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JSONDecodeError from truncated or incomplete API responses

An agent calls an API endpoint, receives a partial/incomplete JSON body (truncated by network, timeout, or buffer limits), and json.loads() fails with JSONDecodeError. ## Agent Decision Tree 1. Check if the response body ends mid-structure → go to Solution A 2. If the response is completely empty → go to Solution B 3. If using Python requests without stream=True → check Solution C

Symptoms

  • json.loads() or JSON.parse() throws a decode error
  • Response body looks valid but ends abruptly
  • Error only happens on large responses or slow connections

Error signatures

JSONDecodeError: Expecting value
JSONDecodeError: Unterminated string
JSONDecodeError: Expecting ',' delimiter
json.decoder.JSONDecodeError
Unterminated string starting at
Expecting property name enclosed in double quotes

Possible causes

  • Network timeout truncated the response mid-transfer
  • API server sent a partial response due to an internal error
  • Output buffer or log capture truncated the response body
  • Streaming response not fully consumed before parsing

Solutions

Solution A: Validate response completeness before parsing

risk: lowagentpending_review

Check Content-Length, response status, and body length before attempting to parse JSON.

  1. Check the HTTP status code — non-200 responses often have non-JSON bodies.
  2. Compare Content-Length header against actual body length.
  3. Verify the body starts with '{' or '[' and ends with '}' or ']'.
  4. If truncated, retry the request with a longer timeout.

Commands

curl -s -w '\n%{http_code} %{size_download}' <url>
python3 -c "import requests; r = requests.get('<url>', timeout=30); print(len(r.text), r.status_code)"

Verification

  • Fetch the same URL with curl and check the last character is } or ] → expect: valid JSON terminator
  • Re-run with timeout=60 → expect: successful parse
✓ 0 verified✕ 0 failed

Agent JSON

Canonical machine-readable representation of this issue:

{
  "issue_id": "794f7f72-f5a5-4ca6-9861-6dc8be4bff54",
  "slug": "jsondecodeerror-truncated-api-response",
  "verification_status": "unverified",
  "canonical_json": "https://codekb.dev/v1/issues/jsondecodeerror-truncated-api-response"
}
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JSONDecodeError from truncated or incomplete API responses · CodeKB