[Bug]: GPD Verification Fails with "Invalid value at 'tools0.function_declarations40...enum0' (TYPE_STRING), 1" Error

[GraveEaterMadison]

Description

Users are encountering an "Invalid value" error when gpd verification is enabled. The specific error message is:
Invalid value at 'tools[0].function_declarations[40].parameters.properties[0].value.properties[1].value.properties[0].value.enum[0]' (TYPE_STRING), 1

Steps to Reproduce

1. Enable gpd verification in opencode.

2. Perform an action that triggers GPD verification (exact action unknown, but typically involves code modifications, phase transitions, or specific GPD commands).

3. Observe the "Invalid value" error.

Expected Behavior

GPD verification should complete without type validation errors, correctly handling all parameter types, including enum values within the lock parameter of gpd-conventions_assert_convention_validate.

But The system throws an "Invalid value" error, specifically (TYPE_STRING), 1, indicating a type mismatch during schema validation, possibly with an enum field expecting a string but receiving an integer.

Runtime

opencode, specifically using the gemini-2.5-flash model (model ID: google/gemini-2.5-flash) but I am facing it with many ai models

Root cause found in src/gpd/mcp/servers/verification_server.py**

The const keyword (JSON Schema draft-6) is not supported by Gemini's OpenAPI 3.0 schema format. Three occurrences:

1. "must_surface": {"const": True} in the reference input schema — fix: "must_surface": {"enum": [True]}

2. "schema_version": {"type": "integer", "const": 1} in _CONTRACT_PAYLOAD_INPUT_SCHEMA — fix: "schema_version": {"type": "integer", "enum": [1]}

3. Same line in _compact_contract_payload_requirement_schema — same fix.

These schemas are used by the run_contract_check and suggest_contract_checks MCP tools in the verification server. When Gemini serializes them, const: 1 gets converted to enum: [1] but the type coercion breaks — the field becomes string-typed while the value 1 remains an integer, producing the (TYPE_STRING), 1 error.

Also latent: src/gpd/contracts.py has schema_version: Literal[1] = 1 in a Pydantic model (Pydantic generates const from Literal). Fix: schema_version: int = Field(default=1, json_schema_extra={"enum": [1]}).

GPD Version

gpd 1.2.2

Operating System

Arch Linux

Screenshots

[Sarfaraz85]

I hit this too and dug into the root cause — it's one level deeper than the const keyword, which matters for the fix.

Why {"type": "integer", "enum": [1]} won't fix it either: the error text gives it away —

Invalid value at 'tools[0].function_declarations[27].parameters.properties[1].value.properties[2].value.enum[0]' (TYPE_STRING), 1

The provider's schema proto types enum as a repeated string field, so any non-string enum member fails re-encoding with the same error. Swapping const: 1 for enum: [1] moves the failure from the const keyword to the enum member type.

Where it comes from: run_contract_check's published schema carries schema_version: {"type": "integer", "const": 1} plus must_surface: {"const": true} fragments inside its if/then surface rules — 10 const occurrences ship across the published schemas today, and the path in the error trace matches properties.contract.properties.schema_version exactly.

Fix approach (PR incoming): rather than spot-fixing each literal, sanitize once at the publish chokepoint (set_published_tool_input_schema), rewriting draft-only constructs into forms valid in both dialects while keeping exactness machine-checkable:

• string const → single-member enum (strings are fine in the restricted dialect)
• numeric const → equal minimum/maximum bounds (avoids the string-typed enum proto entirely)
• boolean const (only occurs inside if/then conditionals) → single-member enum; dialect-restricted clients strip conditional subtrees wholesale, so these never reach the provider proto
• anything unrepresentable → a "Must be exactly …" description note

Internal schemas stay draft-expressive; server-side argument validation remains the enforcement layer. Plus a repo-wide regression test that walks every FastMCP server's published tool schemas and fails on const or non-string enum members outside conditional subtrees — so the class stays fixed, not just these 10 instances.