diff --git a/src/main/services/team/ChangeExtractorService.ts b/src/main/services/team/ChangeExtractorService.ts
index 26e53f0d..a28b758c 100644
--- a/src/main/services/team/ChangeExtractorService.ts
+++ b/src/main/services/team/ChangeExtractorService.ts
@@ -14,6 +14,7 @@ import * as os from 'os';
 import * as path from 'path';
 
 import { JsonTaskChangeSummaryCacheRepository } from './cache/JsonTaskChangeSummaryCacheRepository';
+import { OPEN_CODE_TASK_LEDGER_EVIDENCE_CONTRACT_VERSION } from './opencode/bridge/OpenCodeBridgeCommandContract';
 import {
   getOpenCodeLaneScopedRuntimeFilePath,
   getOpenCodeTeamRuntimeDirectory,
@@ -533,9 +534,25 @@ export class ChangeExtractorService {
             }
           : {}),
       });
+      const evidenceContractVersion =
+        typeof result.opencodeTaskLedgerEvidenceContractVersion === 'number' &&
+        Number.isInteger(result.opencodeTaskLedgerEvidenceContractVersion)
+          ? result.opencodeTaskLedgerEvidenceContractVersion
+          : 0;
+      const hasExpectedEvidenceContract =
+        evidenceContractVersion >= OPEN_CODE_TASK_LEDGER_EVIDENCE_CONTRACT_VERSION;
+      const diagnostics = hasExpectedEvidenceContract
+        ? (result.diagnostics ?? [])
+        : [
+            `OpenCode task ledger evidence contract is unsupported or missing: ${evidenceContractVersion}.`,
+            ...(result.diagnostics ?? []),
+          ];
       void appendOpenCodeTaskChangeDiag({
         event: 'backfill_result',
-        reason: this.classifyOpenCodeBackfillResult(result),
+        reason:
+          !hasExpectedEvidenceContract && result.importedEvents <= 0
+            ? 'unsupported-evidence-contract'
+            : this.classifyOpenCodeBackfillResult(result),
         teamName: input.teamName,
         taskId: input.taskId,
         displayId: input.taskMeta?.displayId ?? null,
@@ -546,6 +563,7 @@ export class ChangeExtractorService {
         deliveryRecordCount: deliveryContextRecords.length,
         deliveryContextFingerprint: deliveryContextPayload.hash,
         result: {
+          opencodeTaskLedgerEvidenceContractVersion: evidenceContractVersion,
           attributionMode: result.attributionMode ?? OPEN_CODE_AUTO_BACKFILL_ATTRIBUTION_MODE,
           outcome: result.outcome,
           dryRun: result.dryRun,
@@ -555,13 +573,14 @@ export class ChangeExtractorService {
           importedEvents: result.importedEvents,
           skippedEvents: result.skippedEvents,
         },
-        diagnostics: (result.diagnostics ?? []).slice(0, 25),
+        diagnostics: diagnostics.slice(0, 25),
         notices: (result.notices ?? []).slice(0, 25),
       }).catch(() => undefined);
       const backfilled =
         result.importedEvents > 0 ||
-        result.outcome === 'imported' ||
-        (result.outcome === 'duplicates-only' && result.candidateEvents > 0);
+        (hasExpectedEvidenceContract &&
+          (result.outcome === 'imported' ||
+            (result.outcome === 'duplicates-only' && result.candidateEvents > 0)));
 
       if (result.importedEvents > 0) {
         await this.invalidateTaskChangeSummaries(input.teamName, [input.taskId], {
@@ -569,7 +588,7 @@ export class ChangeExtractorService {
         });
       }
 
-      if (backfilled || deliveryContextRecords.length === 0) {
+      if ((hasExpectedEvidenceContract && backfilled) || deliveryContextRecords.length === 0) {
         this.openCodeBackfillCache.set(cacheKey, {
           backfilledAt: backfilled ? Date.now() : 0,
           expiresAt: Date.now() + this.openCodeBackfillCacheTtl,
@@ -578,9 +597,9 @@ export class ChangeExtractorService {
         this.openCodeBackfillCache.delete(cacheKey);
       }
 
-      if (result.diagnostics.length > 0 && result.outcome !== 'no-history') {
+      if (diagnostics.length > 0 && result.outcome !== 'no-history') {
         logger.debug(
-          `OpenCode ledger backfill for ${input.teamName}/${input.taskId}: ${result.outcome}; ${result.diagnostics.join('; ')}`
+          `OpenCode ledger backfill for ${input.teamName}/${input.taskId}: ${result.outcome}; ${diagnostics.join('; ')}`
         );
       }
       return { attempted: true, backfilled };
diff --git a/test/main/services/team/ChangeExtractorService.test.ts b/test/main/services/team/ChangeExtractorService.test.ts
index daaa5428..eb0535a3 100644
--- a/test/main/services/team/ChangeExtractorService.test.ts
+++ b/test/main/services/team/ChangeExtractorService.test.ts
@@ -1752,4 +1752,77 @@ describe('ChangeExtractorService', () => {
     );
     expect(backfillOpenCodeTaskLedger.mock.calls[1]?.[0]?.deliveryContextHash).toMatch(/^[a-f0-9]{64}$/);
   });
+
+  it('does not cache duplicates-only OpenCode backfill from an old evidence contract', async () => {
+    tmpDir = await fs.mkdtemp(path.join(os.tmpdir(), 'change-extractor-service-'));
+    setClaudeBasePathOverride(tmpDir);
+    await writeTaskFile(tmpDir, { displayId: 'abc12345', owner: 'bob' });
+    const projectDir = path.join(tmpDir, 'project-dir');
+    const projectPath = path.join(tmpDir, 'repo');
+    await fs.mkdir(projectDir, { recursive: true });
+    await fs.mkdir(projectPath, { recursive: true });
+    await writeOpenCodeDeliveryLedger(tmpDir);
+
+    const backfillOpenCodeTaskLedger = vi.fn(async (input: any) => ({
+      schemaVersion: 1,
+      providerId: 'opencode',
+      teamName: input.teamName,
+      taskId: input.taskId,
+      projectDir: input.projectDir,
+      workspaceRoot: input.workspaceRoot,
+      dryRun: false,
+      attributionMode: input.attributionMode,
+      scannedSessions: 1,
+      scannedToolparts: 1,
+      candidateEvents: 1,
+      importedEvents: 0,
+      skippedEvents: 1,
+      outcome: 'duplicates-only',
+      notices: [],
+      diagnostics: [],
+    }));
+    const workerClient = {
+      isAvailable: vi.fn(() => true),
+      computeTaskChanges: vi.fn(async () =>
+        makeTaskChangeResult(TASK_ID, { content: '', confidence: 'fallback' })
+      ),
+    };
+
+    const service = new ChangeExtractorService(
+      {
+        getLogSourceWatchContext: vi.fn(async () => ({
+          projectDir,
+          projectPath,
+          sessionIds: [],
+        })),
+        findLogFileRefsForTask: vi.fn(async () => []),
+        findMemberLogPaths: vi.fn(async () => []),
+      } as any,
+      {
+        parseBoundaries: vi.fn(async () => ({
+          boundaries: [],
+          scopes: [],
+          isSingleTaskSession: true,
+          detectedMechanism: 'none' as const,
+        })),
+      } as any,
+      { getConfig: vi.fn(async () => ({ projectPath })) } as any,
+      undefined,
+      workerClient as any,
+      { backfillOpenCodeTaskLedger } as any,
+      { getMeta: vi.fn(async () => ({ providerId: 'opencode' })) } as any
+    );
+
+    await service.getTaskChanges(TEAM_NAME, TASK_ID, {
+      owner: 'bob',
+      status: 'completed',
+    });
+    await service.getTaskChanges(TEAM_NAME, TASK_ID, {
+      owner: 'bob',
+      status: 'completed',
+    });
+
+    expect(backfillOpenCodeTaskLedger).toHaveBeenCalledTimes(2);
+    expect(workerClient.computeTaskChanges).toHaveBeenCalledTimes(2);
+  });
 });