diff --git a/internal/api/handler_ri_exchange.go b/internal/api/handler_ri_exchange.go
index 085c2e63..fc6840d0 100644
--- a/internal/api/handler_ri_exchange.go
+++ b/internal/api/handler_ri_exchange.go
@@ -1926,12 +1926,9 @@ func (h *Handler) getExchangeQuote(ctx context.Context, req *events.LambdaFuncti
 		return nil, mapAWSExchangeError("exchange quote failed", err)
 	}
 
-	// Echo the region the SDK chain resolved back to the caller (issue
-	// #238): the frontend modal has no other way to learn it, and
-	// execute rejects a missing region outright since exchanges are
-	// financially irreversible. Carrying the same value the quote used
-	// keeps quote and execute pinned to one region instead of letting
-	// execute re-resolve (and potentially land on a different one).
+	// Return the region the quote was priced in. Execute requires a region,
+	// and callers must send this same one so the exchange runs where it was
+	// priced.
 	return ExchangeQuoteResponse{ExchangeQuoteSummary: quote, Region: region}, nil
 }
 
@@ -2196,11 +2193,9 @@ type ExchangeExecuteResponse struct {
 	ExchangeID string                         `json:"exchange_id"`
 }
 
-// ExchangeQuoteResponse wraps the library's quote summary with the
-// region the SDK chain resolved (issue #238). The embedded pointer
-// promotes the summary's own fields to the JSON top level, matching
-// the shape callers got before this type existed; Region is the only
-// addition.
+// ExchangeQuoteResponse is the library's quote summary plus the region
+// the quote was priced in, which callers must send back on execute. The embedded
+// pointer promotes the summary's fields to the JSON top level.
 type ExchangeQuoteResponse struct {
 	*exchange.ExchangeQuoteSummary
 	Region string `json:"Region"`
diff --git a/internal/api/openapi.yaml b/internal/api/openapi.yaml
index d3611e13..eb8f1200 100644
--- a/internal/api/openapi.yaml
+++ b/internal/api/openapi.yaml
@@ -2321,6 +2321,14 @@ paths:
             application/json:
               schema:
                 type: object
+                required: [Region]
+                properties:
+                  Region:
+                    type: string
+                    description: >
+                      AWS region the quote was priced in. Always set. Send
+                      this same value as `region` on
+                      `/api/ri-exchange/execute`.
         '400':
           $ref: '#/components/responses/BadRequest'
         '401':
@@ -2366,7 +2374,7 @@ paths:
           application/json:
             schema:
               type: object
-              required: [ri_ids, targets, max_payment_due_usd]
+              required: [ri_ids, targets, max_payment_due_usd, region]
               properties:
                 ri_ids:
                   type: array
@@ -2379,6 +2387,12 @@ paths:
                 max_payment_due_usd:
                   type: string
                   description: Spend-cap guardrail (decimal string, e.g. "1000.00")
+                region:
+                  type: string
+                  description: >
+                    AWS region to execute in. Required: a missing region is
+                    rejected with 400 rather than defaulted, since an exchange
+                    cannot be undone. Send the `Region` the quote returned.
       responses:
         '200':
           description: Exchange submitted
diff --git a/internal/api/openapi_ri_exchange_region_test.go b/internal/api/openapi_ri_exchange_region_test.go
new file mode 100644
index 00000000..bfc47c77
--- /dev/null
+++ b/internal/api/openapi_ri_exchange_region_test.go
@@ -0,0 +1,105 @@
+package api
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/LeanerCloud/cloud-commitments-go/pkg/exchange"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"gopkg.in/yaml.v3"
+)
+
+// riExchangeSpecSchema is the slice of an inline openapi.yaml schema these
+// checks read.
+type riExchangeSpecSchema struct {
+	Type       string   `yaml:"type"`
+	Required   []string `yaml:"required"`
+	Properties map[string]struct {
+		Type        string `yaml:"type"`
+		Description string `yaml:"description"`
+	} `yaml:"properties"`
+}
+
+type riExchangeSpecMedia struct {
+	Content map[string]struct {
+		Schema riExchangeSpecSchema `yaml:"schema"`
+	} `yaml:"content"`
+}
+
+type riExchangeSpecOp struct {
+	RequestBody riExchangeSpecMedia            `yaml:"requestBody"`
+	Responses   map[string]riExchangeSpecMedia `yaml:"responses"`
+}
+
+func loadRIExchangeSpecOp(t *testing.T, path string) riExchangeSpecOp {
+	t.Helper()
+	var spec struct {
+		Paths map[string]map[string]yaml.Node `yaml:"paths"`
+	}
+	require.NoError(t, yaml.Unmarshal(openapiSpec, &spec))
+	node, ok := spec.Paths[path]["post"]
+	require.True(t, ok, "post %s missing from openapi.yaml", path)
+	var op riExchangeSpecOp
+	require.NoError(t, node.Decode(&op), "post %s", path)
+	return op
+}
+
+// jsonKeys returns the top-level keys v marshals to, so the spec checks
+// follow the Go json tags rather than a hand-copied field name.
+func jsonKeys(t *testing.T, v any) map[string]bool {
+	t.Helper()
+	data, err := json.Marshal(v)
+	require.NoError(t, err)
+	var m map[string]json.RawMessage
+	require.NoError(t, json.Unmarshal(data, &m))
+	keys := make(map[string]bool, len(m))
+	for k := range m {
+		keys[k] = true
+	}
+	return keys
+}
+
+// TestOpenAPIRIExchangeRegionRequired keeps openapi.yaml in step with the
+// handler on the exchange region (issue #363): execute refuses a request
+// without a region (validateExecuteExchangeBody), and the quote response
+// always carries the region the quote was priced in, which execute must send
+// back.
+func TestOpenAPIRIExchangeRegionRequired(t *testing.T) {
+	// The handler side of the contract: a body that is otherwise valid but
+	// names no region is a 400.
+	err := validateExecuteExchangeBody(ExchangeExecuteRequestBody{
+		RIIDs:            []string{"ri-1"},
+		Targets:          []ExchangeTargetBody{{OfferingID: "4b2293b4-5fbc-4017-9c75-d5a9d3aa8c91", Count: 1}},
+		MaxPaymentDueUSD: "100.00",
+	})
+	var ce *clientError
+	require.ErrorAs(t, err, &ce)
+	assert.Equal(t, 400, ce.code)
+	assert.Contains(t, ce.message, "region is required")
+
+	execKeys := jsonKeys(t, ExchangeExecuteRequestBody{Region: "us-west-2"})
+	require.True(t, execKeys["region"], "execute body's region json key changed; update this test and openapi.yaml")
+
+	execSchema, ok := loadRIExchangeSpecOp(t, "/api/ri-exchange/execute").RequestBody.Content["application/json"]
+	require.True(t, ok, "execute must document an application/json request body")
+	assert.Contains(t, execSchema.Schema.Required, "region", "execute must list region as required")
+	require.Contains(t, execSchema.Schema.Properties, "region", "execute must document the region property")
+	assert.Equal(t, "string", execSchema.Schema.Properties["region"].Type)
+	assert.NotEmpty(t, execSchema.Schema.Properties["region"].Description)
+
+	quoteKeys := jsonKeys(t, ExchangeQuoteResponse{
+		ExchangeQuoteSummary: &exchange.ExchangeQuoteSummary{},
+		Region:               "us-west-2",
+	})
+	require.True(t, quoteKeys["Region"], "quote response's Region json key changed; update this test and openapi.yaml")
+
+	quoteSchema, ok := loadRIExchangeSpecOp(t, "/api/ri-exchange/quote").Responses["200"].Content["application/json"]
+	require.True(t, ok, "quote must document an application/json 200 response")
+	assert.Equal(t, "object", quoteSchema.Schema.Type)
+	assert.Contains(t, quoteSchema.Schema.Required, "Region", "quote response must list Region as always present")
+	require.Contains(t, quoteSchema.Schema.Properties, "Region", "quote response must document Region")
+	assert.Equal(t, "string", quoteSchema.Schema.Properties["Region"].Type)
+	assert.Contains(t, quoteSchema.Schema.Properties["Region"].Description, "execute",
+		"quote Region must tell callers to send it back on execute")
+}