diff --git a/doc/execution-semantics.md b/doc/execution-semantics.md
index d846c688622..bd6e53976b2 100644
--- a/doc/execution-semantics.md
+++ b/doc/execution-semantics.md
@@ -453,6 +453,12 @@ without submitted values. Correct the arguments and retry; the rejection is not
 evidence that the scheduling service is unavailable. Confirm the persisted
 receipt before reporting a scheduled wait.
 
+Native `call_api` text windows use `responseText.limitBytes` from 4 through
+24576 bytes, with a default of 24576. Follow `nextOffsetBytes` until it is null.
+Rejected API fallback arguments return the same bounded schema locations as
+monitor calls, with no submitted values. Correct the arguments before retrying;
+validation does not dispatch an HTTP request or relax route permissions.
+
 A native provider may omit `continuation` (or send `null`) when it reports
 `yielded` after saving a monitor. The controller reads current authorized task
 context and fills the canonical `monitor` continuation only if that same task,
diff --git a/packages/paperclip-runner/runner/crates/runner-core/src/provider_backend.rs b/packages/paperclip-runner/runner/crates/runner-core/src/provider_backend.rs
index 8a8a5935997..660042ee790 100644
--- a/packages/paperclip-runner/runner/crates/runner-core/src/provider_backend.rs
+++ b/packages/paperclip-runner/runner/crates/runner-core/src/provider_backend.rs
@@ -6539,3 +6539,7 @@ mod tests {
         fs::remove_dir_all(directory).unwrap();
     }
 }
+
+#[cfg(test)]
+#[path = "tests/provider_api_validation.rs"]
+mod api_validation_tests;
diff --git a/packages/paperclip-runner/runner/crates/runner-core/src/provider_bridge.rs b/packages/paperclip-runner/runner/crates/runner-core/src/provider_bridge.rs
index 94b5d825780..09cb2fe08da 100644
--- a/packages/paperclip-runner/runner/crates/runner-core/src/provider_bridge.rs
+++ b/packages/paperclip-runner/runner/crates/runner-core/src/provider_bridge.rs
@@ -43,6 +43,7 @@ const COMPLETION_INPUT_SCHEMA_HINT: &str = "Invalid paperclip_finish arguments.
 const QUESTION_INPUT_SCHEMA_HINT: &str = "Invalid request_human_input arguments. Required fields: idempotencyKey, interactionKind, title, prompt, and continuationPolicy. For questions, use payload.version=1 and a complete payload.questionSet matching the authorized schema. Preserve the supplied question and option IDs.";
 const BLOCK_INPUT_SCHEMA_HINT: &str = "Invalid paperclip_block arguments. Required fields: reportedWorkDisposition=blocked, summary, completionClaim, evidence, verification, and blocker. blocker must include reasonCode, owner, unblockAction, and scope.";
 const MONITOR_INPUT_SCHEMA_HINT: &str = "Invalid set_task_monitor arguments. Supply idempotencyKey and monitor: {nextCheckAt: <future UTC timestamp>, notes: <next check>}. Put all scheduling fields inside monitor, or use monitor: null to clear. Omit taskId for the current task. Correct the arguments and retry.";
+const API_INPUT_SCHEMA_HINT: &str = "Invalid call_api arguments. For responseText, use GET with offsetBytes >= 0 and responseText.limitBytes from 4 through 24576 bytes (default 24576). Follow nextOffsetBytes; null means complete. Correct the arguments and retry.";
 
 #[derive(Clone, Debug, Deserialize, Serialize, PartialEq)]
 #[serde(rename_all = "camelCase")]
@@ -246,6 +247,7 @@ impl ProviderBridgeError {
             "paperclip_block" => Some(BLOCK_INPUT_SCHEMA_HINT),
             "request_human_input" => Some(QUESTION_INPUT_SCHEMA_HINT),
             "set_task_monitor" => Some(MONITOR_INPUT_SCHEMA_HINT),
+            "call_api" => Some(API_INPUT_SCHEMA_HINT),
             _ => None,
         };
         Self {
diff --git a/packages/paperclip-runner/runner/crates/runner-core/src/tests/provider_api_validation.rs b/packages/paperclip-runner/runner/crates/runner-core/src/tests/provider_api_validation.rs
new file mode 100644
index 00000000000..6c911f45962
--- /dev/null
+++ b/packages/paperclip-runner/runner/crates/runner-core/src/tests/provider_api_validation.rs
@@ -0,0 +1,90 @@
+use super::*;
+
+#[test]
+fn api_window_rejection_explains_limits_and_allows_corrected_call() {
+    let contract_path = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+        .join("../../../generated/capability/semantic-tool-contracts.json");
+    let contracts: Value =
+        serde_json::from_str(&fs::read_to_string(contract_path).unwrap()).unwrap();
+    let contract = contracts
+        .as_array()
+        .unwrap()
+        .iter()
+        .find(|contract| contract["name"] == "call_api")
+        .unwrap();
+    let operation = crate::provider_bridge::AuthorizedTool {
+        operation_id: "call_api".to_owned(),
+        version: 1,
+        description: contract["description"].as_str().unwrap().to_owned(),
+        input_schema: contract["inputSchema"].clone(),
+        response_schema: contract["outputSchema"].clone(),
+    };
+    let mut bridge = ProviderToolBridge::default();
+    bridge
+        .prepare(AuthorizedToolSet {
+            schema: TOOL_SET_SCHEMA.to_owned(),
+            schema_version: 1,
+            catalog_digest: authorized_tool_catalog_digest(std::slice::from_ref(&operation))
+                .unwrap(),
+            operations: vec![operation],
+        })
+        .unwrap();
+    let mut input = json!({
+        "operationId": "GET /api/assets/{assetId}/content",
+        "pathParams": {"assetId": "PRIVATE_ASSET"},
+        "responseText": {"limitBytes": 60000}
+    });
+    let error = bridge
+        .begin_call(
+            "api-window".to_owned(),
+            "call_api".to_owned(),
+            input.clone(),
+        )
+        .unwrap_err();
+    let result = invalid_tool_call_result("api-window".to_owned(), "call_api".to_owned(), &error);
+    let message = result.result["error"]["message"].as_str().unwrap();
+    assert!(message.contains("responseText.limitBytes"));
+    let window_schema =
+        &contract["inputSchema"]["properties"]["responseText"]["properties"]["limitBytes"];
+    assert!(message.contains(&format!(
+        "from {} through {} bytes (default {})",
+        window_schema["minimum"], window_schema["maximum"], window_schema["default"]
+    )));
+    assert!(message.contains("/properties/responseText/properties/limitBytes/maximum"));
+    assert!(!message.contains("PRIVATE_ASSET"));
+    assert!(message.len() <= 512);
+    assert!(!message.chars().any(char::is_control));
+    assert_eq!(result.result["error"]["code"], "invalid_tool_call");
+    input["responseText"]["limitBytes"] = json!(16000);
+    let mut unknown_field = input.clone();
+    unknown_field["responseText"]["PRIVATE_FIELD"] = json!("PRIVATE_VALUE");
+    let unknown_error = bridge
+        .begin_call(
+            "unknown-api-window".to_owned(),
+            "call_api".to_owned(),
+            unknown_field,
+        )
+        .unwrap_err();
+    let unknown_result = invalid_tool_call_result(
+        "unknown-api-window".to_owned(),
+        "call_api".to_owned(),
+        &unknown_error,
+    );
+    let unknown_message = unknown_result.result["error"]["message"].as_str().unwrap();
+    assert!(unknown_message.contains("/additionalProperties"));
+    for private in ["PRIVATE_FIELD", "PRIVATE_VALUE", "PRIVATE_ASSET"] {
+        assert!(!unknown_result.result.to_string().contains(private));
+    }
+    assert!(unknown_message.len() <= 512);
+    let denied = ProviderToolBridge::default()
+        .begin_call("denied-api".to_owned(), "call_api".to_owned(), json!({}))
+        .unwrap_err();
+    assert_eq!(
+        invalid_tool_call_result("denied-api".to_owned(), "call_api".to_owned(), &denied).result
+            ["error"]["message"],
+        GENERIC_INVALID_TOOL_CALL_MESSAGE
+    );
+    assert!(bridge
+        .begin_call("api-window".to_owned(), "call_api".to_owned(), input)
+        .is_ok());
+}