diff --git a/packages/ai/src/schema/messages.ts b/packages/ai/src/schema/messages.ts
index 8fb12c2ec34..46f5e8aafc5 100644
--- a/packages/ai/src/schema/messages.ts
+++ b/packages/ai/src/schema/messages.ts
@@ -9,7 +9,6 @@ import {
   LanguageModelSchema,
   ProviderOptions,
 } from "./options.js"
-import { isRecord } from "../utils/record.js"
 
 export const MessageRole = Schema.Literals(["system", "user", "assistant", "tool"])
 export type MessageRole = Schema.Schema.Type<typeof MessageRole>
@@ -56,11 +55,6 @@ export const MediaPart = Schema.Struct({
 }).annotate({ identifier: "LLM.Content.Media" })
 export type MediaPart = Schema.Schema.Type<typeof MediaPart>
 
-const isToolResultValue = (value: unknown): value is ToolResultValue =>
-  isRecord(value) &&
-  (value.type === "text" || value.type === "json" || value.type === "error" || value.type === "content") &&
-  "value" in value
-
 const toolResultValueSchema = Schema.Union([
   Schema.Struct({
     type: Schema.Literal("json"),
@@ -80,6 +74,7 @@ const toolResultValueSchema = Schema.Union([
   }),
 ]).annotate({ identifier: "LLM.ToolResult" })
 export type ToolResultValue = Schema.Schema.Type<typeof toolResultValueSchema>
+const isToolResultValue = Schema.is(toolResultValueSchema)
 
 export const ToolResultValue = Object.assign(toolResultValueSchema, {
   is: isToolResultValue,
diff --git a/packages/ai/test/schema.test.ts b/packages/ai/test/schema.test.ts
index 18e6bc466c6..28325eda5fb 100644
--- a/packages/ai/test/schema.test.ts
+++ b/packages/ai/test/schema.test.ts
@@ -21,6 +21,7 @@ import {
   QuotaExceededError,
   RateLimitError,
   RouteID,
+  ToolResultValue,
   TransportError,
   UnknownProviderError,
   Usage,
@@ -83,6 +84,37 @@ describe("llm schema", () => {
   })
 })
 
+describe("ToolResultValue", () => {
+  test("uses the canonical schema guard", () => {
+    const cases: ReadonlyArray<{ readonly value: unknown; readonly expected: boolean }> = [
+      { value: { type: "json", value: { ok: true } }, expected: true },
+      { value: { type: "text", value: "done" }, expected: true },
+      { value: { type: "error", value: "failed" }, expected: true },
+      { value: { type: "content", value: [{ type: "text", text: "done" }] }, expected: true },
+      { value: { type: "content", value: [{ type: "text" }] }, expected: false },
+      { value: { type: "content", value: "done" }, expected: false },
+      { value: { type: "json" }, expected: false },
+      { value: { type: "unknown", value: "done" }, expected: false },
+    ]
+
+    for (const item of cases) {
+      expect(Schema.is(ToolResultValue)(item.value)).toBe(item.expected)
+      expect(ToolResultValue.is(item.value)).toBe(item.expected)
+    }
+  })
+
+  test("accepts canonical results with extra fields", () => {
+    expect(ToolResultValue.is({ type: "json", value: { ok: true }, metadata: { source: "tool" } })).toBe(true)
+    expect(
+      ToolResultValue.is({
+        type: "content",
+        value: [{ type: "file", uri: "https://example.test/result.txt", mime: "text/plain", checksum: "abc" }],
+        metadata: { source: "tool" },
+      }),
+    ).toBe(true)
+  })
+})
+
 describe("AI.Usage", () => {
   test("subtractTokens clamps non-sensical breakdowns to zero", () => {
     // Defense against a provider reporting cached_tokens > prompt_tokens or
diff --git a/packages/ai/test/tool-runtime.test.ts b/packages/ai/test/tool-runtime.test.ts
index f45455b5d35..8930c486afa 100644
--- a/packages/ai/test/tool-runtime.test.ts
+++ b/packages/ai/test/tool-runtime.test.ts
@@ -442,6 +442,33 @@ describe("LLMClient tools", () => {
     }),
   )
 
+  it.effect("projects malformed tagged dynamic output as opaque JSON", () =>
+    Effect.gen(function* () {
+      const malformed = { type: "content", value: [{ type: "text" }] }
+      const dynamic = Tool.make({
+        description: "Return caller-defined JSON.",
+        jsonSchema: { type: "object", properties: {} },
+        execute: () => Effect.succeed(malformed),
+      })
+
+      const dispatched = yield* ToolRuntime.dispatch(
+        { dynamic },
+        LLMEvent.toolCall({ id: "call_1", name: "dynamic", input: {} }),
+      )
+
+      expect(dispatched.result).toEqual({ type: "json", value: malformed })
+      expect(dispatched.output).toEqual({ structured: malformed, content: [] })
+      expect(dispatched.events).toEqual([
+        LLMEvent.toolResult({
+          id: "call_1",
+          name: "dynamic",
+          result: { type: "json", value: malformed },
+          output: { structured: malformed, content: [] },
+        }),
+      ])
+    }),
+  )
+
   it.effect("executes tool calls for one step without looping by default", () =>
     Effect.gen(function* () {
       const layer = scriptedResponses([