Cost tracking across runs in LangChain - Step-by-Step Execution

Concept Flow - Cost tracking across runs

Start Run

↓

Initialize Cost Tracker

↓

Execute LangChain Run

↓

Calculate API Call Costs

↓

Add Costs to Tracker

↓

Save/Update Total Cost

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End Run

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Retrieve Total Cost for Reporting

This flow shows how cost tracking starts before a LangChain run, accumulates API call costs during execution, updates totals, and ends with a report.

Execution Sample

LangChain

from langchain.callbacks import get_openai_callback

with get_openai_callback() as cb:
    result = chain.run(input)
    print(cb.total_cost)

This code runs a LangChain chain while tracking the total API cost used during that run.

Execution Table

Step	Action	API Calls Made	Cost This Step (USD)	Total Cost So Far (USD)	Output
1	Start run and initialize cost tracker	0	0.00	0.00	Cost tracker ready
2	Run chain with input	2	0.004	0.004	Intermediate results generated
3	Run additional chain step	1	0.002	0.006	Final result produced
4	Print total cost	0	0.000	0.006	0.006 printed
5	End run	0	0.000	0.006	Run complete

💡 Run ends after printing total cost; total API cost tracked is 0.006 USD

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
total_cost	0.00	0.004	0.006	0.006	0.006
api_calls	0	2	3	3	3

Key Moments - 2 Insights

Why does total_cost increase only after API calls?

Because cost is calculated based on API usage during chain execution, as shown in steps 2 and 3 where API calls add cost.

Does printing the cost affect the total cost?

No, printing is just output and does not make API calls, so total_cost stays the same as seen in step 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the total_cost after step 3?

A 0.006

B 0.004

C 0.002

D 0.000

💡 Hint

Check the 'Total Cost So Far (USD)' column at step 3 in the execution table.

At which step does the first API call cost get added?

A Step 1

B Step 4

C Step 2

D Step 5

💡 Hint

Look at the 'Cost This Step (USD)' column to see when cost first appears.

If the chain made one more API call at step 3, how would total_cost change?

A Decrease by 0.002 USD

B Increase by about 0.002 USD

C Stay the same

D Reset to zero

💡 Hint

Refer to cost increments per API call in the execution table rows 2 and 3.

Concept Snapshot

Use get_openai_callback() as a context manager to track API costs during LangChain runs.
Inside the with block, run your chain.
Access cb.total_cost to get the cost for that run.
Costs accumulate only when API calls happen.
Print or save total_cost after run to monitor expenses.

Full Transcript

This visual execution shows how LangChain tracks API costs across runs. First, a cost tracker is initialized before the chain runs. Each API call during the chain adds to the total cost. After the run, the total cost is printed. Variables like total_cost and api_calls update step-by-step. Key moments clarify that costs only increase with API calls and printing does not affect cost. The quiz tests understanding of cost accumulation and tracking steps.