diff --git a/src/main/services/team/ChangeExtractorService.ts b/src/main/services/team/ChangeExtractorService.ts
index c615fa30..3d64e58d 100644
--- a/src/main/services/team/ChangeExtractorService.ts
+++ b/src/main/services/team/ChangeExtractorService.ts
@@ -1,4 +1,5 @@
 import { getTasksBasePath, getTeamsBasePath } from '@main/utils/pathDecoder';
+import { appendOpenCodeTaskChangeDiag } from '@main/utils/openCodeTaskChangeDiagLog';
 import { createLogger } from '@shared/utils/logger';
 import { resolveTaskChangePresenceFromResult } from '@shared/utils/taskChangePresence';
 import {
@@ -444,7 +445,8 @@ export class ChangeExtractorService {
       projectDir,
       workspaceRoot,
       cacheKey,
-      deliveryContextRecords
+      deliveryContextRecords,
+      sourceGeneration
     ).finally(() => {
       this.openCodeBackfillInFlight.delete(cacheKey);
     });
@@ -467,7 +469,8 @@ export class ChangeExtractorService {
     cacheKey: string,
     deliveryContextRecords: Awaited<
       ReturnType<ChangeExtractorService['readOpenCodeDeliveryContextRecords']>
-    >
+    >,
+    sourceGeneration: string | null
   ): Promise<boolean> {
     const deliveryContext = await this.createOpenCodeDeliveryContextTempFile(
       input.teamName,
@@ -486,6 +489,31 @@ export class ChangeExtractorService {
         attributionMode: OPEN_CODE_AUTO_BACKFILL_ATTRIBUTION_MODE,
         ...(deliveryContext.filePath ? { deliveryContextPath: deliveryContext.filePath } : {}),
       });
+      void appendOpenCodeTaskChangeDiag({
+        event: 'backfill_result',
+        reason: this.classifyOpenCodeBackfillResult(result),
+        teamName: input.teamName,
+        taskId: input.taskId,
+        displayId: input.taskMeta?.displayId ?? null,
+        memberName: input.effectiveOptions.owner ?? null,
+        projectDir,
+        workspaceRoot,
+        sourceGeneration,
+        deliveryRecordCount: deliveryContextRecords.length,
+        deliveryContextFingerprint: this.hashOpenCodeDeliveryContextRecords(deliveryContextRecords),
+        result: {
+          attributionMode: result.attributionMode ?? OPEN_CODE_AUTO_BACKFILL_ATTRIBUTION_MODE,
+          outcome: result.outcome,
+          dryRun: result.dryRun,
+          scannedSessions: result.scannedSessions,
+          scannedToolparts: result.scannedToolparts,
+          candidateEvents: result.candidateEvents,
+          importedEvents: result.importedEvents,
+          skippedEvents: result.skippedEvents,
+        },
+        diagnostics: (result.diagnostics ?? []).slice(0, 25),
+        notices: (result.notices ?? []).slice(0, 25),
+      }).catch(() => undefined);
       const backfilled =
         result.importedEvents > 0 ||
         result.outcome === 'imported' ||
@@ -516,6 +544,19 @@ export class ChangeExtractorService {
       logger.warn(
         `OpenCode ledger backfill failed for ${input.teamName}/${input.taskId}: ${error instanceof Error ? error.message : String(error)}`
       );
+      void appendOpenCodeTaskChangeDiag({
+        event: 'backfill_exception',
+        reason: 'exception',
+        teamName: input.teamName,
+        taskId: input.taskId,
+        displayId: input.taskMeta?.displayId ?? null,
+        memberName: input.effectiveOptions.owner ?? null,
+        projectDir,
+        workspaceRoot,
+        deliveryRecordCount: deliveryContextRecords.length,
+        deliveryContextFingerprint: this.hashOpenCodeDeliveryContextRecords(deliveryContextRecords),
+        error: error instanceof Error ? error.message : String(error),
+      }).catch(() => undefined);
       if (deliveryContextRecords.length === 0) {
         this.openCodeBackfillCache.set(cacheKey, {
           backfilledAt: 0,
@@ -530,6 +571,27 @@ export class ChangeExtractorService {
     }
   }
 
+  private classifyOpenCodeBackfillResult(
+    result: Awaited<ReturnType<OpenCodeLedgerBackfillPort['backfillOpenCodeTaskLedger']>>
+  ): string {
+    if (result.importedEvents > 0) {
+      return 'imported';
+    }
+    if (result.candidateEvents > 0 && result.outcome === 'duplicates-only') {
+      return 'duplicates-only';
+    }
+    if (result.scannedSessions > 0 && result.scannedToolparts > 0 && result.candidateEvents === 0) {
+      return 'no-write-edit-candidates';
+    }
+    if (result.outcome === 'no-attribution') {
+      return 'no-attribution';
+    }
+    if (result.outcome === 'no-history') {
+      return 'no-history';
+    }
+    return result.outcome;
+  }
+
   private async isOpenCodeTeamCandidate(teamName: string): Promise<boolean> {
     const cached = this.openCodeTeamEligibilityCache.get(teamName);
     const now = Date.now();
diff --git a/src/main/services/team/runtime/OpenCodeTeamRuntimeAdapter.ts b/src/main/services/team/runtime/OpenCodeTeamRuntimeAdapter.ts
index 1f7750ee..78b49714 100644
--- a/src/main/services/team/runtime/OpenCodeTeamRuntimeAdapter.ts
+++ b/src/main/services/team/runtime/OpenCodeTeamRuntimeAdapter.ts
@@ -636,9 +636,24 @@ function buildMemberBootstrapPrompt(
 function buildOpenCodeRuntimeMessageText(input: OpenCodeTeamRuntimeMessageInput): string {
   const replyRecipient = input.replyRecipient?.trim() || 'user';
   const taskRefs = input.taskRefs?.length ? JSON.stringify(input.taskRefs) : null;
+  const deliveryContext =
+    input.messageId && input.taskRefs?.length
+      ? JSON.stringify({
+          schemaVersion: 1,
+          kind: 'opencode-delivery-context',
+          teamName: input.teamName,
+          laneId: input.laneId,
+          memberName: input.memberName,
+          inboundMessageId: input.messageId,
+          taskRefs: input.taskRefs,
+        })
+      : null;
 
   return [
     '<opencode_app_message_delivery>',
+    deliveryContext
+      ? `<opencode_delivery_context>${deliveryContext}</opencode_delivery_context>`
+      : null,
     'You are running in OpenCode, not Claude Code or Codex native.',
     'To make your reply visible in the app Messages UI, call MCP tool agent-teams_message_send (or mcp__agent-teams__message_send if that is the exposed name).',
     `Use teamName="${input.teamName}", to="${replyRecipient}", from="${input.memberName}", text, and summary.`,
diff --git a/src/main/utils/openCodeTaskChangeDiagLog.ts b/src/main/utils/openCodeTaskChangeDiagLog.ts
new file mode 100644
index 00000000..063c90c9
--- /dev/null
+++ b/src/main/utils/openCodeTaskChangeDiagLog.ts
@@ -0,0 +1,90 @@
+/**
+ * Dev-only OpenCode task-change diagnostics.
+ *
+ * The normal logger is intentionally quiet in production. This file writes a
+ * bounded NDJSON trace under Electron's logs dir so local dev sessions can
+ * explain why OpenCode task changes were or were not backfilled.
+ */
+
+import { appendFile, mkdir, stat, truncate } from 'fs/promises';
+import { join } from 'path';
+
+const FILE_NAME = 'opencode-task-change-diag.ndjson';
+const MAX_DIAG_FILE_BYTES = 1024 * 1024;
+
+function getElectronApp(): typeof import('electron').app | null {
+  try {
+    // eslint-disable-next-line @typescript-eslint/no-require-imports -- lazy for tests / non-Electron.
+    const { app } = require('electron') as typeof import('electron');
+    return app ?? null;
+  } catch {
+    return null;
+  }
+}
+
+function isEnabled(): boolean {
+  if (process.env.CLAUDE_TEAM_OPENCODE_TASK_CHANGE_DIAG === '1') {
+    return true;
+  }
+  if (process.env.NODE_ENV === 'development' || process.env.NODE_ENV === 'test') {
+    return true;
+  }
+  const app = getElectronApp();
+  return app !== null && typeof app.isPackaged === 'boolean' && app.isPackaged === false;
+}
+
+function resolveLogsDirectory(): string | null {
+  const app = getElectronApp();
+  if (!app?.getPath) {
+    return null;
+  }
+  try {
+    return app.getPath('logs');
+  } catch {
+    try {
+      return join(app.getPath('userData'), 'logs');
+    } catch {
+      return null;
+    }
+  }
+}
+
+export async function appendOpenCodeTaskChangeDiag(
+  entry: Record<string, unknown>
+): Promise<string | null> {
+  if (!isEnabled()) {
+    return null;
+  }
+  const dir = resolveLogsDirectory();
+  if (!dir) {
+    return null;
+  }
+  const filePath = join(dir, FILE_NAME);
+  let line: string;
+  try {
+    line =
+      JSON.stringify({
+        t: new Date().toISOString(),
+        diagFile: filePath,
+        ...entry,
+      }) + '\n';
+  } catch {
+    return null;
+  }
+
+  try {
+    await mkdir(dir, { recursive: true });
+    try {
+      const st = await stat(filePath);
+      if (st.size > MAX_DIAG_FILE_BYTES) {
+        await truncate(filePath, 0);
+      }
+    } catch {
+      // Missing file is fine.
+    }
+    await appendFile(filePath, line, 'utf8');
+    return filePath;
+  } catch {
+    return null;
+  }
+}
diff --git a/test/main/services/team/OpenCodeTeamRuntimeAdapter.test.ts b/test/main/services/team/OpenCodeTeamRuntimeAdapter.test.ts
index aa0eb900..e6795585 100644
--- a/test/main/services/team/OpenCodeTeamRuntimeAdapter.test.ts
+++ b/test/main/services/team/OpenCodeTeamRuntimeAdapter.test.ts
@@ -361,6 +361,9 @@ describe('OpenCodeTeamRuntimeAdapter', () => {
     expect(sentText).toContain('Include source="runtime_delivery"');
     expect(sentText).toContain('Include relayOfMessageId="msg-1"');
     expect(sentText).toContain('Action mode for this message: delegate.');
+    expect(sentText).toContain('<opencode_delivery_context>');
+    expect(sentText).toContain('"kind":"opencode-delivery-context"');
+    expect(sentText).toContain('"inboundMessageId":"msg-1"');
     expect(sentText).toContain(
       'If your reply is about these tasks, include taskRefs exactly: [{"taskId":"task-1","displayId":"abcd1234","teamName":"team-a"}]'
     );