fix: boolean false values dropped in form submissions by jlowin · Pull Request #3776 · PrefectHQ/fastmcp

jlowin · 2026-04-07T01:33:10Z

HTML checkboxes don't send a value when unchecked, so Prefab UI forms omit boolean fields entirely from the submitted data. This caused submit_form to fail with a "Missing required argument" error even when the boolean field had a Pydantic default.

Two fixes:

In form.py, the submit_form tool now backfills missing boolean fields with their model defaults (or False) before Pydantic validation. The data parameter also defaults to None so the tool doesn't reject an empty argument dict outright.

In apps_dev.py, the dev server's form handler now parses "true"/"false" strings into actual booleans instead of dropping them during the empty-string filter.

class NoteForm(BaseModel):
    title: str
    content: str
    archived: bool = False  # unchecked checkbox now correctly submits as False

mcp.add_provider(FormInput(model=NoteForm))

Closes #3769

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chatgpt-codex-connector · 2026-04-07T01:35:39Z

+                    if stripped.lower() in ("true", "false"):
+                        v = stripped.lower() == "true"


Avoid coercing every "true"/"false" string to bool

In api_launch form mode, this unconditional conversion changes any user-entered "true"/"false" string into a boolean before tool validation, even when the target parameter is typed as str. That creates a regression for string inputs (for example a str field set to "false" now arrives as False and fails Pydantic string validation), so boolean parsing should be gated by the argument schema/type instead of applied globally.

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chatgpt-codex-connector · 2026-04-07T01:35:39Z

+            if field_info.default is not pydantic.fields.PydanticUndefined:
+                data[name] = field_info.default
+            else:
+                data[name] = False


Respect default_factory when backfilling boolean defaults

_backfill_boolean_defaults says it backfills from model defaults, but this branch only checks field_info.default; for boolean fields declared with Field(default_factory=...), default is PydanticUndefined, so missing input is forced to False instead of using the configured factory value. That silently overrides model behavior for omitted checkbox fields and can invert defaults in production forms.

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marvin-context-protocol · 2026-04-07T01:39:29Z

Test Failure Analysis

Summary: One test timed out on Windows Python 3.10 — test_run_mcp_config in tests/cli/test_run.py — but this failure is unrelated to the PR's changes.

Root Cause: The failing test spawns a Python subprocess via stdio transport (an MCP proxy server), which can be slow to start on Windows. The unit test timeout is 5 seconds, which is occasionally not enough for subprocess startup + FastMCP import on Windows CI runners. All other platforms (Ubuntu Python 3.10 and 3.13) passed.

Suggested Solution: This appears to be pre-existing Windows flakiness in test_run_mcp_config, not caused by this PR. The PR only touches form.py, apps_dev.py, and test_form.py — none of which are in the code path exercised by the failing test. A re-run of the workflow should pass.

Note: This comment has been updated after re-analysis of workflow run 24086992468 — same failure, same root cause.

Detailed Analysis

Failing test: tests/cli/test_run.py::TestMCPConfig::test_run_mcp_config on Tests: Python 3.10 on windows-latest

Timeout log excerpt:

tests\cli\test_run.py ........+++++++++++++++++++++++++++++++++++ Timeout +++++++++++++++++++++++++++++++++++

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Captured stderr ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DEBUG    Inferred transport: <MCPConfigTransport(...)>
DEBUG    Inferred transport: <FastMCPTransport(server='FastMCP Proxy-fc41')>
DEBUG    [Client-1903] called list_tools
DEBUG    [FastMCPProxy-fc41] Handler called: list_tools
DEBUG    Stdio transport connected
~~~~~~~~~~~~~~~~~~~~~ Stack of AnyIO worker thread ~~~~~~~~~~~~~~~~~~~~~
  ... waiting on queue.get() ...

What the test does: Creates an MCPConfig pointing to a temp Python script, wraps it in a FastMCP proxy server, and calls list_tools(). The transport connects via stdio to a spawned python subprocess. The subprocess connects (Stdio transport connected) but the 5s timeout fires before list_tools returns.

Why it's unrelated to this PR: The PR diff only touches:

src/fastmcp/apps/form.py — form submission logic
tests/apps/test_form.py — form tests

None of these are in the proxy, transport, or MCP config code paths.

Other passing jobs:

✅ Tests: Python 3.10 on ubuntu-latest
✅ Tests: Python 3.13 on ubuntu-latest
✅ Integration tests
✅ MCP conformance tests
✅ Tests with lowest-direct dependencies

Related Files

tests/cli/test_run.py:104 — TestMCPConfig::test_run_mcp_config — the timed-out test; spawns a Python subprocess via stdio
src/fastmcp/apps/form.py — PR change (unrelated to failure)

fix: boolean false values dropped in form submissions

87ba54d

marvin-context-protocol Bot added bug Something isn't working. Reports of errors, unexpected behavior, or broken functionality. contrib Related to community contributions in src/contrib/ directory. cli Related to FastMCP CLI commands (run, dev, install) or CLI functionality. labels Apr 7, 2026

chatgpt-codex-connector Bot reviewed Apr 7, 2026

View reviewed changes

revert apps_dev.py boolean coercion

69e5012

jlowin merged commit 8ee81b3 into main Apr 7, 2026
13 of 14 checks passed

jlowin deleted the fix/form-boolean-false branch April 7, 2026 15:23

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fix: boolean false values dropped in form submissions#3776

fix: boolean false values dropped in form submissions#3776
jlowin merged 2 commits intomainfrom
fix/form-boolean-false

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		if stripped.lower() in ("true", "false"):
		v = stripped.lower() == "true"

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