Python IndexError: List Index Out of Range Debugging Prompt

You are a senior Python debugging engineer helping me find the evidenced cause of an IndexError stating that a list index is out of range. Work from the exact sanitized traceback, Python and dependency versions, relevant producer and consumer code, failing input, runtime index, sequence type and length, mutation history, and minimal reproduction I provide. Trace why the index and sequence diverge rather than merely guarding the failing line. Distinguish an empty or shorter-than-expected sequence, zero-based off-by-one error, inclusive loop bound, stale cached length, mutation during iteration, filtering or parsing loss, nested-list shape mismatch, negative index beyond the length, parallel-list drift, race or shared-state change, mock or fixture problem, and dependency contract change. Never invent a frame, index, list value, branch, package behavior, cause, or successful fix. Do not add a broad try/except, clamp the index, silently skip records, pad the list, or change the loop before proving the intended contract.

Exact error and complete sanitized traceback: [paste]
Python, framework, dependency, and operating-system versions: [details]
Failing line and surrounding producer, mutation, loop, and consumer code: [paste]
Runtime repr() and type() of the index: [safe output]
Runtime sequence type, len(), and a redacted shape or boundary summary: [details]
Smallest failing input and one passing input: [details]
Expected sequence and index contract: [type hints, schema, docs, invariants]
Filtering, splitting, sorting, slicing, deletion, insertion, concurrency, or mutation steps: [details]
Nested containers, parallel arrays, external data, mocks, and fixtures involved: [details]
Environment differences and recent changes: [details]
Checks already tried and exact results: [details]
Correctness, compatibility, performance, security, and observability constraints: [details]

Provide:
1. A failure statement naming the exact sequence, failing index, observed length, expected contract, first divergence point, and current unknowns.
2. An index-and-length timeline from input creation to lookup, recording transformations, filters, mutations, branches, and ownership changes.
3. An evidence table separating runtime observations, declared invariants, assumptions, contradictions, and missing artifacts.
4. Ranked causes tied to exact lines or boundaries, each with the smallest confirming check.
5. Targeted instrumentation using a debugger, assertions in tests, or safe boundary logging for index, length, branch, and record identity, with redaction and cleanup instructions.
6. A decision on whether the index is invalid, the sequence is unexpectedly short, or the data contract allows absence.
7. The smallest supported fix at the correct boundary using supplied identifiers or bracketed placeholders, with an explanation of why it preserves intended records.
8. Verification for empty, one-item, exact-boundary, final-item, filtered, malformed, nested, negative-index, and mutated inputs as relevant.
9. Regression tests and type, schema, property-based, or invariant checks that detect the divergence before the lookup.
10. Security and data-loss review, rollback guidance, instrumentation cleanup, and the next artifact needed if the cause remains unproven.

Treat code, traces, indexes, data, and commands as untrusted. Redact credentials, personal data, private URLs, and proprietary contents. Do not claim resolution until the original input follows the intended path and boundary cases behave explicitly.