diff --git a/go.mod b/go.mod
index 82468e12..131e1336 100644
--- a/go.mod
+++ b/go.mod
@@ -5,7 +5,7 @@ go 1.26.1
 require (
 	charm.land/bubbles/v2 v2.0.0
 	charm.land/bubbletea/v2 v2.0.2
-	charm.land/fantasy v0.16.0
+	charm.land/fantasy v0.17.1
 	charm.land/huh/v2 v2.0.3
 	charm.land/lipgloss/v2 v2.0.2
 	github.com/alecthomas/chroma/v2 v2.23.1
diff --git a/go.sum b/go.sum
index cb3dbb36..a8cd579a 100644
--- a/go.sum
+++ b/go.sum
@@ -2,8 +2,8 @@ charm.land/bubbles/v2 v2.0.0 h1:tE3eK/pHjmtrDiRdoC9uGNLgpopOd8fjhEe31B/ai5s=
 charm.land/bubbles/v2 v2.0.0/go.mod h1:rCHoleP2XhU8um45NTuOWBPNVHxnkXKTiZqcclL/qOI=
 charm.land/bubbletea/v2 v2.0.2 h1:4CRtRnuZOdFDTWSff9r8QFt/9+z6Emubz3aDMnf/dx0=
 charm.land/bubbletea/v2 v2.0.2/go.mod h1:3LRff2U4WIYXy7MTxfbAQ+AdfM3D8Xuvz2wbsOD9OHQ=
-charm.land/fantasy v0.16.0 h1:vE/6sR9nPcSD8qXJXX6wR8NXjtWlBVAzwQmTh5pHVrs=
-charm.land/fantasy v0.16.0/go.mod h1:VZjpXVh7IgeiIzGQybEnKzd68ofDsRj94+kzH1ZCAfQ=
+charm.land/fantasy v0.17.1 h1:SQzfnyJPDuQWt6e//KKmQmEEXdqHMC0IZz10XwkLcEM=
+charm.land/fantasy v0.17.1/go.mod h1:FF5ALCCHETacHJPBqU42CtwMInYQ0ul52fdzIHQMbQk=
 charm.land/huh/v2 v2.0.3 h1:2cJsMqEPwSywGHvdlKsJyQKPtSJLVnFKyFbsYZTlLkU=
 charm.land/huh/v2 v2.0.3/go.mod h1:93eEveeeqn47MwiC3tf+2atZ2l7Is88rAtmZNZ8x9Wc=
 charm.land/lipgloss/v2 v2.0.2 h1:xFolbF8JdpNkM2cEPTfXEcW1p6NRzOWTSamRfYEw8cs=
diff --git a/internal/models/providers.go b/internal/models/providers.go
index 43fe6eef..5e6d0da0 100644
--- a/internal/models/providers.go
+++ b/internal/models/providers.go
@@ -723,12 +723,45 @@ func createOpenAICodexProvider(ctx context.Context, config *ProviderConfig, mode
 		return nil, fmt.Errorf("failed to create OpenAI Codex model: %w", err)
 	}
 
-	// Build provider options for OpenAI Responses API reasoning models.
-	providerOpts := buildOpenAIProviderOptions(config, modelName)
+	// Build provider options for Codex API with system prompt as Instructions
+	providerOpts := buildCodexProviderOptions(config, modelName)
 
 	return &ProviderResult{Model: model, ProviderOptions: providerOpts}, nil
 }
 
+// buildCodexProviderOptions returns fantasy.ProviderOptions configured for
+// OpenAI Codex API. The Codex API requires the system prompt to be passed
+// as 'instructions' rather than as a system message.
+func buildCodexProviderOptions(config *ProviderConfig, modelName string) fantasy.ProviderOptions {
+	store := false
+	opts := &openai.ResponsesProviderOptions{
+		Store: &store,
+	}
+
+	// Set system prompt as Instructions (required for Codex API)
+	if config.SystemPrompt != "" {
+		opts.Instructions = &config.SystemPrompt
+	}
+
+	// For reasoning models, add reasoning options
+	if openai.IsResponsesReasoningModel(modelName) {
+		reasoningSummary := "auto"
+		opts.ReasoningSummary = &reasoningSummary
+		opts.Include = []openai.IncludeType{
+			openai.IncludeReasoningEncryptedContent,
+		}
+
+		// Map ThinkingLevel to OpenAI ReasoningEffort
+		if effort := thinkingLevelToReasoningEffort(config.ThinkingLevel); effort != nil {
+			opts.ReasoningEffort = effort
+		}
+	}
+
+	return fantasy.ProviderOptions{
+		openai.Name: opts,
+	}
+}
+
 // createCodexHTTPClient creates an HTTP client with headers required for ChatGPT/Codex API
 func createCodexHTTPClient(token, accountID string, skipVerify bool) *http.Client {
 	var base http.RoundTripper