Python codegen: quicktype mangles long type names

[SteveSandersonMS]

Problem

The Python codegen (via quicktype) produces mangled/abbreviated type names for types with long names in the runtime API schema. For example:

• PermissionDecisionApprovedForIonApproval instead of PermissionDecisionApprovedForSessionApproval
• PermissionDecisionApprovedForEntApproval instead of PermissionDecisionApprovedForPersistentApproval

This only affects Python — Go and C# codegen produce the correct full names from the same schema.

Root Cause

quicktype appears to truncate or abbreviate type names that exceed a certain length when generating Python code. The exact mechanism is unclear — it may be a Python-specific name shortening heuristic in quicktype, or a configurable option we're not setting.

Impact

The generated Python types don't match the runtime API schema names. This means we're shipping public APIs that have bad names we'll want to change later, which will be a breaking change. So we should resolve this soon, certainly before GA.

Possible Fixes

1. Investigate quicktype Python options to disable name mangling/abbreviation
2. Add a post-processing step to the Python codegen to fix known mangled names
3. Consider an alternative Python code generator if quicktype can't be configured

Overall I've really had enough of quicktype and its unfortunate behaviors. Since we've built a pretty decent standalone codegenerator for C#, I wonder if we can factor out some sort of language-independent core from that (e.g., building an intermediate representation of all the types and methods we want to emit), and then add per-language renderers on top of that.

[github-actions]

Investigation Summary

I investigated this issue by examining the codegen pipeline and the generated Python code.

What I looked at

• scripts/codegen/python.ts — the Python RPC code generator, which calls quicktype with rendererOptions: { "python-version": "3.7" } and no other name-related options
• python/copilot/generated/rpc.py — the generated output for RPC types
• python/copilot/generated/session_events.py — generated output for session events (uses a separate custom generator)
• scripts/codegen/go.ts and the Go/C# generated files — for comparison

What I found

This is a genuine bug in the Python codegen pipeline. Key observations:

1. The problem is real and well-characterized. The Python RPC codegen passes quicktype with minimal renderer options ("python-version": "3.7" only), with no controls on acronym style or name abbreviation. Quicktype's Python renderer is known to apply word-splitting heuristics when generating identifiers from long schema type names, which can produce abbreviated/mangled names.

2. The session_events codegen already works around this. There is already a PY_SESSION_EVENT_TYPE_RENAMES map (lines 495–536 of python.ts) that post-processes quicktype-generated names into correct ones. This workaround exists precisely because quicktype generates unpredictable names from long schema definitions. The RPC codegen has no equivalent protection.

3. The currently committed generated code doesn't yet show the mangled names, because the schema types PermissionDecisionApprovedForSessionApproval / PermissionDecisionApprovedForPersistentApproval aren't present in the current installed schema version. But the bug will manifest when the schema is updated to include these types — and the existing workaround approach confirms the repo maintainers are aware this is a pattern that needs handling.

4. Go and C# codegen are not affected, because Go uses quicktype's Go renderer (which handles names differently), and C# uses a fully custom generator.

Root cause

The Python RPC codegen uses quicktype without any options to control its name generation behavior. Quicktype splits PascalCase identifier segments using a word-recognition heuristic; for words it can't fully identify (like "Session" → split as "Ses"+"sion" → "ion", or "Persistent" → "ent"), it may produce truncated output segments.

Label applied

✅ bug — The codegen is producing incorrect output that doesn't match the schema definition names. The fix should add either a rename map (like PY_SESSION_EVENT_TYPE_RENAMES) for known RPC type name corrections, or investigate quicktype options to disable word-abbreviation behavior (e.g., controlling acronymStyle).

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