Debugging failed chains in LangChain - Step-by-Step Execution

Concept Flow - Debugging failed chains

Start Chain Execution

↓

Input to Chain

↓

Run Each Chain Step

↓

Error Occurs?

No→Return Output

Yes↓

Catch Error

↓

Log Error Details

↓

Fix Issue or Retry

↓

End or Restart Chain

This flow shows how a chain runs step-by-step, checks for errors, logs them, and then either fixes or retries the chain.

Execution Sample

LangChain

from langchain.chains import SequentialChain

chain = SequentialChain(...)
try:
    result = chain.run(input_data)
except Exception as e:
    print(f"Error: {e}")

This code runs a LangChain chain and catches errors to print them for debugging.

Execution Table

Step	Action	Evaluation	Result
1	Start chain with input_data	Input valid	Proceed to run steps
2	Run first step of chain	Step succeeds	Output produced
3	Run second step of chain	Step fails (e.g. API error)	Exception raised
4	Catch exception	Exception caught	Error message logged
5	Decide to retry or fix	User inspects error	Chain paused or restarted

💡 Execution stops because an exception was raised and caught during chain step

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Exception
input_data	user input	user input	user input	user input
step_output	None	valid output	Exception	Exception
error_message	None	None	API error message	API error message

Key Moments - 3 Insights

Why does the chain stop running after an error?

Because at step 3 in the execution_table, an exception is raised which interrupts normal flow and is caught in step 4.

How can I see what caused the failure?

The error_message variable in variable_tracker shows the exact error caught at step 4, helping identify the problem.

What should I do after catching an error?

As shown in step 5, you should inspect the logged error, fix the issue or retry the chain with corrected input or configuration.

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 3?

A The input data is modified

B The chain completes successfully

C An exception is raised due to a failure

D The chain restarts automatically

💡 Hint

Check the 'Evaluation' and 'Result' columns at step 3 in execution_table

According to variable_tracker, what is the value of error_message after step 2?

A None

B API error message

C Valid output

D User input

💡 Hint

Look at the error_message row under 'After Step 2' column in variable_tracker

If the error_message was empty after step 4, what would that mean?

A The input_data changed

B No error was caught

C The chain failed silently

D The chain output is invalid

💡 Hint

Refer to variable_tracker's error_message value after step 4 and what it represents

Concept Snapshot

Debugging failed chains in LangChain:
- Run chain with try-except to catch errors
- On error, log details for diagnosis
- Inspect error messages to find cause
- Fix issues or retry chain
- Helps keep chain stable and reliable

Full Transcript

This visual execution shows how a LangChain chain runs step-by-step. It starts with input data, runs each step, and checks for errors. If a step fails, an exception is raised and caught. The error message is logged for debugging. Then the user can fix the problem or retry the chain. Variables like input_data, step_output, and error_message change during execution and help track the chain's state. Key moments include understanding why the chain stops on error, how to see the error cause, and what to do next. The quiz questions test understanding of these steps and variable states.