diff --git a/packages/agentic/src/mcp/__tests__/mcp-server.test.ts b/packages/agentic/src/mcp/__tests__/mcp-server.test.ts
index ba1efe25..21b7a9f0 100644
--- a/packages/agentic/src/mcp/__tests__/mcp-server.test.ts
+++ b/packages/agentic/src/mcp/__tests__/mcp-server.test.ts
@@ -1,6 +1,7 @@
 import type { DevframeHost } from 'devframe/types'
 import { Client, InMemoryTransport } from '@modelcontextprotocol/client'
 import { createHostContext } from 'devframe/node'
+import * as v from 'valibot'
 import { describe, expect, it } from 'vitest'
 import { buildMcpServerFromContext } from '../build-server'
 
@@ -165,6 +166,35 @@ describe('mcp adapter (in-memory)', () => {
     }
   })
 
+  it('calls an rpc-backed tool whose return schema has no native converter', async () => {
+    const { ctx, client, cleanup } = await bootPair()
+    try {
+      ctx.rpc.register({
+        name: 'list-things',
+        type: 'query',
+        jsonSerializable: true,
+        args: [],
+        returns: v.array(v.object({ id: v.string() })),
+        agent: { description: 'Lists things.' },
+        handler: () => [{ id: 'a' }, { id: 'b' }],
+      } as never)
+
+      const listed = await client.listTools()
+      const tool = listed.tools.find(t => t.name.endsWith('list-things'))
+      expect(tool).toBeDefined()
+      expect(tool!.outputSchema).toBeUndefined()
+
+      const result = await client.callTool({ name: tool!.name, arguments: {} })
+      expect(result.isError).toBeFalsy()
+      const content = result.content as Array<{ type: string, text: string }>
+      expect(JSON.parse(content[0]!.text)).toEqual([{ id: 'a' }, { id: 'b' }])
+      expect(result.structuredContent).toBeUndefined()
+    }
+    finally {
+      await cleanup()
+    }
+  })
+
   it('coerces non-JSON values returned from a tool', async () => {
     const { ctx, client, cleanup } = await bootPair()
     try {
diff --git a/packages/devframe/src/agent/__tests__/to-json-schema.test.ts b/packages/devframe/src/agent/__tests__/to-json-schema.test.ts
index f7e57272..4fd65665 100644
--- a/packages/devframe/src/agent/__tests__/to-json-schema.test.ts
+++ b/packages/devframe/src/agent/__tests__/to-json-schema.test.ts
@@ -49,7 +49,39 @@ describe('returnToJsonSchema', () => {
       .toEqual({ type: 'object', properties: { ok: { type: 'boolean' } } })
   })
 
-  it('falls back to permissive for validators without a native converter', () => {
-    expect(returnToJsonSchema(v.object({ ok: v.boolean() }))).toEqual(PERMISSIVE)
+  it('yields no schema for validators without a native converter', () => {
+    expect(returnToJsonSchema(v.object({ ok: v.boolean() }))).toBeUndefined()
+    expect(returnToJsonSchema(v.array(v.object({ ok: v.boolean() })))).toBeUndefined()
+  })
+
+  it('yields no schema when the converter cannot express the schema', () => {
+    const throwing = {
+      '~standard': {
+        version: 1,
+        vendor: 'test',
+        validate: (value: unknown) => ({ value }),
+        jsonSchema: {
+          input: () => { throw new Error('unsupported') },
+          output: () => { throw new Error('unsupported') },
+        },
+      } as StandardSchemaV1['~standard'],
+    }
+    expect(returnToJsonSchema(throwing)).toBeUndefined()
+  })
+
+  it('converts the output type, not the input type', () => {
+    const transforming = {
+      '~standard': {
+        version: 1,
+        vendor: 'test',
+        validate: (value: unknown) => ({ value }),
+        jsonSchema: {
+          input: () => ({ type: 'string' }),
+          output: () => ({ type: 'object', properties: { parsed: { type: 'number' } } }),
+        },
+      } as StandardSchemaV1['~standard'],
+    }
+    expect(returnToJsonSchema(transforming))
+      .toEqual({ type: 'object', properties: { parsed: { type: 'number' } } })
   })
 })
diff --git a/packages/devframe/src/agent/to-json-schema.ts b/packages/devframe/src/agent/to-json-schema.ts
index 7f7b4b4a..2571bceb 100644
--- a/packages/devframe/src/agent/to-json-schema.ts
+++ b/packages/devframe/src/agent/to-json-schema.ts
@@ -29,12 +29,28 @@ function safeToJsonSchema(schema: StandardSchemaV1): unknown {
 
 /**
  * JSON Schema for an RPC return value on the agent/MCP surface.
+ *
+ * Unlike args, a return value has no permissive fallback: the schema is
+ * advertised as an MCP `outputSchema`, which obliges the tool to return a
+ * matching object on every call. A validator with no native converter
+ * (e.g. valibot), or one whose converter cannot express the schema, yields
+ * no output schema rather than an unfounded object one, so array- and
+ * primitive-returning tools still work. Conversion uses the converter's
+ * `output`, since a transforming validator returns its output type.
  * @internal
  */
 export function returnToJsonSchema(schema: StandardSchemaV1 | undefined): unknown {
   if (!schema)
     return undefined
-  return safeToJsonSchema(schema)
+  const standard = schema['~standard'] as MaybeJsonSchema
+  if (!standard.jsonSchema)
+    return undefined
+  try {
+    return standard.jsonSchema.output({ target: 'draft-2020-12' })
+  }
+  catch {
+    return undefined
+  }
 }
 
 /**