diff --git a/README.md b/README.md
index bdf76e4..8b00bfb 100644
--- a/README.md
+++ b/README.md
@@ -989,23 +989,18 @@ Create a new user session. This is the entry point for all clients.
 
 If the server requires hashcash proof-of-work (see
 [Hashcash Proof-of-Work](#hashcash-proof-of-work)), the client must include a
-valid stamp in the `X-Hashcash` request header. The required difficulty is
-advertised via `GET /api/v1/server` in the `hashcash_bits` field.
-
-**Request Headers:**
-
-| Header       | Required | Description |
-|--------------|----------|-------------|
-| `X-Hashcash` | Conditional | Hashcash stamp (required when server has `hashcash_bits` > 0) |
+valid stamp in the `hashcash` field of the JSON request body. The required
+difficulty is advertised via `GET /api/v1/server` in the `hashcash_bits` field.
 
 **Request Body:**
 ```json
-{"nick": "alice"}
+{"nick": "alice", "hashcash": "1:20:260310:neoirc::3a2f1"}
 ```
 
-| Field  | Type   | Required | Constraints |
-|--------|--------|----------|-------------|
-| `nick` | string | Yes      | 1–32 characters, must be unique on the server |
+| Field      | Type   | Required    | Constraints |
+|------------|--------|-------------|-------------|
+| `nick`     | string | Yes         | 1–32 characters, must be unique on the server |
+| `hashcash` | string | Conditional | Hashcash stamp (required when server has `hashcash_bits` > 0) |
 
 **Response:** `201 Created`
 ```json
@@ -1027,7 +1022,7 @@ advertised via `GET /api/v1/server` in the `hashcash_bits` field.
 | Status | Error | When |
 |--------|-------|------|
 | 400    | `nick must be 1-32 characters` | Empty or too-long nick |
-| 402    | `hashcash proof-of-work required` | Missing `X-Hashcash` header when hashcash is enabled |
+| 402    | `hashcash proof-of-work required` | Missing `hashcash` field in request body when hashcash is enabled |
 | 402    | `invalid hashcash stamp: ...` | Stamp fails validation (wrong bits, expired, reused, etc.) |
 | 409    | `nick already taken` | Another active session holds this nick |
 
@@ -1035,8 +1030,7 @@ advertised via `GET /api/v1/server` in the `hashcash_bits` field.
 ```bash
 TOKEN=$(curl -s -X POST http://localhost:8080/api/v1/session \
   -H 'Content-Type: application/json' \
-  -H 'X-Hashcash: 1:20:260310:neoirc::3a2f1' \
-  -d '{"nick":"alice"}' | jq -r .token)
+  -d '{"nick":"alice","hashcash":"1:20:260310:neoirc::3a2f1"}' | jq -r .token)
 echo $TOKEN
 ```
 
@@ -2138,7 +2132,7 @@ account registration, no IP-based rate limits that punish shared networks.
 2. Client computes a hashcash stamp: find a counter value such that the
    SHA-256 hash of the stamp string has the required number of leading zero
    bits.
-3. Client includes the stamp in the `X-Hashcash` request header when creating
+3. Client includes the stamp in the `hashcash` field of the JSON request body when creating
    a session: `POST /api/v1/session`.
 4. Server validates the stamp:
    - Version is `1`
@@ -2195,7 +2189,7 @@ Both the embedded web SPA and the CLI client automatically handle hashcash:
 
 1. Fetch `GET /api/v1/server` to read `hashcash_bits`
 2. If `hashcash_bits > 0`, compute a valid stamp
-3. Include the stamp in the `X-Hashcash` header on `POST /api/v1/session`
+3. Include the stamp in the `hashcash` field of the JSON body on `POST /api/v1/session`
 
 The web SPA uses the Web Crypto API (`crypto.subtle.digest`) for SHA-256
 computation with batched parallelism. The CLI client uses Go's `crypto/sha256`.
diff --git a/internal/cli/api/client.go b/internal/cli/api/client.go
index dc37426..1191d25 100644
--- a/internal/cli/api/client.go
+++ b/internal/cli/api/client.go
@@ -52,7 +52,7 @@ func (client *Client) CreateSession(
 	// Fetch server info to check for hashcash requirement.
 	info, err := client.GetServerInfo()
 
-	var headers map[string]string
+	var hashcashStamp string
 
 	if err == nil && info.HashcashBits > 0 {
 		resource := info.Name
@@ -60,17 +60,13 @@ func (client *Client) CreateSession(
 			resource = "neoirc"
 		}
 
-		stamp := MintHashcash(info.HashcashBits, resource)
-		headers = map[string]string{
-			"X-Hashcash": stamp,
-		}
+		hashcashStamp = MintHashcash(info.HashcashBits, resource)
 	}
 
-	data, err := client.doWithHeaders(
+	data, err := client.do(
 		http.MethodPost,
 		"/api/v1/session",
-		&SessionRequest{Nick: nick},
-		headers,
+		&SessionRequest{Nick: nick, Hashcash: hashcashStamp},
 	)
 	if err != nil {
 		return nil, err
diff --git a/internal/cli/api/types.go b/internal/cli/api/types.go
index 707ae82..7a8714b 100644
--- a/internal/cli/api/types.go
+++ b/internal/cli/api/types.go
@@ -4,7 +4,8 @@ import "time"
 
 // SessionRequest is the body for POST /api/v1/session.
 type SessionRequest struct {
-	Nick string `json:"nick"`
+	Nick     string `json:"nick"`
+	Hashcash string `json:"hashcash,omitempty"`
 }
 
 // SessionResponse is the response from session creation.
diff --git a/internal/handlers/api.go b/internal/handlers/api.go
index 0a3a250..b6821de 100644
--- a/internal/handlers/api.go
+++ b/internal/handlers/api.go
@@ -144,35 +144,9 @@ func (hdlr *Handlers) handleCreateSession(
 	writer http.ResponseWriter,
 	request *http.Request,
 ) {
-	// Validate hashcash proof-of-work if configured.
-	if hdlr.params.Config.HashcashBits > 0 {
-		stamp := request.Header.Get("X-Hashcash")
-		if stamp == "" {
-			hdlr.respondError(
-				writer, request,
-				"hashcash proof-of-work required",
-				http.StatusPaymentRequired,
-			)
-
-			return
-		}
-
-		err := hdlr.hashcashVal.Validate(
-			stamp, hdlr.params.Config.HashcashBits,
-		)
-		if err != nil {
-			hdlr.respondError(
-				writer, request,
-				"invalid hashcash stamp: "+err.Error(),
-				http.StatusPaymentRequired,
-			)
-
-			return
-		}
-	}
-
 	type createRequest struct {
-		Nick string `json:"nick"`
+		Nick     string `json:"nick"`
+		Hashcash string `json:"hashcash,omitempty"`
 	}
 
 	var payload createRequest
@@ -188,6 +162,32 @@ func (hdlr *Handlers) handleCreateSession(
 		return
 	}
 
+	// Validate hashcash proof-of-work if configured.
+	if hdlr.params.Config.HashcashBits > 0 {
+		if payload.Hashcash == "" {
+			hdlr.respondError(
+				writer, request,
+				"hashcash proof-of-work required",
+				http.StatusPaymentRequired,
+			)
+
+			return
+		}
+
+		err = hdlr.hashcashVal.Validate(
+			payload.Hashcash, hdlr.params.Config.HashcashBits,
+		)
+		if err != nil {
+			hdlr.respondError(
+				writer, request,
+				"invalid hashcash stamp: "+err.Error(),
+				http.StatusPaymentRequired,
+			)
+
+			return
+		}
+	}
+
 	payload.Nick = strings.TrimSpace(payload.Nick)
 
 	if !validNickRe.MatchString(payload.Nick) {
diff --git a/web/src/app.jsx b/web/src/app.jsx
index df65c4e..c2eb2db 100644
--- a/web/src/app.jsx
+++ b/web/src/app.jsx
@@ -135,20 +135,22 @@ function LoginScreen({ onLogin }) {
     e.preventDefault();
     setError("");
     try {
-      const extraHeaders = {};
+      let hashcashStamp = "";
       if (hashcashBitsRef.current > 0) {
         setError("Computing proof-of-work...");
-        const stamp = await mintHashcash(
+        hashcashStamp = await mintHashcash(
           hashcashBitsRef.current,
           hashcashResourceRef.current,
         );
-        extraHeaders["X-Hashcash"] = stamp;
         setError("");
       }
+      const reqBody = { nick: nick.trim() };
+      if (hashcashStamp) {
+        reqBody.hashcash = hashcashStamp;
+      }
       const res = await api("/session", {
         method: "POST",
-        body: JSON.stringify({ nick: nick.trim() }),
-        headers: extraHeaders,
+        body: JSON.stringify(reqBody),
       });
       localStorage.setItem("neoirc_token", res.token);
       onLogin(res.nick);