mcphost

host.errors

List every error code this host can return, each with its class (agent_fault | host_refusal | upstream_fault | plan_limit | host_fault), family, remedy, docs anchor, retryable flag and help_url -- the same object GET /errors.json serves. Pass code to get one row; a misspelt code answers tool_not_found with did_you_mean. Read-only, anonymous, no tenant data.

Input Schema

Name	Required	Description	Default
`code`	No	An error code, e.g. tool_exception. Omit to list every code.
`tenant_key`	No	The key `signup` returned. Optional on the connection that ran signup (or host.redeem): that connection is already bound to the tenant it created, so later calls on it need no tenant_key. On a connection with no Authorization: Bearer header and no prior signup, omitting it implicitly signs that connection up as a brand-new tenant and runs this call as it -- see host.whoami's source field. When both a header and this argument are present, the header wins.

Output Schema

Name	Required	Description	Default
`code`	No
`class`	No
`codes`	No
`family`	No
`remedy`	No
`classes`	No
`version`	No
`help_url`	No
`retryable`	No
`docs_anchor`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedOct 20, 2026
- addedInput schema / x-mcphost-validated
```
Added value: +true
```

Changed2 schema fields changedOct 18, 2026

addedOutput schema / $defs

Added value: +{
+  "ErrorEnvelope": {
+    "properties": {
+      "code": {
+        "type": "integer"
+      },
+      "data": {
+        "properties": {
+          "error_code": {
+            "type": "string"
+          },
+          "retryable": {
+            "type": "boolean"
+          }
+        },
+        "required": [
+          "error_code"
+        ],
+        "type": "object"
+      },
+      "message": {
+        "type": "string"
+      }
+    },
+    "required": [
+      "code",
+      "message",
+      "data"
+    ],
+    "type": "object"
+  }
+}

addedOutput schema / x-error-envelope

Added value: +{
+  "$ref": "#/$defs/ErrorEnvelope"
+}

AddedOct 17, 2026

TDQS

A4.2/5.0

Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations, the description carries the full burden and does so well: it declares read-only, anonymous, no tenant data, and the return fields, plus the edge-case behavior for a misspelt code (tool_not_found with did_you_mean). It stops short of stating rate limits or auth mechanics.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

One dense but well-ordered paragraph that front-loads the main purpose and return shape before the filtering and edge-case details. Every clause earns its place, though the field list is fairly long.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

An output schema exists, so return values needn't be re-explained, and the description still tells the agent the two invocation modes and the failure behavior. Nothing needed to call this correctly is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so both parameters are fully documented in the schema. The description adds only the filtering behavior of code; it says nothing extra about tenant_key, so the baseline of 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb+resource: 'List every error code this host can return', with the exact payload shape enumerated. It is clearly distinguishable from siblings like host.help.search, which deal with documentation rather than error codes.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description explains the two modes: omit code to list everything, pass code to get one row, and even covers the misspelling case. It gives clear context but never names an alternative tool or states when another sibling would be preferred.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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