fix(prompt_templates): do not mutate messages in-place in function_call_prompt

[mgeorgaklis]

Summary

TLDR: Shallow copies in message validation cause token inflation through duplicate tool definitions across multi-turn conversations

function_call_prompt(messages, functions) in litellm/litellm_core_utils/prompt_templates/factory.py previously mutated the caller's messages list, system message dictionary, or nested content list in-place

Because validate_and_fix_openai_messages(messages) performs a shallow copy of message dictionaries ({**m}), nested content lists remain shared with the caller's conversation history. In multi-turn tool-calling loops that use fallback tool prompting (such as Ollama tool calling or when add_function_to_prompt=True), each turn appended another duplicate copy of all tool definitions to the caller's retained system message, causing Turn N to send N copies of the tool schema prompt

Changes

This change updates function_call_prompt to construct and return fresh message dictionaries and content lists without mutating the caller's input objects in-place. It also adds parametrized multi-turn unit tests (test_function_call_prompt_does_not_inflate_multi_turn_history) built with immutable functional reductions and TypedDict bindings to verify that caller history is preserved and tool schemas are injected once per turn

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge, with the prior mutation-rule finding fully fixed and no new actionable defects identified

Summary

This PR prevents fallback function prompting from mutating retained conversation history and repeatedly inflating tool definitions

• Builds fresh system-message dictionaries and content lists
• Preserves the caller’s original messages and function definitions
• Adds multi-turn regression coverage for validated and unvalidated messages
• Refactors the test to avoid local list mutation

_{Reviews (2) · Last reviewed commit: "style(prompt_templates): eliminate based..."}

[codspeed]

Merging this PR will not alter performance

✅ 31 untouched benchmarks

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_{Comparing mgeorgaklis:fix/function-call-prompt-mutation (64b9daa) with main (356b8d4)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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