[RFC] Cost Tracking Architecture: Subagent Aggregation + Multi-Model Correctness

@fwang

Summary

OpenCode's cost tracking has multiple interrelated gaps that cause inaccurate cost display for users with multi-agent and multi-model workflows. This RFC proposes a unified architecture to address:

Subagent cost aggregation (Cost of session doesn't include cost of subagents #11027) - Parent sessions don't include child session costs
Multi-model correctness (Multi Model Cost Calculations are Bugged #7387) - Costs recalculated with wrong model pricing
Semantic clarity - Single cost field can't distinguish "own" vs "total"
Pricing immutability - Costs can be recalculated, causing decreases
Hidden costs (The token cost generated by session titles, message summary titles, and body summaries has not been accounted for. #7175) - Summary/title generation costs not tracked

Related Issues: #11027, #7387, #7175, #7767, #6989, #485
Related PRs: #7763 (necessary foundation, but insufficient alone)

Problem Statement

The 100-Message Bug (Being Fixed)

PR #7763 addresses the immediate bug where the sidebar only sums the last 100 messages in memory. This is a necessary fix, but not sufficient for accurate cost tracking.

The Subagent Problem (Critical)

When using Task tool to spawn subagents (explore, librarian, oracle, etc.), each creates a child session with parentID linking to the parent. However:

// task.ts - creates child session
const session = await Session.create({
  parentID: ctx.sessionID,  // Links to parent
  // ...
})

// session/index.ts - addCost only updates current session
export async function addCost(sessionID: string, amount: number) {
  await update(sessionID, (draft) => {
    draft.cost = (draft.cost ?? 0) + amount
  })
}
// Parent's cost is NEVER updated!

Result: Users see only their main session's cost, missing all subagent spend.

User Session: $0.50 (displayed)
  ├── @explore: $0.10 (hidden)
  ├── @librarian: $0.20 (hidden)
  └── @oracle: $0.30 (hidden)

Actual total: $1.10
Displayed: $0.50 (55% under-reported!)

The Multi-Model Problem

Issue #7387 reports costs DECREASING when switching models mid-session. Root cause:

// processor.ts - cost calculated with CURRENT model
const usage = Session.getUsage({
  model: input.model,  // Current model, not the model used for THAT call
  usage: value.usage,
  // ...
})

When switching from expensive (opus) to cheap (haiku) model, ALL previous costs are recalculated with the cheaper model's pricing.

Correct behavior: Each LLM call should lock its cost at call time with the actual model/pricing used.

Proposed Solution

Core Principle

"Append-only cost events as source-of-truth + denormalized rollups on sessions"

Schema Changes

// Session schema (enhanced)
interface Session {
  id: string
  parentID?: string
  // ...existing fields...
  
  // NEW: Split cost tracking
  own_cost_micros: number      // This session's direct LLM spend (immutable accumulator)
  total_cost_micros: number    // own + all descendants (cached rollup)
}

// NEW: Cost event record (optional but recommended for audit)
interface CostEvent {
  id: string
  session_id: string
  message_id?: string
  tool_call_id?: string
  
  // Immutable snapshot at call time
  provider_id: string
  model_id: string
  tokens: {
    input: number
    output: number
    cache_read?: number
    cache_write?: number
  }
  pricing_version: string      // Snapshot of rates at call time
  cost_micros: number          // Calculated cost in microdollars (never recalculated)
  
  created_at: number
}

Updated `addCost()` Function

export async function addCost(
  sessionID: string, 
  amount: number,
  metadata?: { model_id: string, provider_id: string, pricing_version: string }
) {
  if (amount === 0) return
  
  // 1. Update own_cost (this session only)
  const session = await update(sessionID, (draft) => {
    draft.own_cost_micros = (draft.own_cost_micros ?? 0) + amount
    draft.total_cost_micros = (draft.total_cost_micros ?? 0) + amount
  })
  
  // 2. Propagate to parent chain
  let currentParentID = session.parentID
  while (currentParentID) {
    const parent = await update(currentParentID, (draft) => {
      draft.total_cost_micros = (draft.total_cost_micros ?? 0) + amount
    })
    currentParentID = parent.parentID
  }
  
  // 3. Optionally emit CostEvent for audit trail
  if (metadata) {
    await CostEvent.create({
      session_id: sessionID,
      ...metadata,
      cost_micros: amount,
      created_at: Date.now()
    })
  }
}

Sidebar Display Changes

// sidebar.tsx
const cost = createMemo(() => {
  const session = session()
  const hasChildren = children().length > 0
  
  // Use total_cost for sessions with subagents, own_cost otherwise
  const amount = session.total_cost_micros ?? session.own_cost_micros ?? 
    messages().reduce((sum, x) => sum + (x.role === "assistant" ? x.cost : 0), 0)
  
  return {
    value: amount,
    label: hasChildren ? "(incl. subagents)" : "",
  }
})

Implementation Plan

Phase 1: Foundation (Builds on PR #7763)

Rename cost to own_cost_micros for semantic clarity
Add total_cost_micros field to Session schema
Add migration to initialize both fields from existing message sums
Update addCost() to propagate to parent chain

Effort: ~2-3 hours | Risk: Low (additive, backward compatible with fallback)

Phase 2: Multi-Model Correctness

Store model/provider per call in processor.ts
Pass metadata to addCost() for audit trail
Never recalculate - costs are immutable once recorded

Effort: ~2-3 hours | Risk: Low

Phase 3: Hidden Costs

Emit cost events for title/summary generation (The token cost generated by session titles, message summary titles, and body summaries has not been accounted for. #7175)
Track all LLM calls that don't go through normal message flow

Effort: ~1-2 hours | Risk: Low

Phase 4 (Optional): Full Audit Trail

Add CostEvent table for complete cost history
Add reconciliation command to verify session costs match events
Support cost breakdown by model in UI

Effort: ~4-6 hours | Risk: Medium (new storage requirements)

Migration Strategy

// migration.ts - run on startup after PR #7763
async function migrateCostFields() {
  const sessions = await Session.list()
  
  // Sort by depth (children first) for correct rollup
  const sorted = sortByDepth(sessions)
  
  for (const session of sorted) {
    // Calculate own_cost from message sums
    const ownCost = session.messages?.reduce(
      (sum, m) => sum + (m.role === "assistant" ? m.cost : 0), 0
    ) ?? session.cost ?? 0
    
    // Calculate total_cost = own + children's total
    const childrenTotal = sessions
      .filter(s => s.parentID === session.id)
      .reduce((sum, s) => sum + (s.total_cost_micros ?? 0), 0)
    
    await Session.update(session.id, (draft) => {
      draft.own_cost_micros = ownCost
      draft.total_cost_micros = ownCost + childrenTotal
    })
  }
}

UX Recommendations

Element	Display	Notes
Sidebar (has children)	`$1.10 (incl. subagents)`	Shows `total_cost`
Sidebar (no children)	`$0.50`	Shows `own_cost`
Optional toggle	"This session only" / "Include subagents"	User preference
Fork display	`$0.00` with lineage note	Don't inherit, show "Forked from [parent]"

Long-Term Vision

┌─────────────────────────────────────────────────────────────┐
│ Session: "Implement auth system"                            │
├─────────────────────────────────────────────────────────────┤
│ Cost: $2.47 (incl. subagents)          [This session only ▼]│
│                                                             │
│ Breakdown by model:                                         │
│   claude-opus-4-5: $1.80 (73%)                              │
│   gpt-5.2: $0.42 (17%)                                      │
│   claude-haiku-4-5: $0.25 (10%)                             │
│                                                             │
│ Subagent costs:                                             │
│   @librarian: $0.45                                         │
│   @explore (×3): $0.32                                      │
│   @oracle: $0.20                                            │
└─────────────────────────────────────────────────────────────┘

Breaking Changes

None. All changes are additive with backward-compatible fallbacks:

New fields default to 0 or calculated from existing data
Migration runs automatically on startup
Existing API contracts unchanged

Acceptance Criteria

Parent sessions display total cost including all descendant sessions
Costs never decrease (monotonically increasing)
Model switching mid-session doesn't affect previously calculated costs
Fork sessions start at $0.00 (no inherited cost)
Sidebar shows "(incl. subagents)" when children exist
Migration correctly backfills existing sessions

Questions for Maintainers

Naming preference: own_cost_micros / total_cost_micros or ownCost / totalCost?
Storage format: microdollars (integers) or dollars (floats)?
CostEvent table: Worth the storage overhead for audit trail?
PR scope: Single PR covering Phases 1-2, or separate PRs?

Happy to contribute a PR for Phase 1 once PR #7763 is merged. Would love feedback on this architecture proposal.

/cc @fwang @rekram1-node

This issue appears to be a comprehensive RFC that consolidates and proposes solutions for several existing issues. Please check these related issues first:

Cost of session doesn't include cost of subagents #11027: Cost of session doesn't include cost of subagents
Multi Model Cost Calculations are Bugged #7387: Multi Model Cost Calculations are Bugged
The token cost generated by session titles, message summary titles, and body summaries has not been accounted for. #7175: The token cost generated by session titles, message summary titles, and body summaries has not been accounted for
session-cost/spent-value under reported for sessions with 100+ messages #7767: session-cost/spent-value under reported for sessions with 100+ messages
[TUI] $ spent in context decreases after request #6989: [TUI] $ spent in context decreases after request
Cost is dropping dramatically when opencode is finishing up. #485: Cost is dropping dramatically when opencode is finishing up

This RFC effectively proposes a unified solution addressing the root causes described in all of these issues. The architectural approach should help resolve the symptoms reported across these multiple tickets.

Thanks for the cross-references! Yes, I've investigated all of these issues before filing this RFC:

Issue	Status	How This RFC Addresses It
#11027	No PR	Primary motivation - `own_cost` + `total_cost` with parent propagation
#7387	No PR	Phase 2 - Store model/pricing per-call, never recalculate
#7175	No PR	Phase 3 - Emit cost events for title/summary generation
#7767	PR #7763	Foundation - PR #7763 fixes 100-msg limit, this RFC builds on it
#6989	PR #7763	Same root cause as #7767
#485	Closed but unresolved	Related symptom - pricing immutability prevents cost drops

The RFC intentionally consolidates these because they share architectural root causes:

No persistence → session-cost/spent-value under reported for sessions with 100+ messages #7767, [TUI] $ spent in context decreases after request #6989 (fixed by PR fix: add persistent cost to prevent under-reporting spent value #7763)
No parent propagation → Cost of session doesn't include cost of subagents #11027 (Phase 1 of this RFC)
Mutable pricing → Multi Model Cost Calculations are Bugged #7387, Cost is dropping dramatically when opencode is finishing up. #485 (Phase 2 of this RFC)
Hidden LLM calls → The token cost generated by session titles, message summary titles, and body summaries has not been accounted for. #7175 (Phase 3 of this RFC)

Happy to update the RFC if I missed any related issues.

@R44VC0RP

Hi @R44VC0RP I noticed that you closed this aggregated cost tracking issue earlier today, may I know if this has been fixed somewhere? Thanks!

@IdrisGit

Following up - I believe this was closed prematurely. The underlying issues this RFC addresses are all still open:

Cost of session doesn't include cost of subagents #11027 (subagent costs not aggregated) - OPEN, no PR
Multi Model Cost Calculations are Bugged #7387 (multi-model cost bugs) - OPEN, no PR
The token cost generated by session titles, message summary titles, and body summaries has not been accounted for. #7175 (hidden title/summary costs) - OPEN, no PR
PR fix: add persistent cost to prevent under-reporting spent value #7763 (100-msg persistence fix) - OPEN, not yet merged

This RFC isn't a duplicate of those issues - it's an architecture proposal that unifies them with a concrete implementation plan. None of these issues have been fixed in recent commits.

@IdrisGit (PR #7763 author) has also expressed interest in collaborating on the subagent cost tracking piece after PR #7763 lands.

Could this be reopened? Happy to contribute a PR once the approach is agreed upon.

cc @thdxr @fwang

Any updates here? Can we have some clarity on whether this feature is part of the short-term roadmap or not :-)

@R44VC0RP

Why is this issue closed as compleated, when its not not compleated? Is Nowone wondering why the displayed costs and the costs in the provider like openai dont matche up? @R44VC0RP whats going on here??

@R44VC0RP

hey @R44VC0RP would you be OK to reopen this? the bug is still here, though kilocode fixed it a few days ago here: Kilo-Org/kilocode@70633f5

This is the same failure mode MartinLoop is trying to handle.

Once coding agents can run unattended it Reinforces the need for comprehensive audit trails that aggregate all child-process costs into a single record. The run needs:

a hard budget cap
a verifier gate
an explicit stop reason
rollback evidence
an inspectable run record

We open-sourced a small runtime around this pattern here:
https://github.com/Keesan12/Martin-Loop

Would value criticism from people running real Claude/Codex/OpenCode loops.

What a very bad surprise to realize that the session that supposedly consumes a few bucks consumed way much more
The cost indicator is very missleading furthermore it's not even real prices but estimated if i do not fumble

please reopen issue. as this is not addressed

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[RFC] Cost Tracking Architecture: Subagent Aggregation + Multi-Model Correctness #12377

Summary

Problem Statement

The 100-Message Bug (Being Fixed)

The Subagent Problem (Critical)

The Multi-Model Problem

Proposed Solution

Core Principle

Schema Changes

Updated `addCost()` Function

Sidebar Display Changes

Implementation Plan

Phase 1: Foundation (Builds on PR #7763)

Phase 2: Multi-Model Correctness

Phase 3: Hidden Costs

Phase 4 (Optional): Full Audit Trail

Migration Strategy

UX Recommendations

Long-Term Vision

Breaking Changes

Acceptance Criteria

Questions for Maintainers

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[RFC] Cost Tracking Architecture: Subagent Aggregation + Multi-Model Correctness #12377

Description

Summary

Problem Statement

The 100-Message Bug (Being Fixed)

The Subagent Problem (Critical)

The Multi-Model Problem

Proposed Solution

Core Principle

Schema Changes

Updated addCost() Function

Sidebar Display Changes

Implementation Plan

Phase 1: Foundation (Builds on PR #7763)

Phase 2: Multi-Model Correctness

Phase 3: Hidden Costs

Phase 4 (Optional): Full Audit Trail

Migration Strategy

UX Recommendations

Long-Term Vision

Breaking Changes

Acceptance Criteria

Questions for Maintainers

Activity

github-actions commented on Feb 5, 2026

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TheFpiasta commented on Apr 1, 2026

norman850 commented on Apr 3, 2026

Keesan12 commented on Apr 29, 2026

Andrflor commented on Aug 12, 2026

ramonskie commented on Aug 21, 2026

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Updated `addCost()` Function