openaq-mcp-server

openaq-mcp-server: dataframe query

openaq_dataframe_query

Read-only

Run a read-only SQL SELECT against the measurement tables openaq_get_measurements staged on a DataCanvas. Reference tables by the name the measurements call returned (measurements_). For aggregation (monthly means, exceedance counts) and cross-sensor comparison over series too large to inline. Only SELECT is allowed — writes, DDL, and file/network table functions are rejected. Responses carry at most 200 rows; aggregate in SQL, or page with ORDER BY plus LIMIT/OFFSET, rather than selecting a whole table.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`sql`	Yes	Read-only SELECT. Reference tables by the names openaq_get_measurements returned (e.g. measurements_1701). Use openaq_dataframe_describe first to see table and column names.
`canvas_id`	Yes	DataCanvas id returned by openaq_get_measurements — minted when a series overflowed the inline preview, or the canvas_id you passed it.

Output Schema

TableJSON Schema

Name	Required	Description
`rows`	No	Result rows, at most 200. Every row here is also rendered in the text output — the two surfaces carry the same set.
`error`	No	Present when the call failed. Absent on success.
`notice`	No	How to reach the rest of the result when the row cap cut it short.
`rowCount`	No	Rows returned in this response, always equal to rows.length. It is the cap (200) when truncated is set, not the size of the full result.
`truncated`	No	True when the query matched more than 200 rows and the response was cut to the cap. Absent when the whole result fit. Page through the rest with ORDER BY plus LIMIT/OFFSET in your own SQL.

Schema Changelog

Changes observed during successful MCP inspections.

Changed5 schema fields changedSep 22, 2026

changedInput schema / properties / canvas_id / description

Previous value: -"DataCanvas id returned by openaq_get_measurements when a series spilled."New value: +"DataCanvas id returned by openaq_get_measurements — minted when a series overflowed the inline preview, or the canvas_id you passed it."

addedOutput schema / properties / notice

Added value: +{
+  "description": "How to reach the rest of the result when the row cap cut it short.",
+  "type": "string"
+}

changedOutput schema / properties / rowCount / description

Previous value: -"Full result count before the row cap."New value: +"Rows returned in this response, always equal to rows.length. It is the cap (200) when truncated is set, not the size of the full result."

changedOutput schema / properties / rows / description

Previous value: -"Result rows (capped at the canvas row limit)."New value: +"Result rows, at most 200. Every row here is also rendered in the text output — the two surfaces carry the same set."

addedOutput schema / properties / truncated

Added value: +{
+  "description": "True when the query matched more than 200 rows and the response was cut to the cap. Absent when the whole result fit. Page through the rest with ORDER BY plus LIMIT/OFFSET in your own SQL.",
+  "type": "boolean"
+}

Changed1 schema field changedSep 20, 2026
- addedInput schema / properties / canvas_id / pattern
```
Added value: +"^[A-Za-z0-9_-]{10}$"
```

Changed6 schema fields changedAug 25, 2026

changedInput schema / $schema

Previous value: -"http://json-schema.org/draft-07/schema#"New value: +"https://json-schema.org/draft/2020-12/schema"

addedInput schema / additionalProperties
```
Added value: +false
```

changedOutput schema / $schema

Previous value: -"http://json-schema.org/draft-07/schema#"New value: +"https://json-schema.org/draft/2020-12/schema"

addedOutput schema / anyOf

Added value: +[
+  {
+    "not": {
+      "required": [
+        "error"
+      ]
+    },
+    "required": [
+      "rows",
+      "rowCount"
+    ]
+  },
+  {
+    "required": [
+      "error"
+    ]
+  }
+]

addedOutput schema / properties / error

Added value: +{
+  "additionalProperties": {},
+  "description": "Present when the call failed. Absent on success.",
+  "properties": {
+    "code": {
+      "description": "JSON-RPC error code for this failure.",
+      "maximum": 9007199254740991,
+      "minimum": -9007199254740991,
+      "type": "integer"
+    },
+    "data": {
+      "additionalProperties": {},
+      "properties": {
+        "reason": {
+          "description": "Machine-readable failure mode. Declared by this tool: `canvas_unavailable`: DataCanvas is not enabled (CANVAS_PROVIDER_TYPE is not duckdb). `canvas_not_found`: The canvas_id is unknown or its canvas has expired. `missing_table`: The SQL references a table that is not staged on this canvas (dropped, expired, or misspelled). Other values are possible when a failure originates below the handler.",
+          "examples": [
+            "canvas_unavailable",
+            "canvas_not_found",
+            "missing_table"
+          ],
+          "type": "string"
+        },
+        "recovery": {
+          "additionalProperties": {},
+          "description": "Actionable next step for the caller.",
+          "properties": {
+            "hint": {
+              "type": "string"
+            }
+          },
+          "required": [
+            "hint"
+          ],
+          "type": "object"
+        },
+        "retryable": {
+          "description": "Whether retrying may succeed.",
+          "type": "boolean"
+        }
+      },
+      "type": "object"
+    },
+    "message": {
+      "description": "Human-readable description of what went wrong.",
+      "type": "string"
+    }
+  },
+  "required": [
+    "code",
+    "message"
+  ],
+  "type": "object"
+}

removedOutput schema / required

Removed value: -[
-  "rows",
-  "rowCount"
-]

First observedJun 16, 2026

TDQS

A4.5/5.0

Behavior5/5

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

Even though readOnlyHint is already true, the description adds valuable behavioral detail beyond the annotation: only SELECT is allowed, writes/DDL/file/network functions are rejected, and responses cap at 200 rows. It also gives pagination guidance with ORDER BY and LIMIT/OFFSET.

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

Conciseness5/5

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

The description is dense but tightly organized, front-loading the core operation and table-referencing rule before restrictions and row limitations. Every sentence carries operational value with no filler.

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?

With annotations, full schema coverage, and an output schema present, the description covers all essentials an agent needs: table naming convention, SELECT-only restriction, row cap, and paging strategy. Nothing critical 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 the input schema already explains both parameters. The description reinforces the canvas_id source but does not add significant meaning beyond the schema's own wording; baseline 3 is appropriate.

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?

The description states a specific verb and resource: 'Run a read-only SQL SELECT against the measurement tables openaq_get_measurements staged on a DataCanvas.' It also distinguishes itself as the SQL/aggregation tool versus the table-describe tool and the raw data access siblings.

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?

It clearly states when to use it: 'for aggregation (monthly means, exceedance counts) and cross-sensor comparison over series too large to inline.' It also points to openaq_dataframe_describe as a precursor step, though it doesn't explicitly name which sibling tools are not appropriate.

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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