diff --git a/src/plugin-interface.ts b/src/plugin-interface.ts
index 5bcc0c364..9aa982808 100644
--- a/src/plugin-interface.ts
+++ b/src/plugin-interface.ts
@@ -8,6 +8,7 @@ import { createCommandExecuteBeforeHandler } from "./plugin/command-execute-befo
 import { createMessagesTransformHandler } from "./plugin/messages-transform"
 import { createSystemTransformHandler } from "./plugin/system-transform"
 import { createEventHandler } from "./plugin/event"
+import { createToolDefinitionHandler } from "./plugin/tool-definition"
 import { createToolExecuteAfterHandler } from "./plugin/tool-execute-after"
 import { createToolExecuteBeforeHandler } from "./plugin/tool-execute-before"
 
@@ -70,6 +71,10 @@ export function createPluginInterface(args: {
       hooks,
     }),
 
+    "tool.definition": createToolDefinitionHandler({
+      hooks,
+    }),
+
     "tool.execute.before": createToolExecuteBeforeHandler({
       ctx,
       hooks,
diff --git a/src/plugin/tool-definition.test.ts b/src/plugin/tool-definition.test.ts
new file mode 100644
index 000000000..23c5167c6
--- /dev/null
+++ b/src/plugin/tool-definition.test.ts
@@ -0,0 +1,63 @@
+import { describe, it, expect } from "bun:test"
+import { createToolDefinitionHandler } from "./tool-definition"
+import { createTodoDescriptionOverrideHook } from "../hooks/todo-description-override/hook"
+import { TODOWRITE_DESCRIPTION } from "../hooks/todo-description-override/description"
+import type { CreatedHooks } from "../create-hooks"
+
+function buildHooks(overrides: Partial<CreatedHooks> = {}): CreatedHooks {
+  return overrides as CreatedHooks
+}
+
+describe("createToolDefinitionHandler (regression for #3705)", () => {
+  describe("#given todoDescriptionOverride hook is registered", () => {
+    describe("#when the tool.definition handler runs for the todowrite tool", () => {
+      it("#then forwards to the hook and rewrites the description", async () => {
+        //#given
+        const handler = createToolDefinitionHandler({
+          hooks: buildHooks({ todoDescriptionOverride: createTodoDescriptionOverrideHook() }),
+        })
+        const output = { description: "opencode core default", parameters: {} }
+
+        //#when
+        await handler({ toolID: "todowrite" }, output)
+
+        //#then
+        expect(output.description).toBe(TODOWRITE_DESCRIPTION)
+      })
+    })
+
+    describe("#when the tool.definition handler runs for any other tool", () => {
+      it("#then leaves the description untouched", async () => {
+        //#given
+        const handler = createToolDefinitionHandler({
+          hooks: buildHooks({ todoDescriptionOverride: createTodoDescriptionOverrideHook() }),
+        })
+        const output = { description: "bash native description", parameters: {} }
+
+        //#when
+        await handler({ toolID: "bash" }, output)
+
+        //#then
+        expect(output.description).toBe("bash native description")
+      })
+    })
+  })
+
+  describe("#given todoDescriptionOverride hook is disabled (null)", () => {
+    describe("#when the tool.definition handler runs for todowrite", () => {
+      it("#then is a no-op", async () => {
+        //#given
+        const handler = createToolDefinitionHandler({
+          hooks: buildHooks({ todoDescriptionOverride: null }),
+        })
+        const output = { description: "opencode default kept", parameters: {} }
+
+        //#when
+        await handler({ toolID: "todowrite" }, output)
+
+        //#then
+        expect(output.description).toBe("opencode default kept")
+      })
+    })
+  })
+})
diff --git a/src/plugin/tool-definition.ts b/src/plugin/tool-definition.ts
new file mode 100644
index 000000000..59f794eff
--- /dev/null
+++ b/src/plugin/tool-definition.ts
@@ -0,0 +1,16 @@
+import type { CreatedHooks } from "../create-hooks"
+
+export function createToolDefinitionHandler(args: {
+  hooks: CreatedHooks
+}): (
+  input: { toolID: string },
+  output: { description: string; parameters: unknown },
+) => Promise<void> {
+  const { hooks } = args
+  return async (input, output) => {
+    const overrideHook = hooks.todoDescriptionOverride
+    if (overrideHook) {
+      await overrideHook["tool.definition"](input, output)
+    }
+  }
+}