codex: fix silently dropping GenOptionText.SystemPrompt

Forward system prompts as developer instructions when starting or
resuming Codex threads, preserving prompt context across requests.

Author: maruel

providers/codex/client.go

@@ -325,7 +325,7 @@ func (c *Client) GenSync(ctx context.Context, msgs genai.Messages, opts ...genai
 	if err := c.ensureBin(); err != nil {
 		return genai.Result{}, err
 	}
-	_, optsErr := parseOpts(opts)
+	co, optsErr := parseOpts(opts)
 	if optsErr != nil {
 		if _, ok := errors.AsType[*base.ErrNotSupported](optsErr); !ok {
 			return genai.Result{}, optsErr
@@ -352,7 +352,7 @@ func (c *Client) GenSync(ctx context.Context, msgs genai.Messages, opts ...genai
 	}()
 
 	sc := newScanner(stdout)
-	newThreadID, err := handshake(stdin, sc, c.model, threadID)
+	newThreadID, err := handshake(stdin, sc, c.model, threadID, co.systemPrompt)
 	if err != nil {
 		return genai.Result{}, err
 	}
@@ -369,7 +369,7 @@ func (c *Client) GenStream(ctx context.Context, msgs genai.Messages, opts ...gen
 	if err := c.ensureBin(); err != nil {
 		return yieldNothing, errFinish(err)
 	}
-	_, optsErr := parseOpts(opts)
+	co, optsErr := parseOpts(opts)
 	if optsErr != nil {
 		if _, ok := errors.AsType[*base.ErrNotSupported](optsErr); !ok {
 			return yieldNothing, errFinish(optsErr)
@@ -397,7 +397,7 @@ func (c *Client) GenStream(ctx context.Context, msgs genai.Messages, opts ...gen
 		}()
 
 		sc := newScanner(stdout)
-		newThreadID, hsErr := handshake(stdin, sc, c.model, threadID)
+		newThreadID, hsErr := handshake(stdin, sc, c.model, threadID, co.systemPrompt)
 		if hsErr != nil {
 			finalErr = hsErr
 			return
@@ -542,15 +542,18 @@ func initAndListModels(stdin io.Writer, sc *bufio.Scanner) ([]ModelInfo, error)
 
 // handshake performs the JSON-RPC initialize → initialized → model/list →
 // thread/start (or thread/resume) sequence. Returns the thread ID.
-func handshake(stdin io.Writer, sc *bufio.Scanner, mdl, resumeThreadID string) (string, error) {
+func handshake(stdin io.Writer, sc *bufio.Scanner, mdl, resumeThreadID, systemPrompt string) (string, error) {
 	if _, err := initAndListModels(stdin, sc); err != nil {
 		return "", err
 	}
 
 	// Send thread/start or thread/resume.
 	var threadReq JSONRPCRequest
 	if resumeThreadID != "" {
-		params, err := marshalJSONRaw(ThreadResumeParams{ThreadID: resumeThreadID})
+		params, err := marshalJSONRaw(ThreadResumeParams{
+			ThreadID:              resumeThreadID,
+			DeveloperInstructions: systemPrompt,
+		})
 		if err != nil {
 			return "", fmt.Errorf("marshal thread/resume params: %w", err)
 		}
@@ -561,7 +564,10 @@ func handshake(stdin io.Writer, sc *bufio.Scanner, mdl, resumeThreadID string) (
 			Params:  params,
 		}
 	} else {
-		params, err := marshalJSONRaw(ThreadStartParams{Model: mdl})
+		params, err := marshalJSONRaw(ThreadStartParams{
+			Model:                 mdl,
+			DeveloperInstructions: systemPrompt,
+		})
 		if err != nil {
 			return "", fmt.Errorf("marshal thread/start params: %w", err)
 		}

providers/codex/internal_test.go

@@ -7,9 +7,12 @@
 package codex
 
 import (
+	"bufio"
+	"bytes"
 	"encoding/json"
 	"errors"
 	"slices"
+	"strings"
 	"testing"
 	"time"
 
@@ -82,6 +85,37 @@ func TestParseOpts(t *testing.T) {
 	})
 }
 
+func TestHandshake(t *testing.T) {
+	responses := strings.Join([]string{
+		`{"id":1,"result":{}}`,
+		`{"id":2,"result":{"data":[]}}`,
+		`{"id":3,"result":{"thread":{"id":"thread"}}}`,
+	}, "\n")
+	var out bytes.Buffer
+	threadID, err := handshake(&out, bufio.NewScanner(strings.NewReader(responses)), "model", "", "write commit messages")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if threadID != "thread" {
+		t.Errorf("thread ID = %q, want thread", threadID)
+	}
+	lines := strings.Split(strings.TrimSpace(out.String()), "\n")
+	if len(lines) != 4 {
+		t.Fatalf("wrote %d messages, want 4", len(lines))
+	}
+	var req JSONRPCRequest
+	if err := json.Unmarshal([]byte(lines[3]), &req); err != nil {
+		t.Fatal(err)
+	}
+	var params ThreadStartParams
+	if err := json.Unmarshal(req.Params, &params); err != nil {
+		t.Fatal(err)
+	}
+	if params.DeveloperInstructions != "write commit messages" {
+		t.Errorf("developer instructions = %q, want write commit messages", params.DeveloperInstructions)
+	}
+}
+
 func TestJSONRPCMessage(t *testing.T) {
 	t.Run("notification", func(t *testing.T) {
 		var m JSONRPCMessage