diff --git a/plugins/workflows/src/json-value.ts b/plugins/workflows/src/json-value.ts
new file mode 100644
index 0000000000..28cce969b9
--- /dev/null
+++ b/plugins/workflows/src/json-value.ts
@@ -0,0 +1,66 @@
+import type { JsonValue } from "./types.js";
+
+export function assertJsonValue(
+  value: unknown,
+  path = "result",
+  ancestors = new WeakSet<object>(),
+): asserts value is JsonValue {
+  if (
+    value === null ||
+    typeof value === "string" ||
+    typeof value === "boolean"
+  ) {
+    return;
+  }
+  if (typeof value === "number") {
+    if (!Number.isFinite(value)) {
+      throw new Error(`${path} contains a non-finite number`);
+    }
+    return;
+  }
+  if (Array.isArray(value)) {
+    if (ancestors.has(value)) throw new Error(`${path} contains a cycle`);
+    ancestors.add(value);
+    for (let index = 0; index < value.length; index += 1) {
+      if (!(index in value)) throw new Error(`${path} contains a sparse array`);
+      assertJsonValue(value[index], `${path}[${index}]`, ancestors);
+    }
+    ancestors.delete(value);
+    return;
+  }
+  if (typeof value === "object") {
+    const prototype = Object.getPrototypeOf(value);
+    if (prototype !== Object.prototype && prototype !== null) {
+      throw new Error(`${path} must contain only plain objects and arrays`);
+    }
+    if (ancestors.has(value)) throw new Error(`${path} contains a cycle`);
+    ancestors.add(value);
+    if (Object.getOwnPropertySymbols(value).length > 0) {
+      throw new Error(`${path} contains symbol properties`);
+    }
+    const object = value as Record<string, unknown>;
+    for (const key of Object.getOwnPropertyNames(object)) {
+      if (key === "__proto__" || key === "constructor" || key === "prototype") {
+        throw new Error(
+          `${path} contains forbidden key ${JSON.stringify(key)}`,
+        );
+      }
+      const descriptor = Object.getOwnPropertyDescriptor(object, key);
+      if (descriptor === undefined || !("value" in descriptor)) {
+        throw new Error(`${path}.${key} must be a data property`);
+      }
+      if (!descriptor.enumerable) {
+        throw new Error(`${path}.${key} must be enumerable`);
+      }
+      assertJsonValue(descriptor.value, `${path}.${key}`, ancestors);
+    }
+    ancestors.delete(value);
+    return;
+  }
+  throw new Error(`${path} is not JSON-compatible`);
+}
+
+export function toJsonValue(value: unknown, path: string): JsonValue {
+  assertJsonValue(value, path);
+  return value;
+}
diff --git a/plugins/workflows/src/runtime.ts b/plugins/workflows/src/runtime.ts
index c9e5e9aa3d..8b8b14a9ac 100644
--- a/plugins/workflows/src/runtime.ts
+++ b/plugins/workflows/src/runtime.ts
@@ -6,6 +6,7 @@ import {
   type QuickJSHandle,
   type QuickJSRuntime,
 } from "quickjs-emscripten-core";
+import { assertJsonValue } from "./json-value.js";
 import type {
   ExecuteWorkflowScriptArgs,
   JsonValue,
@@ -43,66 +44,6 @@ function isAborted(signal: AbortSignal | undefined): boolean {
   return signal?.aborted === true;
 }
 
-function assertJsonValue(
-  value: unknown,
-  path = "result",
-  ancestors = new WeakSet<object>(),
-): asserts value is JsonValue {
-  if (
-    value === null ||
-    typeof value === "string" ||
-    typeof value === "boolean"
-  ) {
-    return;
-  }
-  if (typeof value === "number") {
-    if (!Number.isFinite(value)) {
-      throw new Error(`${path} contains a non-finite number`);
-    }
-    return;
-  }
-  if (Array.isArray(value)) {
-    if (ancestors.has(value)) throw new Error(`${path} contains a cycle`);
-    ancestors.add(value);
-    for (let index = 0; index < value.length; index += 1) {
-      if (!(index in value)) throw new Error(`${path} contains a sparse array`);
-      assertJsonValue(value[index], `${path}[${index}]`, ancestors);
-    }
-    ancestors.delete(value);
-    return;
-  }
-  if (typeof value === "object") {
-    const prototype = Object.getPrototypeOf(value);
-    if (prototype !== Object.prototype && prototype !== null) {
-      throw new Error(`${path} must contain only plain objects and arrays`);
-    }
-    if (ancestors.has(value)) throw new Error(`${path} contains a cycle`);
-    ancestors.add(value);
-    if (Object.getOwnPropertySymbols(value).length > 0) {
-      throw new Error(`${path} contains symbol properties`);
-    }
-    const object = value as Record<string, unknown>;
-    for (const key of Object.getOwnPropertyNames(object)) {
-      if (key === "__proto__" || key === "constructor" || key === "prototype") {
-        throw new Error(
-          `${path} contains forbidden key ${JSON.stringify(key)}`,
-        );
-      }
-      const descriptor = Object.getOwnPropertyDescriptor(object, key);
-      if (descriptor === undefined || !("value" in descriptor)) {
-        throw new Error(`${path}.${key} must be a data property`);
-      }
-      if (!descriptor.enumerable) {
-        throw new Error(`${path}.${key} must be enumerable`);
-      }
-      assertJsonValue(descriptor.value, `${path}.${key}`, ancestors);
-    }
-    ancestors.delete(value);
-    return;
-  }
-  throw new Error(`${path} is not JSON-compatible`);
-}
-
 function parseWorkflowReference(value: JsonValue): WorkflowReference {
   if (typeof value === "string") {
     if (value.length === 0) {
diff --git a/plugins/workflows/src/server-harness.test.ts b/plugins/workflows/src/server-harness.test.ts
index 690dfe0263..e0a307c452 100644
--- a/plugins/workflows/src/server-harness.test.ts
+++ b/plugins/workflows/src/server-harness.test.ts
@@ -61,6 +61,32 @@ describe("workflows plugin", () => {
     await harness.setSettings({ maxActiveRuns: "5" });
   });
 
+  it("registers tool schemas without recursive $refs", async () => {
+    const { bb, harness } = createFakePluginHost({
+      pluginId: "workflows",
+      agentSkillIds: ["workflows"],
+    });
+    hosts.push(harness);
+    await plugin(bb);
+
+    const tools = harness.registrations.agentTools;
+    expect(tools.map((tool) => tool.name)).toEqual(
+      expect.arrayContaining(["bb_workflow_run", "bb_workflow_result"]),
+    );
+    // A self-referential $ref makes some providers reject the whole tool list
+    // before the turn starts, so no tool may ship one.
+    for (const tool of tools) {
+      const schema = JSON.stringify(tool.inputSchema);
+      expect(schema, `tool ${tool.name}`).not.toContain("$ref");
+      expect(schema, `tool ${tool.name}`).not.toContain("$defs");
+    }
+
+    const run = tools.find((tool) => tool.name === "bb_workflow_run");
+    expect(
+      run?.parse({ name: "demo", args: { nested: [1, { deep: null }] } }),
+    ).toMatchObject({ ok: true });
+  });
+
   it("runs a structured workflow asynchronously and notifies its origin", async () => {
     let childCount = 0;
     const { bb, harness } = createFakePluginHost({
diff --git a/plugins/workflows/src/server.ts b/plugins/workflows/src/server.ts
index cbd2768935..ec496dec18 100644
--- a/plugins/workflows/src/server.ts
+++ b/plugins/workflows/src/server.ts
@@ -2,12 +2,14 @@ import type { BbPluginApi, PluginAgentToolResult } from "@get-bb/plugin-sdk";
 import { z } from "zod";
 import { registerWorkflowCli } from "./cli.js";
 import { migrations } from "./data.js";
+import { toJsonValue } from "./json-value.js";
 import { executeWorkflowScript } from "./runtime.js";
 import { createWorkflowService } from "./service.js";
 import {
   DEFAULT_WORKFLOW_SETTINGS,
   registerWorkflowSettings,
 } from "./settings.js";
+import type { JsonValue } from "./types.js";
 import { prepareWorkflowSource } from "./workflow-input.js";
 import { workflowUiRpcContract } from "./ui-contract.js";
 import { buildWorkflowRunView } from "./ui-view.js";
@@ -44,11 +46,16 @@ const sourceInputFields = {
     )
     .optional(),
 } as const;
+// zod 4's `z.json()` compiles to a self-referential `$defs` entry, and some
+// model providers reject an entire tool list that contains a recursive `$ref`
+// before the turn starts. Declare freeform JSON as `unknown` so the wire schema
+// stays flat, then narrow with `toJsonValue` at each call site.
+const freeformJson = z.unknown();
+
 const runInputSchema = z
   .object({
     ...sourceInputFields,
-    args: z
-      .json()
+    args: freeformJson
       .describe(
         "Optional input value exposed to the script as the global `args`, verbatim. Pass arrays/objects as actual JSON values, NOT as a JSON-encoded string — a stringified list breaks `args.filter`/`args.map` in the script. Use for parameterized named workflows (e.g. a research question).",
       )
@@ -65,9 +72,9 @@ const runInputSchema = z
   .strict();
 const resultInputSchema = z
   .object({
-    value: z
-      .json()
-      .describe("The final value matching the requested JSON Schema."),
+    value: freeformJson.describe(
+      "The final value matching the requested JSON Schema.",
+    ),
   })
   .strict();
 
@@ -147,7 +154,7 @@ export default async function plugin(bb: BbPluginApi) {
           projectId: ctx.projectId,
           originThreadId: ctx.threadId,
           source: prepared.source,
-          args: input.args,
+          args: toJsonValue(input.args, "args"),
           resumedFromRunId: input.resumeRunId,
         });
         const previewDirective = `::workflow-preview{run="${run.id}"}`;
@@ -171,7 +178,15 @@ export default async function plugin(bb: BbPluginApi) {
       'Use this tool to return your final response in the requested structured format. You MUST call this tool exactly once at the end of your response with {"value": ...} to provide the structured output.',
     parameters: resultInputSchema,
     async execute({ value }, ctx) {
-      const result = await service.submitStructuredResult(ctx.threadId, value);
+      let parsed: JsonValue;
+      try {
+        parsed = toJsonValue(value, "value");
+      } catch (error) {
+        return errorResult(
+          error instanceof Error ? error.message : String(error),
+        );
+      }
+      const result = await service.submitStructuredResult(ctx.threadId, parsed);
       if (result.ok) return jsonResult({ accepted: true });
       return errorResult(result.error);
     },