fix(#80): unify cost computation — single source of truth

src/renderer/utils/sessionAnalyzer.ts

@@ -298,8 +298,8 @@ export function analyzeSession(detail: SessionDetail): SessionReport {
   // Test progression
   const testSnapshots: TestSnapshot[] = [];
 
-  // Cost tracking
-  let parentCost = 0;
+  // Cost tracking — use the pre-computed cost from calculateMetrics() as single source of truth
+  const parentCost = detail.metrics.costUsd ?? 0;
 
   // Git activity
   const gitCommits: GitCommit[] = [];
@@ -402,7 +402,6 @@ export function analyzeSession(detail: SessionDetail): SessionReport {
 
       const callCost = calculateMessageCost(model, inpTok, outTok, cr, cc);
       stats.costUsd += callCost;
-      parentCost += callCost;
 
       totalCacheCreation += cc;
       totalCacheRead += cr;
@@ -823,14 +822,7 @@ export function analyzeSession(detail: SessionDetail): SessionReport {
     const subagentModel =
       proc.messages.find((m: ParsedMessage) => m.type === 'assistant' && m.model)?.model ??
       'default (inherits parent)';
-    // Compute cost from subagent token breakdown (proc.metrics.costUsd is not populated upstream)
-    const computedCost = calculateMessageCost(
-      subagentModel,
-      proc.metrics.inputTokens,
-      proc.metrics.outputTokens,
-      proc.metrics.cacheReadTokens,
-      proc.metrics.cacheCreationTokens
-    );
+    const computedCost = proc.metrics.costUsd ?? 0;
     return {
       description: desc,
       subagentType: proc.subagentType ?? 'unknown',

test/renderer/utils/sessionAnalyzer.test.ts

@@ -173,6 +173,7 @@ describe('analyzeSession', () => {
         createMockDetail({
           messages,
           session: createMockSession({ messageCount: 4 }),
+          metrics: createMockMetrics({ costUsd: 0.025 }),
         })
       );
 
@@ -188,9 +189,9 @@ describe('analyzeSession', () => {
       expect(report.tokenUsage.totals.cacheCreation).toBe(100);
       expect(report.tokenUsage.totals.grandTotal).toBe(4000);
 
-      // Cost should be positive (sonnet-4 pricing)
-      expect(report.costAnalysis.parentCostUsd).toBeGreaterThan(0);
-      expect(report.costAnalysis.totalSessionCostUsd).toBeGreaterThan(0);
+      // Cost reads from detail.metrics.costUsd (single source of truth)
+      expect(report.costAnalysis.parentCostUsd).toBe(0.025);
+      expect(report.costAnalysis.totalSessionCostUsd).toBe(0.025);
 
       // Message types
       expect(report.messageTypes.user).toBe(2);
@@ -1506,4 +1507,84 @@ describe('analyzeSession', () => {
       expect(report.subagentMetrics.byAgent[0].modelMismatch).toBeNull();
     });
   });
+
+  // -------------------------------------------------------------------------
+  // Unified cost — single source of truth
+  // -------------------------------------------------------------------------
+  describe('unified cost computation', () => {
+    it('parentCostUsd reads from detail.metrics.costUsd', () => {
+      const report = analyzeSession(
+        createMockDetail({
+          metrics: createMockMetrics({ costUsd: 1.2345 }),
+        })
+      );
+      expect(report.costAnalysis.parentCostUsd).toBe(1.2345);
+    });
+
+    it('parentCostUsd defaults to 0 when detail.metrics.costUsd is undefined', () => {
+      const report = analyzeSession(createMockDetail());
+      expect(report.costAnalysis.parentCostUsd).toBe(0);
+    });
+
+    it('subagent cost reads from proc.metrics.costUsd', () => {
+      const processes: Process[] = [
+        {
+          id: 'agent-1',
+          filePath: '/path/to/agent-1.jsonl',
+          messages: [],
+          startTime: new Date('2024-01-01T10:00:00Z'),
+          endTime: new Date('2024-01-01T10:01:00Z'),
+          durationMs: 60000,
+          metrics: createMockMetrics({ totalTokens: 5000, costUsd: 0.15 }),
+          description: 'research task',
+          subagentType: 'explore',
+          isParallel: false,
+        },
+        {
+          id: 'agent-2',
+          filePath: '/path/to/agent-2.jsonl',
+          messages: [],
+          startTime: new Date('2024-01-01T10:01:00Z'),
+          endTime: new Date('2024-01-01T10:02:00Z'),
+          durationMs: 60000,
+          metrics: createMockMetrics({ totalTokens: 3000, costUsd: 0.10 }),
+          description: 'test runner',
+          subagentType: 'code',
+          isParallel: false,
+        },
+      ];
+
+      const report = analyzeSession(createMockDetail({ processes }));
+      expect(report.subagentMetrics.byAgent[0].costUsd).toBe(0.15);
+      expect(report.subagentMetrics.byAgent[1].costUsd).toBe(0.10);
+      expect(report.costAnalysis.subagentCostUsd).toBe(0.25);
+    });
+
+    it('totalSessionCostUsd equals parent + subagent costs', () => {
+      const processes: Process[] = [
+        {
+          id: 'agent-1',
+          filePath: '/path/to/agent-1.jsonl',
+          messages: [],
+          startTime: new Date('2024-01-01T10:00:00Z'),
+          endTime: new Date('2024-01-01T10:01:00Z'),
+          durationMs: 60000,
+          metrics: createMockMetrics({ totalTokens: 5000, costUsd: 0.30 }),
+          description: 'task',
+          subagentType: 'code',
+          isParallel: false,
+        },
+      ];
+
+      const report = analyzeSession(
+        createMockDetail({
+          metrics: createMockMetrics({ costUsd: 0.50 }),
+          processes,
+        })
+      );
+      expect(report.costAnalysis.parentCostUsd).toBe(0.50);
+      expect(report.costAnalysis.subagentCostUsd).toBe(0.30);
+      expect(report.costAnalysis.totalSessionCostUsd).toBe(0.80);
+    });
+  });
 });