diff --git a/README.md b/README.md
index 32f7059..874a4ee 100644
--- a/README.md
+++ b/README.md
@@ -161,6 +161,7 @@ const triage = agent({
 ```
 
 Rig defaults agent tools to `skipPermission: true`, and you can also place plain tool objects in `tools`; rig will convert `s.*` parameter schemas into JSON Schema before creating the Copilot session.
+When `parameters` uses `s.*` schemas, `handler` args are inferred automatically; for plain JSON Schema, pass an explicit generic like `defineTool<{ issue: string }>(...)`.
 
 ## Evaluating agentic performance
 
diff --git a/skills/rig/SKILL.md b/skills/rig/SKILL.md
index 84b8b1c..010b41a 100644
--- a/skills/rig/SKILL.md
+++ b/skills/rig/SKILL.md
@@ -170,6 +170,7 @@ s.nonEmptyArray(s.string)  // string[] with at least one element
 ## Tools
 
 Register custom tools with `defineTool` using an SDK-neutral tool shape. Use `s.*` schemas for `parameters`. Rig defaults tools to `skipPermission: true`.
+When `parameters` uses `s.*` schemas, `handler` args are inferred automatically; for plain JSON Schema, pass an explicit generic like `defineTool<{ issue: string }>(...)`.
 
 ```ts
 import { agent, defineTool, s } from "rig";
diff --git a/skills/rig/rig.ts b/skills/rig/rig.ts
index 00746cd..db0c02b 100644
--- a/skills/rig/rig.ts
+++ b/skills/rig/rig.ts
@@ -540,8 +540,14 @@ export type ToolConfig<TArgs = unknown> = {
   overridesBuiltInTool?: boolean;
   skipPermission?: boolean;
 };
-
-export function defineTool<T = unknown>(name: string, config: ToolConfig<T>): Tool<T> {
+type InferToolArgs<TParameters extends ToolParameters> = TParameters extends Schema ? InferSchema<TParameters> : unknown;
+
+export function defineTool<const TParameters extends ToolParameters>(
+  name: string,
+  config: Omit<ToolConfig<InferToolArgs<TParameters>>, "parameters"> & { parameters: TParameters },
+): Tool<InferToolArgs<TParameters>>;
+export function defineTool<T = unknown>(name: string, config: ToolConfig<T>): Tool<T>;
+export function defineTool(name: string, config: ToolConfig<any>): Tool<any> {
   return {
     name,
     ...normalizeToolConfig(config),
diff --git a/src/rig.test.ts b/src/rig.test.ts
index 9a53d28..b302ad7 100644
--- a/src/rig.test.ts
+++ b/src/rig.test.ts
@@ -52,6 +52,7 @@ vi.mock("@github/copilot-sdk", () => ({
 }));
 
 import { AgentError, PromptBuilder, agent, analyzeResponse, configureAgent, copilotEngine, defineTool, p, s, toJsonSchema } from "rig";
+import type { Tool } from "rig";
 import { oncePerAgent, repair, steering, timeout } from "rig/addons";
 
 beforeEach(() => {
@@ -537,18 +538,19 @@ describe("agent invocation", () => {
   });
 
   it("defines tools with rig schemas using the Copilot SDK helper shape", () => {
-    const handler = vi.fn(async ({ issue }: { issue: string }) => `Issue ${issue}`);
     const lookupIssue = defineTool("lookup_issue", {
       description: "Look up an issue by id.",
       parameters: s.object({ issue: s.string }),
-      handler,
+      handler: vi.fn(async ({ issue }) => `Issue ${issue}`),
     });
+    const expectIssueTool = (_tool: Tool<{ issue: string }>) => true;
+    expect(expectIssueTool(lookupIssue)).toBe(true);
 
     expect(lookupIssue).toEqual({
       name: "lookup_issue",
       description: "Look up an issue by id.",
       parameters: toJsonSchema(s.object({ issue: s.string })),
-      handler,
+      handler: expect.any(Function),
       skipPermission: true,
     });
   });