usgs-water-mcp-server

Water Get Readings

water_get_readings

Read-onlyIdempotent

Get the latest instantaneous (~15-min, real-time) values for up to 100 USGS sites in one call — per-site, per-parameter records with timestamp, value, unit, and provisional/approved qualifiers. Each series returns only its 10 most recent records (totalValues reports the true count; truncated=true if any were capped); use water_get_series for a full date-range series. Sites NWIS returns nothing for are listed in missingSites, not dropped silently. Use water_find_sites first to discover site numbers and available parameters.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`sites`	Yes	One or more USGS site numbers to query. Maximum 100 per call.
`period`	No	ISO 8601 duration for the lookback period (e.g. "PT2H" = last 2 hours, "P1D" = last 1 day, "P7D" = last 7 days). Default: "PT2H" (last 2 hours of readings). Widening it raises totalValues, but each series still returns only its 10 most recent records — use water_get_series to retrieve a full series.	PT2H
`parameterCd`	No	Parameter codes to return. Omit to get all parameters available at each site. Use water_list_parameters to discover codes.

Output Schema

TableJSON Schema

Name	Required	Description
`error`	No	Present when the call failed. Absent on success.
`query`	No	Query parameters used for this request.
`total`	No	Total number of site+parameter time series returned.
`readings`	No	Time series per site+parameter combination.
`truncated`	No	True when at least one series held more than 10 records and was capped. Per-series counts are in readings[].totalValues; use water_get_series for the full series.
`missingSites`	No	Requested site numbers NWIS returned no series for — the site may not exist, or may not measure the requested parameter(s) in the requested period. Empty when every requested site returned data. Verify these with water_find_sites.

Schema Changelog

Changes observed during successful MCP inspections.

Changed6 schema fields changedAug 22, 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": [
+      "readings",
+      "total",
+      "truncated",
+      "missingSites",
+      "query"
+    ]
+  },
+  {
+    "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: `no_data_for_parameter`: NWIS returned no time series — the site(s) may not exist, or may not have data for the requested parameter(s) in the requested period. NWIS returns the same empty response for both cases. `invalid_request`: NWIS rejected the request. Input formats are validated against NWIS-accepted patterns before the call, so this surfaces a value that is well-formed but unacceptable upstream. `upstream_error`: NWIS returned a 5xx error or the request timed out. Other values are possible when a failure originates below the handler.",
+          "examples": [
+            "no_data_for_parameter",
+            "invalid_request",
+            "upstream_error"
+          ],
+          "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: -[
-  "readings",
-  "total",
-  "truncated",
-  "missingSites",
-  "query"
-]

Changed10 schema fields changedJul 16, 2026

addedInput schema / properties / parameterCd / items / pattern
```
Added value: +"^\\d{5}$"
```

changedInput schema / properties / period / description

Previous value: -"ISO 8601 duration for the lookback period (e.g. \"PT2H\" = last 2 hours, \"P1D\" = last 1 day, \"P7D\" = last 7 days). Default: \"PT2H\" (last 2 hours of readings)."New value: +"ISO 8601 duration for the lookback period (e.g. \"PT2H\" = last 2 hours, \"P1D\" = last 1 day, \"P7D\" = last 7 days). Default: \"PT2H\" (last 2 hours of readings). Widening it raises totalValues, but each series still returns only its 10 most recent records — use water_get_series to retrieve a full series."

addedInput schema / properties / period / pattern

Added value: +"^P(?!$)(\\d+Y)?(\\d+M)?(\\d+W)?(\\d+D)?(T(?=\\d)(\\d+H)?(\\d+M)?(\\d+(\\.\\d+)?S)?)?$"

addedInput schema / properties / sites / items / pattern
```
Added value: +"^\\d{8,15}$"
```

addedOutput schema / properties / missingSites

Added value: +{
+  "description": "Requested site numbers NWIS returned no series for — the site may not exist, or may not measure the requested parameter(s) in the requested period. Empty when every requested site returned data. Verify these with water_find_sites.",
+  "items": {
+    "description": "A requested USGS site number that returned no time series.",
+    "type": "string"
+  },
+  "type": "array"
+}

addedOutput schema / properties / readings / items / properties / totalValues

Added value: +{
+  "description": "Number of value records NWIS returned for this site and parameter over the requested period, before the 10-record cap. Equals values.length when nothing was capped.",
+  "maximum": 9007199254740991,
+  "minimum": -9007199254740991,
+  "type": "integer"
+}

changedOutput schema / properties / readings / items / properties / values / description

Previous value: -"Time-ordered value records for this site and parameter."New value: +"Time-ordered value records for this site and parameter, capped at the most recent 10. Compare with totalValues to see whether the period held more; use water_get_series for the full series."

changedOutput schema / properties / readings / items / required

Previous value: -[
-  "siteNumber",
-  "siteName",
-  "parameterCd",
-  "parameterName",
-  "unitCode",
-  "values"
-]New value: +[
+  "siteNumber",
+  "siteName",
+  "parameterCd",
+  "parameterName",
+  "unitCode",
+  "values",
+  "totalValues"
+]

addedOutput schema / properties / truncated

Added value: +{
+  "description": "True when at least one series held more than 10 records and was capped. Per-series counts are in readings[].totalValues; use water_get_series for the full series.",
+  "type": "boolean"
+}

changedOutput schema / required

Previous value: -[
-  "readings",
-  "total",
-  "query"
-]New value: +[
+  "readings",
+  "total",
+  "truncated",
+  "missingSites",
+  "query"
+]

First observedJun 7, 2026

TDQS

A4.9/5.0

Behavior5/5

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

Annotations already declare readOnlyHint, openWorldHint, and idempotentHint. The description goes beyond these by revealing the 10-record cap per series, the truncated flag and totalValues mechanism, and the missingSites behavior — non-obvious operational details an agent must know to interpret results correctly. No contradiction with annotations.

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?

Three dense, information-rich sentences. The core function is front-loaded, followed by the most critical limitation (10-record cap) and the alternative tool, then the missingSites behavior. Every sentence earns its place with zero fluff or repetition.

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?

Given an output schema exists (so return shape is already defined), the description covers the essential operational context: what data is returned, the truncation policy, how missing sites are handled, and the recommended discovery workflow. Nothing an agent needs to call and interpret the tool correctly is missing.

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

Parameters4/5

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

Schema description coverage is 100%, so the schema already documents parameters well (baseline 3). The description adds meaningful behavioral context, particularly for the period parameter — widening it raises totalValues but still caps each series at 10 records — and explicitly directs users to water_get_series for full retrieval. This extra nuance lifts it above the baseline.

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 opens with a specific verb ('Get') and resource ('instantaneous values for up to 100 USGS sites'), and precisely scopes the output (per-site, per-parameter records with qualifiers). It explicitly contrasts itself with water_get_series ('use water_get_series for a full date-range series'), making the purpose unambiguous and distinguishable from siblings.

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

Usage Guidelines5/5

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

It names the alternative tool (water_get_series) for full series and explicitly recommends water_find_sites as a prerequisite ('Use water_find_sites first to discover site numbers and available parameters'). It also clarifies expected behavior for missing sites, providing clear when-to-use and when-not-to-use context.

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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