		},
+		180_000,
+	);
+});
diff --git a/packages/plugin/package.json b/packages/plugin/package.json
index 56efa8c4..ecd61b7b 100644
--- a/packages/plugin/package.json
+++ b/packages/plugin/package.json
@@ -1,6 +1,6 @@
 {
 	"name": "@codeworksh/plugin",
-	"version": "0.2.0",
+	"version": "0.2.1",
 	"description": "SDK for building CodeWork plugins — the plugin, tool, and sandbox contracts shared with @codeworksh/harness.",
 	"keywords": [
 		"agent",
diff --git a/packages/plugin/src/tool.ts b/packages/plugin/src/tool.ts
index 4b786224..a1b4f437 100644
--- a/packages/plugin/src/tool.ts
+++ b/packages/plugin/src/tool.ts
@@ -1,5 +1,5 @@
 import type { Message } from "@codeworksh/aikit";
-import { Effect, type Layer, Schema } from "effect";
+import { Effect, type Layer, Schema, SchemaAST } from "effect";
 import type { TSchema } from "typebox";
 import { ToolExecutionError } from "./tool/error.ts";
 import type { ToolProgress } from "./tool/progress.ts";
@@ -238,8 +238,19 @@ export const provide = <
  * single boundary that crosses into the provider wire; if a provider rejects a
  * construct, fix it here. Draft-07 inlines the root (no top-level `$ref`).
  */
-export const toProviderJsonSchema = (schema: Schema.Top): Record<string, unknown> =>
-	Schema.toStandardJSONSchemaV1(schema)["~standard"].jsonSchema.input({ target: "draft-07" });
+export const toProviderJsonSchema = (schema: Schema.Top): Record<string, unknown> => {
+	// Effect reads `Struct({})` as TS `{}` (any non-null) and emits `{ not: { type: "null" } }`;
+	// providers require an object, so emit one shaped like a non-empty struct's.
+	const encoded = SchemaAST.toEncoded(schema.ast);
+	if (
+		SchemaAST.isObjects(encoded) &&
+		encoded.propertySignatures.length === 0 &&
+		encoded.indexSignatures.length === 0
+	) {
+		return { type: "object", properties: {}, additionalProperties: true };
+	}
+	return Schema.toStandardJSONSchemaV1(schema)["~standard"].jsonSchema.input({ target: "draft-07" });
+};
 
 /**
  * Build the aikit `Message.Tool` wire view from a definition. The derived JSON