fix(cost): snapshot estimatedCostUsd instead of accumulating (#69347)

[MrMiaigi]

The bug: three persist sites accumulated cost instead of snapshotting it like tokens. This caused cost to be inflated 1x-72x on multi-persist sessions because the same cumulative usage was added repeatedly.

Root cause: persistSessionUsageUpdate, updateSessionStoreAfterAgentRun, and the cron isolated-agent run path all used:
estimatedCostUsd = existingCost + runCost

But runCost was already computed from cumulative run usage, so this added the same cost repeatedly on redundant persists.

Fix: snapshot cost directly like tokens already do:
estimatedCostUsd = runCost

Files affected:

• src/auto-reply/reply/session-usage.ts
• src/agents/command/session-store.ts
• src/cron/isolated-agent/run.ts

Tests added:

• session-store.test.ts: verify cost is snapshotted, not accumulated
• session.test.ts: updated existing test to verify snapshot behavior

Fixes #69347

[greptile-apps]

Greptile Summary

This PR correctly fixes a cost-inflation bug where estimatedCostUsd was accumulated across every persist call instead of being snapshotted from the already-cumulative run usage. The fix is applied consistently across all three affected persist sites (session-usage.ts, session-store.ts, cron/isolated-agent/run.ts) and is well-tested.

• P1 (syntax): session.test.ts line 2417 has a malformed comment / Before fix: (missing a /), which is a bare division expression and will cause a parse error in the test file.

Confidence Score: 4/5

Core fix is correct and well-tested, but the test file has a syntax error that will prevent it from compiling.

The production fix is solid. However, the malformed comment on line 2417 of session.test.ts (/ Before fix:) is a division expression, not a comment, and will cause a parse/compile error in the test suite.

src/auto-reply/reply/session.test.ts (line 2417 — malformed comment will break compilation)

Comments Outside Diff (1)

1. src/auto-reply/reply/session-usage.ts, line 135 (link)

   Dead variable left from the old accumulation logic

   existingEstimatedCostUsd is computed but never referenced after the fix — it was previously used in existingCost + runCost. It can be safely removed.

   Prompt To Fix With AI

   This is a comment left during a code review.
   Path: src/auto-reply/reply/session-usage.ts
   Line: 135
   
   Comment:
   **Dead variable left from the old accumulation logic**
   
   `existingEstimatedCostUsd` is computed but never referenced after the fix — it was previously used in `existingCost + runCost`. It can be safely removed.
   
   
   
   How can I resolve this? If you propose a fix, please make it concise.

Prompt To Fix All With AI

This is a comment left during a code review.
Path: src/auto-reply/reply/session-usage.ts
Line: 135

Comment:
**Dead variable left from the old accumulation logic**

`existingEstimatedCostUsd` is computed but never referenced after the fix — it was previously used in `existingCost + runCost`. It can be safely removed.

```suggestion
```

How can I resolve this? If you propose a fix, please make it concise.

---

This is a comment left during a code review.
Path: src/auto-reply/reply/session.test.ts
Line: 2417

Comment:
**Malformed comment (missing leading slash)**

The line starts with `/ Before fix:` instead of `// Before fix:`, making it a division expression rather than a comment. This will cause a syntax/parse error.

```suggestion
    // Before fix: cost would accumulate to $0.0155 (2x)
```

How can I resolve this? If you propose a fix, please make it concise.

_{Reviews (1): Last reviewed commit: "fix(cost): snapshot estimatedCostUsd ins..." | Re-trigger Greptile}

[greptile-apps]

Malformed comment (missing leading slash)

The line starts with / Before fix: instead of // Before fix:, making it a division expression rather than a comment. This will cause a syntax/parse error.

Suggested change

	// Before fix: cost would accumulate to $0.0155 (2x)
	// Before fix: cost would accumulate to $0.0155 (2x)

Prompt To Fix With AI

This is a comment left during a code review.
Path: src/auto-reply/reply/session.test.ts
Line: 2417

Comment:
**Malformed comment (missing leading slash)**

The line starts with `/ Before fix:` instead of `// Before fix:`, making it a division expression rather than a comment. This will cause a syntax/parse error.

```suggestion
    // Before fix: cost would accumulate to $0.0155 (2x)
```

How can I resolve this? If you propose a fix, please make it concise.

[steipete]

Maintainer pass: root cause and production fix look right.

I pushed 8df7230ed7 to fix the red checks:

• removed the now-unused existingEstimatedCostUsd local after switching to snapshot semantics
• fixed the new session-store.test.ts usage-format mock to use a typed provider models[] shape instead of unsafe property access

Local verification on the PR branch:

pnpm check:changed
pnpm test src/agents/command/session-store.test.ts src/auto-reply/reply/session.test.ts

Both pass locally. Waiting on GitHub CI for the updated branch.

[steipete]

Landed via temp rebase onto main.

• Gate: pnpm check; pnpm test; pnpm build; post-rebase pnpm check:changed
• Land commit: c2ddf75
• Merge commit: 9efd2d1

Thanks @MrMiaigi!