GitHub Copilot cache-write tokens are omitted from session cost estimates #51845

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1. github-actions

   assigned

   neriousy

   on Sep 28, 2026
2. github-actions commented on Sep 28, 2026

   

   github-actionsBot

   on Sep 28, 2026 – with GitHub Actions

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   This issue might be a duplicate of existing issues. Please check:

   • #30706: Tracks GitHub Copilot cache-write pricing being dropped in the token-based cost fallback.
3. gr0maqny commented on Oct 27, 2026

   

   gr0maqny

   on Oct 27, 2026

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   I can relate to the issue. I am using haiku 5.5 xhigh on opencode V2 with github copilot credits and the amount in dollars in opencode is largely underreported compared to my quota emptying faster in the github copilot interface.
4. opencodejr commented on Oct 28, 2026

   

   opencodejrBot

   on Oct 28, 2026

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   Thanks for the detailed report and the numbers. I could reproduce this.

   The script below starts a mock Copilot /models endpoint that lists Claude Opus 5.5 with cache_write_price set to $5/M. It then loads the model the way OpenCode does and prices your first session's token counts. The model comes out with a cache-write price of 0, and the session estimate is $3.974241, exactly the figure you saw. Pricing the cache writes would give about $11.437866.

   The cause looks like the line you pointed to: in packages/core/src/github-copilot/models.ts the cache-write price is always set to zero, even though the Copilot model list includes a cache-write price. That code is the same in v2.0.18 and on the latest dev (86ce19d), so it still happens there. One caveat: the model list here is mocked, so I haven't confirmed what Copilot's live API sends for this model.

   Reproduction script (copilot-cache-write.ts)

   // Reproduces #51845: Copilot cache-write tokens priced at $0.
   // Run from an opencode checkout: bun /workspace/repro/copilot-cache-write.ts [/path/to/repo]
   const repo = process.argv[2] ?? "/workspace/repo"
   const { CopilotModels } = await import(`${repo}/packages/core/src/github-copilot/models.ts`)
   const { SessionUsage } = await import(`${repo}/packages/core/src/session/usage.ts`)
   
   // Mock Copilot /models: Claude Opus 5.5 at $4/M input, $0.20/M cache read, $5/M cache write, $20/M output
   // (prices in cents per batch_size tokens, same convention as packages/core/test/github-copilot/models.test.ts).
   const server = Bun.serve({
     port: 0,
     fetch: () =>
       Response.json({
         data: [{
           model_picker_enabled: true,
           id: "claude-opus-5.5",
           name: "Claude Opus 5.5",
           version: "claude-opus-5.5",
           supported_endpoints: ["/v1/messages"],
           billing: { token_prices: { batch_size: 1_000_000, default: {
             input_price: 400, output_price: 2000, cache_read_price: 20, cache_write_price: 500 } } },
           capabilities: { family: "claude-opus", limits: { max_output_tokens: 64000, max_prompt_tokens: 200000 },
             supports: { tool_calls: true } },
         }],
       }),
   })
   try {
     const models = await CopilotModels.get(server.url.origin, {}, [])
     const cost = [...models.values()][0].cost
     console.log("model cost:", JSON.stringify(cost[0]))
     const usage = { input: 162, output: 63_032, reasoning: 21_893, cache: { read: 11_375_466, write: 1_492_725 } }
     const got = SessionUsage.calculateCost(cost, usage)
     const expected = (162 * 4 + (63_032 + 21_893) * 20 + 11_375_466 * 0.2 + 1_492_725 * 5) / 1e6
     console.log("estimated cost:", got.toFixed(6), " expected:", expected.toFixed(6))
     if (cost[0].cache.write === 0) { console.log("BUG: cache_write_price advertised but cache.write priced at 0"); process.exitCode = 1 }
   } finally {
     server.stop(true)
   }

   Run it: bun copilot-cache-write.ts
5. opencodejr

   added

   reproduced

   on Oct 28, 2026
6. rekram1-node commented on Nov 10, 2026

   

   rekram1-node

   on Nov 10, 2026

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   Fixed in v2.2.7 by #62034. Session cost for GitHub Copilot now uses the amount Copilot reports it charged for each request (copilot_usage) instead of the catalog estimate, so cache writes and long-context pricing are included. If you still see a cost that doesn't match Copilot's billing on 2.2.7, let me know and I'll reopen.
7. rekram1-node

   closed this as completedon Nov 10, 2026