LangChain — chain input type mismatch error
LangChain throws a type error because a chain step receives input that does not match its expected schema, often after composing incompatible chains.
Symptoms
- Error: Input of type X is not compatible with expected type Y
- The chain works in isolation but fails when composed
- TypeScript compiles but runtime chain execution fails
Possible causes
- A chain step expects a string but receives an object from the previous step
- The chain was composed without matching input and output schemas
- A RunnablePassthrough or RunnableLambda changes the shape unexpectedly
Diagnosis steps
Trace the chain input types
console.log(chain.inputSchema)Interpretation
LangChain exposes inputSchema and outputSchema for debugging type flow.
Test each step in isolation
await step1.invoke(testInput); await step2.invoke(step1Output)Interpretation
Identify which step produces the wrong output type.
Root cause
A chain step receives an input whose type does not match its expected schema, because the composing step produces a different output shape than the next step expects.
Fix
Add a RunnableLambda to transform the output between steps, use RunnablePassthrough.assign to merge fields, or redesign the chain to match input and output schemas.
Verify the fix
Invoke the full chain with a test input and confirm it completes without type errors; verify the output shape is correct.
Symptom walkthrough
LangChain throws a type error because a chain step receives input that does not match its expected schema, often after composing incompatible chains. The error appears in production but may not surface in local development because the conditions that trigger it depend on the deployment environment or runtime configuration. Understanding the full symptom set helps distinguish this issue from similar errors.
Diagnosis in practice
Follow the diagnosis steps in the frontmatter to isolate the cause. Each step includes a command and an interpretation of the output. The goal is to narrow down whether the issue is in the configuration, the runtime, or the application code. Reproduce the error in a minimal setup before applying a fix.
Root cause
A chain step receives an input whose type does not match its expected schema, because the composing step produces a different output shape than the next step expects.
Fix
Add a RunnableLambda to transform the output between steps, use RunnablePassthrough.assign to merge fields, or redesign the chain to match input and output schemas. After applying the fix, monitor the system to confirm the error does not recur under load or in different environments.
Verification
Invoke the full chain with a test input and confirm it completes without type errors; verify the output shape is correct. If the verification passes, the fix is confirmed; if not, re-examine the diagnosis steps for missed contributing factors.
Diagnostic commands
Run these commands to reproduce and diagnose the issue:
console.log(chain.inputSchema)
await step1.invoke(testInput); await step2.invoke(step1Output)