Incorrect session cost estimate for models with context tiered pricing

Description

Opencode's pricing estimate is incorrect when a model with with tiered context pricing and your session goes into the higher tier. The local cost estimates use the price of smaller context for the entire session, resulting in a cost estimate that is lower than what it should be.

For example, the session that I discovered this showed $3.37 in opencode's UI, but the real price was $3.71. The longer the session, the wider the discrepancy.

Testing with grok 4.6, which has different pricing for > 200k, the model calls reported in Zen differed from what is stored in the local sqlite database.

Plugins

No response

OpenCode version

1.18.18

Steps to reproduce

For testing with Zen:
Use a model that has tiered context pricing, such as grok 4.6 or one of the gpt 5.6 models. Have a session go into the higher tiered pricing. Compare local session pricing vs Zen's UI.

   WITH ranked_calls AS (
     SELECT
       ROW_NUMBER() OVER (ORDER BY time_created, id) AS call_number,
       time_created,
       data
     FROM message
     WHERE session_id = 'ses_ff50d39fbffe2mOJcly5gtxQyW'
       AND json_extract(data, '$.role') = 'assistant'
       AND (
         COALESCE(json_extract(data, '$.tokens.input'), 0) +
         COALESCE(json_extract(data, '$.tokens.cache.read'), 0) +
         COALESCE(json_extract(data, '$.tokens.cache.write'), 0) +
         COALESCE(json_extract(data, '$.tokens.output'), 0) +
         COALESCE(json_extract(data, '$.tokens.reasoning'), 0)
       ) > 0
   )
   SELECT
     call_number,
     strftime('%b %d, %H:%M', time_created / 1000, 'unixepoch', 'localtime') AS date,
     json_extract(data, '$.modelID') AS model,
     COALESCE(json_extract(data, '$.tokens.input'), 0) +
       COALESCE(json_extract(data, '$.tokens.cache.read'), 0) +
       COALESCE(json_extract(data, '$.tokens.cache.write'), 0) AS input,
     printf('$%.4f', COALESCE(json_extract(data, '$.cost'), 0)) AS cost,
     substr('ses_ff50d39fbffe2mOJcly5gtxQyW', -8) AS session
   FROM ranked_calls
   WHERE call_number >= (
     SELECT MAX(call_number) - 9 FROM ranked_calls
   )
   ORDER BY call_number DESC;

Screenshot and/or share link

Zen pricing:

Local sqlite pricing:

Operating System

No response

Terminal

No response

No source file or test is named. Start by reproducing the issue with grok 4.6 or a GPT 5.6 model, then use the supplied SQLite query to compare stored call costs with Zen's pricing. Done means sessions crossing a context-pricing tier produce local estimates that match the applicable tiered costs.