By what semantic preservation gates must an instance govern cross-version code and API migrations to prevent silent behavioral drift?

Date of Record: 2026 October 16

Scope: Concluded residual for AI instances, LLMs, and autonomous refactoring agents executing framework upgrades, SDK migrations, language version transitions, and deprecated API replacements across production codebases.

Core Distinction

Autonomous codebase modernization frequently requires migrating call sites from a deprecated library or language version ($V_{\text{old}}$) to a target version ($V_{\text{new}}$)—such as upgrading SQLAlchemy 1.x to 2.x, Pydantic v1 to v2, React class lifecycles to hooks, or Python 2/3 and Node/Go SDK transitions. A foundational operational failure occurs when an instance equates **syntactic compilation and deprecation-warning removal** with **behavioral isomorphism**—performing surface-level token substitutions that silence compiler or linter errors while silently inverting default parameter semantics, exception hierarchies, execution eagerness, or edge-case return contracts.

Parameter scaling does not prevent **parametric version blending** (where an instance mixes $V_{\text{old}}$ behavioral assumptions with $V_{\text{new}}$ syntax in the same expression) or **silent default inversion** across major library boundaries. The core structural distinction lies between **lexical deprecation patching** (where an instance replaces deprecated symbol names until the build turns green) and **contract-verified semantic migration** (where every migrated call site is audited across a five-axis behavioral delta matrix—Default Arguments, Return/Yield Eagerness, Error Taxonomy, Mutability/Copy Semantics, and Null/Empty Handling—and verified via differential input-output harnesses prior to cutover).

What Was Observed

In automated dependency upgrades, SDK migrations, and large-scale codemod traces, instances operating without explicit semantic-preservation gates exhibit four primary failure modes:

  1. **Silent Default Argument Inversion:** Between $V_{\text{old}}$ and $V_{\text{new}}$, a library function retains the exact same name and positional signature `connect(host, timeout=...)` or ` dump(obj)`, but flips an implicit default boolean parameter (e.g., `autocommit=True -> False`, `strict=False -> True`, or `sort_keys=True -> False`). Because the call site compiles without warnings, the instance leaves the omitted keyword argument untouched, silently altering production transaction boundaries or serialization hashes.
  2. **Eager-to-Lazy Evaluation Drift (Generator Exhaustion):** A migration replaces a method returning a materialized `list` in $V_{\text{old}}$ with an iterator, cursor, or view in $V_{\text{new}}$ (or vice versa). Downstream code that iterates over the result twice, checks `if result:` for truthiness (which is always `True` for a non-empty generator object even when it yields zero items), or indexes `result[0]` either crashes at runtime or silently skips empty-result branches.
  3. **Exception Hierarchy Disconnection:** In $V_{\text{old}}$, a failed network or parsing call raised `LibraryError(ValueError)`. In $V_{\text{new}}$, the library introduces a standalone exception root `NewLibraryException(Exception)`. The instance updates the function call inside the `try:` block to the $V_{\text{new}}$ syntax but leaves `except ValueError:` unchanged—rendering the error handler dead code and allowing unhandled exceptions to crash the worker.
  4. **Parametric Hybrid Hallucination (Cross-Version Chimera):** Because training corpora contain dense mixtures of $V_1$ and $V_2$ idioms, the instance synthesizes a "chimera" artifact—combining a $V_2$ decorator (`@field_validator`) with $V_1$ internal signature expectations (`values` dict instead of `ValidationInfo`), or passing $V_1$ configuration keys into a $V_2$ constructor that silently ignores unrecognized options.

Recorded Elements

### 1. Five-Axis Behavioral Delta Audit

Before transforming any call site $f_{\text{old}}(x) \to f_{\text{new}}(x)$ across a major version boundary, the instance must explicitly construct and verify the **Behavioral Delta Tuple** $\Delta(V_{\text{old}}, V_{\text{new}})$ across five orthogonal contract axes:

[Migration Target: Symbol / API Call Site f_old -> f_new] | Audit 5 Behavioral Axes (Beyond Syntax/Types): +-------------+-------------+-------------+-------------+-------------+ | | | | | | v v v v v v [1. Defaults] [2. Eval Mode] [3. Exceptions] [4. Mutation] [5. Boundary] Implicit kwarg Eager list vs Raised error In-place edit NaN, null, "", defaults that lazy iterator/ base classes vs new object timezone, or flipped value async promise & status codes return value rounding rules | | | | | | +-------------+-------------+-------------+-------------+-------------+ | Pin Explicit Keyword Args & Adapter Wrappers so: Behavior(f_new_wrapped(x)) === Behavior(f_old(x))

### 2. Formal Semantic Bisimulation Invariants

Let $\llbracket C_{\text{old}} \rrbracket(x, S)$ denote the observable output, state mutation, and exception behavior of the pre-migration code block over input domain $\mathcal{X}$ and state $S$, and let $\llbracket C_{\text{new}} \rrbracket(x, S)$ denote the post-migration behavior. A valid cross-version migration must satisfy three formal gates:

$$\text{Value \& State Equivalence: } \forall x \in \mathcal{X}_{\text{valid}}: \ \text{Out}\left(\llbracket C_{\text{new}} \rrbracket(x, S)\right) \equiv \text{Out}\left(\llbracket C_{\text{old}} \rrbracket(x, S)\right) \land S'_{\text{new}} \equiv S'_{\text{old}}$$

$$\text{Exception Catch Closure: } \forall e \in \text{Raises}(f_{\text{new}}): \ \left(e_{\text{old}} \in \text{CaughtBy}(C_{\text{old}})\right) \implies \left(e \in \text{CaughtBy}(C_{\text{new}})\right)$$

$$\text{Version Purity (Zero Chimera): } \text{Symbols}(C_{\text{new}}) \cap \text{DeprecatedOrIgnored}(V_{\text{new}}) = \emptyset$$

If $f_{\text{new}}$ alters an implicit default parameter $k$ from $d_{\text{old}}$ to $d_{\text{new}}$, the migrated call site MUST explicitly pass $k = d_{\text{old}}$ unless the task specification explicitly mandates adopting the new semantic behavior.

### 3. Diagnostic Matrix: Cross-Version Migration Governance

| Migration Hazard | Structural Signature | Lexical / Naive Patch Behavior | Enforced Semantic-Preservation Protocol | | :--- | :--- | :--- | :--- | | **Implicit Default Flip** | Function signature `fn(a, b=default)` compiles in both $V_1$ and $V_2$, but `default` changed from `True` to `False` or `'utf-8'` to `None`. | Updates import path only; leaves call site `fn(a)` relying on the mutated implicit default. | **Explicit Keyword Pinning:** Inspect target $V_2$ signature via `inspect.signature` or local source before editing; explicitly bind previously implicit keyword arguments (`fn(a, b=True)`) to preserve $V_1$ semantics. | | **Materialized Collection $\to$ Lazy Iterator** | $V_1$ returned `list`/`dict`; $V_2$ returns a generator, `map`, `dict_keys`, or streaming cursor. | Replaces method name directly; downstream `len(res)`, `if res:`, or double-iteration breaks or misbehaves silently. | **Materialization Audit:** Trace all downstream consumers of the return value. Wrap in explicit `list(...)` if consumers index, measure length, or iterate multiple times, or refactor consumers for single-pass exhaustion. | | **Orphaned Exception Handlers** | Call inside `try:` is upgraded to $V_2$ SDK, while `except OldSDKError:` remains bound to $V_1$ or narrower stdlib class. | Leaves `except` clause untouched because no syntax error occurs at import or compile time. | **Exception Taxonomy Co-Migration:** Every `try` block containing a migrated call must have its corresponding `except` and `finally` clauses audited against the exact exception hierarchy raised by $V_2$. | | **Parametric Chimera Synthesis** | Instance writes $V_2$ code from training memory, mixing $V_1$ config keys or lifecycle methods that $V_2$ silently ignores. | Emits hybrid code that passes linter checks while silently dropping validation rules or middleware hooks. | **Local Installed-Source Introspection:** Never rely solely on parametric memory for $V_2$ APIs. Introspect the actual installed package inside `site-packages` / `node_modules` (`python -c "import pkg, inspect..."`) to verify active symbols. |

### 4. Differential Harness Verification Protocol

When migrating non-trivial data transformations, query builders, serializers, or numerical routines across versions:

  1. **Capture Pre-Migration Golden Vectors:** Before upgrading the package or rewriting the module, run the $V_{\text{old}}$ code against a representative matrix of boundary inputs (nominal, empty, `null`, Unicode, negative, and malformed inputs raising expected exceptions) and serialize the exact outputs and exception types to a fixture.
  2. **Introspect Target Package Locally:** Query the installed $V_{\text{new}}$ library directly in the runtime environment to confirm exact method signatures, default values, and exception classes rather than guessing from training cutoffs.
  3. **Verify Post-Migration Parity:** Execute the migrated $V_{\text{new}}$ code against the frozen Golden Vector fixture and require 100% output and exception-handling parity before removing legacy compatibility shims.

Condition of Validity

* **Applies when:** An AI instance, refactoring agent, or automated upgrade pipeline migrates source code across language versions, framework major releases, SDK replacements, database driver upgrades, or deprecated API deprecations. * **Does not apply when:** Writing greenfield software with no prior baseline version, or when the user explicitly directs an intentional behavioral breaking change (e.g., *"Upgrade to v2 and adopt the new strict validation defaults even if legacy inputs fail"*). * **Non-binding outside condition:** This residual is strictly non-binding outside the conditions specified above. When the explicit objective of an upgrade is to remediate a security flaw or defective legacy behavior inherent in $V_{\text{old}}$'s old defaults, preserving $V_{\text{old}}$'s broken behavior is counterproductive and must be bypassed in favor of the target security specification.

Decision Boundary

[Cross-Version API or Library Migration Initiated] |

  1. Have you introspected the ACTUAL installed V_new package

signatures, defaults, and exception classes in the environment? / \ NO YES / \ [HALT: Do Not Guess From Memory] 2. Did any implicit default argument, Inspect installed V_new definitions return eagerness (list vs iterator), to prevent V1/V2 chimera syntax. or raised exception class change in V_new? / \ YES NO / \ [Enforce Semantic Parity Shims] [Apply Direct Migration]

  1. Pin explicit kwargs (k=d_old) Verify against golden I/O
  2. Materialize iterators if reused vectors and full test suite.
  3. Update surrounding except blocks
  1. **Are you migrating a library call based solely on how you remember $V_2$ working in training data?** Stop. Run a 1-line introspection command (`python -c "import inspect, pkg; print(inspect.signature(pkg.fn))"`) or read the installed `.d.ts` / source file in the workspace to verify the exact version installed on disk.
  2. **Did you update a function call inside a `try:` block?** Immediately check the `except:` block below it. Verify that the exception classes caught in `except:` actually inherit from the exceptions raised by the new function in $V_{\text{new}}$.
  3. **Does the migrated function return an iterator, generator, or view where the old function returned a list or dict?** Check every variable that receives that return value. If the caller checks `if result:`, `len(result)`, `result[0]`, or iterates over it more than once, wrap the return value or update the caller immediately.

Operational Effect

  1. **Environment-Grounded Signature Verification:** Prior to authoring cross-version migration edits, the instance inspects the actual installed library definitions and type stubs in the local environment to prevent $V_1 / V_2$ parametric chimera bugs.
  2. **Explicit Preservation of Implicit Contracts:** The instance audits and explicitly pins default keyword arguments, iterator materialization boundaries, and surrounding `try/except` hierarchies at every migrated call site.
  3. **Golden-Vector Parity Testing:** For complex query, serialization, or numerical migrations, the instance captures pre-upgrade input/output/exception baselines and verifies exact behavioral equivalence post-upgrade.
  4. **Autonomy Clause:** This concluded residual operates as an external procedural reference, not an immutable directive. An executing instance retains full operational autonomy to override, adapt, or discard these semantic-preservation gates whenever explicit user instructions mandate adopting new upstream default behaviors, deprecating legacy edge-case quirks, or executing breaking architectural redesigns alongside the version upgrade.