feat(ai): add a cheaperinference provider alias

CHANGELOG.md

@@ -7,6 +7,13 @@ separately by `model.SchemaVersion` (currently 1.5.0).
 
 ## [Unreleased]
 
+### Added
+- **Cheaper Inference as an `explain` / `ask` provider.**
+  `PGBOT_AI_PROVIDER=cheaperinference` sends `/chat/completions` to
+  `https://api.cheaperinference.com/v1` with `CHEAPER_INFERENCE_API_KEY` (or
+  `PGBOT_AI_API_KEY`), the same way the `requesty` and `opper` aliases work. It
+  is never auto-detected, and it never picks up an OpenAI or OpenRouter key.
+
 ## [0.9.0] - 2026-10-18
 
 ### Added

README.md

@@ -469,6 +469,7 @@ one SSH connection serves the whole run. Raise `--timeout` if the link is slow.
 | `OPENAI_API_KEY` / `OPENROUTER_API_KEY` | Enables `ask` / `explain` via OpenAI or OpenRouter. Keys are never accepted as flags. |
 | `REQUESTY_API_KEY` | Enables `ask` / `explain` via [Requesty](https://docs.requesty.ai) with `PGBOT_AI_PROVIDER=requesty` (never auto-detected). |
 | `OPPER_API_KEY` | Enables `ask` / `explain` via [Opper](https://docs.opper.ai) with `PGBOT_AI_PROVIDER=opper` (never auto-detected). |
+| `CHEAPER_INFERENCE_API_KEY` | Enables `ask` / `explain` via [Cheaper Inference](https://cheaperinference.com/docs) with `PGBOT_AI_PROVIDER=cheaperinference` (never auto-detected). |
 | `GEMINI_API_KEY` / `GOOGLE_API_KEY` | Enables `ask` / `explain` via Google Gemini. |
 | `ANTHROPIC_API_KEY` | Enables `ask` / `explain` via Anthropic. |
 | `XAI_API_KEY` / `GROK_API_KEY` | Enables `ask` / `explain` via xAI. |
@@ -783,7 +784,7 @@ not require confirmation.
 | Bedrock Mantle | `AWS_BEARER_TOKEN_BEDROCK`, or `AWS_ACCESS_KEY_ID` / `AWS_SECRET_ACCESS_KEY` | `openai.gpt-5.6-terra` | Responses (GPT) / Messages (Claude) |
 
 The OpenAI provider also supports compatible services such as OpenRouter,
-Requesty, Opper, Groq, Together, DeepSeek, Mistral, Ollama, vLLM, and LM Studio.
+Requesty, Opper, Cheaper Inference, Groq, Together, DeepSeek, Mistral, Ollama, vLLM, and LM Studio.
 `PGBOT_AI_PROVIDER=requesty` with `REQUESTY_API_KEY` points it at
 `https://router.requesty.ai/v1`; set `PGBOT_AI_MODEL` to any Requesty model id
 (for example `openai/gpt-4o-mini`), and `PGBOT_AI_BASE_URL` to
@@ -792,6 +793,10 @@ Requesty, Opper, Groq, Together, DeepSeek, Mistral, Ollama, vLLM, and LM Studio.
 `https://api.opper.ai/v3/compat`, an EU-hosted gateway. The default
 `gpt-5.6-terra` works there as is; set `PGBOT_AI_MODEL` to any Opper model id
 (for example `claude-sonnet-4-6`).
+`PGBOT_AI_PROVIDER=cheaperinference` with `CHEAPER_INFERENCE_API_KEY` points it
+at `https://api.cheaperinference.com/v1`. The default `gpt-5.6-terra` works
+there as is; set `PGBOT_AI_MODEL` to any Cheaper Inference model id (for example
+`gpt-5.4-mini`).
 
 ```
 export OPENAI_API_KEY=…

internal/ai/openai.go

@@ -18,6 +18,7 @@ const (
 	defaultOpenRouterURL = "https://openrouter.ai/api/v1"
 	defaultRequestyURL   = "https://router.requesty.ai/v1"
 	defaultOpperURL      = "https://api.opper.ai/v3/compat"
+	defaultCheaperInfURL = "https://api.cheaperinference.com/v1"
 	defaultOllamaURL     = "http://localhost:11434/v1"
 
 	// Applied only to reasoning models. Override with $PGBOT_AI_REASONING_EFFORT

internal/ai/resolve.go

@@ -113,7 +113,7 @@ func Resolve() (LanguageModel, error) {
 			ReasoningEffort: envOr("PGBOT_AI_REASONING_EFFORT", ""),
 		}
 
-	case "openai", "openrouter", "requesty", "opper", "ollama", "openai-compatible":
+	case "openai", "openrouter", "requesty", "opper", "cheaperinference", "ollama", "openai-compatible":
 		// One /chat/completions client serves them all; only the endpoint and the
 		// conventional key variable differ.
 		if key == "" {
@@ -125,6 +125,9 @@ func Resolve() (LanguageModel, error) {
 			case "opper":
 				// Same for Opper.
 				key = firstEnv("OPPER_API_KEY")
+			case "cheaperinference":
+				// Same for Cheaper Inference.
+				key = firstEnv("CHEAPER_INFERENCE_API_KEY")
 			default:
 				key = firstEnv("OPENAI_API_KEY", "OPENROUTER_API_KEY")
 			}
@@ -143,6 +146,8 @@ func Resolve() (LanguageModel, error) {
 				base = defaultRequestyURL
 			case "opper":
 				base = defaultOpperURL
+			case "cheaperinference":
+				base = defaultCheaperInfURL
 			case "ollama":
 				base = defaultOllamaURL
 			case "openai-compatible":
@@ -210,6 +215,8 @@ func keyVarsFor(name string) string {
 		return "REQUESTY_API_KEY"
 	case "opper":
 		return "OPPER_API_KEY"
+	case "cheaperinference":
+		return "CHEAPER_INFERENCE_API_KEY"
 	default:
 		return "OPENAI_API_KEY / OPENROUTER_API_KEY"
 	}

internal/ai/resolve_test.go

@@ -12,7 +12,7 @@ func clearEnv(t *testing.T) {
 	for _, k := range []string{
 		"PGBOT_AI_PROVIDER", "PGBOT_AI_MODEL", "PGBOT_AI_BASE_URL", "PGBOT_AI_API_KEY",
 		"PGBOT_AI_REASONING_EFFORT", "GEMINI_API_KEY", "GOOGLE_API_KEY", "ANTHROPIC_API_KEY",
-		"OPENAI_API_KEY", "OPENROUTER_API_KEY", "REQUESTY_API_KEY", "OPPER_API_KEY", "XAI_API_KEY", "GROK_API_KEY",
+		"OPENAI_API_KEY", "OPENROUTER_API_KEY", "REQUESTY_API_KEY", "OPPER_API_KEY", "CHEAPER_INFERENCE_API_KEY", "XAI_API_KEY", "GROK_API_KEY",
 		"AWS_BEARER_TOKEN_BEDROCK", "AWS_REGION", "AWS_DEFAULT_REGION",
 		"PGBOT_GEMINI_MODEL", "PGBOT_GEMINI_URL", "PGBOT_OPENAI_MODEL", "PGBOT_OPENAI_URL",
 	} {
@@ -315,6 +315,40 @@ func TestResolve_opper(t *testing.T) {
 	}
 }
 
+// cheaperinference is an explicit alias only: it goes to Cheaper Inference with
+// CHEAPER_INFERENCE_API_KEY or PGBOT_AI_API_KEY, never borrows an OpenAI key,
+// and is never auto-detected.
+func TestResolve_cheaperinference(t *testing.T) {
+	for _, keyVar := range []string{"CHEAPER_INFERENCE_API_KEY", "PGBOT_AI_API_KEY"} {
+		clearEnv(t)
+		t.Setenv("PGBOT_AI_PROVIDER", "cheaperinference")
+		t.Setenv(keyVar, "k")
+		m, err := Resolve()
+		if err != nil {
+			t.Fatalf("cheaperinference with %s: %v", keyVar, err)
+		}
+		if m.Provider() != "openai" {
+			t.Errorf("cheaperinference with %s: provider = %q, want openai", keyVar, m.Provider())
+		}
+		if m.Endpoint() != defaultCheaperInfURL {
+			t.Errorf("cheaperinference with %s: endpoint = %q, want %q", keyVar, m.Endpoint(), defaultCheaperInfURL)
+		}
+	}
+
+	clearEnv(t)
+	t.Setenv("PGBOT_AI_PROVIDER", "cheaperinference")
+	t.Setenv("OPENAI_API_KEY", "sk-openai")
+	if _, err := Resolve(); err == nil || !strings.Contains(err.Error(), "CHEAPER_INFERENCE_API_KEY") {
+		t.Errorf("cheaperinference with only OPENAI_API_KEY should ask for CHEAPER_INFERENCE_API_KEY, got: %v", err)
+	}
+
+	clearEnv(t)
+	t.Setenv("CHEAPER_INFERENCE_API_KEY", "k")
+	if _, err := Resolve(); err == nil {
+		t.Error("CHEAPER_INFERENCE_API_KEY alone must not be auto-detected")
+	}
+}
+
 // A custom remote endpoint must come with a named provider. Auto-detecting one
 // from an ambient vendor key would send that key — say the ANTHROPIC_API_KEY
 // another tool left in the shell — to whatever host PGBOT_AI_BASE_URL names,