TOML — type mismatch error during parse

TOML parser throws a type mismatch because a value is assigned to a key that was previously declared with a different type, which TOML strictly prohibits.

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Symptoms

error: Key X has already been defined with a different type
TOML parse fails on the second assignment of a key
The file works after removing the conflicting line

Possible causes

A key is assigned as a string then later as an array or table
A dotted key redefines a parent table with a different type
A table header conflicts with a previously defined inline table

Diagnosis steps

Find the conflicting key assignments

grep -n "key_name" config.toml

Interpretation

Multiple assignments with different types indicate the conflict.

Validate the TOML file

toml-validator config.toml

Interpretation

The validator names the key and the conflicting types.

Root cause

TOML enforces strict typing; a key declared with one type cannot be redeclared with a different type, and the parser rejects the file.

Fix

Use a single type for each key, rename conflicting keys, or restructure the table to avoid type redeclaration.

Verify the fix

Parse the TOML file and confirm no errors; verify the parsed config has the expected values.

Symptom walkthrough

TOML parser throws a type mismatch because a value is assigned to a key that was previously declared with a different type, which TOML strictly prohibits. 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

TOML enforces strict typing; a key declared with one type cannot be redeclared with a different type, and the parser rejects the file.

Fix

Use a single type for each key, rename conflicting keys, or restructure the table to avoid type redeclaration. After applying the fix, monitor the system to confirm the error does not recur under load or in different environments.

Verification

Parse the TOML file and confirm no errors; verify the parsed config has the expected values. 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:

grep -n "key_name" config.toml
toml-validator config.toml