diff --git a/packages/opencode-plugin/README.md b/packages/opencode-plugin/README.md
index e369687d..05ff1d62 100644
--- a/packages/opencode-plugin/README.md
+++ b/packages/opencode-plugin/README.md
@@ -90,6 +90,18 @@ When the env var is set, workflow hooks are skipped and tools throw a clear erro
 
 **When unset**, workflows are active for all agents (default behavior).
 
+### Auto-Compaction
+
+When transitioning to a new phase via `proceed_to_phase`, the plugin automatically triggers a session compaction (summarize) to clear prior-phase context from the LLM window. This is enabled by default.
+
+Set `WORKFLOW_AUTO_COMPACT=false` to disable this behavior:
+
+```bash
+WORKFLOW_AUTO_COMPACT=false npx opencode
+```
+
+**When unset or any value other than `false`**, compaction runs on every successful phase transition (default behavior).
+
 ### Per-Agent Behavior
 
 - **Agent in filter**: Workflow instructions are injected on every message, tools work normally
diff --git a/packages/opencode-plugin/src/tool-handlers/proceed-to-phase.ts b/packages/opencode-plugin/src/tool-handlers/proceed-to-phase.ts
index e1beb1c1..3a9a0b43 100644
--- a/packages/opencode-plugin/src/tool-handlers/proceed-to-phase.ts
+++ b/packages/opencode-plugin/src/tool-handlers/proceed-to-phase.ts
@@ -80,22 +80,33 @@ export function createProceedToPhaseTool(
         });
 
         // Trigger compaction to clear prior-phase context from the LLM window.
+        // Skipped when WORKFLOW_AUTO_COMPACT=false; default is enabled.
         // Fire-and-forget: a failed compaction must never block the phase transition.
         // The summarize API requires providerID + modelID; we use the last-known
         // model from the chat.message hook (cached in the plugin closure).
-        const model = getModel();
-        client.session
-          .summarize({
-            path: { id: context.sessionID },
-            ...(model ? { body: model } : {}),
-          })
-          .catch(() => {});
+        const autoCompact = process.env['WORKFLOW_AUTO_COMPACT']
+          ?.trim()
+          ?.toLowerCase();
+        if (autoCompact !== 'false') {
+          const model = getModel();
+          client.session
+            .summarize({
+              path: { id: context.sessionID },
+              ...(model ? { body: model } : {}),
+            })
+            .catch(() => {});
 
-        logger.info('Triggered compaction after phase transition', {
-          phase: data.phase,
-          sessionID: context.sessionID,
-          hasModel: !!model,
-        });
+          logger.info('Triggered compaction after phase transition', {
+            phase: data.phase,
+            sessionID: context.sessionID,
+            hasModel: !!model,
+          });
+        } else {
+          logger.debug('Skipped compaction: WORKFLOW_AUTO_COMPACT=false', {
+            phase: data.phase,
+            sessionID: context.sessionID,
+          });
+        }
 
         // Build response with instructions (strip whats_next references)
         const lines: string[] = [];
diff --git a/packages/opencode-plugin/test/e2e/plugin.test.ts b/packages/opencode-plugin/test/e2e/plugin.test.ts
index 610fc99b..b8b816f8 100644
--- a/packages/opencode-plugin/test/e2e/plugin.test.ts
+++ b/packages/opencode-plugin/test/e2e/plugin.test.ts
@@ -12,11 +12,14 @@ import { tmpdir } from 'node:os';
 import { WorkflowsPlugin } from '../../src/plugin.js';
 import type { PluginInput, Hooks, Part, UserMessage } from '../../src/types.js';
 
-// Ensure WORKFLOW_AGENTS is unset for the baseline test suite.
-// Tests that need it set/restored manage it themselves in try/finally blocks.
+// Ensure WORKFLOW_AGENTS and WORKFLOW_AUTO_COMPACT are unset for the baseline
+// test suite. Per-test overrides can set them as needed, and shared cleanup below
+// restores the original process environment after each test.
 const _savedWorkflowAgents = process.env.WORKFLOW_AGENTS;
+const _savedWorkflowAutoCompact = process.env.WORKFLOW_AUTO_COMPACT;
 beforeEach(() => {
   delete process.env.WORKFLOW_AGENTS;
+  delete process.env.WORKFLOW_AUTO_COMPACT;
 });
 afterEach(() => {
   if (_savedWorkflowAgents === undefined) {
@@ -24,6 +27,11 @@ afterEach(() => {
   } else {
     process.env.WORKFLOW_AGENTS = _savedWorkflowAgents;
   }
+  if (_savedWorkflowAutoCompact === undefined) {
+    delete process.env.WORKFLOW_AUTO_COMPACT;
+  } else {
+    process.env.WORKFLOW_AUTO_COMPACT = _savedWorkflowAutoCompact;
+  }
 });
 
 // Test utilities
@@ -668,6 +676,52 @@ describe('OpenCode Workflows Plugin E2E', () => {
       expect(summarizeMock).not.toHaveBeenCalled();
     });
 
+    it('does not trigger compaction when WORKFLOW_AUTO_COMPACT=false', async () => {
+      process.env['WORKFLOW_AUTO_COMPACT'] = 'false';
+
+      await setupWorkflowState(testDir, {
+        workflowName: 'epcc',
+        currentPhase: 'explore',
+      });
+
+      hooks = await WorkflowsPlugin(mockInput);
+
+      await hooks['chat.message']!(
+        {
+          sessionID: 'test-session-789',
+          model: {
+            providerID: 'github-copilot',
+            modelID: 'claude-sonnet-4.6',
+          },
+        },
+        {
+          message: {
+            id: 'msg-1',
+            sessionID: 'test-session-789',
+            role: 'user',
+          },
+          parts: [],
+        }
+      );
+
+      const sessionID = 'test-session-789';
+      const proceedResult = await hooks.tool!.proceed_to_phase.execute(
+        { target_phase: 'plan', reason: 'exploration complete' },
+        { sessionID } as never
+      );
+
+      // Verify the transition itself succeeded before checking compaction was skipped
+      expect(JSON.stringify(proceedResult)).toContain('plan');
+
+      // session.summarize should NOT have been called
+      const summarizeMock = (
+        mockInput.client as {
+          session: { summarize: ReturnType<typeof vi.fn> };
+        }
+      ).session.summarize;
+      expect(summarizeMock).not.toHaveBeenCalled();
+    });
+
     it('fails when no workflow is active', async () => {
       hooks = await WorkflowsPlugin(mockInput);