diff --git a/frontend/src/lib/components/ArgInput.svelte b/frontend/src/lib/components/ArgInput.svelte
index 49972d3b59..756b827105 100644
--- a/frontend/src/lib/components/ArgInput.svelte
+++ b/frontend/src/lib/components/ArgInput.svelte
@@ -253,6 +253,9 @@
 
 	let ignoreValueUndefined = $state(false)
 	let error: string = $state('')
+	// Parse error of the JSON editor below, which already displays it. Text that does not parse
+	// never reaches `value`, so without this the field would stay valid on its last parsed value.
+	let jsonError: string = $state('')
 	let isListJson = $state(false)
 	let hasIsListJsonChanged = $state(false)
 
@@ -417,9 +420,12 @@
 		'^(([0-9a-fA-F]{1,4}:){7,7}[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,7}:|([0-9a-fA-F]{1,4}:){1,6}:[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,5}(:[0-9a-fA-F]{1,4}){1,2}|([0-9a-fA-F]{1,4}:){1,4}(:[0-9a-fA-F]{1,4}){1,3}|([0-9a-fA-F]{1,4}:){1,3}(:[0-9a-fA-F]{1,4}){1,4}|([0-9a-fA-F]{1,4}:){1,2}(:[0-9a-fA-F]{1,4}){1,5}|[0-9a-fA-F]{1,4}:((:[0-9a-fA-F]{1,4}){1,6})|:((:[0-9a-fA-F]{1,4}){1,7}|:)|fe80:(:[0-9a-fA-F]{0,4}){0,4}%[0-9a-zA-Z]{1,}|::(ffff(:0{1,4}){0,1}:){0,1}((25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9])\\.){3,3}(25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9])|([0-9a-fA-F]{1,4}:){1,4}:((25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9])\\.){3,3}(25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9]))$'
 
 	function validateInput(pattern: string | undefined, v: any, required: boolean): void {
-		if (nullable && emptyString(v)) {
+		if (jsonError) {
 			error = ''
-			valid && (valid = true)
+			valid && (valid = false)
+		} else if (nullable && emptyString(v)) {
+			error = ''
+			!valid && (valid = true)
 		} else if (
 			typeof v === 'string' &&
 			(v.startsWith('$var:') || v.startsWith('$res:') || v.startsWith('$jsonvar:'))
@@ -563,6 +569,7 @@
 
 	$effect(() => {
 		extra?.['nonEmpty']
+		jsonError
 		let args = [pattern, value, required] as const
 		untrack(() => validateInput(...args))
 	})
@@ -1262,6 +1269,7 @@
 											dispatch('blur')
 										}}
 										code={rawValue}
+										bind:error={jsonError}
 										on:changeValue={(e) => {
 											setNewValueFromCode(e.detail)
 										}}
@@ -1284,6 +1292,7 @@
 									dispatch('blur')
 								}}
 								code={rawValue}
+								bind:error={jsonError}
 								on:change={(e) => {
 									value = e.detail
 								}}
@@ -1373,6 +1382,7 @@
 							dispatch('blur')
 						}}
 						code={rawValue}
+						bind:error={jsonError}
 						on:changeValue={(e) => {
 							setNewValueFromCode(e.detail)
 						}}
diff --git a/frontend/src/lib/components/JsonEditor.svelte b/frontend/src/lib/components/JsonEditor.svelte
index f3d2d4eb2e..36c662733b 100644
--- a/frontend/src/lib/components/JsonEditor.svelte
+++ b/frontend/src/lib/components/JsonEditor.svelte
@@ -40,7 +40,12 @@
 	// Identity for this editor's entry in the unparseable registry, so a caller about to
 	// persist what is on screen can refuse rather than save the last value that parsed.
 	const unparseableKey = {}
-	onDestroy(() => setEditorUnparseable(unparseableKey, false))
+	// Also clears a bound `error`: a parent gating validity on it would otherwise stay invalid
+	// after the editor that reported the error is gone.
+	onDestroy(() => {
+		error = ''
+		setEditorUnparseable(unparseableKey, false)
+	})
 
 	const dispatch = createEventDispatcher()
 	const dispatchIfMounted = createDispatcherIfMounted(dispatch)
diff --git a/frontend/src/lib/components/JsonInputs.svelte b/frontend/src/lib/components/JsonInputs.svelte
index bb39c0aa77..baf2d142d2 100644
--- a/frontend/src/lib/components/JsonInputs.svelte
+++ b/frontend/src/lib/components/JsonInputs.svelte
@@ -1,6 +1,7 @@
 <script lang="ts">
 	import SimpleEditor from '$lib/components/SimpleEditor.svelte'
-	import { createEventDispatcher, untrack } from 'svelte'
+	import { createEventDispatcher, onDestroy, untrack } from 'svelte'
+	import { setEditorUnparseable } from './pendingEditorFlush'
 
 	const dispatch = createEventDispatcher()
 
@@ -31,6 +32,17 @@
 	let userEdited = false
 	let simpleEditor: SimpleEditor | undefined = $state(undefined)
 	let focusTrap: HTMLElement | undefined = $state()
+	let parseError = $state('')
+
+	// Text that does not parse dispatches `undefined`, which callers read as "keep the last args":
+	// registering it lets whoever runs or persists those args refuse instead of using stale ones.
+	const unparseableKey = {}
+	onDestroy(() => setEditorUnparseable(unparseableKey, false))
+
+	function setParseError(message: string) {
+		parseError = message
+		setEditorUnparseable(unparseableKey, message !== '')
+	}
 
 	$effect(() => {
 		const next = initialCode
@@ -48,17 +60,21 @@
 		userEdited = false
 		pendingJson = code
 		simpleEditor?.setCode(code)
+		setParseError('')
 	}
 
 	function updatePayloadFromJson(jsonInput: string) {
 		if (jsonInput === undefined || jsonInput === null || jsonInput.trim() === '') {
+			setParseError('')
 			dispatch('select', undefined)
 			return
 		}
 		try {
 			const parsed = JSON.parse(jsonInput)
+			setParseError('')
 			dispatch('select', parsed)
 		} catch (error) {
+			setParseError(error instanceof Error ? error.message : String(error))
 			dispatch('select', undefined)
 		}
 	}
@@ -88,31 +104,36 @@
 <!-- Add a hidden button that can receive focus -->
 <button bind:this={focusTrap} class="sr-only" tabindex="-1" aria-hidden="true">Focus trap</button>
 
-<div class="h-full rounded-md border">
-	<SimpleEditor
-		bind:this={simpleEditor}
-		on:input={() => (userEdited = true)}
-		on:focus={() => {
-			if (updateOnBlur) {
-				dispatch('focus')
-				updatePayloadFromJson(pendingJson)
-			}
-		}}
-		on:blur={async () => {
-			if (updateOnBlur) {
-				dispatch('blur')
-			}
-		}}
-		on:change={(e) => {
-			if (e.detail?.code !== undefined) {
-				updatePayloadFromJson(e.detail.code)
-			}
-		}}
-		bind:code={pendingJson}
-		lang="json"
-		class="h-full json-inputs-editor"
-		{placeholder}
-	/>
+<div class="h-full flex flex-col">
+	<div class="flex-1 min-h-0 rounded-md border">
+		<SimpleEditor
+			bind:this={simpleEditor}
+			on:input={() => (userEdited = true)}
+			on:focus={() => {
+				if (updateOnBlur) {
+					dispatch('focus')
+					updatePayloadFromJson(pendingJson)
+				}
+			}}
+			on:blur={async () => {
+				if (updateOnBlur) {
+					dispatch('blur')
+				}
+			}}
+			on:change={(e) => {
+				if (e.detail?.code !== undefined) {
+					updatePayloadFromJson(e.detail.code)
+				}
+			}}
+			bind:code={pendingJson}
+			lang="json"
+			class="h-full json-inputs-editor"
+			{placeholder}
+		/>
+	</div>
+	{#if parseError}
+		<span class="text-red-600 dark:text-red-400 text-xs mt-1">Invalid JSON: {parseError}</span>
+	{/if}
 </div>
 
 <style>
diff --git a/frontend/src/lib/components/RunForm.svelte b/frontend/src/lib/components/RunForm.svelte
index e9a96d222d..ef059a74de 100644
--- a/frontend/src/lib/components/RunForm.svelte
+++ b/frontend/src/lib/components/RunForm.svelte
@@ -22,10 +22,11 @@
 	import { argsToJsonPayload } from '$lib/schema'
 	import { triggerableByAI } from '$lib/actions/triggerableByAI.svelte'
 	import InputSelectedBadge from './schema/InputSelectedBadge.svelte'
-	import { untrack } from 'svelte'
+	import { tick, untrack } from 'svelte'
 	import { processSecretArgs } from './secretArgUtils'
 	import { enforceDisabledDefaults, resetKeysToast } from './job_args'
 	import PowerShellCommonParams from './PowerShellCommonParams.svelte'
+	import { anyEditorUnparseable, flushAllPendingEditorChanges } from './pendingEditorFlush'
 
 	let reloadArgs = $state(0)
 	let jsonEditor: JsonInputs | undefined = $state(undefined)
@@ -33,6 +34,13 @@
 	let showInputSelectedBadge = $state(false)
 	let savedPreviousArgs: Record<string, any> | undefined = $state(undefined)
 	let psCommonParams: Record<string, any> = $state({})
+	// Reset on a view switch, where the editor that refused the run is gone, and on a form
+	// validity change, where the field's own error and the disabled button take over.
+	let blockedByUnparseable = $derived.by(() => {
+		void jsonView
+		void isValid
+		return false
+	})
 
 	function extractPsCommonParams(allArgs: Record<string, any>): {
 		scriptArgs: Record<string, any>
@@ -60,6 +68,15 @@
 	}
 
 	export async function run(overrideScheduledForStr?: string | undefined | null) {
+		// An editor whose text does not parse never wrote it to `args`, so running now would send
+		// the last value that did parse. Flush first: a keystroke still inside the editor debounce
+		// has not been parsed yet, and per-field editors parse it in an effect, hence the tick.
+		flushAllPendingEditorChanges()
+		await tick()
+		blockedByUnparseable = anyEditorUnparseable()
+		if (blockedByUnparseable) {
+			return
+		}
 		let processedArgs: Record<string, any>
 		const { args: withDefaults, resetKeys } = enforceDisabledDefaults(args ?? {}, runnable?.schema)
 		if (resetKeys.length > 0) {
@@ -303,6 +320,7 @@
 				<JsonInputs
 					bind:this={jsonEditor}
 					on:select={(e) => {
+						blockedByUnparseable = false
 						if (e.detail) {
 							args = enforceDisabledDefaults(e.detail, runnable?.schema).args
 						}
@@ -374,6 +392,7 @@
 					</Popover>
 				</div>
 			</div>
+			{@render unparseableError()}
 			{#if overrideTag}
 				<div class="flex-row-reverse flex w-full text-primary text-sm">
 					tag override: {overrideTag}
@@ -405,6 +424,7 @@
 			</Button>
 			<div>{@render actions()}</div>
 		</div>
+		{@render unparseableError()}
 	{:else}
 		<Button
 			btnClasses="!px-6 !py-1 w-full"
@@ -415,5 +435,14 @@
 		>
 			{buttonText}
 		</Button>
+		{@render unparseableError()}
 	{/if}
 </div>
+
+{#snippet unparseableError()}
+	{#if blockedByUnparseable}
+		<div class="flex-row-reverse flex w-full text-red-600 dark:text-red-400 text-xs mt-1">
+			Some input is not valid JSON. Fix it before running.
+		</div>
+	{/if}
+{/snippet}