diff --git a/src/__tests__/integration/contextSummarizer.integration.test.ts b/src/__tests__/integration/contextSummarizer.integration.test.ts
index aeb055ea69..4652eb5f09 100644
--- a/src/__tests__/integration/contextSummarizer.integration.test.ts
+++ b/src/__tests__/integration/contextSummarizer.integration.test.ts
@@ -329,7 +329,7 @@ Continue with implementation.`);
 			const logs = [createMockLog({ text: 'Test' })];
 
 			await expect(service.summarizeContext(baseRequest, logs, () => {})).rejects.toThrow(
-				'Context summarization failed'
+				'IPC failed'
 			);
 		});
 	});
diff --git a/src/__tests__/renderer/hooks/useSummarizeHandler.test.ts b/src/__tests__/renderer/hooks/useSummarizeHandler.test.ts
index dd914b2621..bd76302602 100644
--- a/src/__tests__/renderer/hooks/useSummarizeHandler.test.ts
+++ b/src/__tests__/renderer/hooks/useSummarizeHandler.test.ts
@@ -740,7 +740,7 @@ describe('handleSummarizeAndContinue (Tier 3E)', () => {
 			expect(notifyToast).toHaveBeenCalledWith(
 				expect.objectContaining({
 					type: 'error',
-					message: 'Failed to compact context. Check the tab for details.',
+					message: 'Failed to compact context: Summarization failed',
 				})
 			);
 		});
@@ -778,7 +778,7 @@ describe('handleSummarizeAndContinue (Tier 3E)', () => {
 			expect(notifyToast).toHaveBeenCalledWith(
 				expect.objectContaining({
 					type: 'error',
-					message: 'An unexpected error occurred during compaction.',
+					message: 'An unexpected error occurred during compaction: store write failed',
 					sessionId: 'session-1',
 					tabId: 'tab-1',
 				})
diff --git a/src/__tests__/renderer/services/contextSummarizer.test.ts b/src/__tests__/renderer/services/contextSummarizer.test.ts
index 243eab1cc2..1bcef09b7d 100644
--- a/src/__tests__/renderer/services/contextSummarizer.test.ts
+++ b/src/__tests__/renderer/services/contextSummarizer.test.ts
@@ -323,13 +323,16 @@ Continue with implementation.`);
 			expect(consoleLog).toHaveBeenCalledWith('[ContextSummarizer] Received response, length:', 0);
 		});
 
-		it('should handle IPC errors gracefully', async () => {
+		it('should propagate the underlying IPC error', async () => {
+			// Preserving the inner message lets the renderer surface real
+			// causes (e.g. "Agent X is not available") in the failure toast
+			// instead of a generic "Context summarization failed".
 			mockGroomContext.mockRejectedValue(new Error('IPC failed'));
 
 			const logs = [createMockLog({ text: 'Test' })];
 
 			await expect(service.summarizeContext(baseRequest, logs, () => {})).rejects.toThrow(
-				'Context summarization failed'
+				'IPC failed'
 			);
 		});
 	});
diff --git a/src/renderer/hooks/agent/useSummarizeAndContinue.ts b/src/renderer/hooks/agent/useSummarizeAndContinue.ts
index 4f75d65684..5586957096 100644
--- a/src/renderer/hooks/agent/useSummarizeAndContinue.ts
+++ b/src/renderer/hooks/agent/useSummarizeAndContinue.ts
@@ -398,11 +398,18 @@ export function useSummarizeAndContinue(session: Session | null): UseSummarizeAn
 						// Clear the summarization state for this tab
 						clearTabState(targetTabId);
 					} else {
-						// startSummarize returned null (error already set in operationStore)
+						// startSummarize returned null after setting an error state in
+						// operationStore. Pull the actual error so the toast can
+						// surface it directly instead of pointing users at a "tab"
+						// that never renders details.
+						const tabState = useOperationStore.getState().summarizeStates.get(targetTabId);
+						const detail = tabState?.error?.trim();
 						notifyToast({
 							type: 'error',
 							title: 'Compaction Failed',
-							message: 'Failed to compact context. Check the tab for details.',
+							message: detail
+								? `Failed to compact context: ${detail}`
+								: 'Failed to compact context.',
 							sessionId: sourceSessionId,
 							tabId: targetTabId,
 						});
@@ -410,10 +417,13 @@ export function useSummarizeAndContinue(session: Session | null): UseSummarizeAn
 				})
 				.catch((err) => {
 					console.error('[handleSummarizeAndContinue] Unexpected error:', err);
+					const detail = err instanceof Error ? err.message : String(err);
 					notifyToast({
 						type: 'error',
 						title: 'Compaction Failed',
-						message: 'An unexpected error occurred during compaction.',
+						message: detail
+							? `An unexpected error occurred during compaction: ${detail}`
+							: 'An unexpected error occurred during compaction.',
 						sessionId: sourceSessionId,
 						tabId: targetTabId,
 					});
diff --git a/src/renderer/services/contextSummarizer.ts b/src/renderer/services/contextSummarizer.ts
index b77304ad03..9a64c0af2e 100644
--- a/src/renderer/services/contextSummarizer.ts
+++ b/src/renderer/services/contextSummarizer.ts
@@ -208,9 +208,14 @@ export class ContextSummarizationService {
 				originalTokens,
 				compactedTokens,
 			};
-		} catch {
-			// The groomContext API handles its own cleanup - rethrow
-			throw new Error('Context summarization failed');
+		} catch (err) {
+			// Preserve the underlying error message so callers can surface it to
+			// the user. Wrapping with a generic string here used to swallow the
+			// real cause (e.g. "Agent X is not available", spawn ENAMETOOLONG)
+			// and the resulting "Compaction Failed" toast told users to "check
+			// the tab" — but no tab actually showed details.
+			if (err instanceof Error) throw err;
+			throw new Error(typeof err === 'string' ? err : 'Context summarization failed');
 		}
 	}