diff --git a/src/plugin/normalize-tool-arg-schemas.test.ts b/src/plugin/normalize-tool-arg-schemas.test.ts
index 27f148995..8a9247dda 100644
--- a/src/plugin/normalize-tool-arg-schemas.test.ts
+++ b/src/plugin/normalize-tool-arg-schemas.test.ts
@@ -6,7 +6,7 @@ import { tmpdir } from "node:os"
 import { dirname, join } from "node:path"
 import { pathToFileURL } from "node:url"
 import { tool } from "@opencode-ai/plugin"
-import { normalizeToolArgSchemas } from "./normalize-tool-arg-schemas"
+import { normalizeToolArgSchemas, sanitizeJsonSchema } from "./normalize-tool-arg-schemas"
 
 const tempDirectories: string[] = []
 
@@ -95,3 +95,36 @@ describe("normalizeToolArgSchemas", () => {
     expect(afterQuery?.examples).toEqual(["issue 2314"])
   })
 })
+
+describe("sanitizeJsonSchema", () => {
+  it("rewrites bare $ref values to $defs JSON pointers", () => {
+    // given
+    const schema = {
+      type: "object",
+      properties: {
+        new_encoding: { $ref: "Encoding" },
+        existing_pointer: { $ref: "#/$defs/AlreadyValid" },
+      },
+      $defs: {
+        Encoding: { type: "string" },
+        AlreadyValid: { type: "string" },
+      },
+    }
+
+    // when
+    const sanitized = sanitizeJsonSchema(schema)
+
+    // then
+    expect(sanitized).toEqual({
+      type: "object",
+      properties: {
+        new_encoding: { $ref: "#/$defs/Encoding" },
+        existing_pointer: { $ref: "#/$defs/AlreadyValid" },
+      },
+      $defs: {
+        Encoding: { type: "string" },
+        AlreadyValid: { type: "string" },
+      },
+    })
+  })
+})
diff --git a/src/plugin/normalize-tool-arg-schemas.ts b/src/plugin/normalize-tool-arg-schemas.ts
index 0f626b546..52813bc64 100644
--- a/src/plugin/normalize-tool-arg-schemas.ts
+++ b/src/plugin/normalize-tool-arg-schemas.ts
@@ -47,6 +47,14 @@ function isRecord(value: unknown): value is Record<string, unknown> {
   return typeof value === "object" && value !== null && !Array.isArray(value)
 }
 
+function normalizeJsonSchemaRef(value: string): string {
+  if (value.startsWith("#") || value.includes(":") || value.startsWith("/")) {
+    return value
+  }
+
+  return `#/$defs/${value}`
+}
+
 export function sanitizeJsonSchema(value: unknown, depth = 0, isPropertyName = false): unknown {
   if (Array.isArray(value)) {
     return value.map((item) => sanitizeJsonSchema(item, depth + 1, false))
@@ -67,6 +75,11 @@ export function sanitizeJsonSchema(value: unknown, depth = 0, isPropertyName = f
       continue
     }
 
+    if (!isPropertyName && key === "$ref" && typeof nestedValue === "string") {
+      sanitized[key] = normalizeJsonSchemaRef(nestedValue)
+      continue
+    }
+
     const childIsPropertyName = key === "properties" && !isPropertyName
     sanitized[key] = sanitizeJsonSchema(nestedValue, depth + 1, childIsPropertyName)
   }