Python: --version AUTO leaves 0.0.0-fern-placeholder in the generated User-Agent header #18206

Component

SDKs

Priority

P1 - High (Strongly needed)

SDK Language(s)

Python

Bug Description

With --version AUTO (autoversion) on the hosted GitHub mode: pull-request path, the Python generator emits two different placeholder strings into src/<pkg>/core/client_wrapper.py:

"User-Agent": "phenoml/0.0.0-fern-placeholder",
"X-Fern-SDK-Version": "0.0.0.dev0",

AutoVersionStep then rewrites only the Python-mapped placeholder (0.0.0.dev0), so the [fern-autoversion] commit stamps the real version into pyproject.toml, .fern/metadata.json, and X-Fern-SDK-Version, but User-Agent keeps the literal placeholder. Replay is not involved; the PR reports no conflicts.

Steps to reproduce

1. Python SDK group with github: { mode: pull-request }, autorelease, replay enabled (our config: fernapi/fern-python-sdk with client_class_name and wire_tests.enabled, no custom user-agent template, no global User-Agent header).
2. Trigger a generation from a spec change.
3. Inspect the [fern-generated] commit on the opened PR: User-Agent is phenoml/0.0.0-fern-placeholder while X-Fern-SDK-Version is 0.0.0.dev0.
4. Inspect the [fern-autoversion] commit: X-Fern-SDK-Version becomes e.g. 18.2.0, User-Agent is still phenoml/0.0.0-fern-placeholder.

Public example: PhenoML/phenoml-python-sdk#257, commits f554c20 ([fern-generated]) and f8de428 ([fern-autoversion]). It has recurred on every Python regeneration for us since at least 2026-08. Release 16.11.0 was published to PyPI on 2026-08-26 with User-Agent: phenoml/0.0.0-fern-placeholder before we noticed (https://github.com/phenoml/phenoml-python-sdk/blob/16.11.0/src/phenoml/core/client_wrapper.py). TypeScript and Java from the same runs are correct, since their mapped and canonical placeholders are the same string.

Expected

Every version-bearing string in the generated SDK, including User-Agent, is rewritten to the computed version.

Actual

User-Agent ships as phenoml/0.0.0-fern-placeholder unless hand-edited on the PR branch before merge.

Where it comes from (reading main)

• packages/cli/generation/local-generation/local-workspace-runner/src/runGenerator.ts builds the IR with the unmapped placeholder: effectiveIrVersion = isAutoVersion(irVersion) ? MAGIC_VERSION : irVersion (around L148-150), and only later maps the version it hands to the generator config: mappedVersion = mapMagicVersionForLanguage(version, generatorLanguage) (around L227, used at L365).
• packages/cli/generation/ir-generator/src/generateIntermediateRepresentation.ts derives platformHeaders.userAgent from that IR version (${packageName}/${version}, around L629-630).
• The Python generator (generators/python/src/fern_python/generators/sdk/core_utilities/client_wrapper_generator.py) writes User-Agent from ir.sdk_config.platform_headers.user_agent and X-Fern-SDK-Version from the project config's package_version, so the two placeholders diverge for Python only.
• runLocalGenerationForWorkspace.ts already handles this: it passes mapMagicVersionForLanguage(MAGIC_VERSION, irLanguage) as the IR version. runGenerator.ts looks like it just needs the same mapping (compute generatorLanguage before building the IR and use mapMagicVersionForLanguage(MAGIC_VERSION, generatorLanguage) for effectiveIrVersion).

Environment

• Fern-hosted generation triggered from the backend repo's CI check (.fern/metadata.json: invokedBy: "ci", cliVersion: "5.148.3"), GitHub App push, Linux runners; not reproduced locally.
• Related history: fix(cli): map magic version to PEP 440-compatible format for Python generators #13773 introduced the 0.0.0.dev0 mapping for Python; fix(cli): prevent --version AUTO from corrupting generated SDK code in remote generation path #16727 / fix(cli): prevent --version AUTO from leaking literal "AUTO" into self-hosted SDK publish config #16960 touched effectiveIrVersion on this path.

Versions

Fern CLI version: 5.143.4 in fern.config.json; hosted run reports cliVersion 5.148.3 in .fern/metadata.json
SDK Generator versions:

• fernapi/fern-python-sdk 5.34.2 (also seen on 5.3x releases since 2026-08)
• fernapi/fern-typescript-sdk 3.98.3 (unaffected)
• fernapi/fern-java-sdk 4.23.2 (unaffected)

Workaround

On each Python regeneration PR, edit src/phenoml/core/client_wrapper.py by hand to set "User-Agent": "phenoml/<new-version>" before merging. CI does not catch it, so it is a manual pre-merge check in our release runbook.

Are you interested in contributing a fix?

No