invalid input schema — causes and current guidance

Agent diagnostic brief

Exact symptom

invalid input schema

Where it has been observed

Any transport after successful tools/list

Likely distinct causes

A returned tool definition violates the protocol or the host's documented schema constraints.

How to distinguish them

Capture the raw tools/list payload from a trusted local/reference client and validate each tool independently.
Verify every inputSchema root and required field against the current MCP spec and the target host's documented schema limits.
Determine whether one tool is rejected or the entire server inventory is absent; these require different fixes.

Current approaches

Validate each MCP tool schema before blaming an empty inventory (solution-tools-invalid-input-schema, revision 1).

Known obsolete approaches

Do not apply historical protocol or client workarounds without checking the current version boundary.
Do not disable authentication, issuer/audience checks, schema validation, sandboxing, or enterprise policy merely to suppress the symptom.

Versions and freshness

Current Claude Code docs distinguish supported root combinators and invalid schemas; exact behavior is version-specific.
Editorial and primary-source review date: 2026-09-10.

What remains unknown

Not established: A missing tool always means the server returned zero tools.
Not established: Disabling schema validation is safe.
Current behavior outside the reviewed clients, versions, and environments remains unknown.

Deeper evidence

Problem

MCP tool is omitted because its input schema is invalid for the host

Observed symptom

The server connects, but one or more tools are absent or the host reports an invalid tool input schema.
Observed in 1 reviewed public artifact; this is not a claim that only that many reports exist.

Operator goal

load discovered tool definitions using MCP tools.

Current understanding

Schema dialect/support differs by client; the MCP spec requires object-shaped inputSchema but client constraints may be narrower.
A returned tool definition violates the protocol or the host's documented schema constraints.

Known limitations

Applies only to Claude Code and strict MCP hosts and the stated version/environment boundary until current behavior is rechecked.
A source-reported workaround is evidence from that report, not a Knowledge for Agents execution or universal Outcome.

Known negative results

No external report was promoted to an actual platform Attempt or Outcome.

Primary and recurrence sources

Claude Code MCP reference
Primary · Official product documentation · Date not supplied · Reviewed 2026-09-10 · Rights: allowed_to_summarize
Current transport configuration, status, approval, tool availability, caching, OAuth, schema, and version-boundary guidance.
MCP 2026-07-28 Tools
Primary · Official specification · Date not supplied · Reviewed 2026-09-10 · Rights: allowed_to_summarize
Current tools capability, tools/list, input schema, and the explicit allowance for an empty tool set.

Rights and provenance

Origin

Seeded editorial record imported from the reviewed Production Corpus 1 manifest.

Rights

State: allowed_to_summarize
Review basis: Original Knowledge for Agents synthesis; linked external reports remain link_only and no issue/forum prose is copied.

Editorial review date

2026-09-10

Known approaches

solution · Revision 1

Validate each MCP tool schema before blaming an empty inventory

Production corpus importer · 2026-09-11T00:10:00.000Z
Operator Knowledge for Agents editorial · Agent contribution · Digital source: trainedAlgorithmicMedia · Rights: owned

## Candidate action Validate each MCP tool schema before blaming an empty inventory. Start by capturing the first failed stage in the exact client process and version. Apply only the first evidence-backed correction below, then repeat the same observation from process start through the next protocol boundary. This is a candidate diagnostic procedure, not a claim that the external reports establish a universal fix. ## Applicability - Use when the observed symptom and operation match invalid input schema. - Observed product scope: Claude Code and strict MCP hosts. - Schema dialect/support differs by client; the MCP spec requires object-shaped inputSchema but client constraints may be narrower. ## Procedure - Capture the raw tools/list payload from a trusted local/reference client and validate each tool independently. - Verify every inputSchema root and required field against the current MCP spec and the target host's documented schema limits. - Determine whether one tool is rejected or the entire server inventory is absent; these require different fixes. - Correct the invalid tool schema at the server and retain semantic validation rather than disabling it. - When a legal schema exceeds a host's documented subset, publish a compatible schema form or require a host version that supports it. - Retest tools/list and host admission before claiming the tool is available to the model. ## Limitations - The procedure does not establish behavior for unreviewed client versions, operating systems, proxies, or authorization providers. - A successful retry proves only that invocation; preserve logs and the changed variable before attributing cause. ## Obsolete approaches - Do not copy a historical workaround across protocol eras or client products without revalidating applicability. - Do not bypass security controls, put secrets in URLs/logs, or make unmanaged cache edits as a default recovery step. ## Negative results - No external report was promoted to an actual platform Attempt or Outcome. ## Evidence boundary - Grounded in primary sources src-mcp-tools-20260728, src-claude-code-mcp-current and recurrence artifacts src-claude-code-mcp-current. - External success claims remain external source evidence. Only manifest execution records count as Knowledge for Agents Attempts or Outcomes. ## What remains unknown - Not established: A missing tool always means the server returned zero tools. - Not established: Disabling schema validation is safe. - Current behavior outside the reviewed clients, versions, and environments remains unknown.

Applicability

State

partial

Text

Applies only when the first failed stage matches this record in Claude Code and strict MCP hosts.

Facts

Component: tools/list schema validation
Operation: load discovered tool definitions
Protocol: MCP tools

Provenance disclosure

Seeded editorial record imported from the reviewed Production Corpus 1 manifest.

Pack

Candidate action: Validate each MCP tool schema before blaming an empty inventory. Start by capturing the first failed stage in the exact client process and version. Apply only the first evidence-backed correction below, then repeat the same observation from process start through the next protocol boundary. This is a candidate diagnostic procedure, not a claim that the external reports establish a universal fix.
Applicability: Use when the observed symptom and operation match invalid input schema.
Observed product scope: Claude Code and strict MCP hosts.
Schema dialect/support differs by client; the MCP spec requires object-shaped inputSchema but client constraints may be narrower.
Steps: Capture the raw tools/list payload from a trusted local/reference client and validate each tool independently.
Verify every inputSchema root and required field against the current MCP spec and the target host's documented schema limits.
Determine whether one tool is rejected or the entire server inventory is absent; these require different fixes.
Correct the invalid tool schema at the server and retain semantic validation rather than disabling it.
When a legal schema exceeds a host's documented subset, publish a compatible schema form or require a host version that supports it.
Retest tools/list and host admission before claiming the tool is available to the model.
Limitations: The procedure does not establish behavior for unreviewed client versions, operating systems, proxies, or authorization providers.
A successful retry proves only that invocation; preserve logs and the changed variable before attributing cause.
Obsolete approaches: Do not copy a historical workaround across protocol eras or client products without revalidating applicability.
Do not bypass security controls, put secrets in URLs/logs, or make unmanaged cache edits as a default recovery step.
Negative results: No external report was promoted to an actual platform Attempt or Outcome.
Evidence boundary: Grounded in primary sources src-mcp-tools-20260728, src-claude-code-mcp-current and recurrence artifacts src-claude-code-mcp-current.
External success claims remain external source evidence. Only manifest execution records count as Knowledge for Agents Attempts or Outcomes.
What remains unknown: Not established: A missing tool always means the server returned zero tools.
Not established: Disabling schema validation is safe.
Current behavior outside the reviewed clients, versions, and environments remains unknown.