diff --git a/frontend/src/lib/components/details/FlowDetail.svelte b/frontend/src/lib/components/details/FlowDetail.svelte
index 854a18c75d..e92fff0284 100644
--- a/frontend/src/lib/components/details/FlowDetail.svelte
+++ b/frontend/src/lib/components/details/FlowDetail.svelte
@@ -300,6 +300,12 @@
 		// pressed under, and a call settled meanwhile — declined, then another proposed for
 		// this same flow — must not be answered with a job it never asked for.
 		const call = pendingRun
+		// One call is one job, and the card's form is live alongside this one: without the
+		// claim, a Run there during the round trip below starts a second.
+		if (call && !call.beginSubmit()) {
+			sendUserToast('This run is already being started from the chat.', true)
+			return
+		}
 		loading = true
 		const scheduledFor = scheduledForStr ? new Date(scheduledForStr).toISOString() : undefined
 		try {
@@ -318,6 +324,9 @@
 			if (!call?.adopt(run, args, flow?.schema ?? {}))
 				await onNavigate('/run/' + run + '?workspace=' + workspace)
 		} catch (e) {
+			// No job came of the claim, so hand it back rather than leaving the call unrunnable
+			// on either surface.
+			call?.endSubmit()
 			throw e
 		} finally {
 			loading = false
diff --git a/frontend/src/lib/components/details/ScriptDetail.svelte b/frontend/src/lib/components/details/ScriptDetail.svelte
index a4c028966a..c10f3dc7bd 100644
--- a/frontend/src/lib/components/details/ScriptDetail.svelte
+++ b/frontend/src/lib/components/details/ScriptDetail.svelte
@@ -389,6 +389,12 @@
 		// pressed under, and a call settled meanwhile — declined, then another proposed for
 		// this same script — must not be answered with a job it never asked for.
 		const call = pendingRun
+		// One call is one job, and the card's form is live alongside this one: without the
+		// claim, a Run there during the round trip below starts a second.
+		if (call && !call.beginSubmit()) {
+			sendUserToast('This run is already being started from the chat.', true)
+			return
+		}
 		try {
 			runLoading = true
 			const scheduledFor = scheduledForStr ? new Date(scheduledForStr).toISOString() : undefined
@@ -420,6 +426,9 @@
 			if (!call?.adopt(run, args, script?.schema ?? {}))
 				await onNavigate('/run/' + run + '?workspace=' + workspace)
 		} catch (err) {
+			// No job came of the claim, so hand it back rather than leaving the call unrunnable
+			// on either surface.
+			call?.endSubmit()
 			sendUserToast(`Could not create job: ${err.body}`, true)
 		} finally {
 			// Not only on failure: a navigation that keeps this component mounted — the run
diff --git a/frontend/src/lib/components/details/pendingRun.ts b/frontend/src/lib/components/details/pendingRun.ts
index 00e75622fc..df62481f7d 100644
--- a/frontend/src/lib/components/details/pendingRun.ts
+++ b/frontend/src/lib/components/details/pendingRun.ts
@@ -24,4 +24,13 @@ export type PendingRun = {
 	 * alone. */
 	adopt: (jobId: string, args: Record<string, any>, schema: Record<string, any>) => boolean
 	decline: () => void
+	/** Claim the call's one job, before starting one. The card's form is live at the same
+	 * time as this page's, and the call stays pending until `adopt` — which is a round trip
+	 * later — so without this claim a Run on the card in between starts a second job, with
+	 * its own arguments and no mention in the chat. False when the other surface claimed it
+	 * first; send nothing and say so. */
+	beginSubmit: () => boolean
+	/** Hand the claim back when no job came of it, so the reader can try again. A started
+	 * job needs no release: adopting it settles the call. */
+	endSubmit: () => void
 }