docs(design): FastMCP native registration investigation (#209)#250
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Recommendation: stay with generation layer. FastMCP native path cannot produce the flat tool-arg surface AdCP requires without rebuilding the generation logic at registration time. Writeup documents the wire-shape mismatch, behavior that would need reimplementation, and re-open triggers. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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KonstantinMirin
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May 13, 2026
Upstream spec added a cluster of breaking changes — most Asset classes
went from 'discriminator is implicit' to 'asset_type is required'.
Pydantic honors 'required' literally, breaking ergonomic construction:
# Before upstream change: TextContent(content='hello')
# After upstream change, pre-fix:
TextContent(asset_type='text', content='hello') # must repeat tag
Fix: inject_literal_discriminator_defaults() in post_generate_fixes.py.
Pattern-matches AnnAssign nodes whose annotation is Literal['x'] (bare
or Annotated-wrapped) with no default, appends ` = 'x'` on the same
line. Touched 687 fields across 615 classes in 105 files — Asset
types, pricing options, webhook types, catalog types, all spec
discriminator fields.
Wire consumption unchanged. Literal type still rejects other values,
so validation strength preserved. Wire dicts with the tag present
validate as before; dicts without the tag now also work (fall through
to default) — useful for minimal clients.
Pattern-based, not value-specific: robust to spec-value churn as long
as the single-value-Literal discriminator shape holds.
Also pulls upstream PRs adcontextprotocol#250 (design doc) and adcontextprotocol#251 (A2A contextId,
+22 integration tests). All green.
+16 new tests in tests/test_literal_discriminator_defaults.py. Covers
Asset defaults, VAST/DAAST dual-discriminator defaults, validation-
strength preservation, wire-format compatibility, injector meta-tests
(skips multi-value literals, extracts single-value, skips non-Literal).
2102 passing (was 2086), mypy clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Summary
@tool()with a pydantic-typed param nests args under the param name; AdCP needs request fields flat at the tool-argument level. No unwrap option in FastMCP.docs/design/mcp-native-registration.md— covers wire-shape mismatch, behavior that would need reimplementation (_inline_refs,additionalProperties: true, MRO-walk filtering, per-instance handler binding), startup-cost comparison, and re-open triggers.ADCP_TOOL_DEFINITIONS+_tool_to_requestinto one declarative table, and dropping unused hand-craftedinputSchemastubs.Closes #209.
Test plan
🤖 Generated with Claude Code