diff --git a/packages/worker/src/mcp/capabilities/define-capability.node.test.ts b/packages/worker/src/mcp/capabilities/define-capability.node.test.ts
new file mode 100644
index 0000000000..f505ec0ecd
--- /dev/null
+++ b/packages/worker/src/mcp/capabilities/define-capability.node.test.ts
@@ -0,0 +1,76 @@
+import { expect, test, vi } from 'vitest'
+import { z } from 'zod'
+import { McpCallerError } from '#mcp/caller-error.ts'
+import { createMcpCallerContext } from '#mcp/context.ts'
+import { defineCapability } from './define-capability.ts'
+
+function createCapabilityContext() {
+	return {
+		env: {} as Env,
+		callerContext: createMcpCallerContext({
+			baseUrl: 'https://heykody.dev',
+			user: {
+				userId: 'user-1',
+				email: 'user@example.com',
+			},
+		}),
+	}
+}
+
+test('Zod capability validation failures identify the capability, fields, and repair path', async () => {
+	const inputHandler = vi.fn(async () => ({ package_id: 'github' }))
+	const inputCapability = defineCapability({
+		name: 'package_get',
+		domain: 'packages',
+		description: 'Get a package.',
+		inputSchema: z.object({ package_id: z.string() }),
+		outputSchema: z.object({ package_id: z.string() }),
+		handler: inputHandler,
+	})
+
+	const inputError = await inputCapability
+		.handler({ kody_id: 'github' }, createCapabilityContext())
+		.catch((error: unknown) => error)
+
+	expect(inputError).toBeInstanceOf(McpCallerError)
+	expect(inputError).toMatchObject({
+		message: expect.stringContaining(
+			'Invalid input for capability "package_get".',
+		),
+		cause: expect.any(z.ZodError),
+	})
+	expect(inputError.message).toContain('package_id')
+	expect(inputError.message).toContain(
+		'Call search({ entity: "package_get:capability" }) for the exact input shape.',
+	)
+	expect(inputHandler).not.toHaveBeenCalled()
+
+	const outputHandler = vi.fn(async () => ({ package_id: 123 }) as never)
+	const outputCapability = defineCapability({
+		name: 'package_save',
+		domain: 'packages',
+		description: 'Save a package.',
+		inputSchema: z.object({ package_id: z.string() }),
+		outputSchema: z.object({ package_id: z.string() }),
+		handler: outputHandler,
+	})
+
+	const outputError = await outputCapability
+		.handler({ package_id: 'github' }, createCapabilityContext())
+		.catch((error: unknown) => error)
+
+	expect(outputHandler).toHaveBeenCalledOnce()
+	expect(outputError).toBeInstanceOf(Error)
+	expect(outputError).not.toBeInstanceOf(McpCallerError)
+	expect(outputError).toMatchObject({
+		message: expect.stringContaining(
+			'Capability "package_save" returned an invalid output shape.',
+		),
+		cause: expect.any(z.ZodError),
+	})
+	expect(outputError.message).toContain('package_id')
+	expect(outputError.message).toContain(
+		'This is a capability implementation error; changing the input shape will not repair it.',
+	)
+	expect(outputError.message).not.toContain('Call search')
+})
diff --git a/packages/worker/src/mcp/capabilities/define-capability.ts b/packages/worker/src/mcp/capabilities/define-capability.ts
index 87c38f66ea..57958872e3 100644
--- a/packages/worker/src/mcp/capabilities/define-capability.ts
+++ b/packages/worker/src/mcp/capabilities/define-capability.ts
@@ -4,6 +4,7 @@ import {
 	errorFields,
 	logMcpEvent,
 } from '#mcp/observability.ts'
+import { McpCallerError } from '#mcp/caller-error.ts'
 import {
 	type Capability,
 	type CapabilityDefinition,
@@ -118,7 +119,12 @@ export function defineCapability<
 			try {
 				parsedArgs = inputParser(args) as InferCapabilitySchema<TInputSchema>
 			} catch (error) {
-				const { errorName, errorMessage } = errorFields(error)
+				const callerError = createCapabilityValidationError(
+					error,
+					definition.name,
+					'input',
+				)
+				const { errorName, errorMessage } = errorFields(callerError)
 				logMcpEvent({
 					category: 'mcp',
 					tool: 'capability',
@@ -131,9 +137,9 @@ export function defineCapability<
 					failurePhase: 'parse_input',
 					errorName,
 					errorMessage,
-					cause: error,
+					cause: callerError,
 				})
-				throw error
+				throw callerError
 			}
 
 			let result: Awaited<ReturnType<typeof definition.handler>>
@@ -172,7 +178,12 @@ export function defineCapability<
 				})
 				return finalized
 			} catch (error) {
-				const { errorName, errorMessage } = errorFields(error)
+				const validationError = createCapabilityValidationError(
+					error,
+					definition.name,
+					'output',
+				)
+				const { errorName, errorMessage } = errorFields(validationError)
 				logMcpEvent({
 					category: 'mcp',
 					tool: 'capability',
@@ -185,14 +196,42 @@ export function defineCapability<
 					failurePhase: 'parse_output',
 					errorName,
 					errorMessage,
-					cause: error,
+					cause: validationError,
 				})
-				throw error
+				throw validationError
 			}
 		},
 	}
 }
 
+function createCapabilityValidationError(
+	error: unknown,
+	capabilityName: string,
+	valueKind: 'input' | 'output',
+) {
+	if (!(error instanceof z.ZodError)) return error
+
+	if (valueKind === 'input') {
+		return new McpCallerError(
+			[
+				`Invalid input for capability "${capabilityName}".`,
+				z.prettifyError(error),
+				`Repair: Call search({ entity: "${capabilityName}:capability" }) for the exact input shape.`,
+			].join('\n'),
+			{ cause: error },
+		)
+	}
+
+	return new Error(
+		[
+			`Capability "${capabilityName}" returned an invalid output shape.`,
+			z.prettifyError(error),
+			'This is a capability implementation error; changing the input shape will not repair it.',
+		].join('\n'),
+		{ cause: error },
+	)
+}
+
 function mergeKeywords(
 	keywords: Array<string> | undefined,
 	tags: Array<string> | undefined,