diff --git a/.changeset/compaction-threshold-toggle.md b/.changeset/compaction-threshold-toggle.md
new file mode 100644
index 000000000..d5233c97e
--- /dev/null
+++ b/.changeset/compaction-threshold-toggle.md
@@ -0,0 +1,5 @@
+---
+'@truefoundry/trueforge-ui': patch
+---
+
+Make the compaction threshold an Auto/Custom selector: Auto omits `trigger` (runtime derives ~80% of the model context window); Custom reveals a number input defaulting to 50000.
diff --git a/packages/trueforge-ui/src/atoms/draft/AgentRuntimeConfigFields.tsx b/packages/trueforge-ui/src/atoms/draft/AgentRuntimeConfigFields.tsx
index e50fd8cdc..647345439 100644
--- a/packages/trueforge-ui/src/atoms/draft/AgentRuntimeConfigFields.tsx
+++ b/packages/trueforge-ui/src/atoms/draft/AgentRuntimeConfigFields.tsx
@@ -1,8 +1,9 @@
 'use client';
 
-import type { AgentRuntimeConfig } from '../../server/types.js';
+import type { AgentCompactionConfig, AgentRuntimeConfig } from '../../server/types.js';
 import { cn } from '../lib/cn.js';
 import { auiInputClass } from '../lib/inputClasses.js';
+import { PopoverSelect } from '../primitives/PopoverSelect.js';
 import { Switch } from '../primitives/Switch.js';
 import { Tooltip } from '../primitives/Tooltip.js';
 
@@ -30,6 +31,14 @@ function parsePositiveInteger(raw: string): number | null {
 }
 
 const NO_SANDBOX_PROVIDER_HINT = 'No sandbox provider yet, add one in Settings → Sandbox';
+const DEFAULT_COMPACTION_THRESHOLD_TOKENS = 50_000;
+
+const COMPACTION_THRESHOLD_MODE_OPTIONS = [
+  { value: 'auto', label: 'Auto' },
+  { value: 'custom', label: 'Custom' },
+] as const;
+
+type CompactionThresholdMode = (typeof COMPACTION_THRESHOLD_MODE_OPTIONS)[number]['value'];
 
 type RuntimeSwitchField = {
   label: string;
@@ -50,6 +59,16 @@ export function AgentRuntimeConfigFields({
   layout = 'compact',
   onChange,
 }: AgentRuntimeConfigFieldsProps) {
+  const withCompaction = (compaction: AgentCompactionConfig): AgentRuntimeConfig => ({
+    ...value,
+    contextManagement: {
+      ...value.contextManagement,
+      compaction,
+      largeToolResponse: value.contextManagement?.largeToolResponse ?? { enabled: true },
+    },
+  });
+  const applyCompaction = (compaction: AgentCompactionConfig) => onChange(withCompaction(compaction));
+
   const capabilityFields: RuntimeSwitchField[] = [
     {
       label: 'Dynamic sub-agents',
@@ -72,6 +91,8 @@ export function AgentRuntimeConfigFields({
   ];
   const sandboxEnabled = value.sandbox?.enabled ?? false;
   const compactionEnabled = value.contextManagement?.compaction?.enabled ?? true;
+  const thresholdMode: CompactionThresholdMode =
+    value.contextManagement?.compaction?.trigger != null ? 'custom' : 'auto';
   const webSearchField: RuntimeSwitchField | null = webSearchAvailable
     ? {
         label: 'Web search',
@@ -104,16 +125,9 @@ export function AgentRuntimeConfigFields({
   };
   const compactionField: RuntimeSwitchField = {
     label: 'Context compaction',
-    description: 'Summarize older turns as the context window fills.',
+    description: 'Summarize older turns as context fills.',
     checked: compactionEnabled,
-    update: enabled => ({
-      ...value,
-      contextManagement: {
-        ...value.contextManagement,
-        compaction: { ...value.contextManagement?.compaction, enabled },
-        largeToolResponse: value.contextManagement?.largeToolResponse ?? { enabled: true },
-      },
-    }),
+    update: enabled => withCompaction({ ...value.contextManagement?.compaction, enabled }),
   };
   const largeToolResponseField: RuntimeSwitchField = {
     label: 'Large tool response offloading',
@@ -131,7 +145,8 @@ export function AgentRuntimeConfigFields({
     }),
   };
   const runtimeFields = [sandboxField, fileDownloadsField, compactionField, largeToolResponseField];
-  const compactionThreshold = value.contextManagement?.compaction?.trigger?.value ?? 50_000;
+  const compactionThreshold =
+    value.contextManagement?.compaction?.trigger?.value ?? DEFAULT_COMPACTION_THRESHOLD_TOKENS;
 
   const switchField = ({
     field,
@@ -215,43 +230,59 @@ export function AgentRuntimeConfigFields({
         </section>
         <section className="py-4">
           {switchField({ field: compactionField, className: rowClassName })}
-          <label
+          <div
             className={cn(
               'mt-3 flex border-l-2 border-primary-button-bg/50 pl-3',
               rowClassName,
               compactionEnabled ? '' : 'opacity-50',
             )}
           >
-            <span className="min-w-0">
+            <span className="min-w-0 flex-1">
               <span className="text-text-primary block text-sm font-medium">Compaction threshold tokens</span>
               <span className="text-text-secondary mt-0.5 block text-xs leading-snug">
-                Input-token threshold that triggers compaction.
+                {thresholdMode === 'auto'
+                  ? "Automatically trigger compaction at 80% of model's context window"
+                  : `Trigger compaction when input reaches ${compactionThreshold.toLocaleString()} tokens`}
               </span>
             </span>
-            <input
-              type="number"
-              min={1}
-              disabled={disabled || !compactionEnabled}
-              value={compactionThreshold}
-              className={auiInputClass('h-8 w-28 shrink-0 disabled:opacity-60')}
-              onChange={event => {
-                const threshold = parsePositiveInteger(event.target.value);
-                if (threshold === null) return;
-                onChange({
-                  ...value,
-                  contextManagement: {
-                    ...value.contextManagement,
-                    compaction: {
-                      ...value.contextManagement?.compaction,
+            <div className="flex shrink-0 items-center gap-2">
+              {thresholdMode === 'custom' ? (
+                <input
+                  type="number"
+                  min={1}
+                  disabled={disabled || !compactionEnabled}
+                  value={compactionThreshold}
+                  aria-label="Compaction threshold tokens"
+                  className={auiInputClass('h-8 w-28 disabled:opacity-60')}
+                  onChange={event => {
+                    const threshold = parsePositiveInteger(event.target.value);
+                    if (threshold === null) return;
+                    applyCompaction({
                       enabled: compactionEnabled,
                       trigger: { type: 'input_tokens', value: threshold },
-                    },
-                    largeToolResponse: value.contextManagement?.largeToolResponse ?? { enabled: true },
-                  },
-                });
-              }}
-            />
-          </label>
+                    });
+                  }}
+                />
+              ) : null}
+              <PopoverSelect
+                aria-label="Compaction threshold mode"
+                value={thresholdMode}
+                options={COMPACTION_THRESHOLD_MODE_OPTIONS}
+                disabled={disabled || !compactionEnabled}
+                className="w-28"
+                onValueChange={mode =>
+                  applyCompaction(
+                    mode === 'custom'
+                      ? {
+                          enabled: compactionEnabled,
+                          trigger: { type: 'input_tokens', value: DEFAULT_COMPACTION_THRESHOLD_TOKENS },
+                        }
+                      : { enabled: compactionEnabled },
+                  )
+                }
+              />
+            </div>
+          </div>
         </section>
         {switchField({ field: largeToolResponseField, className: cn('py-4', rowClassName) })}
       </div>
diff --git a/packages/trueforge-ui/test/atoms/draft/AgentConfigEditors.test.tsx b/packages/trueforge-ui/test/atoms/draft/AgentConfigEditors.test.tsx
index 85bc53aff..70b7a3926 100644
--- a/packages/trueforge-ui/test/atoms/draft/AgentConfigEditors.test.tsx
+++ b/packages/trueforge-ui/test/atoms/draft/AgentConfigEditors.test.tsx
@@ -482,8 +482,10 @@ describe('AgentConfigEditors', () => {
 
     expect(screen.getByRole('switch', { name: 'File downloads' })).toBeDisabled();
     expect(screen.getByRole('switch', { name: 'File downloads' })).toHaveAttribute('aria-checked', 'false');
-    expect(screen.getByRole('spinbutton', { name: /Compaction threshold tokens/ })).toBeDisabled();
-    expect(screen.getByRole('spinbutton', { name: /Compaction threshold tokens/ })).toHaveValue(42_000);
+    expect(screen.getByRole('button', { name: 'Compaction threshold mode' })).toBeDisabled();
+    expect(screen.getByRole('button', { name: 'Compaction threshold mode' })).toHaveTextContent('Custom');
+    expect(screen.getByRole('spinbutton', { name: 'Compaction threshold tokens' })).toBeDisabled();
+    expect(screen.getByRole('spinbutton', { name: 'Compaction threshold tokens' })).toHaveValue(42_000);
 
     fireEvent.click(screen.getByRole('switch', { name: 'Context compaction' }));
     expect(onChange).toHaveBeenCalledWith({
@@ -501,6 +503,93 @@ describe('AgentConfigEditors', () => {
     });
   });
 
+  it('enables a custom compaction threshold with a 50000-token default', () => {
+    const onChange = vi.fn();
+    render(
+      <SlotsProvider>
+        <AgentConfigEditors
+          editor="runtime"
+          spec={{ model: { name: 'openai/gpt' } }}
+          models={[]}
+          connectors={[]}
+          skills={[]}
+          loading={false}
+          error={null}
+          sandboxAvailable
+          onChange={onChange}
+          onClose={vi.fn()}
+        />
+      </SlotsProvider>,
+    );
+
+    expect(screen.getByRole('button', { name: 'Compaction threshold mode' })).toHaveTextContent('Auto');
+    expect(screen.getByText("Automatically trigger compaction at 80% of model's context window")).toBeInTheDocument();
+    expect(screen.queryByRole('spinbutton', { name: 'Compaction threshold tokens' })).not.toBeInTheDocument();
+
+    fireEvent.click(screen.getByRole('button', { name: 'Compaction threshold mode' }));
+    fireEvent.click(screen.getByRole('option', { name: 'Custom' }));
+    expect(onChange).toHaveBeenCalledWith({
+      model: { name: 'openai/gpt' },
+      config: {
+        contextManagement: {
+          compaction: {
+            enabled: true,
+            trigger: { type: 'input_tokens', value: 50_000 },
+          },
+          largeToolResponse: { enabled: true },
+        },
+      },
+    });
+  });
+
+  it('clears the compaction trigger when the threshold mode is set to Auto', () => {
+    const spec: AgentSpec = {
+      model: { name: 'openai/gpt' },
+      config: {
+        contextManagement: {
+          compaction: {
+            enabled: true,
+            trigger: { type: 'input_tokens', value: 42_000 },
+          },
+          largeToolResponse: { enabled: true },
+        },
+      },
+    };
+    const onChange = vi.fn();
+    render(
+      <SlotsProvider>
+        <AgentConfigEditors
+          editor="runtime"
+          spec={spec}
+          models={[]}
+          connectors={[]}
+          skills={[]}
+          loading={false}
+          error={null}
+          sandboxAvailable
+          onChange={onChange}
+          onClose={vi.fn()}
+        />
+      </SlotsProvider>,
+    );
+
+    expect(screen.getByRole('spinbutton', { name: 'Compaction threshold tokens' })).toHaveValue(42_000);
+    expect(screen.getByText('Trigger compaction when input reaches 42,000 tokens')).toBeInTheDocument();
+
+    fireEvent.click(screen.getByRole('button', { name: 'Compaction threshold mode' }));
+    fireEvent.click(screen.getByRole('option', { name: 'Auto' }));
+    expect(onChange).toHaveBeenCalledWith({
+      ...spec,
+      config: {
+        ...spec.config,
+        contextManagement: {
+          compaction: { enabled: true },
+          largeToolResponse: { enabled: true },
+        },
+      },
+    });
+  });
+
   it('selects the first mounted MCP on open, otherwise the first connector', async () => {
     const loadMcpTools = vi.fn(async (connectorId: string) => [
       { id: `${connectorId}.tool`, name: `${connectorId}.tool` },